The Fungi That Cause Rot in Sweetpotatoes

Harold T. Cook.

Diseases destroy 20 to 40 percent of the sweetpotato crop in the field, in storage, and in transit to market. The average annual production from 1939 to 1948 of nearly 62 million bushels had a farm value of about 119 million dollars. The losses thus might be set at 24 million to 48 million dollars each year.

More than 40 fungus diseases attack sweetpotatoes. Most of the damage is caused by seven of them. Diseases that affect the growing crop reduce the yield and the proportion of choice size potatoes and cause disfigurations and skin blemishes that reduce their salability. Those that affect the potatoes in storage, in transit, and at the market cause decay, shriveling, and unattractive appearance.

The greatest monetary losses arise from damage in storage, in transit, and at the market. Sweetpotatoes are extremely perishable after they are dug and many decay before they reach the consumer. The later the decay occurs, the greater the monetary losses because of the added costs of harvesting, storing, grading, cleaning, packing, shipping, and marketing.

Upper left: Sections of sweetpotato roots affected with internal cork, caused by a virus. Upper right: Black rot of sweetpotato principally a storage rot. Lower left: Potato tuber infected with ring rot, a bacterial disease. Lower right: Fusarium tuber rot of Irish potato, a destructive disease in the United States.

STEM ROT is one of the worst field diseases. It is found in all our growing areas. It destroys more than 50 percent of the plants in some fields. It lowers total yield and the number of medium size, uniformly shaped sweet-potatoes preferred for marketing.

Stem rot is most conspicuous about 2 weeks after the sprouts are set in the field. By that time many of the diseased sprouts are dead or are yellow and dying. The inside of the stem and vines of infected plants are brown or black instead of the normal white color. The stem is often split near the ground level and decayed. Some of the diseased plants develop new roots above the decayed part of the stem and survive the entire growing season.

The potatoes produced by the infected plants are usually small and decayed at the stem end. Their vascular tissues are brown. Sometimes a tuft of new sprouts grows from the stem end at harvest. Diseased sprouts growing from diseased seed potatoes in the plant bed sometimes can be recognized by a faint purplish tint, which shows through the white part of the stem and is caused by the dark tissues.

Stem rot is caused by Fusarium oxysporum f. batatas or F. hyperoxysporum. The fungi can live indefinitely in the soil. They also overwinter in stored sweetpotatoes that are diseased. They are usually spread from one locality to another by the exchange or sale of diseased seed potatoes or plants. They also may be carried in soil that adheres to farm implements.

Varieties of the Jersey group, including Big Stem Jersey, Little Stem Jersey, Maryland Golden, and Nancy Hall, are very susceptible to stem rot. Porto Rico is intermediate in resistance. Southern Queen, Triumph, and Yellow Strassburg are quite resistant but most of the more resistant varieties have poor table quality. A breeding program has been started to develop resistant kinds of good eating quality.

BLACK ROT is serious in the seedbed and field. It is especially destructive in storage and transit. It causes some reduction in yield, and some of the potatoes have visible black rot spots on them at harvest. Frequently, however, potatoes that appear free from black rot at harvest are so badly diseased at the end of storage or on arrival at market that they are nearly a total loss.

The most conspicuous symptom is the circular, dark-brown or black spots on the potatoes. The spots vary in size from mere specks to 1 or 2 inches in diameter. The blackening usually extends into the potato only as far as the vascular ring, but in some varieties it extends much farther. Sometimes small, black bristles, the fruiting bodies of the fungus, develop at the center of the spots.

Black rot may cause small, round black spots on the white underground stem of the sprouts or it may cause :blackening and decay of most of the base of the stem, a condition that is sometimes referred to as black shank.

Black rot is caused by the fungus Ceratostomella fimbriata, which may remain alive in the soil for several years. It also lives from one season to the next on infected sweetpotatoes in storage. It is usually spread from one farm area to another by the exchange or purchase of plants and seed potatoes. Harvested potatoes become infected by spread of the fungus spores during grading, brushing, and washing. When lots containing a few infected potatoes are washed, the wash water becomes contaminated by the fungus spores and nearly all of the potatoes become infected. Such potatoes may appear sound when they are packed for shipping after washing, but they may be heavily infected when they arrive at the market.

The fungus grows best at temperatures of 73 to 81 F., but also makes good growth at 55 , the lowest temperature at which sweetpotatoes should be stored. Little or no growth Occurs at temperatures above 95 . When infected sweetpotatoes are held at 110 for a day, the fungus is killed. Most infection takes place through wounds.

No commercial variety of sweet-potatoes is entirely immune to black rot, but several unnamed seedling varieties have been discovered to be highly resistant.

SOFT ROT is the most serious disease in storage and transit. It may start soon after the potatoes are stored and destroy nearly all of the potatoes before the end of the storage period or it may not affect the potatoes until they have been removed from storage and cleaned, packed, and shipped to market.

It is easily recognized by the soft, mushy decay and the accompanying fuzzy gray and black mold. The affected tissues are moist, brown, and stringy. The decaying potatoes have a rather pleasant aromatic odor, which is quite noticeable in storage houses when potatoes are affected with the disease. The affected potatoes lose moisture rapidly and finally become dry and brittle.

Soft rot is usually caused by the common bread mold fungus, Rhizopus nigricans, but several other species of Rhizopus also may cause it. Rhizopus spores are almost universally present, because the bread mold fungus grows on a wide variety of vegetable material.

Species of Rhizopus responsible for soft rot grow well at the usual storage temperatures and even better at higher temperatures. Infection rarely takes place except at wounds, such as those where the potato is broken from the stem and at cuts and bruises caused in digging, cleaning, and packing. Less infection occurs in bruised potatoes that are held at a high relative humidity and about 85 F. temperature for a few days immediately after wounding, because the injuries heal so rapidly at this temperature and high humidity that the fungus cannot get established.

All varieties are susceptible to soft rot, but some decay more rapidly than others. Southern Queen and Nancy Hall are quite resistant. Porto Rico, Big Stem Jersey, and Triumph are intermediate. Yellow Jersey is very susceptible.