Biological control of potato Verticillium wilt under controlled and field conditions using selected bacterial antagonists and plant extracts

Five pre-selected bacterial isolates and four plant extracts (canola, rapeseed, seaweed, and Canada milkvetch) were tested, under both growth room and field conditions during two growing seasons at two locations, for their ability to protect two potato cultivars (Russet Burbank, moderately susceptible and Kennebec, highly susceptible) against Verticillium dahliae. In growth room trials, Pseudomonas fluorescens Biotype F isolate DF37 significantly reduced the incidence, severity, and vascular discoloration of Verticillium wilt in both cultivars Kennebec and Russet Burbank over two seasons. Bacillus pumulis isolate M1 reduced wilt parameters only on cultivar Kennebec. Among the plant extracts tested, Canada milkvetch extract (MVE) was the most effective in reducing the wilt (55–84% reduction relative to the control). MVE reduced disease regardless of the method of application (incorporation into soil versus seed coating). In the first year of field testing, bacterial treatment DF37 and plant extract MVE were effective in reducing Verticillium wilt on Russet Burbank and Kennebec, respectively. Reductions in percent infection and vascular discoloration were estimated at 26% and 67% relative to the non-treated control for DF37 and 45% and 55% for MVE, respectively. In the second year of field trials, bacterial isolates DF37 and M1, and plant extract MVE reduced all wilt parameters by percentages ranging from 19% to 31% and increased yield (18%) on cultivar Kennebec. Bacterial isolate DF37 also reduced wilt (29–43%) and increased yield (24%) on cultivar Russet Burbank.