Necrosis in the tuber starts at the stem end in the tissues just under the skin. At that stage the skin over the necrotic areas becomes purplish to silver. The diseased areas continue to enlarge and may eventually extend into all tissues. Discolored areas often occur as pockets. The necrotic tissues frequently develop in rings or spiral patterns. Affected tissues are deep brown and dry and are corky or mealy. In seriously affected potatoes the surface over necrotic areas becomes dry, cracked, and sunken. The sunken areas in the skin may be irregular or in the form of rings or half moons.

Necrosis is usually present at harvest, but often deepens and spreads in storage. Occasionally potatoes that appear sound at digging time become severely affected after 6 weeks of storage. All varieties tested (Netted Gem, Pontiac, Mohawk, Chippewa, Teton, and Houma) have been found susceptible.

BEAN PLANTS are subject to a number of viruses that dwarf and disfigure them, interfere with their normal function, and reduce the yield. They generally are classed as mosaics. Greasy pod, southern bean mosaic, pod mottle, and pod-distorting mosaic produce symptoms on the pods that detract from their appearance. They are most likely to be seen on the market.

Symptoms on beans affected by viruses vary with the severity of the disease. A mottled appearance, typical of symptoms of certain viruses, may result from irregular areas of dark green, which appear water-soaked in contrast to the grayish green of the normal areas on the same pod. Pods that are mottled, malformed, or lumpy and have many irregular, sunken areas generally may be suspected of having a virus disease.

Greasy pod is caused by a strain of the common bean mosaic virus. The pods are most severely affected when temperatures are high. The disease is marked by the shiny or greasy appearance of the pods. Toperop, Rival, Contender, Tenderlong 15, and Improved Tendergreen are resistant to common bean mosaic.

Southern bean mosaic was first reported in Louisiana, but is now known to occur in Illinois, California, Maryland, Georgia, Mississippi, and other States. It has become more and more serious. Affected pods are strongly mottled and malformed. No fully resistant variety has been found.

Pod mottle virus produces symptoms much like those produced by southern bean mosaic virus, but the two diseases are distinct. The pod mottle virus was first found in South Carolina in 1945. Little is known about its distribution or economic importance. Affected pods are strongly mottled. The surface may appear roughened or irregularly sunken. Many of the popular varieties of green-podded bush and pole snap beans are resistant. Some varieties of the green-podded bush type, such as Full Measure, Landreth Stringless, Green Pod, Longreen, Plentiful, and Tendergreen, are susceptible, however. . The pod-distorting mosaic virus is a strain of yellow bean mosaic. It also affects sweetclover, red clover, and gladiolus. Beans may become infected if grown near those plants if they are carrying the virus. The affected pods are distorted and malformed. The surface is lumpy and irregular and has many sunken areas. No variety is resistant to yellow bean mosaic, but the disease is seldom found in the Southern States.

INTERNAL CORK, a virus disease of sweetpotatoes, was first reported in South Carolina -in 1944. It occurs in Georgia, North Carolina, Virginia, Tennessee, Mississippi, Maryland, Louisiana, Alabama, and Texas.

It causes serious losses in some places, but because of its obscure nature it is probably often overlooked in the commercial sale of sweetpotatoes. It causes hard, brown or nearly black, corky Spots in the flesh of the roots. The corky spots do not appear on the surface. To find them one has to cut the roots, preferably in thin transverse slices. The corky spots may appear singly or in groups anywhere in the flesh. Occasionally a group of spots merge into a large, dark, corky area. The only external evidence of the disease is an occasional pit or sunken Place in the surface over a corky spot. If affected roots are cut and the spots exposed to the air, the color fades to a medium brown. If they are exposed for several days, the drying and shrinking of the healthy tissues cause the corky spots to appear raised. Seriously affected roots are unsatisfactory for food because the corky spots remain firm and are bitter.

TOMATO PLANTS and fruits are affected by several viruses and may be affected by more than one virus at the same time. Virus diseases may occur anywhere that tomatoes are grown. All varieties are susceptible. Virus diseases most frequently found on tomatoes in the market are tomato mosaic, double-virus streak, and spotted wilt.

Fruits affected by certain strains of the mosaic virus show a pattern of mottled color, which is retained after they are ripened. That detracts from their appearance and causes wastage through paring because the affected parts do not ripen thoroughly. The fruits are not necessarily a total loss, however, and the disease does not make them more subject to decay. The mottled pattern caused by tomato mosaic is entirely on the surface. Affected areas do not collapse, become sunken, or turn brown. The most typical diseased fruits as seen in a market are green with cream-colored marks radiating from the stem scar over the shoulder area. Those marks may be continuous streaks or may be broken up by small areas of green tissues. The margin between the green and cream-colored areas is usually distinct. When affected tomatoes ripen, the green portion becomes red, but the cream-colored areas turn yellow, so that the mottled appearance remains. Shoulder markings are the most typical symptoms, but the mottled places also may extend over any part of the fruit. Mosaic-affected fruits that are found on the market were affected when shipped; the only change that occurs in transit is the change in color.

Fruits affected with double-virus streak have many small, irregular, or angular spots anywhere on the surface. Glossy at first, they turn brown and usually are slightly sunken. The spots often merge into larger, irregular, brown ones. Sinking and discoloration of the affected areas in creases with time and can be expected to become more conspicuous in transit. Fruits showing the disease are unattractive. The most seriously affected ones may be a total loss.

The virus disease known as spotted wilt has caused serious losses in some sections of California and Oregon because of the presence there of a damaging strain of the virus. The disease also occurs in Texas and in some of the east North Central and Atlantic States, but generally it does not cause serious losses there. Affected fruits in the market usually show several or many ring patterns, often with a mottled condition, which is due to the lack of normal ripening in the surrounding tissues. The ring appearance is due to a slight sinking and shriveling of the tissues around the margin of the affected area which causes the center of the ring to appear raised. The affected areas may be practically normal in color or may appear bronzed or reddish brown.

STONY PIT, a virus disease of pears, may occur anywhere pears are grown. Commercially important losses, however, appear to be confined to the Pacific coast area. The Bosc variety is most subject to stony pit, but the disease also affects Anjou, Winter Nelis, Hardy, and Forelle.

In affected fruit, masses of stone cells occur in the flesh of pears at the bottoms of dimple-like depressions. The lumps of stone cells are so hard that it is almost impossible to cut them. When the pitting is severe and the pits are numerous, fruits may be greatly distorted. Seriously affected fruits are worthless. Fruits that are only slightly pitted, however, are often marketed.

LACY P. MCCOLLOCH is a pathologist of the Bureau of Plant Industry, Soils, and Agricultural Engineering.