Grasses and Legumes

The Many Ailments of Clover

Earle W. Hanson, Kermit W. Kreitlow.

There are some 250 described species of true clovers (Trifolium) but only four species red, alsike, white (including Ladino), and the crimson, are widely grown and of great importance.

The sweetclovers (Melilotus) are not true clovers. Twenty-two species of sweetclover are recognized. Three species white, yellow, and sour clover are of importance in agriculture.

All clovers are subject to injury from diseases. All parts of the plant are attacked and sometimes destroyed the roots, crowns, stems, leaves, and inflorescences. Fungi, bacteria, and the viruses all can damage the clovers.

Some of the pathogens infect only specific organs of the plant, such as the leaves or roots. Others attack several or all parts of a plant. The pathogens differ also in parasitism. Some infect only certain species of clover. Others have a broad range of hosts and can attack nearly all clovers and many other hosts as well.

Upper left: Leaves of healthy (upper) and virus-infected (lower) red clover. Upper right: Leaves of red clover infected with powdery mildew (Erysiphe polygoni). Lower left: Leaf spot of Ladino white clover, caused by the fungus Cercospora zebrina. Lower right: Stems of red clover discolored by the spring black stem fungus.

AN IMPORTANT PROBLEM in producing clovers, the establishment and maintenance of stand, involves several factors. One is the root and crown disease complex, which includes the seedling blights, root rots, and crown rots.

Those diseases probably are the most important of all clover diseases. They occur wherever clovers are grown. They are caused by a complex of soil inhabiting fungi. The fungi may be widely distributed or occur only locally. Some are virulent pathogens that can attack vigorous plants. Others are weak pathogens that cause damage only after the plants have been weakened by winter injury, nematodes, insects, drought, unfavorable soil conditions, or improper management. Some are primarily seedling pathogens. Others attack clover plants of any age. Some are primarily root pathogens. Others are primarily crown pathogens. Several organisms may attack a plant simultaneously, or one may follow another in sequence. Thus the difficult problem of root and crown disease is one that must receive greater attention if productive stands are to be maintained.

CROWN WART of clover, caused by Urophlyctis trifolii, occurs in central Europe on red clover (Trifolium pratense), white clover (T. repens), and some others. In the United States the disease is of minor importance. It occurs mainly in the South Central States and on excessively wet soils. It is similar to the more important crown wart of alfalfa. Its characteristic symptom is the formation of irregularly shaped galls around the crown of the plant, at and just below the soil level. The galls first become noticeable in late spring and increase in size as summer advances. Infected plants wilt in hot weather. Leaves of white clover are sometimes distorted.

Sclerotinia crown and stem rot is caused by Sclerotinia trifoliorum, and is widely distributed, especially in the regions of mild winters or heavy snow cover. It has long been recognized as one of the most destructive diseases of clover in northern Europe. It occurs also in the Soviet Union and Canada. In the United States it is of considerable economic importance in the southern and central clover belts and causes extensive damage in the Pacific Northwest and in the Northeast. Rarely does it occur in the north central part of the northern clover belt. The disease spreads and develops most rapidly during Cool-55 to 65 F. wet weather, but the fungus that causes it can grow and infect plants at temperatures ranging from below freezing to 75 .

Sclerotinia trifoliorum has a broad range of hosts, which include all important true clovers and the sweet-clovers, alfalfa (Medicago sativa), black medic (M. lupulina), birdsfoot trefoil (Lotus corniculatus), sainfoin (Onobiychis viciaefolia), and many other legumes and nonlegumes, including numerous weeds. Red clover, crimson clover (Trifolium incarnatum), and alsike clover (T. hybridum) are all very susceptible. White clover is generally considered to be less susceptible but not immune. Some other species of Sclerotinia may also occasionally infect clovers.