Leaf Diseases of Winter Wheat

Agdex#:	112/600
Publication Date:	07/93
Order#:	83-046
Last Reviewed:	07/93
History:	(Reprinted January 1993)
Written by:	J.C. Sutton - University of Guelph; L.A. Hunt - University of Guelph

Table of Contents

1. Seasonal Appearance of Leaf Diseases
2. Powdery Mildew
3. Leaf Rust
4. Septoria Leaf Spot
5. Tan Spot or Yellow Leaf
6. Snow Molds
7. Stem Rust
8. Management of Leaf Diseases
9. 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

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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.

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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

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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.

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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.

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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.

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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.

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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.

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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.

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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.

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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%.

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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.

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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.

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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.

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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.

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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.

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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.

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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.

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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.

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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.

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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:

1. 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.
2. 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.
3. 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.
4. 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).
5. 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.
6. Future possibilities. Varieties with improved disease resistance, and fungicide sprays effective for controlling leaf diseases, likely will become available to farmers in Ontario.
7. 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

• The Agronomy Guide: Oat-Cyst Nematodes

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