As the stems and petioles are killed, a mass of white mycelium grows over them. Some of the masses of mycelium then change into small, hard, black, cartilaginous bodies the sclerotia. They are attached to the surface of (or imbedded in) the dead stems, crowns, and roots or in the soil near the roots. Some are as small as a clover seed. Some are larger than a pea seed.

When the affected plant parts decay, the sclerotia remain in the soil as a future source of infection. Sclerotic are the chief means by which the fungus survives from year to year.

They can remain viable in soil for several years. In the fall, if conditions are right, the sclerotia germinate and produce one or more small, disk-shaped, pinkish-buff, mushroomlike fruiting bodies called apothecia, which are borne individually on slender stalks. The apothecia are one-sixteenth to one-fourth of an inch in diameter. They produce millions of spores, which spread to the leaves and petioles of nearby plants, causing infection and repeating the cycle.

The greatest reduction in stands occurs in late winter and early spring. The damage therefore is sometimes confused with winter killing. The disease usually occurs in patches throughout a field, but when conditions are very favorable the patches may become so numerous as to merge and cause extensive damage to a stand. In the South a few days of warm weather sometimes checks the disease, and the plants recover.

Control is difficult. Clean cultivation, deep plowing to bury the sclerotia beyond their capacity to send up apothecia, and long rotations are helpful. Care should be taken not to distribute the sclerotia with clover seed. Grazing or clipping in late fall sometimes removes infected leaves and reduces the amount of foliage that may become infected and mat down on the crowns during the winter. Adapted varieties are more resistant than non adapted strains. The most promising method of control appears to be the breeding of resistant strains.

Common root rot is a group of root diseases caused by species of Fusarium and several other soil fungi that produce similar symptoms and frequently attack plants simultaneously. The relative prevalence and importance of the fungi vary with the locality, kind of clover, age of plant, season of the year, Soil type, and management practices. Mostly they are weak pathogens and cause damage after the plants have been weakened or injured. Most of them are widely distributed and cause damage wherever clovers are grown.

Scientists have learned a lot about the problem, but relatively little is understood concerning many of its phases. The field symptoms of common root rot of red clover are well known, for example, but attempts to reproduce them under controlled conditions often are unsuccessful. Research men have demonstrated that some isolates of the fungi can attack clover seedlings.

Species of Fusarium have been most frequently reported as causing root rot.

Symptoms of the disease are a localized or general rotting of any part of the root system. Taproot, secondary roots, and even the crown may be attacked. The color of the diseased areas ranges from light brown to black. The rotting may be limited to the cortical areas around the exterior of the root, the vascular core may be discolored, or the entire root may be affected. Secondary roots are constantly pruned away by the rots and new secondary roots are formed to compensate, but the replacement process is usually the slower, so that by the end of the second year most plants have left only a few short secondary roots. The lower part of the taproot often is destroyed completely. Such destruction causes wilting and a gradual dying of the plant.

Common root rot kills plants in all stages of development. Effects on stand are most conspicuous during the second year, but losses up to 45 percent during the first year are not uncommon. Stand losses occasionally occur in the spring when the plants are weak because of low food reserves or winter injury. Diseased stands frequently produce a fair first crop of hay but fail to recover and to produce a second crop. Most clovers and sweetclovers are susceptible to root rot.

Besides Fusarium, Rhizoctonia, Phoma, and other organisms may be associated with root rots. Plenodonius meldoti and Cylindrocarpon ehrenbergi are of primary importance following the winter dormancy period on sweetclover in Alberta, Canada.

Control is difficult, but any practice that improves the general vigor of the plant is helpful. Proper liming, fertilization, and crop rotation are important. Only adapted varieties should be grown. No varieties available in 1953 had high resistance when conditions favor the disease. Plant breeders have under way a project to develop resistant strains of red clover.

Phytophthora root rot, caused by Phytophthora cactorum, is a widespread disease of sweetclover in North America, notably Ohio, Indiana, Illinois, and Missouri. It occurs in Alberta and Ontario. The fungus attacks individual plants or small groups of plants in fields or along roadsides. It is most abundant in low, wet parts of fields, where in seasons of heavy spring rainfall and cool temperatures it may kill most of the plants.

Its presence is first noted in the spring, when infected plants wilt, die, or are generally unthrifty, When their roots are examined, the upper portions usually are found to be rotted. The decay generally is limited to the upper 3 or 4 inches but may extend as much as 8 inches below the crown. The decayed places usually are soft and watery. The color changes but little at first. Later they may become discolored and shrunken.

Crop rotation and the use of well-drained fields are helpful control measures. It should also be possible to develop resistant varieties because resistant plants are known to exist.

Seedling blights, caused by Pythium, Rhizoctonia, and other fungi, are the most destructive seedling diseases. They occur wherever clovers are grown. Sometimes they seriously reduce the stands. Three types of injury occur. Preemergence killing starts shortly after the seed is sown and develops rapidly, so that the seedlings are destroyed before they emerge from the soil. In postemergence damping-off, infection commonly occurs before emergence, although the rate of disease development is slower and the seedlings emerge only to be killed soon thereafter. Root and hypocotyl rotting causes varying degrees of stunting, in which plants survive the early seedling stages, after which some recover and some die.

Seedling blights are caused by a complex of fungi, including several species of Pythium, Rhizoctonia, Fusarium, Gliocladium, Phoma, and others. One of the most virulent is Pythium debaryanum. It would seem that seed treatment might help control this group of diseases, but the results of field tests have given limited encouragement for this method of control.

THE STEM diseases attack the supporting and conducting systems of the plant. Often they cause serious losses. Usual symptoms are stem discoloration, withering and dying of attached leaves and petioles, and general wilting and stunting of the plant. Frequently stems break off or crack open at the site of infection. Several of the major diseases of clover are included in this group.

Northern anthracnose, caused by Kabatiella caulivora, is a major disease of red clover in the cooler areas of North America, Europe, and Asia. It develops best at 68 to 77 F. and is checked by continuous hot dry weather: In the United States it is important only in the northern clover regions but there it frequently causes damage occasionally exceeding 50 percent of the crop in some fields. Losses as high as 50 to 60 percent have been reported in Germany. Complete crop failures have been observed in the Netherlands. Seed production and hay yield and quality are greatly reduced in badly infected fields.

The disease is serious only on red clover. It may occur on alsike, white, crimson, and Persian (Trifolium resupinatum) clovers and possibly others. It has never been found on alfalfa, but has been reported on black medic and on sainfoin in the Netherlands. The species of fungus inciting the disease consists of a large number of physiologic races, which differ in their capacity to infect different species of clover,and different strains of a particular species. No red clover strain yet developed is immune to the disease, but wide differences in resistance exist among European and American strains. Varieties developed in the southern part of the United States are more susceptible than those developed in the northern clover areas.

Symptoms are confined mostly to the petioles and stems. Infection also occurs on the petiolules small stalks connecting the leaflets to the petiole and occasionally on the leaflets themselves. The first symptoms noticed in the field are usually dark-brown or black spots on the petioles. The spots soon cut off translocation to the parts above them the upper part of the petiole and the leaf causing them to wilt, turn grayish brown, and die. The petiole bends downward at the site of the lesion to form the familiar "shepherd's crook." Stem lesions are most characteristic. They develop first as small, dark spots, which soon lengthen to form lesions with dark margins and light-colored centers. As the stem grows, a crack often appears in the center of the lesion. Stems finally may be girdled and killed. Plants in a badly infected field look as if they were scorched with fire, because of the abundance of blackened and broken stems, withered petioles, and brown, dead leaves. The name scorch has been aptly used in Britain to denote the disease.

Southern anthracnose, caused by Colletotrichum trifolii, is a major disease of red clover in the southern clover belt of the United States. It has been recorded as far north as southern Canada, but is primarily a high-temperature disease that flourishes at about 82 F. It is of little economic importance in the northern clover areas. It is confined mostly to North America, although it has been reported on alfalfa in South Africa and in Europe. It occurs occasionally on crimson clover, sub clover (Trifolium subterraneum), bur-clover (Medicago hispida)) and white sweetclover (Mellilotus alba). It has not been observed on white clover. Alsike clover is practically immune.

Southern anthracnose has been regarded as the most destructive disease of red clover in the Southern States. It reduces yields of hay and seed and can destroy stands of clover. A resistant variety, Kenland, is available. Most European and American strains developed in regions where the disease does not occur are susceptible; hence it is important to grow only locally adapted strains or strains known to be resistant.