Field and laboratory studies were conducted in 1985–1986 to evaluate the relative severity ofRhizoctonia solani damage among 26 clones and five potato (Solanum tuberosum) cultivars. A test with true potato seeds showed that resistance toR. solani could be identified in two years. One highly resistant line was identified by this method. Moderate degrees of resistance were found in lines from the USDA Potato Breeding Program (BARC) with russet types producing a higher percentage of highly resistant lines than round whites. A laboratory test revealed that apical sprout damage (nipping) was variable within and between lines and that it was an important phase of disease development to measure when determining resistance because of its effects on the resulting plant growth and production of marketable tubers. It was also determined that to accurately define resistance toR. solani in potato, all phases of the disease must be evaluated to insure against resistance “breakdown” at some point in plant development.

Selected potato (Solanum tuberosum L.) genotypes were grown in the field from May to September during 1984 and 1985, to study their growth and gas exchange responses in a warm climate. The parameters measured were leaf area index (LAI), canopy photosynthesis on ground area basis (CPn-Ga), stomatal conductance, dry matter partitioning and yield. The LAI ranged from 1.40 for Dakchip to 6.60 for Pungo during August 1984. The LAI also differed significantly among the potato genotypes for the three samplings during 1985. Atlantic, Chipbelle and DTO-33 showed no decline in their LAI up to 73 days after planting (DAP), indicating a better heat-stress tolerance response than the other genotypes. During both years, CPn-Ga differed significantly among the genotypes and Pungo had higher CPn-Ga than all the other genotypes. Mean CPn-Ga rates were 1.72 and 4.34 g CO2 m−2hr−1 during 1984 and 1985, respectively. Mean adaxial and abaxial stomatal conductances were 0.86 and 1.46 cm sec−1, during 1984, and stomatal conductances were similar for both years. Stomatal conductance did not appear to limit gas exchange in potato leaves. Dry matter partitioning to tubers ranged from 8.9% for Pungo to 55.5% for Atlantic 67 DAP during 1984. At final harvest, July 19, 1985, dry matter partitioning to tubers varied from 47.5% for Pungo to 69.9% for Chipbelle. The tuber yield ranged from 9.6 to 27.8 MT/ha. This study indicated that Atlantic and La Chipper have potential for growing in a warm climate.

Inability to produce large quantities of high quality, relatively diseasefree potato seed is a major constraint to providing economic support to a highly important and rapidly expanding potato production industry in Pakistan. Studies were undertaken for 5 years to evaluate productivity of seed of cultivars Red Bad, Ajax, Atica, Patrones, Montana, Cosima, Desiree, and Cardinal grown at five environmentally diverse locations in Pakistan versus seed of the same cultivars imported from Europe. The productivity trials were conducted during the autumn and spring seasons at the Ayub Agricultural Research Institute, Faisalabad (Punjab Plains). The results from the 5-year test period indicate that seed of each of the cultivars produced in the high hills (Baluchistan, Northwest Frontier Province) and during autumn (September to December) in the Punjab Plains, the latter cold stored until the following September, are equally as productive as seed imported from Europe.

We have found that by using water agar as an isolation medium, V-8 juice agar as a growth substrate, and sterilized rye grains as a storage medium we can: 1) produce copious amounts ofHelminthosporium solani conidia that were successfully used as inoculum and: 2) the sterilized rye grains could be used to maintain viable cultures of the five isolates ofH. solani used in this study for 16 months. All of the isolates were pathogenic on tubers of the potato cultivars Katahdin and Superior.

Red LaSoda produces a very attractively colored tuber and several growers are anxious to obtain seed tubers. It has proven its adaptability in the seed-producing areas of the North and also its superior yielding ability over Triumph in the southern potato-producing areas. The Red LaSoda variety will yield equally as well as LaSoda and the tubers do not require added coloring to make an attractive product. The quality of the tubers is equally as good as most potato varieties grown in the southern states.

Several varieties of potatoes that are grown in the south have been evaluated for their suitability for processing.

Results of these studies of isolates of the potato leaf roll virus from 56 various seed potato areas of the United States and Canada, covering a wide geographical range, substantiate the existence of distinct strains of the virus. In addition to the 3 strains from Wisconsin previously reported, one additional very severe strain, designated as strain 4, exists in at least one area in South Dakota. The strains are distinguished most readily by the initial symptom type and relative disease severity onP. floridana. The response ofP. floridana infected with strain 4 at 24° and 28° C, is directly opposite to that of the other 3 strains at the same temperatures.