Verticillium wilt of potato – the pathogen, disease and management †

Verticillium wilt is a major disease of potato caused by either Verticillium dahliae or Verticillium albo-atrum. Both species are soilborne fungi that invade xylem elements, disrupt water transport in plants and cause vascular wilt in a variety of hosts. Despite the broad host range of V. dahliae, a degree of host adaptation occurs with some isolates exhibiting different levels of aggressiveness depending on the host species and even cultivar within a species. In addition, isolates can be separated into vegetative compatibility groups (VCGs) based on the ability to undergo hyphal anastamosis with other isolates. Distinct sub-specific groups have been recognized which contain host-adapted pathotypes that are aggressive and prevalent on particular hosts. Isolates of V. dahliae collected from potato stems, tubers and potato fields in the USA mostly belong to VCG 4A and VCG 4B, with the VCG 4A isolates being highly aggressive on potato compared with isolates of VCG 4B and other VCGs. In contrast, isolates collected from potatoes grown in Israel were predominantly VCG 2A and VCG 4B. Initial inoculum of V. dahliae is primarily soilborne microsclerotia. Verticillium dahliae can be transmitted in certified seed tubers, but tuber-borne inoculum appears to have little effect on verticillium wilt symptoms and potato yields. Vascular infection by V. dahliae of seed tubers of the moderately susceptible potato cultivar ‘Russet Burbank’ resulted in negligible effects on the development of verticillium wilt symptom, did not significantly contribute to aboveground stem infection or the formation of microsclerotia in debris, and did not significantly contribute to progeny tuber infection. An importance of tuber-borne inoculum is the potential to introduce inoculum to soils not previously used to grow potato or where a management practice such as fumigation has been applied to reduce soilborne inoculum. Management strategies should focus on reducing initial inoculum in soil and restricting vascular infection and pathogen development within the host. Tactics that may reduce initial inoculum include crop rotation, soil fumigation, green manures, soil solarization, and bio-fumigants. Cultivar resistance restricts pathogen infection and colonization of plants. Integration of several management tactics and a better understanding of soil health will be needed for lasting and economic management of the disease.