Diseased seeds often are a source of primary root infection and are important in relation to seedling blight. Cereal seeds grown in the more humid parts of the United States are commonly attacked by a large group of fungi and bacteria. Fungi of more than 25 genera and several kinds of bacteria have been isolated from wheat and barley kernels. Many are saprophytic or weakly pathogenic. Species of Alternaria, which usually are non-pathogenic, are the most common organisms associated with kernels of grain, but Helminthosporium species and Fusarium species are the most common virulent root-rotting pathogens prevalent in the seed. Preemergence killing, seedling blight, root rot, basal stem rot, and lowered yields frequently result when such seed is sown.

Striking differences exist in the prevalence of seed-borne parasitic fungi, especially Helminthosporium and Fusarium, not only in different seasons but in different localities in the same year. Seed of wheat, oats, and barley with 10 to 25 percent infection with Fusarium and Helminthosporium are fairly common some seasons. Seed lots with more than 5o percent infection are not infrequent. The percentage of seed infected with Helminthosporium species and Fusarium species is closely associated with the percentage of germination, stand, amount of seedling blight, and the number of stunted and deformed plants.

Various kinds of mechanical injuries, such as cracking and chipping of the seed coat during threshing, may be the indirect cause of poor stands from seeds that are relatively free from root-rotting organisms. The injuries may not be enough to reduce germination in laboratory tests. Sometimes the cracks are so small that they can be seen only through a magnifying glass. When such seed is planted, especially under conditions unfavorable for germination, however, soil-inhabiting fungi may enter through the breaks and rot the seed before it can germinate. Treatment with a suitable disinfectant places a layer of protective fungicide over the openings and prevents the entrance of root-rotting organisms. The more extensive the seed-coat injuries, however, the greater the need for seed treatment and the greater the benefits from it.

MANY SPECIES OF INSECTS and nematodes attack the roots and basal portion of the stems of cereals and grasses. The bluegrass billbug, Calendra parvula, has received the greatest attention of technicians. It attacks the roots, crown, and lower internodes of the stem, often weakening the plants and providing avenues of entrance for rot-inducing fungi and bacteria.

As the external symptoms of insect injury are seldom conspicuous, the relationship of insects to the rot is apt to be overlooked. Decay usually starts around the wounds the insect makes when it feeds and deposits its eggs. It is most common on wheat, but it also attacks barley, rye, and many grasses. In 1939 the amount of infested wheat plants in the hard red spring wheat region ranged from a trace to 96 percent. In southern Minnesota and eastern South Dakota, fields with 25 to 50 percent infestation were not uncommon.

Studies made in the north central region of the United States by Earle W. Hanson, of the Department of Agriculture in cooperation with the Minnesota Agricultural Experiment Station, indicate that the greater the infestation of billbugs the greater the amount of root rot. Obviously symptoms of root rot do not necessarily indicate insect infestation, since root rots may develop in the absence of insects, but invariably the insect-infested plants are more severely rotted than the noninfested plants. Furthermore, varieties somewhat resistant to root rots may develop severe infection when injured by insects.

Some of the root-rotting organisms are relatively weak parasites. They can cause damage only when the plants are weakened or when entrance to the plant is provided through wounds, such as those caused by insects, nematodes, and mechanical injuries. The frass deposit within insect tunnels in the base of the plant makes a good medium for growth and multiplication of many fungi. The movement of the insect inside the plant helps to distribute the fungus organisms within the host.

The injured tissues are readily attacked even by semi-weak parasites. Therefore rots that follow insect injuries usually involve much more tissue than that destroyed by the insect. Insect injuries, in a sense, predispose plants to root rot and also greatly complicate the development of varieties of cereals resistant to root rots.

PHYSIOLOGIC SPECIALIZATION is a common phenomenon among parasitic fungi, including those that cause root rots of cereals. Not only are the root rots of cereals caused by many distinct species of fungi but most of the species comprise many parasitic races. For example, Helminthosporium sativum consists of many distinct cultural and parasitic races. The races vary greatly in their parasitic capabilities on cereals and grasses. Some races of H. sativum are very virulent on wheat, barley, and rye. Others are moderately virulent. Still others are only weakly virulent. They also differ in host range. Some attack wheat, barley, rye, and oats. Others do not attack oats. Some attack corn readily. Others are nonvirulent on corn.

Similar genetic variation in parasitism has been reported in other root-rotting fungi Calonectria graminicola, Fusarium culmorum, F. graminearum, Ophiobolus graminis, and Rhizoctonia solani.

Root-rotting fungi are not static. New races may arise by means of hybridization between races or by Mutation. Differences in virulence of races are of great importance because they complicate the study of inheritance of resistance to root rots and the selection of varieties resistant to root rots.

New varieties may be resistant in one locality in North America, but susceptible in another because of the prevalence of different pathogens and the existence of different races of the pathogens in different regions. Moreover, root rot problems are continually changing because new pathogens are introduced or new virulent races of old pathogens are developed or introduced from the other regions or countries. Sometimes problems change because relatively minor root diseases become major diseases. Changes in cropping systems and the introduction of new varieties also may create new conditions. All new varieties therefore should be tested for resistance to root rot before they are made available to the growers.

One of the best means of selecting varieties resistant to root rots is to test them in pathological nurseries disease gardens in the regions in which they will be grown. Disease gardens were established in Minnesota in 1919 for testing the varietal reaction of wheat to diseases. The procedure is now common for testing different crops against root rot.

A disease garden is a "sick" plot of land in which varieties of the same crop are grown in successive years and subjected to the most severe disease tests that can be devised. The soil in the disease gardens is inoculated with all available physiologic races of seed-borne and soil-inhabiting pathogens and potentially important root-rotting pathogens that occur in the region in which the varieties are to be grown. The gardens actually are the critical testing grounds for the selection of varieties and hybrids resistant to rot.

Different species of fungi or different physiologic races of some fungi may be widespread in some seasons but not others. If one wants to be certain of resistance, he must test the variety in the disease garden for several years. The production of new disease-resistant varieties takes at least 10 years, usually much longer. Root rot tests should be continued during that period.

THE USE OF RESISTANT varieties and good farming practices that promote strong plants are the best ways to reduce damage caused by root rots. Control measures generally do not eliminate the disease but tend to reduce the amount of infection. Specific control measures, therefore, are restricted at best to localities. To control or limit root rots one should plant sound seed; treat the seed with suitable fungicides; plant early, shallow, and at recommended rates; use good cultural methods; practice sanitation and good cropping sequence; and grow resistant varieties.

Seeds used to be treated with fungicides primarily to control smuts. Since 1949 many varieties of cereals have been developed that are resistant to smuts, so that treatment of their seed appears to be unnecessary. Nevertheless, seed treatment of the new smut-resistant varieties makes for increases in yield, chiefly because the newer fungicides are effective in preventing seed decay and the seedling blight.

Seedling blight can be controlled or reduced by treating seed with fungicides and growing the seed in relatively disease-free soil. The treatments retard primary seed-borne infection and thus reduce the amount of root rot inoculum early in the season. The early protection of the seedlings affords an easy way to increase crop yields. Seed treatments do not reduce the amount of secondary infection, as plants grown in infested soil may become infected any time between germination of seed and maturity.

Increases in yield of wheat, oats, and barley of 5 to 10 percent or more are common. H. Murphy, of the Department of Agriculture, obtained increases up to 60 percent in yields of oats when he treated seed heavily infected with H. victoriae with New Improved Ceresan and sowed it in experimental plots at Ames, Iowa.

Deep-planted seeds of cereals are more subject to seedling blight than shallow-sown seeds. That is especially true if seed lots are not disease-free, if the seedbed is not well prepared, and if weather conditions become unfavorable for normal germination. The deeper the seeds are planted, the longer the succulent tissues are exposed to attack by seed and soil-borne organisms. Field and greenhouse tests indicate that deep seeding increases root rot and lowers stand and yield. Specific depth of seeding cannot be given, as the recommended depth of seeding varies with the crop, soil type, precipitation, and region in which the crop is grown. As a general rule, seed should be sown just deep enough to provide sufficient moisture for good germination.

Fertilizers apparently have little effect on the development of most root rots unless the fertility level is so low that the hosts do not grow well. Most attempts to control common rots by application of commercial fertilizers have been unsuccessful. On the other hand, H. R. Rosen and J. A. Elliott in Arkansas were able to control take-all by using commercial fertilizers. S. D. Garrett in England recommended acid phosphates for the control of the disease. T. C. Vanterpool, in Alberta, Canada, found that an unbalanced phosphorus-nitrate relationship predisposes wheat seedlings to attack by Pythium.