Booklet 50_clips

3
Eyespot disease of small grains
50 Video Clips
on Fungal Diseases of Cereals
THE BIOLOGY OF FUNGAL PATHOGENS
Department of Plant Pathology, Christian-Albrechts University,Kiel
Prof. Dr. Joseph-Alexander Verreet, Dr. Holger Klink (eds.)
Bunt and smut diseases of cereals
Barley leaf spots
Leaf rust and other rusts of cerealsPowdery mildew of wheat
Fusarium diseases of wheat
Tan spot of wheatSeptoria leaf blotch of wheat
DVD-ROM
for Mac and
Windows PC
With the kind support of
DVD-ROM
for Mac and
Windows PC
Stiftung
Schleswig-Holsteinische
Landschaft
Editors-in-Chief
Prof. Dr. Joseph-Alexander Verreet,
Dr. Holger Klink
Department of Plant Pathology,
Christian-Albrechts University, Kiel
Hermann-Rodewald-Str. 9,
D-24118 Kiel, Germany
Phone: +49.431.880.2996
or +49.431.880.4586
Fax: +49.431.880.1583
E-mail: javerreet@phytomed.uni-kiel.de
or hklink@phytomed.uni-kiel.de
Video Production and Design
STUMM-FILM Dr. Rolf Stumm
Medien GmbH, Martin-Luther-Str.55,
D-71636 Ludwigsburg, Germany
E-mail: info@stummfilm.de
www.stummfilm.de
Reordering Address
The American Phytopathological Society
3340 Pilot Knob Road
St. Paul, Minnesota 55121
United States of America
Phone: +1.651.454.7250;
Fax: +1.651.454.0766
www.apsnet.org
© 2008
Institut für Phytopathologie
der Christian-Albrechts-Universität Kiel
& STUMM-FILM Dr. Rolf Stumm
Medien GmbH, Ludwigsburg
Terms of use
This Digital Video Clip Collection is protected by
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Booklet_50_Clips_Final.qxp 06.03.2008 10:51 Seite 2

32
Preface
Using The Biology of Fungal Pathogens: 50 Video Clips on Fungal Diseases of Cereals,
edited by Dr. Joseph-Alexander Verreet and Dr. Holger Klink, University of Kiel, will be a
delight for both teacher and student.The wonder of biology is clearly evident in these com-
pelling video clips – students will be both fascinated and educated.
This clip collection is a treasure trove for the teacher who understands that many differ-
ent media are necessary to keep students engaged and learning. These illustrations will
lock in a student’s understanding of material studied in textbooks by giving them a par-
ticularly vivid visual demonstration of structures and their functions. The interactions
between fungi and plants come alive via the artistry of these clips.
Although the clips may certainly be used to teach details of specific important diseases of
cereal crops, they are also especially desirable for use in general biology classes, as well
as for classes in plant pathology and mycology. Cooperative Extension educators will be
able to use these clips as highlights in their presentations to commercial growers and in
Master Gardener training.
This booklet will allow you to choose the perfect illustrations of topics you would like to get
across to your students: formation of sexual and asexual fungus spores, fungal attacks on
plants, epidemic development, haustorium formation, and disease resistance, for example.
The voice-over narration gives the story; replays without the soundtrack can be used to
emphasize the points appropriate to the individual instructor’s purposes. By so vividly illus-
trating interesting stories on fungal biology, these clips will greatly help students to grasp
complex principles and new scientific vocabulary.
Margery Daughtrey
Editor-in-Chief, APS PRESS
The fifty short movie clips on this DVD-ROM were taken from eight videos in the DVD series
“The Biology of Fungal Pathogens” edited by Prof. Dr. Joseph-Alexander Verreet and
Dr. Holger Klink, Christian-Albrechts University, Kiel. So far, four DVD volumes are available
through APS PRESS. Each volume includes one to three teaching videos focusing on major
fungal diseases of cereals:
Vol. 1: Fungal pathogens and diseases of cereals (1)
– Septoria leaf blotch of wheat (8:30 min)
– Tan spot of wheat (8:30 min)
– Powdery mildew of wheat (10:50 min)
– Leaf rust and other rusts of cereals (15:30 min)
– Fusarium diseases of wheat (10:50 min)
– Barley leaf spots – fungal disease or stress response? (14:00 min)
– Eyespot disease of small grains (13:00 min)
– Bunt and smut diseases of cereals (10:00 min)
How to use the clips
On the DVD-ROM there is a folder named “50_clips”. It contains fifty video clips encoded
in the MPEG-1 format, one of the most compatible video compression formats available;
it is playable in almost all computers (PC and Mac). Simply ‘drag-and-drop’ clips from the
DVD-ROM to your systems's hard drive to incorporate them into Microsoft PowerPoint™
presentations, for example.
Please note: when you insert an mpg file into a PowerPoint™ presentation, you will need
to keep the original mpg file with the PowerPoint file in order to play the video within the
PowerPoint presentation successfully. On Macs, always keep the PowerPoint file along with
the mpg file in one folder so that they can be moved together. With Windows, it may be
easiest to place the PowerPoint and the mpg files in the same directory before the video
is inserted into the presentation. Always transfer the original mpg file along with the
PowerPoint file in the same relative directories or folders if you are moving your presenta-
tion to a different computer – otherwise the video will not run and the presentation may crash.
Where the clips come from

54
Clips available on the DVD-ROM
Clips from “Septoria leaf blotch of wheat”
Clip_01 Ascogonium and antheridium
Clip_02 Ascus formation
Clip_03 Ascospore release
Clip_04 Infection by ascospores
Clip_05 Pycnidia
Clip_06 Infection by pycnospores
Clips from “Tan spot of wheat”
Clip_07 Pseudothecia
Clip_08 Conidiospores
Clip_09 Infection by ascospores
Clip_10 Drechslera tritici-repentis and Drechslera teres
Clip_11 Infection by DTR conidiospores
Clips from “Powdery mildew of wheat”
Clip_12 Primary pustules
Clip_13 Host infection
Clip_14 Conidia formation
Clip_15 Epidemic
Clip_16 Resistance to mildew
Clip_17 Mildew’s genetic adaptability
Clip_18 Teleomorph
Clips from “Leaf rust and other rusts of cereals”
Clip_19 Reproductive cycle of urediniospores
Clip_20 Formation of teliospores
Clip_21 Formation of basidiospores
Clip_22 Formation of pycniospores and aeciospores
Clip_23 Complete life cycle (summary)
Clip_24 Brown rust of rye, barley leaf rust, stem rust of wheat
Clip_25 Stripe or yellow rust
Clips from “Fusarium diseases of wheat”
Clip_26 Seedling infection
Clip_27 Foot rot
Clip_28 Formation and spread of conidia
Clip_29 Formation and spread of ascospores
Clip_30 Infection of heads
Clip_31 Mycotoxins
Clips from “Barley leaf spots – fungal disease or stress response?”
Clip_32 Barley leaf spots (an overview)
Clip_33 The fungus Ramularia collo-cygni
Clip_34 Fungal infection of the host plant
Clip_35 Physiological leaf spots (PLS)
Clip_36 The plant’s antioxidant protection system
Clip_37 Oxidative stress
Clip_38 Antioxidant effects of fungicides
Clips from “Eyespot disease of small grains”
Clip_39 Conidia formation
Clip_40 Conidial spread and germination
Clip_41 Host infection
Clip_42 Disease development
Clip_43 Genetic recombination
Clip_44 Formation and spread of ascospores
Clips from “Bunt and smut diseases of cereals”
Clip_45 Common bunt of wheat
Clip_46 Contamination of healthy grains
Clip_47 Germination of bunt spore
Clip_48 Seedling infection
Clip_49 Loose smut of barley
Clip_50 Embryo infection

7
“The fungus Septoria tritici causes leaf blotch of wheat,
and can reduce harvest yields by as much as 50%.
After harvest, the fungus survives the winter in the
stubble. There, the sexual form – or teleomorph – of
Septoria tritici develops: Mycosphaerella graminicola.
The fungus germinates, and within a short time its
mycelium permeates the dry plant tissue.
From sexually differently determined mycelia the female
gametangium, the ascogonium develops. It contains
numerous cell nuclei. The male gametangium, the
antheridium, develops from another mycelium. It too is
full of nuclei.The ascogonium forms a receptive hypha,
the trichogyne, which is necessary for fertilization. Via
the trichogyne, the male cell nuclei migrate into the
ascogonium. Male and female nuclei pair up, but do not
yet fuse.”
Running time: 1:08 min
6
Clip_01
Ascogonium and antheridium
Septoria leaf blotch of wheat
“Hook-shaped tubes – or ‘croziers’ – grow out of the
ascogonium. Each pair of nuclei divides synchronously
in the ultimate cell of the crozier. Only now can fusion
of the nuclei take place. In the subsequent reduction
division or meiosis, the chromosome set is halved.
Haploid nuclei result. In this way the genetic variability
of the fungus is increased.The new genetic variants are
multiplied. Two groups of four genetically identical
nuclei develop. In ascus parent cells they develop into
spores – the sexual ascospores. They lie side by side
in spore cases, the asci, enveloped by an ascocarp.
This type of fruiting body, open at the top, is termed a
pseudothecium.The sexual reproduction spores remain
in it until weather conditions are favorable.”
Clip_02
Ascus formation

9
“Through a slit-like opening, the stoma, the germinal
hypha penetrates into the interior of the leaf. So far,
there are no visible symptoms of the disease. The
young plants that are affected look lush and green.
But subsequently, the products of metabolism of the
developing fungal mycelium kill more and more of the
leaf cells. The cells collapse, and the chlorophyll –
necessary for photosynthesis – is destroyed.As a result,
the leaves become increasingly yellow and later
brown.”
Clip_04
Infection by ascospores
8
“Rain falls, or the relative humidity is high. The asci
swell. The increased osmotic pressure causes the asci
to burst.The spores are ejected into the air. In this way,
Mycosphaerella graminicola spores are carried by the
wind from plant to plant and from one field to the next.
Thus young wheat plantations are infected by the asco-
spores at an early stage. Only if the leaves stay moist
for a prolonged period will the ascospore germinate
and form a germ tube.”
Clip_03
Ascospore release

11
“For successful infection of the new leaves, leaf humid-
ity must exceed 98% for at least 48 hours. Only under
these optimal conditions can the pycnospores germi-
nate and form a germ tube. With a germinal hypha,
Septoria tritici penetrates into the substomatal cavity
and the intercellular spaces. An extensive mycelium
develops in the interior of the leaves.”
Clip_06
Infection by pycnospores
10
“In this necrotic leaf tissue, the pycnidia develop, asex-
ual fruiting bodies of the anamorph, Septoria tritici. In
the pycnidia, asexual pycnospores have formed. Water
uptake causes them to swell. During dry weather, the
surrounding tissue shrinks faster than the pycnidia.This
mechanism causes the pycnospores – in the form of
mucilaginous tendrils – to be pressed out of the opening
in the pycnidium, the ostiolum.
The impact of raindrops breaks up the tendrils. The
pycnospores contained in the splashes of rain are cata-
pulted onto adjacent leaves and those directly above.
In this way the infection reaches further tiers of leaves.”
Clip_05
Pycnidia

13
“As soon as the sexual ascospores have left the
pseudothecium, asexual spores often develop directly
on the empty fruiting body, between its setae: conidio-
spores of the anamorph or asexual stage Drechslera
tritici-repentis. Unlike the heavy ascospores of the
sexual form, these conidiospores are highly mobile.
They are easily blown off by the wind and carried to
neighboring wheat fields – and thus to their primary
host.”
Clip_08
Conidiospores
Tan spot of wheat
12
“A young planting of winter barley in early April.Among
the young barley plants there are still some residues of
the previous crop that has been plowed under – wheat.
On these stubble residues, magnification reveals dark
fruiting bodies topped by a ring of setae. This is the
teleomorph – that is, the sexual reproductive stage – of
the DTR tan spot pathogen Pyrenophora tritici-repentis.
In this sexual stage, ascospores develop.
If moisture in the form of rain or dew now penetrates
into the fruiting body – the pseudothecium – which is
open at the top, the mature ascospores are actively re-
leased.These very large ascospores are usually carried
only a few centimeters. However, the young barley
plants in the field are not a host for this fungus, which
is primarily specific to wheat. Only couch grass – along
with some other grasses – may also act as an alterna-
tive host.”
Clip_07
Pseudothecia
Tan spot of wheat

1514
“Other fungal diseases of wheat often show very similar
leaf necroses. For this reason, Drechslera tritici-repentis
can only be diagnosed reliably on the basis of its char-
acteristic conidia, which form on the necroses. With
sufficient magnification it can be seen that each conid-
ium rests on a separate carrier cell. The DTR conidia
are often slightly curved and, typically, long.
By contrast, the conidiospores of Drechslera teres – the
causal agent of net blotch of barley – are noticeably
more squat and rounded at both ends.
Net blotch, a disease of major economic importance, is
characterized by a brown reticular pattern of the necro-
ses, surrounded by longish, lighter yellow regions.
Besides this more common ‘net type’ of the disease,
there is also a second, less common form, known as
the ‘spot type’. These two types of infection are seen
almost exclusively in barley.”
Clip_10
Drechslera tritici-repentis and Drechslera teres
Tan spot of wheat
“However, where agriculture is characterized by mini-
mum tillage and single-crop wheat farming, there is a
potential risk of Drechslera tritici-repentis causing
serious or even catastrophic tan spot epidemics,
because wheat stubble is an ideal substrate for the
pseudothecia.
In early spring countless ascospores set out on their
short journey. When the subsequent crop – as in this
case – is also wheat, most of the spores find a suitable
host in the immediate neighborhood. In warm, humid
weather the ascospores germinate rapidly. The germ
tube attaches itself to the leaf surface with an appres-
sorium. From this point, intracellular hyphae penetrate
the interior of the leaf, the mesophyll. The developing
mycelium releases toxic substances that kill the green
vegetative tissue. At the site of infection, the effect of
these mycotoxins manifests itself first as a small black
spot around which a yellowish halo forms – a chlorosis.
Finally, the infected leaf tissue dies.”
Clip_09
Infection by ascospores
Tan spot of wheat
net typenet type
spot typespot type
DD. ter. teresesDTRDTR

1716
“The basis for a mildew epidemic is created in the early
stages of a plant’s development. Often, a high percent-
age of the young plants are already infected prior to
bolting. However, the level of disease intensity is still
low: only one or two mildew pustules per leaf. On these
primary pustules, at higher magnification, typical spore
chains can be detected – the anamorph or asexual
stage, Oidium monilioides. Each spore chain consists of
eight conidiospores. The terminal spore is the first to
mature. It’s detached by the wind and carried away.”
Clip_12
Primary pustules
Powdery mildew of wheat
“In particular in warm, dry weather, the conidiospores
of Drechslera tritici-repentis are readily detached.Aided
by air currents, the pathogen passes rapidly from the
lower to the upper leaves, which at this stage are espe-
cially active, supplying the assimilates required for
grain fill. For this reason, infection of the upper leaves
has a highly negative impact on the grain yield.
Following infection, it takes only about a week for a new
generation of conidia to reach maturity. So a typical
characteristic of DTR proliferation is horizontal spread
through the crop, over long phases of vegetation. As
development of the grain progresses, the number of
conidia increases exponentially – and with it the likeli-
hood of infection.”
Clip_11
Infection by DTR conidiospores
Tan spot of wheat

1918
“With secondary hyphae, the fungus now infects the
neighboring host cells. Being an ectoparasite, powdery
mildew forms its mycelium on the outside of the plant.
Its conidia develop from an onion-shaped parent cell.
This grows in length and forms a transverse wall, thus
isolating the tip of the hypha. By multiple repetition of
this process a chain of spores is created – asexual
conidia, which gradually mature. Borne by the wind, the
conidia can cover long distances. The result is a slowly
increasing, horizontal spread of the pathogen through-
out the entire wheat planting.”
Clip_14
Conidia formation
“If this windborne spore makes contact with a new
host, it rapidly germinates, especially in hot and humid
weather. The short germ tube attaches itself with its
appressorium to the surface of the leaf. From this
adhesive organ, an infection hypha penetrates the wall
of the host cell. This hypha releases enzymes, which
facilitate the passage of the hypha through the cuticle
and the outer cell wall. With a mechanical pressure of
more than 20 kilopascals the infection hypha indents the
inner wall layer of the host cell. From this indentation,
a membrane develops which covers the haustorium like
a sheath. Via this nutritional organ the fungus obtains
nourishment directly from the host plant. The infected
cell remains vital.”
Clip_13
Host infection

2120
“In the selection of new wheat varieties, resistance to
mildew is an important goal. Mildew-resistant varieties
differ from mildew-prone varieties in having certain
resistance genes in their genome. So far, around twenty
such resistance genes to powdery mildew are known.
They trigger biologic defense reactions in the plant,
which prevent the mildew pathogen from reproducing.
One such defense reaction, for instance, is the ‘sacrifice’
of healthy cells in the tissue immediately surrounding
the site of infection. With this hypersensitive defense
response, the challenged plant deprives the pathogen
of the nutrients it requires to survive: the obligate bio-
trophic parasite finds no more nourishment and dies.
This is referred to as a defense necrosis.”
Clip_16
Resistance to mildew
“The weather has an overriding influence on the onset,
course, and severity of mildew infestation.
Warm weather promotes spore production. Likewise,
high relative humidity, though without rain, encourages
the development of epidemics. That’s why river mead-
ows and mist-prone valleys tend to be prime mildew
locations.
Some agronomic practices, such as excessive nitrogen
fertilization, also contribute to the development of pow-
dery mildew. The plants grow tall faster, the leaf density
in the canopy increases: a warm, moist microclimate
develops – an ideal environment for the fungus.
If the basic conditions for powdery mildew are favorable,
the relatively long phase of gradual horizontal spread
ends abruptly, and is followed by rapid spread of the
pathogen to upper levels of the plants. This very quick-
ly leads to the full-blown form of a mildew epidemic –
with massive infection of the upper leaves which are
important for yield, and even of the ears themselves.”
Clip_15
Epidemic

2322
“In the adaptation of mildew to constantly changing
environmental conditions – and these include the use
of fungicides – the sexual unification of compatible
mycelia plays a key role. The nuclei of the two partner
cells fuse. Fertilization is followed by several divisions,
which recombine the genetic material in many different
ways. Thus virulence genes also are constantly varied
– and new races are created.
This sexual recombination is the cause of the large
genetic variety of the ascospores of powdery mildew.
The ascospores of Blumeria graminis – the teleomorph,
or sexual stage of the fungus – mature in fruiting
bodies which form on older leaves in the summer.
These spherical cleistothecia have characteristic attach-
ments known as appendices. Initially, the fruiting bodies
are hyaline to pink; later they turn brown-and-black.”
Clip_18
Teleomorph
“However, mildew also exploits its genetic variability.
Because powdery mildew of wheat occurs in numerous
physiological strains – known as races – which differ from
one another due to the presence of specific virulence
genes. One cause of the high genetic variability of these
races is the exchange of genetic information between
individual races.
Another cause is mutation. Several different types of
mutations can occur, such as base-pair substitution. So
it’s only a question of time before a newly cultivated
mildew-resistant wheat variety is confronted with a race
that has corresponding virulence genes against that
variety, and thus overcomes its resistance barriers.”
Clip_17
Mildew’s genetic adaptability

2524
“Toward the end of the vegetative period,on rust-infected
wheat leaves, and usually on the underside of the leaf,
a second spore type forms: With these teliospores the
sexual stage in the life cycle of Puccinia recondita
begins.
A young, still immature teliospore: it consists of two
cells.The cells each contain a pair of sexually compatible
nuclei, that is, they are dikaryotic. The two nuclei carry
different genetic information. In the process of matura-
tion of the teliospores, each pair of nuclei merges to
form a single, diploid nucleus: a nucleus with a double
chromosome set. Following this nuclear fusion, or
karyogamy, the spore wall thickens and becomes dark-
colored. Usually, this maturation process of the telio-
spores is completed by harvest time. On stubble resi-
dues, the teliospores survive the dormancy period, the
fall and the winter.”
Clip_20
Formation of teliospores
Leaf rust and other rusts of cereals
“The urediniospores are produced asexually. Like the
cells of the mycelium, each urediniospore has two
nuclei. During maturation, the spore walls thicken and
take on their characteristic structure and color. The
mature urediniospores are easily detached and carried
to other wheat leaves by the wind. Here they remain
until dew or rain wets the leaf followed by a warm night.
Under these optimal conditions, the urediniospores ger-
minate rapidly. The germ tube grows toward a stoma, a
slit-like opening in the leaf. Over the stoma an adhesive
organ now forms: From this appressorium, a penetra-
tion hypha grows into the cavity beneath the stoma. In
the cavity, the tip of the hypha dilates. From this vesicle
new hyphae emerge, forming a mycelium that grows
between the plant’s cells.With small, spherical organs,
the haustoria, the parasite now extracts the nutrients it
needs for its own development from the surrounding
plant cells. But the infected host cells stay alive.
In warm weather, it takes just one week following infec-
tion for a new uredinium to break open on the leaf sur-
face. This pustule in turn releases countless uredinio-
spores which the wind spreads through the crop and
far beyond it: often over entire regions, countries, even
continents.
From the moment of infection through formation of the
mycelium inside the leaf, the asexual development of
urediniospores to the bursting open of new uredinia fol-
lowed by spore distribution, the reproductive cycle of
the urediniospores is repeated several times while the
wheat is growing.This explains the exponential increase
in the severity of infection, particularly when rust-prone
varieties are grown during warm weather. ”
Clip_19
Reproductive cycle of urediniospores

2726
“A young meadow rue plant is infected by a basidio-
spore.After some time a fungal organ called a pycnium,
or spermagonium, develops. If the infecting basidio-
spore was of the ‘+’ mating type, all of the cells in this
pycnium are likewise of the ‘+’ type. Other pycnia of
the ‘–’ mating type may also develop, originating from
a basidiospore of this second mating type.
In the interior of the pycnia, minute haploid pycnio-
spores are produced. The pycniospores are exuded in
a sticky drop of honeydew.The honeydew lures insects,
to which the pycniospores adhere. It’s primarily in this
way that pycniospores of the ‘+’ type are transferred
to pycnia of the ‘–’ type – and vice versa.
The pycniospores thus transferred are taken up by spe-
cial hyphae – the ‘receptive hyphae’. The cytoplasm of
the two partner cells merges. This is plasmogamy, the
beginning of fertilization. The nucleus of the pycnio-
spore migrates into the receptive hypha and sub-
sequently – inside strands of mycelium – deep into the
mesophyll of the meadow rue’s leaf. Here the nucleus
reproduces itself many times over. Two nuclei of each
mating type pair up. These cells, which are now di-
karyotic, become parent cells for the aeciospores of
leaf rust. The aeciospores develop in the form of
chains. They are produced within an open, cup-like
organ, the aecium.”
Clip_22
Formation of pycniospores and aeciospores
“In spring, and especially in regions with warm climates,
the teliospores germinate.The germ tube bends, form-
ing a metabasidium. The diploid nucleus migrates into
it. In two reduction divisions – or meiosis – the genetic
information is recombined, resulting in four haploid
nuclei. They are in two pairs, belonging to the ‘+’ and
‘–’ mating types respectively.
Now the metabasidium divides into four cells. On each
of them a sterigma emerges.The nuclei migrate through
it into the developing basidiospores. As a complete
entity, this fungal organ is designated a basidium.
Similar basidia are found in most mushrooms. For this
reason, they are classified together with the rusts and
smuts as Basidiomycetes.
The mature basidiospores of Puccinia recondita are
forcibly ejected. However, these haploid basidiospores
of leaf rust do not infect the young wheat plants;
because Puccinia recondita is a heteroecious parasite,
which requires an appropriate alternate host to com-
plete its life cycle. The primary such host for leaf rust
of wheat is Thalictrum, or meadow rue.”
Clip_21
Formation of basidiospores

2928
“Rye also is often affected by leaf rust. In fact in many
regions, leaf rust is the most important disease of rye.
Rye leaf rust is caused by the forma specialis recondita
of Puccinia recondita. In order to complete its life cycle,
this fungus also needs an appropriate alternate host –
asperifoliate plants like Anchusa or Echium.
Leaf rust of barley can likewise cause significant yield
losses.The uredinia are typically very small. Barley leaf
rust is caused by Puccinia hordei.
Stem rust – caused by Puccinia graminis – is another
important cereal disease, especially in warm climates.
Stem rust is also known as black rust, because of the
black telia filled with teliospores. These telia form on
stems and leaf sheaths at the end of the vegetation
period. The alternate host in this case is common bar-
berry.”
Clip_24
Brown rust of rye, barley leaf rust, stem rust of wheat
“After being released, the aeciospores are carried by
the wind back to the other host, the cereal. This is
where the dikaryotic aeciospore germinates.The infec-
tion leads to the development of dikaryotic mycelium,
from which, in turn, dikaryotic urediniospores and –
later on in the year – teliospores originate.The dikaryo-
tic phase ends with the basidiospores, which are once
again haploid. So the complete life cycle of leaf rust
includes five spore stages – Puccinia recondita is
macrocyclic.”
Clip_23
Complete life cycle (summary)

3130
“Large quantities of plant residues that are infested
with plant-pathogenic species of Fusarium remain in
the field after the harvest. If this crop debris isn’t covered
with soil there will be plenty of nutrition for these patho-
gens since they can survive as saprophytes.
In the soil, many Fusarium species form thick-walled
resting spores. With these chlamydospores, they can
survive underground for years. When wheat is sown in
a seedbed infested with these pathogens, development
of the seedlings is at risk. The delicate seed leaf – the
cotyledon – is enclosed in the firmer germ sheath, the
coleoptile. However, the coleoptile does not provide suf-
ficient protection against the Fusarium mycelium.
The roots of the seedling also are an attractive target
for these pathogens. Once the hyphae have penetrated
into the root cells, the seedling usually dies – either
while still underground or soon after emerging. This
results in areas with only slight or patchy emergence
of the seed.”
Clip_26
Seedling infection
“The characteristic feature of stripe – or yellow – rust
is that the bright yellow uredinia are arranged in rows.
Stripe rust is caused by Puccinia striiformis. In contrast
to leaf rust, stripe rust prefers a moist, cool climate. It
infects cereals, and above all wheat, early in the year,
long before emergence of the heads. A warning for the
farmer are nest-like lighter areas in the field - foci of
early stripe rust infection. From here the epidemic will
spread throughout the crop.
In advanced stages, the heads are commonly affected.
Toward the end of the vegetation period, stripe rust pro-
duces in addition to urediniospores teliospores from
which, later, basidiospores develop. However, there is
no known alternate host for these basidiospores, so
they have no known biological function. The uredinio-
spores survive by infecting volunteers in fall and then
passing the winter as a mycelium or as uredinia in the
young seed. Being a biotroph, stripe rust – like the
other rusts – has to have a green host plant.”
Clip_25
Stripe or yellow rust

3332
“These necroses provide Fusarium species with ideal
conditions for sporulation.Their asexually formed conidia
are usually spindle- or sickle-shaped, like these macro-
conidia of Fusarium culmorum.Along with the cells that
produce them, the conidia form a cushion-like cluster
– a sporodochium.
Development of the conidia, especially in hot, humid
conditions, usually is complete by the time heading and
flowering occur. Now, if there’s prolonged rainfall,
splashes of rain that hit the sporodochia carry the
mature conidia from the stem base to higher leaves –
right up to the flowering head.The pollen sacs suspended
from the florets, the anthers, are particularly rich in
nutrients. In warm, rainy weather spores adhering to
the anthers germinate rapidly.”
Clip_28
Formation and spread of conidia
“Even for plants that are still undamaged at the tillering
stage, Fusarium species continue to pose a threat,
especially in dry soil. From now on, the crown roots are
a major gateway through which soil-borne Fusarium
species enter the plant. The mycelium advances into
the crown node and from there, on into the stem base
restricting the flow of nutrients from the roots into the
above-ground parts of the plant. The weakened plants
turn brown – first on the leaf sheaths, then in the lower
parts of the stem. The lesions at the base of the plant
accelerate necrotization of the lower leaves.”
Clip_27
Foot rot

3534
“Starting from the anthers, the hyphae often develop
toward the filaments, to which the anthers are attached
and along these, penetrate deep into the interior of the
wheat floret – passing the remnants of the stigma and
the style. Meanwhile, development of the kernel has
already begun.
From an infected filament, hyphae can spread to the
seed coat, the pericarp. The hyphae penetrate into the
outer cells of the endosperm. Right now, in the weeks
after flowering, the endosperm is being filled intensively
with assimilates from the leaves.
Fusarium species release a number of toxic substances
from their hyphae.These mycotoxins can facilitate colo-
nization of the plant tissue by the fungus – in this case
the starch-containing storage cells.
Increasingly, the mycelium also penetrates into the inte-
rior of the central axis – the rachis. This is where the
vascular system is located that carries water and
nutrients to the spikelets.The intruding mycelium inter-
rupts the flow of nutrients: the affected spikelets are
starved. They bleach out – and die prematurely. Within
the rachis, the fungus usually proliferates downward.
And thus, gradually, the characteristic picture of head
blight evolves.
Finally, on the glumes, and in particular on their edges,
a salmon-colored spore coating develops.”
Clip_30
Infection of heads
“The complete life cycle of Fusarium graminearum
includes, besides the asexual conidial stage – the ana-
morph – a sexual stage during which the ascospores
are formed – the teleomorph Gibberella zeae. The
ascospores mature in a fruiting body open at the top,
a perithecium. The perithecia are found primarily on
harvest residues, like this corn debris.
If perithecia are wetted by rain the tube-like asci swell.
As they are ejected from bursting asci, the ascospores
are taken up by air currents. Thus they’re carried
directly to the heads and the anthers. Here, like the
conidia, the ascospores quickly germinate.”
Clip_29
Formation and spread of ascospores

3736
“Numerous factors – both biotic and abiotic – damage
the leaf tissue, thus impairing photosynthesis.
The biotic factors include, above all, fungi like
Pyrenophora teres which causes net blotch, barley
scald, due to Rhynchosporium secalis and – particularly
widespread in subtropical regions – brown spot disease,
or spot blotch.
Abiotic factors are responsible for Mlo gene-related
spots that frequently occur on certain varieties of spring
barley and for the increasingly common physiological
leaf spots. These are attributed to stress response in
the plant. It’s easy to confuse these stress-induced leaf
spots with biotic lesions caused by the fungus
Ramularia collo-cygni. This disease also is rapidly
becoming more important in many parts of the world.”
Clip_32
Barley leaf spots (an overview)
Barley leaf spots – fungal disease or stress response?
“Not only are the quantity and size of the kernels re-
duced, but above all, due to contamination with
numerous, chemically very different mycotoxins, the
quality of the grain suffers. If these mycotoxins find
their way into the gastrointestinal tracts of mammals,
they may cause acute or chronic poisoning.A variety of
symptoms can occur, depending on which mycotoxins
predominate in the food chains of humans and animals.
Elimination of lightweight seed reduces the mycotoxin
contamination; but even after storage, mycotoxins may
continue to be produced, especially if the relative
humidity is high.”
Clip_31
Mycotoxins

3938
“The ripe conidia are detached from the conidiophores
by air currents. Due to their small size and rough sur-
face, Ramularia conidia are carried by the wind. They
can reach distant fields. Given sufficient moisture –
especially dew formation – the spores germinate.
Through a stoma, they infect the leaf. Inside the leaf, a
mycelium develops rapidly with the aid of toxins, kills
the cells and proliferates through the entire leaf tissue.
Patches of necrosis develop. Starting from the blades
of the leaves, the sheaths are soon also affected, fol-
lowed by the stems. Finally, the fungus reaches the
heads – including the awns. The grain can also be
affected, although only externally. Spread of the patho-
gen with the seed is only of minor importance.”
Clip_34
Fungal infection of the host plant
“Over a century ago, botanist Fridiano Cavara described
this fungus, calling it Ophiocladium hordei. The curved
shape of the conidiophores resembles a swan’s neck –
in Latin, collum cygni. When the fungus was reclassi-
fied in 1988, this feature gave it its name.
On the tip of each conidiophore spores are formed –
conidia. The conidiophores grow in clusters out of the
stomatal openings,often on the undersides of the leaves.
Since the stomata are arranged along the veins of the
leaves, the conidiophore clusters form long rows, like
strings of pearls.
The leaf tissue in which the spores develop forms
necrotic spots. The lateral margins of these necroses
are strictly delimited by the veins of the leaf. The dark
brown patches are surrounded by a yellow – chlorotic –
halo.”
Clip_33
The fungus Ramularia collo-cygni

4140
“Recent research has shown that in the development
of physiological leaf spots, oxygen radicals – so-called
reactive oxygen species – play a major role and in par-
ticular the superoxide anion O2-. Unlike molecular
atmospheric oxygen, the superoxide anion has a sur-
plus electron.
Every photosynthetically active plant cell produces,
besides molecular oxygen, oxygen radicals – though
only in small quantities. In contrast to molecular oxy-
gen, reactive oxygen species like the superoxide anion
react readily.Their indiscriminate reaction with all kinds
of molecules endangers cell function.That’s why, in the
course of evolution, cells have developed an antioxidant
protection system. In this system, certain enzymes
catalzye detoxification reactions.Thus, in several steps,
superoxide and other oxygen radicals are detoxified –
to molecular oxygen and water.”
Clip_36
The plant’s antioxidant protection system
“This infection of young barley is usually very incon-
spicuous and remains – until well into the spring – a
symptom that’s easy to overlook: isolated patches on
lower leaves.
More conspicuous than these early symptoms of
Ramularia infection are very similar looking leaf spots
that develop early in the year on the upper leaves,
which are exposed to direct sunlight. With increasing
solar radiation and plant development, the number of
these leaf spots multiplies rapidly – in contrast to the
Ramularia spots before head emergence.
These so-called ‘physiological leaf spots’ – PLS – are
also dark brown and relatively small. However, they lack
the chlorotic halo typical for Ramularia spots. Moreover,
the physiological spots are not delimited by the leaf
veins. Nonetheless, it is often difficult to distinguish
reliably between physiological leaf spots and Ramularia
lesions. In many cases, differentiation is only possible
in the laboratory: The decisive criterion for diagnosing
Ramularia collo-cygni is the presence of characteristic
fungal structures. The absence of fungal structures or
other biotic pathogens identifies physiological leaf
spots.”
Clip_35
Physiological leaf spots (PLS)

4342
“One finding that at first sight seems surprising is that
the application of certain fungicides – in particular
strobilurins and azoles – can reduce the oxidative
stress. This effect is not due to elimination of fungal
pathogens. Instead, the fungicides taken up by the
plant in small quantities activate the antioxidant pro-
tection system: Superoxide anions and other oxygen
radicals are thus detoxified much more efficiently. This
is why barley plants treated with fungicides usually stay
green longer and are more vigorous than untreated
plants. And they’re less susceptible to stress-induced
leaf spots.”
Clip_38
Antioxidant effects of fungicides
“The high yields and qualities of modern barley varie-
ties are only possible with maximum photosynthesis
activity. But, the higher the photosynthetic activity, the
more toxic oxygen radicals are produced.When oxygen
radicals can no longer be adequately detoxified by the
antioxidant protection system, the resulting oxidative
stress puts the cell at risk.
There are numerous external factors that can increase
this stress: drought, heat or cold, nutrient deficiency or
air pollutants. Powerful light at photosynthetically effec-
tive wavelengths is an especially active stress factor.
In sensitive barley cultivars, the interaction of different
stress factors causes oxidative damage to vitally im-
portant membranes of the plant cells. These membrane
lesions are among the main causes of physiological leaf
spots. The leaf proteins important for photosynthesis
are also denatured; bonds are broken by oxygen radi-
cals. Additionally, oxidative stress causes increased
production of the ripening hormone ethylene.The com-
bination of increased ethylene formation and protein
decomposition leads to premature senescence of the
leaves, diminishing the plant’s yield potential.
The extent to which irradiation increases oxidative
stress is demonstrated by shading experiments. Leaves
protected from direct exposure to sunlight have signif-
icantly fewer spots than unshaded leaves.”
Clip_37
Oxidative stress

4544
“Rain falls: the impact of raindrops breaks off numer-
ous conidia. With the impact of rain drops, the conidia
are splashed away, onto adjacent young wheat plants.
The more often infection conditions like this prevail
during the wheat’s early development, the more spores
will land on the young plants. On the coleoptile, which
envelops the young plant’s cotyledon, the conidium
absorbs water and swells.
Conidia of Helgardia herpotrichoides germinate at their
tips. From this germ tube a hypha develops that grows
along depressions over the surface of the coleoptile. In
some places the hypha thickens to form blister-like
adhesion organs, appressoria.
Conidia of Helgardia acuformis germinate in several
places at once. The hyphae of this species are some-
what shorter, and overgrow the coleoptile irregularly, in
no particular direction. In this case, the appressoria
form at the tips of the hyphae.”
Clip_40
Conidial spread and germination
“Straw residues left behind in the field after harvest are
a potential source of infection by Helgardia. In these
harvest residues, the fungus survives in the form of a
dense mycelium, the stroma. In fall, when the young
wheat plants are getting established, conditions are
usually ideal for reproduction of the fungus. In humid
weather and at temperatures between 5 and 15 °C, the
fungus forms asexual spores, conidia. Given the right
conditions, this sporulation may be repeated several
times.
In cereals, two different species of the pathogen occur:
these are the conidia of Helgardia herpotrichoides, a
species of the fungus that infects in particular wheat
and barley. The conidia of Helgardia acuformis are
somewhat longer and straighter. Besides wheat and
barley, this species also infects rye.”
Clip_39
Conidia formation

4746
“Little by little, the infection cushions solidify and turn
brown. From their undersides, numerous infection
hyphae penetrate into the leaf. Metabolic products
released by the fungus cause the infected tissue to
turn brown. The outer leaf sheaths tear. In prolonged
periods of humid weather, the stems remain tightly
enveloped by the infected leaf sheaths: Growth of the
pathogen continues. At the base of the infected stem,
the characteristic sign of the disease appears – the
indistinctly outlined eyespot.
Ultimately, the fungus penetrates into the stem of the
plant.This is where water and nutrients are transported
to the aerial parts of the plant. A dense, grayish-white
mycelial mat forms in the interior of the stem.
Gradually, the enzymes released by the fungus de-
compose the stem tissue, thus destroying the vascular
system. The transportation of water and nutrients is
obstructed. If the supply of assimilates to the head is
already insufficient during grain fill the typical symptom
of premature spike senescence develops. Often, the
affected heads are completely white. Under windy or
rainy conditions, the rotten stems fall down and lodge.”
Clip_42
Disease development
“Originating from an appressorium, a slender infection
hypha penetrates the cell wall and grows through the
cells into deeper cell layers. Helgardia herpotrichoides
also uses the appressoria to advance into the tissue.
The hyphae of this species grow between the cells of
the coleoptile. Gradually, between the cells, dense
mycelial aggregations develop.
Finally, the fungus grows right through the coleoptile.
An elongated ‘runner hypha’ reaches the surface of the
underlying leaf sheath. Here, the hypha begins to
branch out.The characteristic Helgardia infection cush-
ions develop. Those of the species ‘herpotrichoides’
consist of loosely associated hyphae, while those of
Helgardia acuformis are somewhat more compact and
symmetrical. By means of a simple dye test, the infec-
tion cushions can be made visible and used to identify
the fungal pathogen.”
Clip_41
Host infection

4948
“After some time, small, cuplike fruiting bodies devel-
op on the straw residues – the apothecia. The sexual
form of Helgardia herpotrichoides is known as
Oculimacula yallundae, the teleomorph of the species
‘acuformis’ is called Oculimacula acuformis. There is
no visible difference between the sexual forms of the
two species.
Tightly packed on the cuplike apothecia of Oculimacula
are the asci.They contain the product of sexual recom-
bination – the ascospores. Under sufficiently humid
conditions they are ejected from the asci. The asco-
spores are carried by the wind – sometimes over long
distances – to other cereal crops, where they infect
more plants.”
Clip_44
Formation and spread of ascospores
“After the harvest, infected stubble residues remain in
the field and initiate a new life cycle of the fungus.
Besides asexual reproduction, the fungus can also go
through a sexual reproduction cycle in which male and
female gametangia are formed. The fusion of these
sexual organs initiates fertilization.
In the further course of the sexual phase, an exchange
of genetic material occurs. As a result of this genetic
recombination, new characteristics can develop. They
enable the pathogen to adapt to changed environmental
conditions.”
Clip_43
Genetic recombination

5150
“During the harvest, the infected heads are mixed with
the healthy grain in the threshing cylinder. In the cylinder
the bunt balls are shattered, and release their infectious
contents.The microscopically small bunt spores spread
like a cloud of dust through the interior of the harvester.
Spores lodge in the fine hairs of the healthy grains.
When infection is severe, the entire batch takes on the
fishy odor of the bunt spores and becomes inedible.”
Clip_46
Contamination of healthy grains
“The early stages of infection are inconspicuous: The
infected plants are, externally, hardly any different from
the healthy ones. Only at a late stage do clear symp-
toms of infection manifest themselves:The heads of the
infected plants are spread open abnormally. On closer
observation, their ‘grains’ turn out to be compact fruiting
structures – sori – of the fungus Tilletia caries, known
as bunt balls. In their interior, closely packed, there are
millions of spores.The dark color distinguishes the bunt
spores. Simple nitrogen compounds such as trimethyl-
amine cause the disease’s characteristic fish-like odor
from which the popular name ‘stinking smut’ derives.”
Clip_45
Common bunt of wheat

5352
“While numerous fungal hyphae grow over the kernel,
the grain begins to germinate. The shoots are infected.
Through the coleoptile, the hyphae penetrate into the
interior of the plant. As the young wheat plant grows,
the mycelium spreads farther and farther inside it.
The heads are also colonized by the fungus. Instead
of endosperm, a dense mycelium forms beneath the
teguments of the affected heads. Gradually, the hyphal
cells become constricted and rounded. Once again,
new bunt spores are formed. Protected by the seed
coat, the spores of Tilletia caries survive until the next
harvest.”
Clip_48
Seedling infection
“In the soil, the bunt spores, which remain capable of
germinating for years, awake to new life.
The two genetically different nuclei in the interior of the
spore fuse. In a two-stage meiotic division the genetic
matter is redistributed. Finally, subsequent mitotic divi-
sions result in eight haploid nuclei. The bunt spore
begins to germinate. A promycelium grows out of it. At
the end of the promycelium, eight finger-like hyphal
structures form – the primary sporidia. A haploid
nucleus enters each sporidium.
The sporidia form conjugation pegs and fuse pairwise.
During conjugation, one of the two nuclei migrates into
the adjacent sporidium. From the conjugated sporidia,
a dikaryotic mycelium forms.”
Clip_47
Germination of bunt spore

5554
“Loose smut of barley is caused by Ustilago nuda.
Unlike the hard bunt balls of the common bunts, the
sori of the smuts are covered only by a thin membrane.
As the healthy barley plants start to flower, the sori on
the infected heads burst open, one after another. The
spores are released and are carried away by the wind.
Many of them land on the flowering heads of the healthy
plants.The spores get caught on the stigma of the floret.”
Clip_49
Loose smut of barley
“Here, they begin to germinate. A septate promycelium
forms consisting of four cells each with one nucleus.
The nuclei of adjacent cells are genetically different.
Pairs of cells with different nuclei fuse. Starting from
these fused cells, a dikaryotic mycelium forms.
This mycelium grows through the outer layers of the
developing grain. Finally, the hyphae reach the embryo
and infect it. Following infection of the embryo, which
is characteristic of the smuts, the fungus goes into a
dormancy stage.The interior of the ripening heads har-
bors a new generation of infected barley plants.”
Clip_50
Embryo infection

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