Root and Crown Rots of the Grasses

Roderick Sprague.

Range and pasture grasses during various stages of their life are subject to attack by soil-borne parasitic fungi.

Some parasitic fungi can stand considerable drought, but most of the relatively delicate organisms need plenty of moisture. During the early life of the grass host, the fungi are well favored because the grass seeds germinate in moist soil, where the fungi are waiting. Later in its life grass may form mats of foliage, which help to keep the humidity high and therefore favor root and crown rots.

Root rots and crown rots are in total probably the most destructive group of parasites of the grass family. In parts of the Great Plains, for instance, they were one of the chief reasons why regrassing after the drought years was so difficult. Losses up to 100 percent from seedling blight, for instance, were general.

Most of the soil-borne fungus parasites of grasses develop fine filamentous mycelium without producing any conspicuous fruiting bodies. A few develop small, hard, black sclerotic, which later give rise to small fruiting bodies about one-third inch high. A number of them produce colored or obscure masses of spores on diseased parts. Others form microscopic or nearly microscopic fruiting bodies in diseased parts.

The identification of the various root rots and crown rots must depend to some extent on microscopic examination of the diseased tissue and frequently only after isolation of pure cultures. Certain symptoms, however, aid in distinguishing some of the diseases without great difficulty.

SOME OF THE FUNGI in the soil can survive alone and are called soil inhabiting. Most of them, however, depend on humus in the soil to survive during part of their life. Such fungi can grow through the soil for some distances away from the nearest particle of humus, but they must retain a life line of mycelium with the piece of humus from which they started. Such fungi are called soil invading.

Most of the root rot fungi are soil invading and can therefore be starved out by prolonged fallowing. They need nitrogen, phosphorus, and other chemicals to grow, the same as plants do. They tend to accumulate in the area next to the roots in the soil the rhizosphere. The region teems with activity from fungi, bacteria, and the plant roots. As the available supply of soil nutrients may become depleted, the fungi sometimes invade living cells in the roots of the grasses in search of food.

If they maintain a mild form of parasitism with mutual exchange of. nutrients, both host and fungus may benefit. Actually these mycorhizal fungi serve as root hairs. They grow out into the soil, seeking nutrients which they carry through their mycelial filaments into the cells of the plant. The mycorhizal fungi take from the plant but give food from the soil in exchange. If the parasitism is favorable to both fungus and plant it is called symbiosis.

Usually in grasses, however, when fungi invade the roots they are destructively parasitic. In young plants death may quickly follow. In older ones the process may be prolonged as a slow necrosis and "going-out" of old stands. All soil fungi are not parasitic. Some cannot attack living plant tissue. They are called saprophytes. Some are weakly parasitic or are starved into invading living tissue. Sometimes some of the Pythium species appear to act in this way. Other fungi prefer the parasitic life. They are especially to be feared.

Species of grasses differ in their resistance to root rots and crown rots. Sometimes the resistance may be due to some mechanical feature of the plant, such as stout cell walls, but more likely the resistance lies in some chemical antagonism within the cytoplasm of the cell itself. We also have some instances of strain differences in grass species in their tolerance to certain fungi, but in relation to root rots these instances are all too few.

The adaptability of the host to the environment greatly influences its ability to withstand attacks by soil fungi. If the grass, for instance, is a warm-temperature-loving grass from the southern plains, it will often be wiped out by relatively weak soil-borne parasites if sown in cold, wet soil in the northern Great Plains. If the grass prefers a particular soil acidity, it may die from root rots if grown in a place where the acidity is not favorable for the growth of the grass. A desert grass such as Indian ricegrass will soon damp-off or decay if grown in areas of high humidity and frequent rain. Also, grasses that are not hardy are sometimes so weakened that they are more subject to common root rots than are vigorous ones. Winter injury, however, is not a great factor in the ecology of root rot. Much of the injury is to young spring-seeded plants. Injury to older plants is sometimes associated with winter-injured plants or with plants growing in infertile soil. These cases are usually associated with weak parasites, such as certain species of Fusarium, Curvularia, and Gloeosporium bolleyi. Some strains of those fungi, however, are strongly parasitic, so that their presence does not necessarily mean that the root rot injury is on weakened plants.

THE SYMPTOMS the various fungi cause on the host are more interesting to the general reader than the exact identity of the organisms. We can classify the many rots into a few general groups on the basis of symptoms with a minimum of reference to the causal organism. It is important, however, to know something about the various species and genera of the fungi that cause the rots.

The same organism may cause more than one type of decay. For instance: Helminthosporium sativum may cause a seed rot or seedling blight if it occurs on moldy, untreated seed; several years later it may be involved in a crown rot of older plants.

1. Preemergence rots. The plant or seed is killed as the seed starts to germinate or just before it emerges from the soil. The rootlets are often rotted to short stubs or collapse in a soft decay. Seed rot in the soil is most frequent in early spring or late fall in cold, wet soil or later in the year if the seed is planted just before a heavy, beating rain. Warm-temperature grasses or grasses with small seeds are especially subject to preemergence rot. The loss of seed from this cause alone averages well above 25 percent of all seed planted. Much of the loss is so common as to be taken for granted.

The most common causes of seed rot are certain species of bacteria and some fungi, including Pythium debaryanum, P. ultimum, Fusarium culmorum, and Rhizoctonia solani.

Control of preemergence decay includes the judicious selection of appropriate seeding dates in well-drained soil; the use of viable, clean seed of recent harvest; and seed free from seed-borne molds. The seed should be treated with some seed-treatment material such as Arasan or Ceresan M.

2. Root necrosis, common root rots, and damping-off. Sometimes the plants that escape preemergence rots will develop a rapid soft rot and fall over soon after emergence. That is damping-off. Such fungi as Rhizoctonia solani and Pythium debaryanum are usually the cause of damping-off. Damping-off is common in ornamental and vegetable seedlings but less common in grasses. Sometimes one will encounter damping-off in prolonged rainy periods in thick stands that had emerged earlier during weather that was more favorable.

Grass roots are more likely to suffer from a slower decay, or necrosis. Necrosis due to Pythium debaryanum is likely in poorly drained soil or soil saturated by prolonged rains. The roots are often stubbed back by a slow and complete decay. Plants affected with pythium necrosis tend to recover rapidly when good growing conditions return.

The common root rot of maturing cereal plants has its counterpart in grasses. The grass roots are slowly killed by the action of Fusarium culmorum or Helminthosporium sativum and associated molds, sometimes alone, often competing in the roots of the same host. The Fusarium may cause pink or rosy colors on the dead roots, especially at the ground line, where masses of spores form.

Head scab of cereals and sometimes of grasses is caused by Fusarium (part of the F. roseum complex). This organism or group of organisms can also cause a seedling blight.

The associated fungus Helminthosporium sativum causes a brown blotch and root decay of wheatgrasses and, in fact, on many range and pasture grasses. Some seedling blight may result from it. It can also cause pre-emergence rot when its spores are borne on the seed of grasses raised in areas with high rainfall in summer. Such seed should be treated before seeding. Grass seed grown in the drier western regions, however, are usually free of seed-borne parasitic molds.

3. Root browning and seedling blights. In root browning, the root decay is represented by a firm, brown rot, usually of young rootlets. Its action is slow but often deadly. The diseased seedlings gradually fail to keep pace with healthy ones; about 6 weeks after seeding in the spring, they shrivel and die. They soon disappear in the winds that sweep across the plains and are replaced by weeds.

The main cause of seedling blight in grasses in the Great Plains and far West is another species of Pythium (P. graminicola). Its action differs from that of root necrosis caused by P. debaryanum in that the plants fail to recover when good weather returns. In fact, the damage often occurs during relatively favorable growing conditions. Some conditions favor its prevalence. It seems to be especially active in grass planted on old plowed sod. It outlasts the bacteria in summer fallow and can only be starved out by several years of continuous fallow. It is possibly less serious on ground that has last grown a crop of corn, oats, or potatoes.

Damage from the root browning fungus is sometimes lessened by the use of a balanced fertilizer that supplies nitrogen and phosphorus to the host and probably to the fungus. The same organism destroys the roots of mature plants in old stands, especially when the stands are sod-bound and have too little nitrogen.

Fall seeding, where practical, is helpful in checking loss from seedling blight. By the following June the plants have passed their most susceptible stage.

Seed treatment does little to check root browning. The sphere of control that seed-borne chemicals can effect is in a small zone close to the seed. Seed treatment helps control seed rot, but it is useless in controlling decay that originates on the rootlets just beyond the old seed.

In South Dakota long search for strains of grasses resistant to seedling blight has resulted in a few strains of grass said to have some tolerance to Pythium graminicola.

Other fungi besides P. graminicola cause seedling blights, but they are considerably less widespread and less often reach the devastating proportions that this disease assumes. The common root rot Fusaria, Helminthosporium, Curvularia, and Rhizoctonia sometimes kill seedlings.