Postharvest Virus Diseases

Lacy P. McColloch.

Virus diseases are widespread and affect many food crops, but the ones that produce marks or blemishes on otherwise marketable fruits and vegetables are fewer than those that attack the plants in the field.

CABBAGE MOSAIC is occasionally found on vegetables in the market, but usually it is not serious. It became generally prevalent in southeastern Wisconsin in 1934. Similar diseases have since been reported in California and Washington. The cabbage head is not so readily affected as the plant leaves.

The affected head has many small, sunken, brown or black spots, which usually occur on the tissues between the large veins. The spots may merge with one another. Sometimes the spotting is confined to a few outer head leaves, but it may be scattered throughout the head. Often only a part of the head is affected from exterior to center. Spotting develops either as the head approaches maturity in the field or later in transit or storage. An indication of the disease in stored heads is the untimely separation of the leaves from the core. Seriously affected heads are worthless for market.

A NUMBER of viruses attack celery, but only two are important as market diseases southern celery mosaic and western celery mosaic. Southern celery mosaic is confined largely to Florida, but a similar disease has been reported from several other States. Affected plants appear stunted and the leaves are yellowed and mottled. As growth continues, however, the new shoots are only faintly mottled, and that is usually unnoticed after blanching. The leafstalks often show brownish or buff-colored, sunken, water-soaked spots, and may eventually become brown and shriveled. Seriously affected bunches are worthless, but plants showing only moderate mottling of the leaves are marketable.

Western celery mosaic is the most serious virus disease of celery grown in California. Rust-colored spots and streaks appear on the upper surface of older leaves. The stalks are unaffected. As the blemishes are confined to the upper leaves, which are removed in preparation for market, losses are less serious than if the stalks of the celery were affected.

MOSAIC, one of the most serious diseases of cucumbers, may occur anywhere that they are grown in greenhouse and field. The mosaic virus causes a mottling of the fruits and sometimes prevents the development of the normal green color. Other symptoms are wartlike protuberances, distortion, and stunting of the fruits. The most severely affected fruits are eliminated in grading and packing. Mottled fruits are less attractive than normal ones, but are salable.

SPOTTED WILT, a virus disease of lettuce, has become increasingly important in some of the coastal districts of California. It is more prevalent in the late-summer and early-fall crops than in others. Spotted wilt has not been found in the Imperial Valley of California or in Arizona.

Typical symptoms of spotted wilt on head lettuce are the presence of brown, sunken streaks and spots, especially on the lower part of the midrib. Any of the leaves, however, including the heart leaves, may be affected. The leaves often show some curvature of the basal attachment. Seriously affected heads are worthless.

GARDEN PEAS are attacked by a number of viruses in the field. Two are important factors in the quality of marketed peas.

Mosaic is the most serious virus disease of peas in the market. It may occur wherever peas are grown, but the tall varieties grown in California and Washington for market are more susceptible than the varieties grown mainly for canning. American Wonder, Perfection, Canners Gem, Dwarf White Sugar, Little Marvel, Wisconsin Early Sweet, and Surprise are among the varieties that are resistant to the common mosaic.

The virus affects the pods and causes distortion, mottling, and sometimes dark-green ridges, between which are sunken, yellowish-green areas. Affected pods are poorly filled. Seeds are more yellow than normal. Affected pods do not change very much during transit. The mosaic condition detracts from their appearance and may lower the quality.

The spotted wilt virus affects peas less often than the mosaic virus, but it may occur wherever the insect vector that spreads it is found. Pea pods affected with it may have characteristics similar to those of mosaic, but generally the pods show spots of irregular brown patterns with concentric markings. The pods may also be stunted, distorted, and collapsed. The condition may become somewhat worse during transit, but the disease was present when the peas were shipped.

THE PRINCIPAL VIRUS DISEASES affecting peppers on the market are tobacco mosaic, tobacco etch, and certain strains of cucumber mosaic. Some of them occur together in the same plant and make identification difficult.

Pepper pods that have a spotted, lumpy, or ring-marked surface probably have a virus disease.

A yellow strain of tobacco mosaic virus that causes a spotting of pepper pods is widespread and may affect peppers wherever they are grown. The spotted appearance is due to cream-colored spots and streaks, which occur in the green-colored tissues of the pod. The cream-colored areas are not sunken. The spotted appearance is present when the peppers are shipped and does not increase in transit. Affected peppers are unattractive, but unless they are very seriously spotted they would not be discarded on the market.

When the tobacco etch virus occurs alone, it causes little or no markings of the pepper fruits. When it occurs in combination with the tobacco mosaic virus, the complex causes serious damage to affected pods. The disease appears first as round, granular spots, which appear submerged as seen through the normal waxy coating of the fruits. The tissues surrounding the spots collapse later and dry out, causing the spots to appear as raised lumps. Only one side of the pod is usually affected. Affected pods may change during transit and appear much worse than when packed. Seriously affected pods are a total loss.

The common cucumber mosaic virus is widespread, but it causes little damage in pepper pods. A strain of the virus that causes a ring spot on pepper pods is becoming increasingly important in peppers on the market, however. Ring spot makes round markings of various sizes on the pod. The smallest spots involve all the tissues within the ring. They may retain the green color after the rest of the pod is red or they may appear reddish, but paler than the normal red tissues. Large spots have a circular, greenish-brown ring, which outlines the spots. The tissues within the ring usually have normal color but gradually become raised, because of the collapsing of the affected tissues that form the ring. Pods are affected when shipped, but may change during transit and appear worse on arrival. Seriously affected pods are worthless.

OF THE MANY virus diseases that affect potato plants in the field, net necrosis and tuber necrosis virus are most likely to be found on marketed potatoes.

Net necrosis is a name applied to a disorder of tubers produced by plants affected by the leaf roll virus. The virus enters the tuber at the stem end and kills the strands that conduct food and water. The brown color of the network of killed strands makes the netted appearance. The appearance varies, however, with the angle at which the tuber is cut and may appear as dots or streaks. The netted appearance is best brought out if one pares the potato lengthwise. The discoloration may extend throughout the tuber or only a short distance at the stem end.

Often infected potatoes do not show net necrosis at harvest, but the condition develops in storage or transit if the temperature is favorable. At the optimum temperature for development, 50 F., a maximum amount of net necrosis develops in 90 days.

The development of net necrosis in tubers of the Green Mountain variety can be prevented largely by holding freshly dug potatoes at 70 for 60 days or at 32 to 36 for 30 to 60 days. Chippewa, Katandin, and other new varieties, although subject to leaf roll, do not develop net necrosis.