| Scald is an important fungus disease of barley, rye,
smooth bromegrass, and orchardgrass worldwide in cool, humid, largely
temperate climates. Other grasses that may become infected in Illinois
are Canada and southern wildrye, reed canarygrass, redtop, Hordeum
distichon, quackgrass, and cheat.
Severe attacks of scald have caused yield losses as high as 20
percent in cultivated crops. Attacks at the late boot or early heading
stage reduce the size of the grain and the number of kernels formed
per head.
Damage is most severe in prolonged cool, damp weather in the early
spring and late autumn when the temperatures are 39 to 82 F (4 to
28 C). A soil temperature of 60 F (16 C) is optimum for seedling
infection of barley. Infection decreases sharply at 68 F (20 C),
and is absent at 71 F (22 C).
Hot, dry weather checks the spread of scald. The disease damages
grass and cereal plants by destroying the leaf tissues, thus reducing
the food-making ability of the plant.
Scald is caused by two widely distributed fungi in the genus Rhynchosporium
(R. secalis and R. orthosporum). Each species is subdivided into
a number of highly specialized races. Each one of these is restricted
to a single or closely related grass or cereal species.
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Figure
1. Barley scald. Many of the lesions have fused.
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SYMPTOMS
Scald is easily identified from the oval to lens-shaped or elongated
spots (lesions) which develop mostly on the leaves and leaf sheaths. At
first, the lesions are water-soaked, with a dark bluish green to pale
grayish green color. Later they dry out, the centers become light tan
or straw brown to grayish white, and are surrounded by prominent, dark
brown to reddish brown borders (straw-colored on rye) that are often wavy.
With time the lesions enlarge (ranging from 1/2 to 2 inch or more long),
merge, and form elongated, irregular blotches of various shapes and sizes
that girdle the leaves. The tip of the leaf beyond the lesions collapses
and dies (Figures 1 and 2). Sometimes older lesions have a "zoned"
appearance. The lower leaves often have a "scalded" look and
die prematurely. The scald symptoms are strikingly similar on all cereals
and forage grasses.
Under severe conditions, inconspicuous, medium brown lesions occasionally
appear on the awns and tips of barley glumes. Lens-shaped lesions, which
are dark blue to pale grayish green, may appear at the base of the kernels.
DISEASE CYCLE
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The Rhynchosporium fungi overseason as mycelia on dead or living
leaves of infected plants and on other crop debris. During prolonged
periods of cool, moist weather in the spring, the scald fungi resume
growth on fall-infected tissues and produce large numbers of colorless,
two-celled, microscopic spores called conidia (Figure 3), which
develop in a thin layer of slime on the surface of the lesions from
a stroma of spore-producing mycelia. The conidia are carried by
splashing rains and air currents to new growth, where the leaves,
leaf sheaths, and seedlings become infected.
Spore production and infection occur repeatedly during cool, moist,
humid periods that last at least 12 hours in the spring and early
summer, and continue until the crop ripens. Scald is checked during
hot, dry, summer weather. New infections occur in the fall when
cool, damp weather returns. The scald fungi are not carried within
seed, but can be carried on the seed. Large numbers of conidia are
produced on the seed and may infect seedlings when the soil temperature
is around 60 F (16 C). The Rhynchosporium fungi can survive on grass
and cereal stubble for up to a year.
Scald lesions provide suitable sites for the growth of the variety
of facultative or saprophytic fungi.
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Figure
2. Scald of barley leaves caused by Rhynchosporium secalis.
Courtesy G.H. Boewe).
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CONTROL
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1. Sow only certified, disease-free seed of improved, well-adapted
varieties of cereal grains and grasses recommended by University
of Illinois Extension Agronomists and your nearest Extension adviser.
Refer to the Illinois Agronomy Handbook and Illinois Extension Circular
1231, Condensed Plant Disease Management Guide for Field Crops.
Both publications are revised annually.
2 Grow scald-resistant varieties of barley and smooth bromegrass
when they are available and are recommended. If scald becomes serious
enough, it should be possible to develop resistant varieties of
other cereals and forage grasses. Physiologic races of each species
of Rhynchosporium must be considered in the development of resistant
varieties.
3. Treat the seed, where feasible, with a suggested seed-treatment
fungicide.
4. Plow under cover crops, severely infected stands, volunteer
rye or barley, and plant debris. Where practical, seed a mixture
of forage species. Do not grow pure, dense stands of a single grass
variety.
5. Follow recommended mowing and grazing practices. Cut early and
remove from the field any hay crop that becomes heavily infected.
This practice reduces losses in hay quality and removes inoculum
that may threaten future cuttings. Do not leave a heavy mat of hay
on the grass during cool, damp weather. Leaf-blighting diseases,
such as scald, are seldom destructive in frequently cut or closely
grazed pastures.
6. Keep down weed grasses by cultural or chemical means. Follow
suggestions of University of Illinois Extension Agronomists.
7. A carefully controlled burning of dead grass in the early spring
may be warranted if pastures are severely affected. This ancient
practice destroys organic matter, but kills leaf-blighting fungi
and bacteria on over-wintering leaves and other crop residues.
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Figure
3. Rhynchosporium secalis, one of the two causes of scald of
cereals and grasses, as it appears under a high-power microscope.
Note the two-celled, colorless conidia borne on a stroma at the
surface of a lesion (drawing L. Gray).
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