The Northern Forage Grasses

Kermit W. Kreitlow.

Many of the organisms that attack western and southern grasses occur on northern grasses. I mention them briefly and discuss in more detail the other serious diseases on northern grasses.

Redtop (Agrostis alba) is a perennial species that occurs most commonly in poorly drained areas but grows well on poor, acid soils. The most widespread and important disease of redtop, brown stripe, is discussed in the section on needlegrasses, page 258.

Redtop also is attacked by rust and smut. In the humid northeastern and north central sections, redtop is damaged by several species of Helminthosporium. The most common, H. erythrospilum, is termed red leaf spot. It also attacks other species of Agrostis.

Small spots, with round, water-soaked areas, develop on leaves. They turn straw-colored, with a reddish or brownish-red border. As the spots lengthen and merge, they form streaks and cause the leaves to wither and turn brown. Sometimes the leaves wither immediately without developing leaf spots. The injury then resembles the effect of drought.

Infection occurs from spores spread by wind and rain. Heavy infection follows periods of warm, wet weather. The fungus overwinters on old, dead leaves. Resistant plants exist within the species, and it should be possible to develop resistant varieties.

Tall oatgrass (Arrhenatherum elatius) is not so widely grown as some species, but it is important in mixtures for hay or pasture in some regions. Brown stripe (Scolecotrichum graminis) is one of its most important foliar diseases. It is also susceptible to anthracnose (Colletotrichum graminicola), one of the common pathogens on a great many grasses. Anthracnose injures many cereals and grasses in the humid north central area in late summer or autumn as the plants approach maturity. If conditions are favorable for development of the disease, the fungus attacks seedlings, causing stunting and wilting. In older plants, the fungus attacks the culm or leaf sheath, but it may spread into the crown and roots of perennial grasses, frequently causing stands to die out in the second or third season, especially in infertile places. Early attacks cause a general reduction in vigor and premature ripening or dying. The disease is especially destructive to Sudangrass, which it attacks at the height of vegetative vigor in midsummer.

Lesions on leaf sheath or stem are usually light tan with a darker border of red or brown. The center of the lesion usually contains small, dark specks, the fruiting organs of the fungus. Sometimes stems are attacked early at the nodes. The result is premature ripening and shriveling of seed. Affected stems usually bleach and turn brown at the base. On leaves of large grasses like Sudangrass, the fungus forms small round or oval spots, straw-colored with a pigmented border or zonate. The spots eventually form long bleached streaks on which black fruiting organs are scattered.

The fungus is spread by spores and fungus threads (mycelium), which live saprophytically on crop residues. Spores of the fungus can germinate and penetrate uninjured leaves to establish infection. In some grasses the disease is seed-borne and infects seedling roots and crowns. The fungus thrives at 80 F. the reason for its prevalence in midsummer.

General control measures include adequate soil fertility, crop rotation (but not a sequence of closely related species), complete coverage of old plant residues, and the use of resistant varieties.

Smooth brome (Bromus inermis) and related species are severely damaged at times by several diseases. One, brown spot, caused by Helminthosporium bromi, is common. It appears as small, dark-brown, oblong spots on the first leaves to develop in spring. The spots later merge and form large, yellowed areas on leaves. Older spots are generally dark purple to brown, surrounded by a yellow border or halo. Affected leaves turn brown from tip to base, then wither and die. Fruiting bodies (perithecia) of the fungus form in diseased leaves in summer and carry the fungus through the winter.

In early spring when adequate moisture is present, ascospores are discharged from the fruiting bodies developed in old leaves the previous summer and are carried by wind and rain to initiate infection on new leaves. All perithecia do not discharge spores at one time, and a continuous supply of inoculum is provided throughout spring and early summer by successive liberation of spores from the fruiting bodies. The disease also may be seed-borne.

Brown spot develops best during cool, wet weather. In the Northern States it reaches its peak about the first week in June. It spreads very little during hot, dry periods of midsummer. It becomes prevalent again in the fall.

Improved varieties of smooth brome resistant to brown leaf spot are being developed at several agricultural experiment stations. They should help reduce losses.

Another disease of smooth brome is leaf scald, caused by Rhynchosporium secalis. That fungus also attacks barley, rye, and many other grasses. A different species, Rhynchosporium orthosporum, parasitizes orchardgrass and a few other grasses. Leaf scald is widespread in the cooler humid sections of North and South America, Europe, and Asia. It occurs primarily on leaf blades and less extensively on the sheaths. It is most destructive in spring and autumn.

The symptoms, similar on the different hosts, consist first of small, water-soaked, bluish-gray, ovate spots that enlarge to form irregular, light-gray, conspicuous scaldlike blotches. The blotches usually have darker brown margins. Symptoms may develop on the first green leaves. If conditions are favorable, the disease becomes progressively more destructive as the season advances. Scalded leaves are often killed, and almost complete defoliation results. The hot, dry weather of midsummer retards the disease, but it develops again in the fall.

The leaf scald fungus overwinters on dead leaves in old crowns and (in milder climates) in lesions on perennial grasses. Abundant spores are produced in old lesions. Wind carries them to infect new leaves during cool, wet periods. The best temperature for infection lies between 60 and 70 F., but the spores can germinate over a range of 40 to 80 .

A number of specialized races of the fungus must be considered in developing resistant varieties. Resistant lines of smooth brome exist, and new varieties are being developed that contain resistance to the pathogen. Crop rotation, elimination of old plant residues, and spring burning help control the disease.

Leaf spot (Selenophoma bromigena), although destructive, is more restricted in its range. It occurs primarily in the Central and Western States, Canada, and parts of Europe. The fungus can attack other species of Bromus. It has started to become increasingly prevalent on mountain brome (Bromus marginatus) in the West.

The first lesions appear as small, brown flecks on leaves early in spring. The lesions usually remain localized, but if conditions are good they may enlarge and merge. The spots generally are gray and round or irregular. They have narrow brown borders. Infection often spreads to the stem and seed heads. Infected leaves turn yellow and die; defoliation, stunting, and death result. Tiny fruiting bodies of the fungus develop in the discolored lesions; they frequently drop out of the tissue, leaving small holes in the lesions.

The spores inside the fruiting bodies can remain alive at least 18 months. Probably they overwinter there. Wind and rain carry the fruiting bodies to other plants. The fungus also is carried on the seed.

High humidity and temperatures of 60 to 70 F. favor infection. Heavily infected leaves of smooth brome have been reported at St. Paul, Minn., as early as March 15. The disease develops rapidly in April, May, and early June but is relatively inactive in summer. It appears again in September and October.

Crop rotation and spring burning are the best temporary means for control. Resistant varieties may be developed eventually.

Smooth brome and other species of Bromus sometimes are seriously damaged by a bacterial disease (Pseudomonas coronafaciens var. alropurpurea); brown stripe (Scolecotrichum graminis); and septoria leaf spot (Septoria bromi).

Orchardgrass (Dactylis glomerate) harbors many diseases. Some, such as brown stripe (Scolecotrichum graminis), scald (Rhynchosporium orthosporum), and the rusts, are widespread. Others, such as the leaf spots caused by Mastigosporium rubricosum and Stagonospora maculata, are more prevalent and destructive in the Northwest and Northeast, respectively. The effects of brown stripe, scald, and rust are discussed on pages 258, 254, and 276; the leaf spot diseases caused by Mastigosporium rubricosum and Stagonospora maculata are dealt with here. Mastigosporium rubricosum attacks several grasses besides orchardgrass. The leaf spot fungus is commonly found in spring and autumn in the more humid parts of the Northwest. Nurilerous dark-purple or brownish flecks form on the leaves and enlarge to elliptical spots with ashy-gray or fawn-colored centers. Later the spots become gray with red or purple borders. When lesions are numerous, leaves are killed, and the plants are largely defoliated.

Spores produced in old lesions spread the disease. The spores help the fungus survive the winter and summer. The fungus also can survive in a vegetative state in old leaves.

Infection develops most extensively during rainy, foggy weather and is largely checked during long periods of low humidity. Near the west coast, the disease is most evident during the winter and is largely checked in summer.

Very little has been done to control the disease. It may be possible to select resistant lines.

Stagonospora maculata is a fungus that causes effects similar to the mastigosporium leaf spot, but it occurs primarily in the Northeast. It attacks only orchardgrass but related species can attack many grasses.

It produces small, somewhat elongated, dark-brown or dark-purple lesions on leaves of orchardgrass. Leaves of some plants are so heavily spotted that they turn brown, wither, and die prematurely. Sometimes the brown area develops first at the tip of a leaf and gradually works back toward the base until the leaf is completely brown. On other leaves long, brown streaks develop along the margins.