Leaf Diseases of Winter Wheat
Table of Contents
- Seasonal Appearance of Leaf Diseases
- Powdery Mildew
- Leaf Rust
- Septoria Leaf Spot
- Tan Spot or Yellow Leaf
- Snow Molds
- Stem Rust
- Management of Leaf Diseases
- Related Links
Discoloration, spotting, and blighting are symptoms commonly observed
on leaves of winter wheat. The symptoms often are produced by adverse
weather or soil conditions including air pollutants, cold winds,frost,
ice, drought, flooding, nutrient deficiencies and excesses, soil compaction
and poor soil structure. Sometimes insects chew, suck or burrow into stems
and leaves. Most frequently, however, the symptoms are caused by microscopic
fungi, viruses or bacteria that may affect virtually all parts of the
wheat plant. Fungal diseases affecting wheat leaves probably reduce yields
of Ontario winter wheat by 15 to 25% in most seasons. Diseases may reduce
yields directly and by predisposing the wheat plants to other stresses
such as drought. This Factsheet is intended to aid recognition and management
of major fungal diseases affecting leaves of winter wheat
Seasonal Appearance of Leaf Diseases
The various leaf diseases appear on winter wheat at different times of
the year. In autumn, powdery
mildew and leaf rust are common, Septoria leaf spot is present in some
years, and pink snow mold may be active in cool, wet weather During winter,
snow mold may develop under snow cover when temperatures are favorable,
and damage becomes evident as the snow melts. In early spring symptoms
of wheat spindle streak mosaic, a viral disease, may appear on the leaves.
A few weeks before heading, powdery mildew and leaf rust often develop
again and may increase in severity until the wheat begins to mature. Outbreaks
of tan spot and Septoria leaf spot are most severe at heading. Stem rust
also may appear near maturity. The kinds and severity of diseases in various
wheat-
growing areas differ in accordance with local climate, soil conditions
and farming practices.
Figure 1. Powdery mildew
on wheat in the field. Numerous patches of white fungal growth appear
on the leaves. The lower and middle leaves are most severely affected.
Powdery Mildew (Figure 1)
Symptoms
The disease may affect all above-ground portions of the plant, but is
usually most severe on
the lower and middle leaves. Elliptical patches of white fungal growth
appear on both leaf surfaces.
The patches may enlarge, and run together to form a dense mat. The fungal
growth later becomes gray or reddish brown. The cottony growth becomes
powdery when the fungus produces spores. Tiny brown or black fungal bodies
(perithecia) commonly appear on the fungal mat. Beneath the fungal growth,
the leaves or heads remain green for a while, but turn yellow or brown
prematurely
Cause
Powdery mildew is caused by a fungus (Etysiphe graminis f. sp.
tritici) which attacks only wheat. There are several races of the
fungus, which look alike, but produce different disease reactions on different
wheat cultivars. The mildew fungus produces two types of spores; one kind
on the white fungal growth, and another kind in the black perithecia.
Disease development
Wind-blown spores produced in infested stubble or straw, or on volunteer
wheat, initiate disease in young wheat in the fall or when growth resumes
in spring. Several generations of mildew growth and spores are produced
on the wheat in the growing season. The abundant spores are wind-blown
locally or regionally and serve to disperse the fungus and thus spread
disease. Mildew flourishes when the weather is humid and moderately warm
(15 to 226C), but develops slowly in cool weather (5 to 140C). Hot weather
(above 250C) retards the disease. Mildew may thrive on seedlings and adult
plants especially when growth is lush and dense, but develops slowly on
maturing wheat.
Mildew occurs in most years, but is often not seen because of its presence
deep in the leaf canopy rather than on the upper leaves. Growth of foliage
and roots is reduced and affected plants senesce prematurely. Mildew commonly
results in fewer tillers, heads, and grains per head, and the grains may
be poorly filled.
Leaf Rust (Figure 2)
Figure 2. Leaf rust on
winter wheat. Numerous orange-red spots are present mainly on the upper
surface of the leaf blades.
Symptoms
Numerous orange-red spots (uredial pustules) appear mainly on the upper
surface of the leaf blades. Each spot is less than 1.5 mm in diameter
Affected leaves turn brown or yellow-brown prematurely and sometimes dry
out. In maturing wheat small black spots (telial pustules) each less than
1.5 mm, may appear mostly on the underside of leaf blades or on leaf sheaths.
Cause
The fungus Puccinia recondita f. sp. tritici causes leaf
rust mainly in wheat. The fungus occurs in numerous forms or races that
can be distinguished only according to disease reactions on a range of
wheats. The fungus is spread as wind-blown spores (uredospores) produced
in the orange-red spots.
Disease development
Epidemics may be started in young wheat shortly after emergence m autumn
by spores blown from volunteer wheat or wheat straw. The rust probably
overwinters in the wheat crop. The relative importance of spores blown
from the U.S.A. as a source of infection is not known.
Disease develops rapidly between 15 and 220C when dew periods are 6 to
8 hours or more. Several generations of spores are required for serious
disease to occur. However, in favorable weather, new generations of spots
and spores occur every 7 to 10 days. Disease progresses slowly as the
wheat matures.
Leaf rust is present m most years, but becomes severe only every 3 to
5 years. The disease reduces plant vigor and decreases seed filling. Moisture
loss from affected plants greatly exceeds that of healthy plants, thus
increasing the risk of drought stress.
Septoria Leaf Spot (Figures 3 and 4)
Figure 3. Septoria leaf
spot (Septoria tritici) on winter wheat in the heading stage. Note
the brown spots and yellowing of areas between the spots.
Symptoms
Yellow flecks first appear on the lower leaves. Later, yellow to red-brown
or gray-brown spots or blotches may develop on all above-ground parts
of wheat. On the leaves, the spots range up to 15 mm long and 5 mm wide.
Spots near the base of the leaf blade may kill the leaf. Spots and blotches
on leaves, culms and heads characteristically become speckled with tiny,
brown or black fungal bodies (pycnidia)
Figure 4. A magnified view
of a spot produced on a wheat leaf by Septoria tritici.
The spot was about 1 cm in length and is speckled with black fungal bodies
called pycnidia.
Causes
In any particular field, Septoria leaf spot may be caused by one to three
related fungi (Septoria tritici, Septoria nodorum, and Septoria
avenae f. sp. triticea; respective sexual states are Mycosphaerella
graminicola, Leptosphaeria nodorum, and Leptosphaeria avenaria
f. sp. triticea). Microscopic examination of spores is required
to distinguish these fungi. Septoria tritici is possibly most common
in Ontario. All three fungi attack mainly wheat.
Disease development
Sources of the fungus are infested stubble and straws volunteer wheat,
and seed (S. nodorum and S. tritici only). Spores from the speck-like
pycnidia, and sexual spores (ascospores) initiate disease in winter wheat
in the autumn or spring. Spores are produced abundantly on diseased wheat
crops throughout the growing season when the weather is wet. Splashing
rain disperses spores in the crop.Disease is most severe at the heading
stage. Wet, windy weather with temperatures of 15 to 270C favor disease
outbreaks. Dry weather halts disease progress. The disease is moderately
common in Ontario, and may reduce grain yields 10 to 20%.
Tan Spot or Yellow Leaf (Figure 5)
Figure 5. Tan spot disease
on a wheat leaf. Several tan-brown spots have expanded and run together.
The blackish cast on the coalesced spots is due to sporualation of a tan
spot fungus.
Symptoms
Oval-shaped tan spots up to 12 mm in length appear on the leaves. Areas
around the spots turn yellow: The spots may run together and result in
blighting of large areas of the leaf. Small, dark brown areas often appear
on the tan spots.
Cause
The disease is caused by a fungus Pyrenophora trichostoma: conidial
state is Drechslera, also known
as Helminthosporium tritici-repentis), which also causes leaf spots
on grasses, including bromegrass and wheat grass. Small black bodies form
in autumn on wheat and grass debris and release spores (ascospores) in
spring. During late spring and throughout the summer a second kind of
spore (conidium) is produced abundantly on leaf spots and straw.
Disease development
Spores blown by wind from wheat or grass debris to wheat crops initiate
disease in spring
Infections occur earlier and are more numerous when the wheat is close
to the debris. Numerous spores produced on diseased leaves spread the
fungus in the crop during the growing season. Tan spot is most severe
after heading. Disease outbreaks are favored by warm weather with rains
or prolonged heavy dews.
Occurrence
Tan spot occurs in most areas of Ontario where winter wheat is grown.
When moderate or severe, the disease hastens ripening and reduces yields.
Snow Molds (Figure 6)
Figure 6. Snow mold on
winter wheat just after snow melt. Whitish fungal growth appears on brown
bleached and green wheat leaves.
Symptoms
Irregular patches of dead or damaged wheat are evident after snow melt.
Conspicuous masses of pinkish, whitish or gray fungal growth on brown,
bleached, or green leaves and on the soil surface is a positive indication
of snow mold. Sometimes affected plants are speckled with brown-black
fungal bodies (sclerotia). Leaves may be partly or entirely killed and
appear brown or bleached. When the crowns are attacked, the plants are
usually killed. When the crowns are unharmed, new leaves emerge among
the damaged leaves and the wheat plants often recover. Snow mold-affected
plants usually are found in large patches that correspond roughly with
areas of persistent snow cover such as where drifts had occurred near
fencerows.
Causes
Two "low temperature" tungi seem to be the main cause of snow
mold disease in winter wheat, other winter cereals, and certain grasses
in Ontario.Pink snow mold (Fusarium nivale) is common, but speckled
snow mold (Typhula species) is also found. Pink snow mold tends
to appear pinkish and produces masses of microscopic spores on the pink
growth. Speckled snow mold is grey-white and affected plants tend to bleach
and to shatter easily. The speckling (sclerotia) may or may not be present.
These fungi survive in dead leaves of affected plants, or as fungal bodies
(sclerotia) in soil.
Disease development
The snow mold fungi are active when temperatures under the snow cover
are around the freezing point. Under deep snow: temperatures may be near
freezing even when air temperatures are much lower Beneath deep snow,
wheat plants become weakened and more susceptible to snow mold. The pink
snow-mold fungus can attack wheat also in the absence of snow during cool
wet weather in autumn
and spring.
Occurrence
Snow molds appear to be an important factor in the survival of winter
wheat in some areas of Ontario where snow fall is heavy Wheat plants with
injured leaves often recover and yield well. If many crowns are injured,
however the stands may be too thin for satisfactory yields.
Stem Rust
Stem rust, caused by a fungus (Puccinia graminis f. sp. tritici),
is not normally a serious disease on winter wheat in Ontario. It is characterized
by reddish-brown spots with powdery masses of spores (uredospores) on
the leaves, stems and heads. These spores are windblown locally and regionally
and produce disease outbreaks in summer Black streaks, containing another
kind of spore (teliospores) appear abundantly on maturing wheat. Since
the fungus alternates on wheat and common barberry, destruction of barberries
is an important control measure, and has been a major factor in reducing
the importance of stem rust in Ontario. More information on stem rust
is given in OMAF Factsheet Leaf and Head Diseases of Barley, Agdex
114/632.
Management of Leaf Diseases
Outbreaks of leaf diseases in the fall reduce crop growth and many increase
susceptibility of wheat to winter injury and snow mold damage. Leaf disease
in early spring weaken and stunt growth. This affects early development
of heads and can cause death of tillers that would otherwise form heads.
After heading, leaf diseases reduce grain filling, resulting in ripened
grains of low test weight. Overall, leaf diseases cause financial losses
through reduced yield, reduced grades, or because fields severely damaged
by snow molds require reseeding with spring cereals. Careful attention
to control measures is thus well warranted.
Disease occurs only when the disease causing fungus is present in a crop
of susceptible wheat and the weather is favorable for the fungus to attack
and spread in the crop. The longer the fungus is present and conditions
are favorable, the more severe the disease becomes. Disease severity may
be reduced by appropriate control practices:
- Reduce amounts of disease-causing fungi at planting time.
Plow down or otherwise destroy
all infested stubble, straw and volunteer wheat on your farm as soon
as is practical before
planting time. Rotate wheat with crops other than small-grain cereals.
These practices help
to (a) destroy disease fungi; (b) delay or prevent disease outbreaks;
and (c) reduce disease severity.
- Choice of variety. Currently recommended varieties
have some resistance to certain leaf diseases. Fredrick is partially
resistant to leaf rust and should be used where rust reduces test weights.
Yorkstar, and the new variety Gordon, appear less susceptible to snow
mold injury and should be grown in areas where snow mold is a serious
problem.
- Seed selection and treatment. Disease organisms carried
on seed include Septoria, seedling blight fungi, and smut fungi.
Use of disease-free seed is recommended. Broad spectrum protectant fungicides
such as Maneb or Thiram help control Septoria and seedling blights,
but not smut diseases. A systemic fungicide, carbathiin (Vitaflo 250)
is recommended for control of smut diseases, but not for Septoria
or seedling blights. A seed treatment containing both
a protectant fungicide and carbathiin is available (Vitaflo 280). Seed
buyers should specify the treatment they require.
- Seeding date. Snow mold injury is less severe in
wheat that is well-established at freeze-up. Timeseeding to allow 7
to 10 weeks of growth in the fall, and to produce plants with about
10 tillers,especially in areas of heavy snow fall (over 200 cm).
- Fungicide Sprays. Research in Ontario has shown that
certain fungicides sprayed on wheat once or twice during the spring
will control several diseases and usually will increase yields by 10
to 30%. One or more fungicides may be registered for
use on wheat in 1984 or later. Fungicides normally are used with a growth
regulator and specified levels of nitrogen.
- Future possibilities. Varieties with improved disease
resistance, and fungicide sprays effective for controlling leaf diseases,
likely will become available to farmers in Ontario.
- Related Publications and Factsheets. Consult the
current issue of OMAF Publication 296, Field Crop Recommendations,
for recommended fungicide treatments, and information on disease resistance
in all recommended varieties.The following publications give information
on other diseases and disorders affecting wheat leaves: The Agronomy
Guide: Oat-Cyst Nematodes; and Agriculture Canada Publication 1353 Watch
for the cereal leaf beetle.
Related Links
For more information:
Toll Free: 1-877-424-1300
Local: (519) 826-4047
E-mail: ag.info.omafra@ontario.ca
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