Diseases caused by fungi

Title: Diseases of small grains in North and Central Florida
Full Citation
Permanent Link: http://ufdc.ufl.edu/UF00066902/00001
 Material Information
Title: Diseases of small grains in North and Central Florida
Series Title: Plant Pathology Fact Sheet PP-38
Physical Description: Book
Language: English
Creator: Kucharek, Tom
Affiliation: University of Florida -- Florida Cooperative Extension Service -- Department of Plant Pathology -- Institute of Food and Agricultural Sciences
Publisher: Florida Cooperative Extension Service, Institute of Food and Agricultural Sciences, University of Florida
Publication Date: 1988
Spatial Coverage: North America -- United States of America -- Florida
 Record Information
Bibliographic ID: UF00066902
Volume ID: VID00001
Source Institution: Marston Science Library, George A. Smathers Libraries, University of Florida
Holding Location: Florida Agricultural Experiment Station, Florida Cooperative Extension Service, Florida Department of Agriculture and Consumer Services, and the Engineering and Industrial Experiment Station; Institute for Food and Agricultural Services (IFAS), University of Florida
Rights Management: All rights reserved by the source institution and holding location.

Table of Contents
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    Diseases caused by fungi
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Full Text

Plant Pathology Fact Sheet

Diseases of Small Grains in North and

Central Florida
Tom Kucharek, Professor and Extension Plant Pathologist, Department of Plant
Pathology, University of Florida, Gainesville, F L 32611. 1988; Revised 2001.

Florida Cooperative Extension Service/ Institute of Food and Agricultural Sciences/ University of Florida/ Christine Waddill, Dean

Diseases of small grains (wheat, oats,
rye, triticale, barley) are some of the most im-
portant plant diseases on a worldwide basis.
Because of small profit margins for these crops,
the primary control techniques have been re-
sistant varieties, basic tillage techniques, and
relatively inexpensive seed treatments. In re-
cent years, emphasis on high yields coupled
with new chemical control strategies offer po-
tential for less traditional approaches. Effective
use of the new wave of plant disease control
techniques requires accurate diagnosis of the
causal agent (fungus, virus, bacterium, non-
parasite) of disease. Fungal and viral diseases
have been the primary limiting factors for yield
and quality of small grains grown in Florida.


Soilborne wheat mosaic virus (SBWMV)
was first found in Florida in 1970 in Escambia
County. Since then, it has been found in Santa
Rosa, Okaloosa and Madison Counties. Al-
though the virus can be spread mechanically
(by hand), natural spread occurs by moving soil
that contains the soilborne fungus vector,
Polymyxagraminis. Wheat that is infected by this
fungus but not the virus sustains no known
damage. Because the virus is associated with a
soilborne organism, the initial distribution of
the virus in a field is usually patchy or in streaks
due to soil tillage operations (Fig. 1). SBWMV
is sometimes found where the tillage equip-

ment is first used in the field. In 2000, SBWMV
or a closely related virus was found in rye in
an experimental rye planting in Gadsen County.

Wheat roots are infected by the zoospore
(swimming) stage of the fungus. After entering
the root, the fungus proceeds with a complex
life cycle, simultaneously releasing the virus
into the root. Virus multiplication and spread
in the plant follows. Later, the fungus forms
thick-walled spores for survival in soil. These
spores are capable of carrying the virus during
the survival period. The fungus will become
active again during cool, wet weather. Trans-
mission of the virus through seed has not been

Patches of infected plants show various
degrees of symptom severity depending on
time of infection, temperature, and the amount
of virus within a plant (Fig. 1). Symptoms in-
clude stunting of plants, reduced tillering, de-
layed heading, shriveled seed, seed abortion,
and vein-restricted, yellow to brown to white
streaks or mosaic patterns on leaf blades and
sheaths (Figs. 1 & 2). Some varieties may dis-
play oval-like leaf scalding symptoms. Also,
root systems are reduced. In Florida, leaf symp-
toms are most vivid during late February to
early April. Rosetting (severe stunting with
abnormally high amount of leaf area to stem
lengths) prior to stem jointing may occur in
some susceptible varieties.


The most effective control is the use of
resistant varieties. Equipment (tractors, imple-
ments, soil tube samplers, etc.) should be
washed to remove clinging soil after its use in
an infested field. Crop rotation with oats will
offset damage because wheat, rye, barley and
triticale are hosts of this virus. The fungus vec-
tor with the virus can live in the soil for years.

Oat Soilborne Mosaic Virus (OSBMV)
is thought to occur in Florida but such has not
been ascertained. Symptoms, affects on yield,
and control measures would be expected to be
similar to that of WSBMV.

Barley Yellow Dwarf Virus (BYDV) oc-
curs in Florida and has been severe in some
years. This virus is spread by many species of
aphids and is capable of causing disease in
wheat, barley, oats, and many grasses. Aphids
acquire BYDV after feeding on infected plants
for 30 minutes to 30 hours. One to four days
later, they can transmit BYDV and some aphids
may be able to transmit the virus for the rest of
their lives. As the acquisition feeding time is
increased the latent period of one to four days
may be reduced. BYDV is not passed to the egg
or nymph stages of the aphid. BYDV is not

Symptoms of BYDV vary from none to
brilliant leaf yellowing in wheat and barley or
red to purple coloring of oat leaves (Fig. 3). Such
off-colors tend to be associated with leaf mar-
gins at first, but later envelops the entire leaves.
Leaves tend to curl or twist. Infected plants have
reduced root systems, stiffer leaves, and possi-
bly, darkened phloem within infected tissue.

Control BYDV with resistant varieties.
In addition, control of aphids with insecticides
has reduced this viral disease.

Wheat Spindle Streak Virus (WSSV)
has been identified in Georgia and Alabama but
not yet in Florida. It is spread by the same fun-
gus vector as that for SBWMV but the two vi-

ruses are distinct. WSSV infects both wheat and
rye. WSSV has been reported to occur in patchy
areas within a field, often in low wet areas, or
throughout the entire field. Symptoms of WSSV
are similar to that of SBWMV except that the
linear chlorotic streaks between leaf virus tend
to be tapered at their ends. Control for this vi-
rus would be similar to that for SBWMV.


Glume Blotch (GB) is caused by
Stagnospora nodorum (Septoria nodorum). This fun-
gus can live in old wheat debris for at least two
years. Also, the fungus can be transmitted by
infected seed. Survival of the fungus for seven
years in seed has been reported. Weed hosts
are not known to be sources of inocula (spores).
Wheat, barley, rye, and blue grass are suscep-
tible, but wheat is the principle host and the
disease is most severe during spring months
in Florida. Small amounts of GB occur during
the fall and winter. GB reduces yield by reduc-
ing seed number, seed size (grade), and tillers.

Stagnospora nodorum is capable of pro-
ducing ascospores that result from sexual mat-
ing, and pycnidospores that do not require
mating for formation. Pycnidospores are the the
spores present in Florida. These spores are
formed in flask-shaped structures (pycnidia)
embedded within infected tissue (Fig. 4), and
are formed in leaves 10-16 days after infection.
Slightly longer intervals occur in stem tissues.
Spores are splashed by rain or irrigation water
from nearby pycnidia located on old crop refuse
or growing plants. Penetration occurs through
the cuticle of leaves by germ tubes growing
from the spores. At least six hours of moisture
on the leaf is essential for the penetration pro-
cess but is not required for growth of the fun-
gus within the leaf after penetration. Survival
of spores outside of the pycnidia can occur but
reports are varied as to how long.

Three to seven days after penetration,
light-colored, chlorotic (yellow) to necrotic

(brown) blotches appear on leaves. From 10-16
days after penetration, the tiny, black, dot-like
pycnidia mature and form spores internally.
Adding a drop of water to dried lesions or
blotches will swell pycnidia and allow the
spores to ooze out in a tendril. This life cycle is
shortest when temperatures are between 680 to
810F. This fungus is capable of growth between
40 and 900 F. Mature lesions on leaves are foot-
ball shaped, tan-colored in the center (Fig. 5),
and vary in size. The leaf spot may be sur-
rounded by a yellow halo. On glumes, awns,
and stem parts, blotches rather than distinct
spots are formed. On glumes, these blotches
tend to be limited to the upper half of the glume
(Fig. 4). Leaf spots of GB may appear similar to
those caused by Helminthosporium sativum
(Bipolaris sorokiniana) and sometimes one spot
may have both organisms.

Control of GB is best achieved by two
years of crop rotation plus a seed treatment or,
better yet, use of seed not infected with this fun-
gus. In many growers' situations, however, this
ideal combination can not be achieved. There-
fore, the use of a fungicide spray program as
outlined in Plant Protection Pointer 27 offers a
method to control GB as well as
Helminthosporium leaf spot and leaf rust. Numer-
ous cultural techniques (fertility, planting
depth, row and plant spacing, and irrigation
frequency) have been tested for their affects on
severity of GB, but no clear results were found.
Later planting dates in the fall would be ex-
pected to reduce the amount of spores avail-
able for spring months when GB epidemics are
normally seen in Florida.

Helminthosporium Leaf Spot or Spot
Blotch (HLS) is caused by Helminthosporium
sativum (Bipolaris sorokiniana). This disease has
been more of a problem on wheat in the rye-
growing areas of Florida, as HLS is common
on rye and triticale. This fungus has a wide host
range on many grass species, but does not in-
fect corn, sorghum, or oats. This fungus sur-
vives on crop debris and is carried in or on seed.

All plant parts are susceptible; that is, this fun-
gus causes spots or blotches on aboveground
parts as well as stem and root rots and seed-
ling blight.

Two kinds of spores are formed by H.
sativum: ascospores from a mating process and
conidia without a mating process. Conidia are
the primary inocula of HLS in Florida. Conidia
form on the surfaces of lesions and thus are
suitable for spread by wind. With the aid of a
hand lens the mass of spores appears as a min-
iature forest. If they land upon susceptible tis-
sue that is wet with dew or rain water, they ger-
minate, form germ tubes, and with special struc-
tures, the host tissue is penetrated. The life cycle
varies from three to 20 days, depending upon
temperature. Growth of the fungus occurs be-
tween 33 and 990 F, but temperatures from 71
to 850 F are optimal for infection and symptom

HLS may appear as dark, pin-point spots
or may be up to one inch in length (Fig. 6). Spots
are somewhat linear, paralleling the leaf veins,
and are dark green, black, or brown. A yellow
halo may or may not surround the dark lesion.
Glume blotch lesions are lighter in color than
HLS. Where flower parts are infected, the heads
appear dirty green to black (Fig. 7) in contrast
to dirty brown heads affected by glume blotch.

Control of HLS is similar to that of glume
blotch. In addition, wheat plantings should be
separated from rye plantings to the extent pos-
sible. Differences in susceptibility between crop
varieties exist, but resistant varieties to HLS are
not available.

Leaf Blotch (LB) of oats is caused by
Helminthosporium avenue (Drechslera avenacea)and
other plant species are not known to be sus-
ceptible. The life cycle of this fungus and con-
trol of this disease are similar to those for
Helminthosporium leaf spot of wheat.

LB of oats is primarily a disease of the

leaves and panicles in Florida, although a black
stem phase has occurred. Leaf symptoms in-
clude linear, reddish brown to black spots, usu-
ally 1/2 to one inch long, which are water-
soaked (greasy) when small (Fig. 8). Spots are
narrow, being somewhat limited by leaf veins,
but they can be blotchy. With a hand lens, dark
fuzzy spore masses may be seen near the cen-
ter of the spots. Black stem symptoms of this
disease display pinkish-brown blotches on the
leaf sheaths or large black areas on the stem,
the latter usually being associated with nodal
tissue. A white mold growth occurs in the stem
cavity near the blackened tissues. Stem break-
age may occur. Control of LB is done with re-
sistant varieties (if available), seed treatment,
and crop rotation.

Powdery Mildew (PM) is caused by
Erysiphe graminis (Blumeria graminis). In Florida,
the only strain of PM of consequence on small
grains is the one in wheat, usually during the
spring months. On some varieties, PM can be
severe, covering the entire leaves with a white,
powdery mold growth (Fig. 9). PM can cause
yield loss on susceptible varieties that main-
tain high disease severities from prior to joint-
ing until flag leaves (uppermost leaf on a ma-
ture plant) form. Excess nitrogen predisposes
plants to PM.

The fungus causing PM produces two
spore types: one is produced after a mating
process ascosporee) and the other is produced
in large amounts without a mating process
conidiaa). Conidia are formed among the white
powdery growths on leaves and appear to be
the main source of inocula for PM in Florida.
Upon germination, a conidium produces a
germ tube from which an infection peg forms
and penetrates directly through the leaf cuticle.
Conidia of PM are unique as they do not re-
quire free moisture for germination, but ger-
mination is most rapid when free moisture is
present. Conidia germinate from 34 to 860 F,
but disease development is most rapid from
59 to 720 F. Above 770 F, disease progression is

slowed; this is why PM is not as severe on up-
per leaves of fully jointed wheat in Florida. PM
consists of numerous races which differ in abil-
ity to cause disease in different varieties. The
PM fungus survives from one season to another
on volunteer wheat plants, in susceptible wild
grasses, as a mycelial mat (dried up white fun-
gus growth) and possibly by the ascospore
within a cleistothecium ( a fungus encapsula-

Control is best achieved with varieties
resistant to the current prevalent races. Some
new fungicides may be available for control of
PM, but discuss their use with your county
agent first. Use a balanced soil fertility; do not
use excess amounts of nitrogen.

Leaf Rust (LR) of wheat is caused by
Puccinia recondita f. sp. tritici, and is a problem
on susceptible varieties or previously resistant
varieties infected by new or different races of
this fungus. In Florida, LR occurs from late win-
ter through spring. Occasionally, rye or barley
may be infected by this fungus. This fungus
produces three kinds of spores in wheat,
urediospores, teliospores, and sporidia. The
urediospore, contained in orange to brown
uredia (Fig. 10), is thought to be the only spore
of consequence for spread of LR in Florida.
Uredial pustules can form on all parts of the
plant above ground level. They first appear on
lower leaves and with time the disease spreads
to upper plant parts. When a urediospore ger-
minates, it produces a germ tube from which
structures are produced that penetrate the
breathing pores stomataa) on plant tissue. A
minimum of four hours of leaf wetness is
needed for penetration to occur but longer wet-
ting periods will result in more penetrations.
Within 7 to 14 days, or more if temperatures or
varieties are not ideal for the fungus, new pus-
tules are formed in areas that began as yellow
to tan flecks on susceptible tissue. If the tissue
is less susceptible, no pustule is formed (highly
resistant reaction) or the original fleck will be
partially covered by a uredium (intermediate

responses). The life cycle is the shortest when
temperatures are from 59 to 810 F.

Telia are black and contain teliospores.
Typically, telia form on older infected leaves.
When teliospores germinate, they produce spo-
ridia that infect the alternate hosts (meadow rue
and others). On the alternate host, pycnia and
aecia are formed, and each containing their re-
spective spore types. However, in Florida, di-
rect pertinence of spore stages other than the
urediospores is probably negligible. The sig-
nificance of the alternative host relates to the
potential for new races to form and aeciospores
are infectious to wheat.

Control of LR is by use of resistant vari-
eties or with foliar sprays during spring
months; see Plant Protection Pointer No. 27.

Stem Rust (SR) is caused by Puccinia
graminis. Nearly all small grains are susceptible
to different "form-species" of this fungus. The
form-species tritici infects wheat; the form-spe-
cies avenae infects oats; etc. Although SR can be
devastating, the SR epidemic in 1974 in wheat
is the only known case in Florida and this was
related to the widespread planting of a highly
susceptible variety. Normally, SR is seen in
trace amounts each year in Florida.

The SR fungus has a life cycle and tem-
perature responses similar to leaf rust of wheat,
but the alternate hosts (barberry, and others) are
different. The length of the life cycle on wheat
has been as short as five days and as long as 85
days, depending on temperature and variety-
race combination. A seven to 15 day life cycle
would be typical.

SR can appear similar to leaf rust on
leaves of wheat, but often the uredial pustules
are red to brown, rather than orange as with leaf
rust. Should SR be seen on stems, including leaf
sheaths, long red to brown uredia will be evi-
dent (Fig. 11). With a microscope, the two rusts
can be distinguished by urediospore shape;

spores of SR are oblong whereas those of leaf
rust appear round or block-shaped.

Control SR with resistant varieties. Fo-
liar sprays may be useful, but this depends
upon the type of fungicide used.

Leaf Rust and Stem Rust of Rye can oc-
cur but only the former has been seen on occa-
sion in Florida. These rusts have similar symp-
toms to their respective rusts on wheat. The al-
ternate hosts for SR on rye is the same as for
wheat, but the alternate host for LR on rye is
different (Aichliisa spp.).

Crown Rust (CR) of oats is caused by
Puccinia coronata. It has a life cycle similar to
that of leaf rust of wheat, except its alternate
host is Rhamnus spp. When free moisture is
present, urediospores will germinate from near
freezing to 860 F. Temperatures near 770 F
shorten the life cycle to five days, depending
upon the variety. A nine to 15 day life cycle is
typical during the spring months in Florida.
Penetration of leaf tissue by infection structures
may occur in three hours near 700 F or take 12
hours at 860 F or 24 hours at 410 F .

Symptoms and control for CR are simi-
lar to those for leaf rust of wheat, except a foliar
spray program is usually not advised.

Loose Smuts (LS) of wheat, oats, and
barley have occurred in Florida but none of
these fungal diseases has been severe except
on occasions in some fields. LS is caused by
Ustilago tritici, U. avenue, U. nuda on wheat, oats,
and barley, respectively. LS on wheat and bar-
ley are seedborne within normal-appearing
seed. Upon seed germination, the fungus grows
systemically within the plant and after flower-
ing, black spore masses are present within flo-
rets instead of seed (Fig. 12). These black spores,
called teliospores, are spread by wind to other
flowers within the crop where they germinate
and infect the female portion of the flower. The
seed then forms normally.

With LS on oats, the life cycle is the same
on occasion, but most infections occur in the
field during the early seedling stage. The black
teliospores, which are carried on seed, germi-
nate to form sporidia that in turn germinate. If
a fusion occurs between the germ tubes of cer-
tain sporidia, the fused-germ tube is capable
of penetrating the seedling. Infection is favored
by somewhat dry conditions and is best at 64
to 720 F.

Control of LS is done with resistant vari-
eties and seed treatments that are highly selec-
tive for control of smuts. Also, seed sources
should never be from fields that had smut.

Anthracnose of rye, caused by
Colletotrichum graminicola, appears during warm
(770 F) and wet periods in the spring. Prema-
ture ripening and bleaching of the lower stem
occur. Later, the lower stem turns purple to
brown where dark fungal structures with spines
may be seen with a hand lens. Root rot, culm
rot, lodging, and flower sterility are further
symptoms. This disease is most apt to occur in
fields with acidic soil or low in phosphorus.
Broom sedge fields are indicators of such con-
ditions. Crop rotation with non grass crops is

Pythium Root Rot (PRR) of small grains
is caused by several species of Pythium which
are common in soils in Florida. These fungi pro-
duce swimming spores which germinate in
water. From the germ tube, structures are pro-
duced that are capable of infecting root tissue.
Also, several types of resting spore types may
form that allow these fungi to survive harsh
conditions in the soil or in old plant debris.

Symptoms of PRR include lower leaf
yellowing (similar to nitrogen deficiency).
Eventually, all leaves on a plant may be yel-
low followed by stunting, leaf browning, and
death of the plant (Fig. 13). Although entire
plantings may display these symptoms, this
disease occurs commonly in patches or linear
streaks in the field (Fig. 14). These fungi may
also cause a pre- or post-emergent seedling
blight. After jointing occurs, PRR occurs but is
less damaging than early season infections.

Control of PRR on small grains is with
seed treatment, avoidance of planting prior to
the middle of November, and avoidance of
deep planting. After stand establishment, soil
moisture, nitrogen levels, and overall fertility
levels should be adequate to promote a consis-
tently growing plant. Plants are more apt to be
infected when the root system is not expand-
ing. Where the crop is to be used for grazing
and planted early or in warm soils, the use of
an effective seed treatment is highly important.

Figure 1. Stunting of wheat caused by soil-
borne wheat mosaic virus.

Figure 3. Leaf symptoms in oats caused by
barley yellow dwarf virus.

Figure 2. Mosaic symptoms in wheat caused
by soilborne wheat mosaic virus.

Figure 4. Lesions of glume blotch in a wheat

Figure 5. Leaf lesions of glume blotch in

Figure 6. Leaf lesions in wheat caused by
Helminthosporium sativum.

Figure 7. Blotches on wheat spikes caused by Figure 8. Leaf lesions and effects of leaf
Helminthosporium sativum. blotch in oats.

Figure 9. Powdery mildew of wheat. Figure 10. Pustules of wheat leaf rust.

Figure 11. Pustules of stem rust in stems and Figure 12. Loose smut of wheat.
leaves of wheat.

Figure 13. Pythium root rot in young rye plants.

Figure 14. Pythium root rot in wheat.

--L~je~?rp~ I I 1

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