Some selections ofS. tuberosum (S. andigenum) were found to possess a high level of resistance to mechanical and graft inoculation with viruses A and X. Nine of the selections proved highly resistant to inoculation with both viruses; graft inoculated plants developed no symptoms nor could either virus be recovered from them. Apparent immunity to graft inoculation with virus X was found in selections ofS. acaule, S. sucrense and 1 species hybrid. Four selections ofS. tuberosum and 1 selection ofS. maglia developed symptoms of virus X infection but were poor hosts for the virus as indicated by low titer upon subinoculating to indicator hosts. One selection ofS. cardiophyllum, 4 ofS. chacoense, 3 ofS. demissum, 2 ofS. hougasii, 1 ofS. phureja, 1 ofS. tuberosum, 8 ofS. stoloniferum and 2 species hybrids were apparently immune from mechanical inoculation with virus Y. Nine of these selections were apparently immune from graft inoculation with virus Y. Apparent immunity to graft inoculation with virus A was found in selections ofS. chacoense, S. demissum, S. hougasii, S. maglia, S. phureja, S. stoloniferum and 8 species hybrids. About half of the selections developed symptoms of virus A infection but the virus could not be recovered from them. No selection was found to be highly resistant to all three viruses.

Procedures for the efficient and effective evaluation of potato seedling varieties for field immunity from potato virus A in the greenhouse were developed. Plants of seedling varieties selected from parents possessing field immunity are inarch-grafted with virus-A-infected plants of potato seedling 41956 to eliminate from further tests seedling varieties that show top necrosis and are, thus, field-immune from the virus. Subinoculations to plants ofSolanum demissum P. I. 175404 (local-lesion host) from mottled or symptomless plants about 35 days after grafting will detect, in 4 to 6 days, all seedling varieties susceptible to virus A. These susceptible seedling varieties are replanted, and when the plants are 1 to 2 inches tall they are infested with 25 to 50 virus-A-infective aphids for a 24-hour-period. After an incubation period of 30 to 35 days, diagnosis of infection is made by subinoculation from each plant to the indicator host. Seedling varieties not infected by aphid inoculation are considered field-immune from virus A.

Virus X-infected scions of Green Mountain grafted to stocks of S 41956 (virus-X-immune) when held under an 8-hour day at 65° to 70° F. formed aerial tubers more consistently than when held under 12- and 16-hour day lengths at this temperature range. Similarly grafted plants held at 70° to 75° F. under a 12-hour daylength wilted rapidly and died. Aerial tubers did not form and wilting did not occur when scions of S 41956 were grafted to stocks of Green Mountain and subjected to the conditions described. Under an 8-hour day and at the lower temperatures, the strain of virus X which Green Mountain scions were carrying when grafted to immune stocks, had no effect on formation of the aerial tubers.

Samples of the late blight fungus were obtained from infected plants growing on 15 potato-dump heaps in Aroostook County, Maine, during June 1955. Inoculation of differential hosts showed the presence of 11 races. Of the 15 samples, 10 were mixtures of 2 or more races. Race 0 alone was identified in only one sample. Races of the blight fungus pathogenic to Kennebec were found on 11 of the 15, or 73.3 per cent, of the refuse heaps. Consequently, Kennebec and other named varieties highly resistant to, or immune from, race 0 only should be adequately sprayed with a fungicide.

Comparative studies of strain 1 (mild) of the leaf roll virus and an isolate from diseased plants of potato seedling X927-3 were made. These studies show the X927-3 isolate of the virus to differ from strain 1 in its rate of becoming systemicDatura stramonium in the degree of symptom development, and in the erratic incubation period in potato varieties Katahdin, Chippewa, Red Warba, Russet Burbank, and potato seedling X927-3 and 41956. Cross-protection tests indicate that the X927-3 isolate of the virus will prevent reinfection of infected potato plants with strain 1. It is proposed that the X927-3 isolate of the virus be known as strain 5 of the leaf roll virus.

Plants of potato seedling X927-3 that had been exposed to natural infection with the leaf roll virus over a 12-year period before becoming diseased were found to be infected with it. Initial symptoms of leaf roll in plants of X927-3 were persistent and resembled initial infection of potato plants with some “yellows” viruses. The second-generation symptom types expressed by diseased plants were shown to be induced, in part, by the leaf roll virus. All virus isolations from diseased plants of seedling X927-3 induced a response inP. floridana similar to that caused by the mild strain 1. Evaluation of the stock source of the leaf roll virus maintained in Katahdin showed 3 virus strains to be present in the inoculum material. Comparative studies of the X927-3 isolate with the known strains indicated that the X927-3 isolate and the mild strain 1 caused similar reactions inP. floridana plants ; however, the X927-3 isolate was not as easily transmitted by aphids or by grafting, and it did not become systemic as rapidly in certain hosts as did the remaining 3 virus strains.