Distribution—Pacific Northwest
Cause—Barley Yellow Dwarf Virus (BYDV) is trans- mitted by several aphid species. The disease is more severe under cool and moist conditions.
Symptoms and effects—The most common symptom is discoloration of the leaf tip, which can turn yellow to reddish-purple. Older leaves are almost always un- evenly discolored, with bright yellow blotches along the leaf margin. Symptoms can include shortened leaves or curled margins at the leaf tip. Affected leaves may die prematurely under high temperatures and water stress. Plants are stunted or dwarfed, with an un- derdeveloped root system that makes them more sus- ceptible to drought stress and soil-borne pathogens. Foliar symptoms can be confused with nutrient defi- ciencies, aster yellows, or stress, so the presence of BYDV needs to be confirmed by laboratory testing.
Control—Control BYDV indirectly by altering the sowing date to establish plants before or after aphid flights or by controlling aphids with systemic insecti- cides. Provide infected plants with adequate fertilizer and water to reduce stress.
Species affected—BYDV affects more than 150 grass species, including timothy, perennial ryegrass, Ken- tucky bluegrass, orchardgrass, and fescues.
Brown stripe
Distribution—Pacific Northwest
Cause—Cercosporidium graminis, a fungal pathogen surviving in diseased plant material and debris. Infec- tion is promoted by cool, moist weather.
Symptoms and effects—Brown to gray, elongated leaf spots appear in midspring or fall (plate 9.2). They are followed by the appearance of gray, blackish groups of spore-bearing structures (conidiophores) that extend beyond the leaf openings (stomata). Early- season infection can contribute to significant leaf loss.
Control—Chemical control may be necessary if envi- ronmental conditions are favorable for disease devel- opment. Apply foliar fungicides during stem
elongation. Do not allow livestock to graze in treated areas or feed treated plant parts to livestock.
Species affected—All cool-season forage grasses Net blotch
Distribution—Pacific Northwest
Cause—The fungal pathogens, Drechslera spp. (syn.
Helminthosporiumspp.), overwinter in infected plant material. They are transmitted by equipment, wind, rain, and irrigation water. Infection is favored by cool and wet conditions in the spring. Free moisture is nec- essary for infection.
Symptoms and effects—Initial lesions are small and water soaked, but change to reddish brown to purplish black. Lesions are oriented both parallel and crosswise to the leaf axis. As they elongate, they develop a net- like appearance (“net blotch”). Individual lesions may coalesce, and leaves may die.
Control—Avoid prolonged leaf wetness. Chemical control is suggested only for crops grown for seed pro- duction when disease pressure is high and prolonged conducive conditions are forecast. No chemical con- trols are available for pastures.
Species affected—Cool-season grasses Purple eyespot
Distribution—Pacific Northwest
Cause—The fungal pathogen, Mastigosporium rubri-
cosum, survives in infected leaves. Cold, wet condi- tions during winter and early spring favor infection.
Symptoms and effects—Initial symptoms include small, dark purple to brownish spots. Spots merge to
form elliptical lesions with gray to brownish centers and red or purple borders. Severely infected leaves may die early, and the overall quality of the crop may be reduced.
Control—Seldom needed.
Species affected—Orchardgrass, bentgrass, and timothy
Scald and leaf blotch
Distribution—Pacific Northwest
Cause—The fungal pathogens, Rhynchosporium spp., survive in living or dead plants. Disease development is favored by prolonged cloudy, wet, cool spring weather. Infection is followed by a latent period of 10 to 14 days before symptoms develop.
Symptoms and effects—First symptoms include oval to elongated lesions ranging in color from dark to pale to bluish gray. Lesions may appear water soaked. If in- fection progresses, lesions develop a light gray, tan, or white center and a dark brown edge (plate 9.3). A chlorotic region surrounds the lesion. Centers also dry out.
Control—Remove or burn diseased and dead plants to minimize primary inoculum. Harvest or graze earlier to prevent disease spread. If growing crops for seed production, apply foliar fungicides during wet condi- tions in the spring. No chemical controls are available for pastures.
Species affected—Orchardgrass Rusts
Distribution—Pacific Northwest, but the incidence fluctuates yearly. Leaf rust is seldom of economic im- portance in Idaho.
Cause—Puccinia spp. can cause stripe, leaf, or stem rust. These fungal pathogens are very host-specific; they attack only one or, if an alternate host is needed, two hosts. All Puccinia spp. need free moisture and temperatures above 50°F to infect the host. The stripe rust pathogen, P. striiformis, survives on infected plants. P. triticina, the leaf rust pathogen, may over- winter on perennial grasses, but an alternate host (meadow rue) is necessary to complete its life cycle. Stem rust is caused by P. graminis subsp. gramini-
cola, which survives in overwintering plants. How- ever, the absence of alternate hosts does not translate
into an absence of disease in grassy forages, because rust can overwinter in infected grasses.
Symptoms and effects
Stripe rust—Symptoms appear earlier in the season than those of leaf and stem rust. Yellow to orange pus- tules develop in a linear pattern on leaf blades and sheaths (plate 9.4). When infection is severe, forage yield can be reduced considerably.
Leaf rust—Small, scattered, circular to oval, orange- red pustules develop on the upper surface of leaf blades and sheaths (plate 9.5).
Stem rust—Brick-red pustules develop on all above- ground plant parts (plate 9.6). The pustules rupture the plant’s epidermis, causing the lesions to appear ragged and the plant surface to feel rough. Depending on the degree of infection, pustules are scattered or coalesce, especially later in the season. Older pustules contain overwintering black spores that cause no damage to grass plants.
Control—Use resistant varieties when available. Inter- rupt the rust life cycle by eradicating alternate hosts. Labeled foliar fungicides can be used to control rusts.
Species affected—Orchardgrass, Kentucky bluegrass, fescues
Septoria and Stagonospora leaf spots
Distribution—Pacific Northwest
Cause—The fungal pathogens, Septoria spp. and
Stagonosporaspp., can survive unfavorable conditions in plant debris. Infection occurs mainly during cool, wet weather in the spring and fall, when free moisture is present on leaves.
Symptoms and effects—Infection may start near the leaf tip. Small lesions may appear as stripes, spots, or blotches. Lesions may be gray, gray-green, light or dark brown, or dark purple. Older lesions may enlarge and fade to a straw color. Leaf tips can turn chlorotic and mottle.
Control—No fungicides are labeled for control of Sep-
toriaspp. and Stagonospora spp.
Species affected—Bluegrass may be seriously dam- aged. Infections of tall fescue and ryegrass are rarely severe.
Slime molds
Distribution—Pacific Northwest
Cause—Slime molds are caused by myxomycetes. These organisms live in the soil or thatch, and their de- velopment is favored by wet conditions and abundant leaf litter. Their appearance on plants is not considered a disease, and only severely affected plants may show a reduced rate of photosynthesis.
Symptoms and effects—Slime molds cover leaves with a grayish, white, or purplish brown mass of small, round fruiting bodies.
Control—Not necessary.
Species affected—All grasses Smuts and bunts
Distribution—Pacific Northwest
Cause—Head, kernel, stem, and stripe smuts, as well as common and dwarf bunts, are caused mainly by fun- gal pathogens belonging to the genera Ustilago,
Tilletia, and Sphacelotheca. These pathogens may be soil-borne or seed-borne. Only the stem and stripe smuts are of relevance to forage grasses.
Symptoms and effects—The plant parts affected de- pend on the species of smut. Stem smuts produce dark brown to black masses of smut spores on stems. Plants infected with stripe smuts have long, narrow, grayish or black stripes on their leaves and stems. Infected plants may die, especially during the summer.
Control—Pathogen-free seeds and seed treatment with labeled fungicides may reduce the incidence of some smuts during the first year of production. Avoid high nitrogen applications, which may increase dis- ease.
Species affected—Certain species of wheatgrass, rye- grass, and bluegrass are most commonly affected. Snow molds/Fusarium patch
Distribution—Probably the entire Pacific Northwest
Cause—Snow molds can affect many grasses if snow cover persists for at least 100 days and soils are lightly frozen or nonfrozen. Pink snow mold and Fusarium
patchdescribe different phases of this disease; both are caused by Microdochium nivale. Pink snow mold is mostly associated with snow melt, while Fusarium patch describes the occurrence of M. nivale without snow cover. This disease can occur year-round and is
promoted by wet, cool conditions. Gray snow mold (also known as Typhula blight) is caused by Typhula spp. It can coexist with pink snow mold, but requires snow cover throughout the winter.
Symptoms and effects—M. nivale can cause circular patches. Under snow cover, plant leaves may exhibit a fluffy, white mycelium that turns pinkish when ex- posed to sunlight (plate 9.7). Symptoms caused by Ty-
phulaspp. become apparent at snow melt. Leaves are matted together and sometimes covered with a light to dense, white to gray mycelium. Mycelium disappears when grass dries, but leaves in infected areas become grayish to silver white and brittle.
Control—In general, avoid heavy fertilization in the fall and snow compaction. Promote fast drying and good drainage.
Species affected—Many grasses
Diseases of clover
ROOT DISEASES OF CLOVER Damping-off and seedling blightDistribution—Pacific Northwest
Cause—Various species of the soil-borne fungi and fungus-like organisms Rhizoctonia spp., Pythium spp.,
Phytophthoraspp., and Fusarium spp.
Symptoms and effects—Two types of damping-off occur: pre-emergence and post-emergence.
Pre-emergence damping-offis often referred to as seed decay because poor plant emergence leads one to believe the seed decayed. This disease reduces the stand.
Post-emergence damping-off occurs while the plants are emerging or soon thereafter. A lesion develops on the stem, which becomes discolored and collapses. The plant dies quickly, and a stand can be nearly de- stroyed within 2 or 3 days. Surviving plants may be weak and yield poorly.
Control—Use quality seed treated with a fungicide ef- fective against Pythium. Plant into a good seedbed in well-drained soil. Do not overirrigate, especially when plants are small.
Root and crown rots
Distribution—Pacific Northwest. In Idaho, these dis- eases are most severe from the Rupert-Burley area to western Idaho.
Cause—Depending on weather conditions, various species of soil-borne fungi and fungus-like organisms, such as Fusarium spp., Rhizoctonia spp., Pythium spp., and Phoma spp., can attack plants in all develop- mental stages, but are more pronounced during the second year of plant development. Crown infections are enhanced by wounds created by winter injury, root insects, livestock, machinery, frequent cutting, or des- iccation.
Symptoms and effects—Foliar symptoms can in- clude curled leaf edges, gray color, and wilting. In- fected plants are stunted and yellowish during dry, hot conditions, and they require more frequent irrigation due to their shallow roots. Root symptoms include the absence of lateral and hair roots, as well as pale yel- low, brown, or black streaks on and in the roots. Inter- nal rot may be restricted or can extend over the length of the taproot (plate 9.8). Infected roots of older plants are extensively branched, resulting in shallow-rooted plants. The entire center of the crown may exhibit dry rot, leaving a whorl of buds at the extremity of the crown. Yields are reduced, and infected plants are more susceptible to cold and winter injury.
Control—Use resistant varieties when available. Im- prove plant health with adequate nutrients, uniform soil moisture, and recommended harvest intervals. Control root insects. Avoid damage to roots and crowns from late-fall grazing and spring harrowing.
Species affected—Most clovers; may be severe in red clover.
Sclerotinia crown rot and wilt
Distribution—Pacific Northwest
Cause—Sclerotinia trifoliorum, a fungus that survives as hard, black sclerotia within infected plant debris or in the soil. Sclerotia are similar in size to wheat kernels and can survive for multiple years. Infection is favored by moist, cool fall weather. Damage occurs during win- ter when conditions are mild or snow cover is present.
Symptoms and effects—Symptoms caused by S. tri-
foliorumbegin in late fall as small, brown leaf and stem spots. Infected leaves drop and are covered with a dense, white fungal mass. The disease spreads to the crown and root system. In spring, infected crowns
develop a soft, watery rot. New growth wilts, dies, and may be covered with fungus. From March to June, dis- eased plants occur individually or as patches in a field. In severe cases, individual patches merge and form large areas of dead plants. Stands can be reduced con- siderably during early spring.
Control—Plant disease-free seed and resistant vari- eties if available. Rotate with non-legumes for 4 years. Avoid excessive nitrogen fertilization. Plow deeply to bury sclerotia.
Species affected—All clovers
Nematodes (clover-cyst, root-knot)
Distribution—Pacific Northwest
Cause—Clover-cyst nematode (Heterodera trifolii) and root-knot nematode (Meloidogyne hapla)
Symptoms and effects—Plants look stunted. Under magnification, female clover-cyst nematodes look like miniature brown lemons attached to the root. Root- knot nematodes cause visible knots and galls on the roots.
Control—See “Root diseases” under “Diseases of grasses.”
Species affected—May affect white clover and other legumes.
DISEASES AFFECTING STEMS OF CLOVER