Three common varieties of rust attack clovers. They cannot be distinguished on the basis of symptoms but can be differentiated by differences in their capacity to infect the Various clovers. For example, the variety of rust on alsike clover (Uromyces trifolii hybridi) infects only alsike, while the rust on red clover (U. trifollifallens) infects red, zigzag, crimson, Berseem, and several other clovers. The rust on white clover (U. trifolii trifolii-repentis) does not infect red or zigzag clovers but does infect crimson and Berseem, besides white clover. These rust fungi differ from those causing the cereal rusts in that they can complete their entire life cycle on a clover species and do not require an alternate host.

The most conspicuous symptom of clover rust is the uredial, or brown rust, stage, in which round or irregular, pale-brown pustules, surrounded by the torn epidermis, appear on the lower surface of leaves and on the petioles and the stems. Sometimes in winter in the South and early spring in the North, small swollen whitish-to-yellow clusters of tiny cuplike structures occur on the stems, petioles, and large Veins of the leaves. These are called the accia and may cause distortion of the affected leaves and petioles. Rust fungi in the telial, or black spore, stage overwinter on the debris of diseased plants.

Fungicides such as sulfur can be used to control the clover rusts, but rarely is it practical to use them. Resistant plants exist in present varieties and resistant strains can be developed if the importance of the disease warrants it.

ALL COMMON CLOVERS are susceptible to several viruses, some of which are widely distributed. Most of the legume viruses have a wide range of hosts so that clover viruses infect not only clovers but also other legumes, and viruses of other legumes attack clovers. Some viruses from nonlegume hosts such as tobacco, gladiolus, potato, and some weeds can readily infect clovers. That means that many of the virus diseases spread from one crop to another. The extent of the spread and amount of infection usually depend on the kinds and numbers of insects present. Aphids are probably the most important carriers.

Symptoms vary with the virus and host. Most of the clover viruses are systemic that is, they are present in all parts of the plant. The most conspicuous symptoms are usually found in the leaves. They include vein chlorosis, mild to severe mottling, chlorotic patches between the veins, and other abnormal combinations. Sometimes the leaves are curled, puckered, or ruffled. Some viruses cause a reduction in vigor as indicated by a general stunting of the plant. Others have no apparent effect on vigor. Viruses that have little effect on one clover may kill another. Symptoms of most clover virus diseases are conspicuous during the cooler periods of the growing season and sometimes disappear temporarily or are masked during hot weather. Weakening caused by virus disease may predispose plants to attack by other pathogens (especially those that cause root rots) or prevent them from surviving severe winters or prolonged droughts.

The virus diseases reported to occur on the more important clovers are: Red clover: Red clover vein-mosaic, common pea mosaic, yellow bean mosaic, potato yellow dwarf, American pea streak, New Zealand pea streak, Wisconsin pea streak, pea mottle, pea wilt, alfalfa mosaic, sub clover mosaic, ring spot, broadbean common mosaic, broadbean mild mosaic, and cucumber mosaic.

Alsike clover: Alsike clover mosaic, red clover vein-mosaic, sub clover mosaic, common pea mosaic, pea mottle, pea wilt, and New Zealand pea streak.

White clover: Alfalfa mosaic, red clover vein-mosaic, yellow bean mosaic, pea mottle, pea wilt, American pea streak, New Zealand pea streak, and broad-bean mild mosaic.

Crimson clover: Alfalfa mosaic, red clover vein-mosaic, sub clover mosaic, alsike clover mosaic, common pea mosaic, yellow bean mosaic, pea mottle, pea wilt, American pea streak, potato yellow dwarf, and broadbean common mosaic.

Sub clover: Sub clover mosaic and yellow bean mosaic.

Sweetclovers: Alfalfa mosaic, alsike clover mosaic, red clover vein-mosaic, sub clover mosaic, common pea mosaic yellow bean mosaic, pea mottle, pea wilt, American pea streak, tobacco streak, tobacco ring spot, and broad-bean mild mosaic.

Little has been done to control the virus diseases of clovers. Some of the newer insecticides kill the insect vectors. When possible, clover should not be grown close to other legumes, particularly peas or beans. The ultimate solution is to develop varieties of clovers resistant to the most prevalent and injurious virus diseases. That remains to be done.

SEVERAL PATHOGENS can attack the floral parts of the clovers under some conditions. They are of importance only when they interfere with seed production.

Anther mold (Botrytis anthophila) was first reported from Russia in 1914 and has since been found to be widely distributed in Europe. In the United States it has been found to a limited extent in Oregon. It has no apparent effect on the plants until flowering time, when it destroys the normally yellow pollen grains, replacing them with the gray spores of the fungus. If abundant it can reduce seed setting.

EARLE W. HANSON, a native of Minnesota, joined the Department of Agriculture in 1937. From 1937 to 1946 he was employed by the division of cereal crops and diseases of the Bureau of Plant Industry, Soils, and Agricultural Engineering doing research at the Minnesota Agricultural Experiment Station on the diseases of hard red spring wheats and the development of disease-resistant varieties of wheat. Since 1946 he has been employed jointly by the division of forage crops and diseases of the Bureau and the University of Wisconsin.

KERMIT W. KREITLOW is also a member of the division of forage crops and diseases and is stationed at Beltsville, Md. He is a graduate of the University of Minnesota and Louisiana State University. Dr. Kreitlow has been engaged in work on forage crop diseases since 1941.