Food and Feed Grains

Root Rots of Wheat, Oats, Rye, Barley

J. J. Christensen.

The root rots of wheat, oats, barley, and rye are among the least conspicuous diseases of cereals, but they are among the most destructive.

They are caused by many species of fungi, which are widespread and live on or in seed, soil, and dead plant refuse. They attack a large number of hosts and thrive under a wide range of conditions.

To the many types of root rots in cereal crops many names have been given seedling blight, take-all, root rot, basal stem rot, foot rot, snow mold, Victoria blight, the helminthosporium blight, stem break. I use the term root rot here to include all diseases that affect roots and basal portions of the culms, although occasionally I refer to special types of root rot.

Root rots of cereals were prevalent in the United States many years ago, but they did not attract attention until about 1900. H. L. Bolley in North Dakota in 1909 emphasized the importance of root rots on cereals in the United States. He proved that the root rots of wheat were caused by several fungi that accumulated in the soil, particularly if crop rotation was not practiced. He attributed the unproductiveness of certain wheat lands in the Northwest to root rots, instead of depleted soil fertility or to toxins. The discovery of wheat rosette (a virus disease) in Illinois in 1919, originally attributed to root rot fungi, and the severe outbreak of root rots in the rust nursery at St. Paul, Minn., in 1919 and 1920 stimulated further investigations.

In the United States the prevalence and severity of root diseases depends largely on the particular kinds of crop and varieties. In general, take-all is the most important root disease in winter wheat areas, cercosporella root rot is prevalent only in the Columbia Basin of the Pacific Northwest, and helminthosporium and fusarium root rots are usually more destructive in the spring wheat region. The diseases are not restricted to any region, however. For instance, root rot of oats, caused by Helminthosporium victoriae, has been destructive in all districts where varieties from Victoria crosses are grown.

Root rots probably are present every year in all fields of cereals in the United States. Sometimes they cause enormous losses to wheat, oats, and barley. In some places they are the limiting factor in crop production and prevent the growing of certain varieties of cereals in a region, such as barley varieties susceptible to common root rot in Minnesota and oat varieties susceptible to helminthosporium blight in the United States.

The amount of damage they cause varies greatly from season to season and from one field to another and often is difficult to determine because of the nature of the diseases.

Root rots usually are not conspicuous, like the rusts and smuts. Infected plants commonly reach maturity, although various organisms may injure their roots. Unless damage is severe and plants are actually killed in definite places, as frequently happens in fields infected by the take-all and snow mold organisms, damage caused by root rots frequently is overlooked. Infected plants yield less. Their seeds are fewer, smaller, sometimes shriveled, and of poorer quality than those from healthy plants.

Take-all is a major disease of wheat in Australia. Losses may be as much as 60 percent. Heavy losses also have been reported in the United States and Europe.

Two destructive epidemics of root rots of barley occurred in 1943-1944 in the upper Mississippi Valley. Barley production in Minnesota dropped from about 50 million bushels to less than 13 million. Yields dropped more than 30 percent.

Helminthosporium blight of oats was first observed in Iowa in 1944. By 1946 it was prevalent in every important oats-growing State, and ruined many millions of bushels of oats. In Iowa and Illinois it caused an estimated loss of at least 20 percent of the potential oats crop more than 20 million bushels in Iowa alone. The disease was so destructive in 1947 that the growing of varieties such as Vicland and Tama that were released to growers because of their resistance to crown and stem rusts actually had to be discontinued.

THE SYMPTOMS of some root rots, such as typhula and take-all, can be diagnosed readily. But it is nearly impossible to distinguish the symptoms of root rots caused by some organisms, and one must use laboratory procedures to identify the cause. The difficulty arises because the symptoms caused by different pathogens often overlap. They may be almost identical under some conditions. Two or more pathogens may be associated on a given host. Saprophytes maybe present and may modify the symptoms.

Root rots generally are characterized by seedling blight, stunting of plants, yellowing and bleaching of foliage, discoloration of roots and bases of the stems, and premature killing of adult plants. The same root-rotting organisms also may attack the upper part of the plants and cause head and seed blight, foliage lesions, and rotting of nodes.

Take-all, caused by Ophiobolus graminis, is a common root rot of wheat. It also attacks rye, barley, and oats. The diseased plants frequently are stunted, ripen prematurely, and occur in patches, which stand out among the normal plants. The black lesions of the rotted tissue, sometimes with conspicuous mycelium at the base of culms, are easy to spot.

The root rot that is caused by cercosporella herpotrichoides is called foot rot or stem break. It makes eyespot lesions at the base of culms. Infected plants have a tendency to fall over as the heads fill and make patches of lodged and tangled plants. The pathogen attacks wheat, barley, and rye, but not oats. It may be the same as Leptosphaeria herpotrichoides, the cause of the destructive stem break of winter wheat in northern Europe.

The root rot symptoms caused by species of Pythium vary and are influenced largely by the crop and the organism: In Japan they are associated with certain types of snow mold on wheat; in Canada the disease is called "browning" because of the brown color that follows the gradual dying of foliage; in the United States Pythium causes root rot of mature plants and seedling blight of wheat, oats, and barley. The symptoms produced by Pythium may be confused with other types of root rot. The leaves of infected plants, particularly the lower ones, tend to lose their green color. Infected roots develop reddish-brown lesions.

Snow mold is common in winter cereals in northern Europe. It is caused by several fungi, among them Calonectria graminicola, Typhula itoana, and Pythium species. The outstanding symptom is a conspicuous mycelial growth, which covers the affected seedling as the snow disappears in spring.

The root rot caused by Colletotrichum graminicola is an important disease on oats. The fungus produces a distinct black stromatic formation (cushion-like) at the base of the stem near the surface of the soil.

The so-called common root rot fungi on wheat, oats, barley, and rye produce few distinguishing symptoms. They generally attack all underground Parts of the plant and cause varying degrees of rots on roots, crown, and basal parts of stems. The diseased tissue often has large light-brown to black areas. Common root rot may kill plants at any time from germination until maturity. They may or may not be stunted. Seedling blight commonly occurs. Symptoms usually are not apparent on adult plants unless their bases are examined. Common root rot is caused by many species of fungi. Helminthosporium sativum, H. victoriae, H. avenge, Fusarium graminearum, F. culmorum, F. nivale, and Rhizoctonia solani are the more common ones.

ROOT-ROTTING FUNGI usually cause most damage when conditions are unfavorable for the development of the host plants. Temperature, moisture, the amount of inoculum, and mechanical injuries of the host, such as insect damage, have a bearing on the severity of root rots. Rusts and foliage diseases may predispose plants to root rots. Sometimes that effect is indirect severe rust in the fall on winter grain may make plants more subject to winter injury, which, in turn, predisposes the plants to root rot.

Soil temperature affects both host and pathogen, but its effect apparently is more important on the host. High temperatures, which are detrimental for normal development of wheat, oats, barley, and rye, favor the growth of many of the root-rotting fungi. Seedling blights and root rots generally are most severe at relatively high temperatures. Early seeding of spring-sown grain therefore reduces the incidence of root rot and increases yield. The organisms associated with snow mold, however, are most destructive at relatively cool temperatures and cause the greatest damage while the hosts are covered with snow. Winter injury to fall-sown grain may predispose plants to root rots.