The brown stain is only skin deep. It is usually worse at the stem end of the potato. It occurs as scattered blotches on lightly infected potatoes, but large areas may be brown or nearly black on badly infected ones.

Scurf is caused by Monilochaetes infuscans, a fungus that overwinters on potatoes in storage. It persists in the soil for several years. If potatoes are bedded, the fungus grows from the mother potato up to the base of the sprouts. After the sprouts are planted it spreads down to the new potatoes. Most of the infection apparently comes from the seed potatoes and is carried to the field on the sprouts.

Scurf is worse on heavy soils and those containing a large quantity of organic matter than on sandy soils.

Pox, or soil rot, is a widely distributed field disease, but generally it is less important than stem rot or black rot. It reduces the yield and the proportion of salable potatoes.

Its most conspicuous symptom is the pits it causes on the potatoes. The pits are one-fourth inch to more than an inch in diameter and have a jagged margin. In the early stages the infected spots are dark-colored and water-soaked. Later the skin of the potato covering the pit breaks and the contents fall out, leaving an empty cavity. The disease also kills the young feeding roots and causes dark lesions on the part of the stem below the soil line. Affected plants are usually stunted and have yellow leaves. Many of them die early.

Streptomyces ipomoea 'the causal fungus, lives indefinitely in the soil and is spread in soil adhering to farm machinery or plants, by wind-blown soil, and by floodwaters. It likes soils that are less acid than pH 5.2. The addition of sulfur to the soil to make it more acid has reduced damage from pox in some places.

INTERNAL CORK, a new virus disease, was discovered in South Carolina in 1944. Since then it has been found in sweetpotatoes in many other areas, but it is most serious in South Carolina, Georgia, and North Carolina.

Sweetpotatoes affected with internal cork appear normal externally, but have dark brown to blackish corky spots scattered irregularly through the flesh. The corky tissues remain firm during cooking and have a slightly bitter taste. They vary in size up to about one-tenth inch in diameter and one-fifth inch in length. Closely grouped corky spots may affect large areas in the flesh. Leaves of plants affected with internal cork are sometimes marked with purplish ring spots.

Only a little internal cork is found when the sweetpotatoes are dug but corky spots increase in number and size during storage. The rate of increase is more rapid at 70 F. than at the recommended storage temperatures of 55 and 60 .

JAVA BLACK ROT ranks next to soft rot and black rot in importance as a storage disease. It occurs in all parts of the United States where sweet-potatoes grow and in many other countries. It was named Java black rot because it was first discovered on sweetpotatoes sent from Java. It is caused by a fungus, Diplodia tubericola.

It causes a dry rot of the roots. The decayed tissues, brown at first, turn black and hard. The fungus forms black protuberances on the surface of the potato. The decay usually starts at the ends, but sometimes at breaks on other parts of the potato. It progresses slowly and there is little evidence of the disease until about a week after infection. The potato rots in 4 to 8 weeks.

SURFACE ROT, a common storage disease, causes shallow, circular, depressed spots on the surface and a gradual drying out. It progresses so slowly that a great deal of damage may be done before the extent of the disease is recognized. The spots usually are not more than three-fourths inch in diameter and seldom penetrate below the vascular ring. They usually are grayish brown, but sometimes they may be so dark-colored that they resemble black rot.

The disease is caused by the fungus Fusarium oxysporum. Infection apparently takes place through the small rootlets that are damaged when the potatoes are harvested. The disease does not become conspicuous until the potatoes have been in storage for 2 months.

Surface rot is worse on potatoes harvested when the ground is wet and when the potatoes are in the storage house several days before the curing process is started.

All varieties are affected, but Big Stem Jersey and Little Stem Jersey are more susceptible than some of the darker skinned varieties.

THE MOST EFFECTIVE and practical control of the field diseases is to plant only healthy seed potatoes and plants in healthy seedbeds and fields.

Except for the fungi that cause stem rot and pox, none of the fungi that cause major diseases of sweetpotatoes remains alive in the soil for more than 2 or 3 years. A crop rotation of about 4 years generally rids a field of the fungi that cause black rot and scurf.

Plant beds should be thoroughly cleaned and filled with new soil or sand or new plant beds prepared.

Plants free of black rot or scurf can be had even from affected sweetpotatoes by planting vine cuttings or sprout cuttings instead of pulled sprouts. The cuttings will be free of those diseases because black rot and scurf do not affect the above-ground parts. It is more practical to use sprout cuttings than vine cuttings because they can be obtained and Planted as early as pulled sprouts; vine cuttings cannot be made until much later. The sprout cuttings should he made by cutting the sprouts about 1.5 inches above the soil line of the Plant bed without disturbing the below-ground parts. Once black rot and scurf have been eliminated from the sweetpotato seed stock, it is all right to use pulled sprouts in succeeding years.

Because the fungi that cause stem rot and pox live indefinitely in the soil, neither the use of clean planting stock nor crop rotation will eliminate those diseases from a farm. The most practical control is by the use of resistant varieties when suitable ones have been developed. The addition of sulfur to the soil to lower the reaction to slightly less than pH 5.2 reduces the amount of pox.

Much of the losses caused by black rot and scurf in storage and transit may be avoided by storing and shipping only potatoes that are free of those diseases. Black rot and scurf may be eliminated from the potato stocks by use of clean plants and crop rotation.

Losses in storage and transit caused by soft rot, Java black rot, and surface rot can be greatly reduced by proper curing so that wounds caused in harvesting, handling, washing, and packing will heal rapidly and wall out the decay fungi. Temperature and humidity are important in curing sweetpotatoes. The temperature should be about 85 F. and the humidity should be high. Only enough ventilation should be used to keep the potatoes from becoming wet. Frequently outside temperatures are high enough to favor healing when the sweet-potatoes are harvested and additional heat in the storage house is not necessary. Sweetpotatoes may be cured to reduce decay after they are taken from storage and washed and packed for shipment as well as when they are placed in storage in the fall. Storing sweetpotatoes at temperatures even slightly below 55 F. causes chilling injuries that make the sweetpotatoes more likely to decay.

HAROLD T. COOK is a pathologist in the Bureau of Plant Industry, Soils, and Agricultural Engineering.