In very moist weather, bacteria in the soil invade freshly cut or poorly healed seed pieces a possible explanation for the more general appearance of blackleg in wet than in dry seasons in some places.

Because cuts, bruises, and other injuries permit the entrance of rot-inciting organisms, tubers should be handled carefully to avoid bruising. Storage should be provided with favorable temperatures and humidity for healing over (or suberizing) injured tissues. Removing from the seed potatoes all tubers showing rot and storing 9 cut seed potatoes immediately after cutting at about 70 F. and 80 percent humidity to favor adequate healing of the cut surfaces have been effective control measures in some places.

We know of no varieties that are immune to blackleg, but some contract blackleg less easily than others. In 1947 Carl Stapp, a bacteriologist in Germany, reported that Carnea, Flava, Johanna, Priska, Robusta, Sickingen, and Starkeragis were more resistant to blackleg than the 56 other German varieties he included in his test.

RING ROT, caused by Corynebacterium sepedonicum, is a bacterial disease. It is recognized by wilt of the foliage and rot of the vascular ring of the tubers. Chlorosis, or yellowing, and marginal browning and wilting of the leaves are symptoms. In the tubers the disease is detected by a light-yellow discoloration of the vascular elements, which break down and exude a cheesy bacterial and cellular ooze when a tuber is squeezed. When other micro-organisms enter, the lesions enlarge, and Soft rot of the tuber results. Cankers or fissures often appear on the surface of infected tubers in the later stages.

Bacterial ring- rot was found in Maine in 1932, a year after it was reported in eastern Canada. By 1940 it was detected in most of the potato States. In some areas it caused severe losses.

Ring rot apparently is not harbored in the soil, but infected tubers over-wintering in the soil may develop infected volunteer plants, which may serve as sources of infection. The disease is spread from infected to healthy tubers by the seed-cutting knife, planting machines, grading equipment, and contaminated hands, gloves, bags, baskets, barrels,. and bins that have come in contact with diseased potatoes.

Control involves prevention, sanitation, and the use of resistant varieties.

Control of ring rot is primarily the responsibility of seed-potato growers, .who should propagate and maintain their own seed potatoes. Potatoes that are free from ring rot can be kept so if no infected seed potatoes are introduced from other sources. If a grower must bring in seed potatoes from other farms and localities, however, he will be safe if he plants them in separate fields for a few years or until he is absolutely certain that they are free from ring rot.

Seed-potato certifying agencies in Most States set up a rigid zero tolerance for ring rot in certified seed potatoes in Largely as a result of those standards, the spread and extent of ring rot has been reduced greatly, and growers of seed potatoes have access to an adequate supply of healthy potatoes even if they cannot grow their own seed stock.

Sanitation involves selling for table stock the crop that harbors ring rot and disinfecting all equipment, machinery, containers, and bins that in any way were in contact with the diseased potatoes. Bins, crates, barrels, and bags can be treated with copper sulfate (25 pounds to 100 gallons of water). Machinery can be treated with formaldehyde (1 part in 10). Ring-rot free seed potatoes, such are produced by foundation seed-potato growers or on State foundation seed-potato farms, should be planted. By following this clean-up program many potato growers have eradicated ring rot from their farms.

Although no variety is immune to ring rot, some varieties contract the disease less easily than others. Teton, introduced in 1946, is an example.

VIRUS DISEASES of potatoes have been reported in every country. Leaf roll and latent mosaic apparently occur most widely.

The nature and cause of virus diseases have been under investigation for more than 100 years. Many investigators now regard them as filterable proteins that multiply in living cells.

The virus diseases of potatoes can be classified as mosaics and yellows. Distinguishing characters of the mosaic group are mottling and the ability to be transmitted in the sap. A mark of the yellows group is chlorosis; it cannot be transmitted in sap. The mosaic group includes several viruses, some of which have several strains or races that differ in pathogenicity. The yellows group includes the viruses that cause leaf roll, witches'-broom, and aster yellows.

As with most of the fungus diseases, the potato virus diseases are designated by common names that indicate the response or reaction of the plant to the virus. The reactions or symptoms are affected by temperature, host, and the strain or race of the virus. Mottling usually is absent at higher temperatures. Roll of the leaves is favored by relatively high temperatures. Some varieties manifest the effects of a virus; another variety might act as a symptomless carrier of the same virus. One race of a virus can cause a reaction in the same variety that differs from the reaction induced by another race. Most of the potato viruses are transmitted by sucking insects (aphids and leafhoppers) but some are spread also by chewing insects. Some viruses are spread by mere contact of diseased plants with healthy plants and by mechanical agents, such as the seed-potato cutting knife and planting machines.

ALTHOUGH SEVERAL MOSAIC diseases incited by as many different viruses affect the potato, I shall describe only three latent mosaic, virus A, and vein banding mosaic, which occur in many potato regions.

Latent mosaic, caused by virus X, is so called because many varieties of potatoes are symptomless carriers. Synonyms of the latent mosaic virus are potato virus X, potato-acronecrosis virus, simple mosaic virus, potato virus 16, Solanum virus 1, and Annulus dubius.

Latent mosaic virus is harbored by potato varieties more generally than any of the other potato mosaic viruses. Almost every plant in most of the older varieties harbors the virus. Many varieties manifest the milder strains of this mosaic by slightly light-green and rugose leaves when the weather is cool.

Latent mosaic virus has several races or strains that induce a range of reactions, varying from weak or no apparent symptoms to severe necrosis. A plant harboring one race may be protected against infection from other races of this virus. More than one race may be harbored by a plant, suggesting mutation or simultaneous infection by more than one race.

Other plants in the nightshade family that are susceptible to latent mosaic are tomato, tobacco, pepper, jimsonweed, petunia, physalis, henbane, woody nightshade, and black nightshade. Jimsonweed is a good one for detecting different races of latent mosaic virus. It reacts to the races by manifesting no symptoms for the weaker races to severe necrosis for the more virulent races. Globe amaranth, or bachelor's button, which manifests red local lesions, also is a good host for detecting latent mosaic virus.

Latent mosaic is easily transmitted mechanically. Contact of diseased with healthy plants and the cutting knife spread the disease. Some sucking insects, such as aphids, fail to transmit latent mosaic, but grasshoppers are reported to transmit it.

Reductions in yield of 10 to 30 percent are reported for the common races of latent mosaic. Virulent races can cut yields more than 50 percent. Because most of the old varieties harbor latent mosaic, it is possible that yields from them are 10 or 15 percent lower than the yields of healthy stock.

VIRUS A often induces slightly light-green and rugose leaves, symptoms that are similar to those induced by virus X, the cause of latent mosaic. The symptoms are favored by cool temperatures and are masked at high temperatures. Some varieties merely show light-green foliage. Others develop dead spots in the foliage.

Although tobacco and some other species in the nightshade family are susceptible to virus A, the symptoms often are so like those induced by virus X that diagnosis is difficult.

Virus A is transmitted by aphids. It can also be transmitted in the sap. It is transmitted mechanically less easily than virus X. Virus A apparently involves several races, which vary in degree of reaction they induce on their host plants.

Virus A reduces yields by 15 to 25 percent in some varieties, apparently by causing plants to mature too early. It does not affect the set of tubers.

Since most of the old varieties generally harbor virus X, they carry a composite infection after contracting the aphid-transmitted virus A. On many varieties the two viruses induce more pronounced symptoms than those of one of them, such as distinct mottling and crinkling. This composite infection is recognized as mild mosaic, a term first used to designate mildly affected plants, as distinguished from severely dwarfed, curled, rolled, and necrotic plants.

VEIN BANDING MOSAIC, caused by virus Y, is distinguished by rugosity, or wrinkled surface of the leaf, and vein clearing in early stages and by necrotic spots and streaks and leaf drop in the later stages. The symptoms vary in different varieties, ranging from slight rugosity to severe dwarfing, curling, and necrosis. Synonyms for vein banding mosaic virus are potato virus Y, Solanum virus 2, and Marmor upsilon.

Vein banding mosaic occurs in leading potato areas of every country. In some places a high percentage of the crop is infected.

Susceptible hosts, besides potato, are tomato, black nightshade, henbane, woody nightshade, petunia, Solanum nodiflorum, Datura innoxia, and tobacco.

The several races of vein banding mosaic vary in virulence. Vein banding mosaic virus is readily transmitted by sap inoculation, grafting, and aphids. It is responsible for a greater reduction in yield than that caused by either virus X or virus A. Infected plants die prematurely; the result is undersized tubers. Usually less than half a crop is obtained from diseased Plants.

Because the older potato varieties generally harbor virus X, they harbor two viruses after contracting vein handing mosaic virus (virus Y). This Composite infection is known as rugose mosaic, which often causes more severe host reactions than either one Of the viruses. On some varieties, however, the vein banding mosaic virus induces as severe reactions as the composite infection.