In the warmer areas of the Southwestern States, Rhizoctonia solani causes a root canker of alfalfa. The disease is characterized by dark, sunken areas, which sometimes have a brownish border. The diseased areas are usually circular, but sometimes they are oblong and extend part way around the root. Lesions generally occur where young roots emerge from larger ones. They often develop inwardly to the central region of the root, but the root usually is not completely rotted off.

The disease is seasonal in its development and closely correlated with warm temperatures. Lesions develop mainly in June, July, August, and September, when soil temperature, at 3 inches below the soil surface, varies from about 70 to 95 F., whereas there is practically no disease development during the winter months when soil temperature at 3 inches below the soil surface may be as low as 41 to 50 . Under controlled conditions lesions develop abundantly on roots grown in soil at a temperature of 77 to 86 , whereas they develop very little, if at all, on roots grown at soil temperature of 61 to 64 The economic importance of the disease is not well defined. In regions where it is known to occur, alfalfa stands are of short duration. Plant mortality is highest during the summer when the disease is most prevalent. It causes the death of many small roots and some large ones.

As R. solani is soil-borne and has a wide host range, development of resistant varieties offers the best means of control. Technicians have found little resistance in the many varieties of alfalfa that they have studied, but there is need for a more thorough search among varieties.

Rhizoctonia crocorum causes a root rot of alfalfa. Usually the fungus spreads in all directions through the soil, killing the plants as it progresses. Top growth of infected plants turns yellow, wilts, and eventually dies. Infected roots have a mat of reddish-brown or violet fungus threads on the Outside of the bark. The threads penetrate the roots. Later the roots begin to decay, and the bark sloughs off. The disease is more prevalent in poorly drained fields. Since no resistance to the causal fungus is known to occur in alfalfa, crop rotation and drainage are the most practical ways to control it.

LET US NOW CONSIDER the diseases caused by nematodes.

The stem nematode (Ditylenchus dipsaci) occurs on alfalfa in Canada, Europe, South America, and the United States. It is destructive only in rather restricted areas where high moisture and especially flood irrigation favor its development.

The extent of damage by this eel-worm varies from year to year. Its development is favored by cool, moist conditions in fall and spring. Sometimes it kills a stand of alfalfa in 2 or 3 years after seeding.

The stem nematode may infect alfalfa at any age of the plant. In very young seedlings it infects the plant at the cotyledonary node, causing it to become greatly swollen. Unifoliolate and trifoliolate leaves produced by the plant have short petioles, which are somewhat swollen and distorted. Seedling plants are dwarfed and seldom recover from infection.

On established plants, injury is mainly in the crown, and young buds and stem bases are affected. Infected buds become thickened and deformed and usually do not elongate into stems. As spring advances, they turn dark and rot off. A condition best described as a crown rot is produced. As infected buds usually do not elongate, infected plants have a reduced number of stems. Later in the season, stems often become infected at the base, become dark brown or black, and break off easily. The interior is decayed. Occasionally colonies of nematodes become established a foot or more above the ground and cause swollen areas on the stems.

The causal organism is a slender, nearly colorless eellike worm, which is about one-twentieth of an inch long. Those that first enter the plant are usually the preadult form. After feeding for a few days they mature, mate, and begin producing eggs. The eggs hatch soon after being laid. The young begin feeding, and the colony soon contains nematodes in all stages of development. Several generations occur in a single season. They pass the winter in an inactive state in infected buds and in soil and rubbish about the crowns of plants.

The alfalfa stem nematode does not adapt itself readily to many host plants in the United States. It is known to occur naturally on alfalfa, sweetclover, and white clover. Two biologic races are known to occur on alfalfa.

Resistance in alfalfa plants was first noted in some that were introduced from Turkistan. Now known as Nemastan, at least 85 percent of these plants are highly resistant or immune to the stem nematode. More recently, two strains were introduced from Iran; about 90 percent are highly resistant or immune. Other searches for resistant material have disclosed occasional resistant plants in some of the varieties Commonly grown for hay in the United States. The introductions from Turkistan and Iran have a high degree of resistance to the stem nematode, but they are not high in seed and forage production and are extremely susceptible to some of the foliage diseases. They have been used in Nevada as a source of resistance in developing a variety which will be high in forage and seed production and maintain its high degree of resistance to the stem nematode.

The root knot nematode (Meloidogyne species) has characteristic root galls, which are sometimes found upon alfalfa in the region where this nematode is abundant upon other crops. It is not reported as causing serious disease.

The meadow nematode (Pratylenchus pratensis) is known to occur on roots of alfalfa and other legumes in Kentucky. Presumably it is widespread.

FRED R. JONES has been engaged in research on diseases of forage crops since 1917, for 33 years with the Department of Agriculture in cooperation with the Wisconsin Agricultural Experiment Station and later as associate in research in the University of Wisconsin.

OLIVER F. SMITH has been engaged in research on forage legumes since 1934 from 1934 to 1940 with the Department of Agriculture in cooperation with the Wisconsin Agricultural Experiment Station and since 1940 with the Department of Agriculture in cooperation with the Nevada Agricultural Experiment Station.

Leaf scald of tall fescue.