Finding the Fighters

This article appears in the September 2017 issue of Potato Grower. 

Verticillium wilt of potato, also known as early dying disease, is caused by two soil-borne fungi of Verticillium: V. dahliae and V. albo-atrum. Of the two, V. dahliae is found in most of the U.S., whereas V. albo-atrum is found primarily in the northern U.S. and southern Canada. The fungus is capable of long-term survival in the soil and infects plants through the root system. From there it spreads to the vascular tissue, where it interferes with water and nutrient transport. Disease symptoms on potato leaves include wilting, chlorosis and eventually necrosis. Stems of infected plants show vascular discoloration. Tubers develop necrosis in the vascular tissue, which substantially reduces their quality. Verticillium wilt can also markedly decrease yield. Soil fumigation with methyl bromide can control Verticillium wilt, but this fungicide is being phased out because it depletes the ozone layer. 

Breeding for improved resistance to Verticillium wilt is highly desirable. No potato cultivars are completely resistant, but some show a fairly high level of partial resistance. Potato growers need more information about partial resistance to identify cultivars suitable for production. Potato breeders need such information to select parental materials to develop resistant cultivars.   

Cultivar variations in plant maturity can make it difficult to separate out symptoms of the disease from natural senescence. Early-maturing cultivars are generally more susceptible to the disease than late-maturing cultivars. In our study, we evaluated 274 cultivars for their reaction to infection with V. dahliae by inoculating the soil around two-week-old potted plants growing in the greenhouse. We also evaluated the same 274 cultivars for maturity based on the development of buds and flowering in the greenhouse. The experiment was repeated a second year.  

There was a moderate correlation between resistance and maturity both years, indicating, as expected, that early-maturing cultivars were generally more susceptible to Verticillium wilt than later-maturing cultivars. The most resistant cultivars were Abnaki, Cariboo, Houma, Navajo, Reddale and Russette. The most susceptible cultivars were Irish Cobbler, Pungo and Russet Norkotah. These results indicate that plant maturity should be taken into consideration when evaluating potato resistance to Verticillium wilt.

A maturity-adjusted resistance rating was calculated from residuals of the regression between resistance and maturity. The maturity-adjusted resistance rating minimizes the effect of maturity on the rating for resistance. However, even after adjusting resistance ratings for maturity, there were still significant differences among the cultivars, indicating that a large part of the resistance cannot be explained by maturity alone.  With maturity-adjusted resistance, both short-season and long-season cultivars with partial resistance to Verticillium wilt can be identified. This information will benefit growers in choosing cultivars for their particular growing environment and also enable breeders to select cultivars for use as parents with resistance less influenced by maturity.