Survival of Phytophthora infestans in surface water, movement of spores in soil and characterization of resistance to late blight in selected potato lines

Survival of Phytophthora infestans in surface water; effects of tuber depth, soil moisture and fungicides in soil on infection of potato tubers by P. infestans; and, resistance to P. infestans in potato tubers, stems, leaflets, and flowers of selected cultivars and advanced potato lines were quantified. Coverless petri dishes with water suspensions of sporangia and zoospores were used to quantify longevity of spores in water under natural conditions. Spores in water survived 0--16 days in sunlight and 2--20 days in shade. Spore survival increased 1.7--5.8 days when soil was present in the water. Maximum survival of spores in water exposed to sunlight without soil was 2 to 8 days. Significantly fewer spores chilled at 10 C for two hours survived for extended periods than those not chilled. Infection of potato tubers decreased with increasing soil depth; infection was rare below 5 cm. Less tuber infection occurred in a Shano silt loam than in medium and fine sands. Soil depth at which tubers became infected did not differ between fine and medium sands and increased soil moisture did not significantly increase depth of tuber infection. Metiram and mancozeb on soil were fungicidel to spores, and protected tubers from infection by P. infestans for up to three days. In the Columbia Basin, all analyzed commercial cultivars were susceptible to late blight. Seven advanced breeding lines had high foliar levels of resistance to P. infestans in the field. Screening for resistance in foliage was more selective in the greenhouse than the field. Upper canopy leaflets of commercial cultivars were more resistant to lesion expansion than lower canopy leaflets, but leaflets in the upper canopy produced more sporangia. Tubers with high levels of resistance in the laboratory were not always resistant in the field. Level of resistance in tubers and leaflets was not correlated among clones. Plant parts were ranked in order of increasing resistance to infection by P. infestans in the following way: flowers, lower leaflets, upper leaflets, tubers and stems. Flowers of cultivars with high levels of resistance were susceptible to infection.