Reports on Plant Diseases |
| RPD No. 311 -
Selenophoma Leaf Spot or Speckle of Forage Grasses |
June
1980 |
[ Symptoms
] [ Disease Cycle ] [ Control
]
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Selenophoma leaf spot, speckle, blotch, or eye spot is caused by
the fungi Selenophoma bromigena and S. donacis. The disease is common
and is destructive in Illinois on smooth bromegrass and other Bromus
species. Many grasses are susceptible, including timothy, fescues,
orchardgrass, bluegrasses, reed canarygrass, wild ryegrasses, switch
grass, common reed, tall oatgrass, needlegrasses, prairie Junegrass,
squirreltailgrass, dropseed grasses, sandreed, and California oatgrass.
Selenophoma is not important on oats, rye, wheat, and barley.
The disease is most damaging when the weather is cool (60 to 70
F, or 16 to 21 C) and moist during the spring and autumn. Hot, dry
weather checks the disease. Infected leaves turn yellow, die, and
fall off. Severely infected plants may be stunted or killed prematurely.
Seed set may be reduced greatly. In orchardgrass, significant reductions
in crude protein and carbohydrate can occur when the severity of
the disease is only moderate.
A number of distinct groups, strains, or races of the Selenophoma
fungi are known. They differ in their ability to infect the various
grasses and cereal grains.
Symptoms
Small flecks or speckles that are brown to purple form on the leaves,
leaf sheaths, and stems (culms) in the early spring. If the weather
is cool and damp, the spots may enlarge later and merge to form
lesions that are round to lens-shaped or irregular and are gray
to straw-colored with narrow borders that are brown, red, or purple
(Figure 1). Infection often spreads from the leaves to the culms
and seed heads. Small fungus fruiting bodies (pycnidia) that are
golden-brown to black develop in light-colored centers of the older
lesions (Figure 2). The mature pycnidia commonly drop out, leaving
small holes in the lesions.
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Figure
1. Selenophoma leaf spots or "speckles" on smooth
bromegrass (courtesy University of Wisconsin).
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Disease Cycle
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Figure 2.
Extreme closeup of Selenophoma leaf sot on smooth bromegrass. Note
dark spore-bearing structures (Selenophoma pycnidia) within the
lesions (courtesy University of Wisconsin).
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Air currents and splashing rains carry the pycnidia, with their
enclosed spore masses, to other plants. The spores inside the fruiting
bodies can remain alive for at least 18 months and probably overwinter
there. The spores, germinating in free water on the leaves and culms,
produce infection in cool, damp weather during the spring or early
summer and again in the fall. The Selenophoma fungi are seedborne.
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1. Sow only disease-free seed of improved, well-adapted grass varieties,
as recommended by University of Illinois agronomists and the area Extension
adviser. Resistant varieties of smooth bromegrass and orchardgrass have
been developed; however, the adaptability of these resistant cultivars
to Illinois is not yet known. Plant certified seed whenever available.
2. Treat the seed, where feasible, with a thiram- or captan-containing
seed disinfestant. For details, see Report on Plant Diseases No. 1001,
"Seed Treatments for Field Crops." Seed treatment helps prevent
the introduction of Selenophoma fungi, carried with the seed, to new fields.
3. If warranted, employ a careful, controlled burning of dead grass in
the early spring if pastures are affected severely. This ancient practice
destroys organic matter, but kills leaf-blighting fungi and bacteria in
the overwintering leaves, stems, and other crop refuse. Check local EPA
regulations about open burning.
4. Rotate with nongrass crops where practical. Rotation helps prevent
buildups of the disease.
5. Keep down weed grasses by cultural or chemical means.
6. Avoid the following:
a. Excessive rates of fertilizers high in quickly available nitrogen.
b. Pure, dense stands of a single grass variety. Where practical, seed
a mixture of forages.
c. Close grazing and clipping. Follow recommended mowing and grazing
practices.
d. Leaving a heavy mat of hay on the grass during cool, damp weather.
7. Maintain adequate soil fertility, especially of potassium and phosphorus,
based on a soil test.
8. Cut early and remove from the field any hay crop that becomes heavily
infected.
9. Plow-under cleanly the cover crops and plant debris.
In the future, resistant varieties of smooth bromegrass and orchardgrass
should become available.
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For further information concerning diseases of crucifers
and other vegetables, contact Mohammad Babadoost, Extension Specialist in Fruit
and Vegetable Pathology, Department of Crop Sciences, University of Illinois
at Urbana-Champaign.
University of Illinois Extension provides equal
opportunities in programs and employment.
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