APS Homepage

Poster: Biology & Disease Mgmt: Chemical Control


Sensitivity of Alternaria spp. causing diseases in tomato and potato to boscalid and azoxystrobin
C. VRISMAN (1), M. Islam (1), S. Miller (1) (1) The Ohio State University, U.S.A.

Tomato and potato growers in Ohio have recently reported reduced efficacy of fungicides used to control diseases caused by Alternaria. We aimed to assess the sensitivity of Alternaria isolates to two commonly used fungicides. Thirty isolates were collected from symptomatic tomato and potato plants in 2015, 28 of which were identified as A. alternata and two as A. solani. Isolates were screened for sensitivity to commercial formulations of azoxystrobin and boscalid. Mycelial discs were transferred to potato dextrose agar plates containing 0.0, 0.01, 0.1, 1, and 10 µg boscalid/mL; or 0.0, 0.01, 0.1, 1, 10, and 100 µg azoxystrobin/mL amended with SHAM (100 µg/mL). The diameter of fungal colonies was measured after 8 days of incubation. The effective dose for 50% inhibition (EC50) of mycelial growth was calculated for each isolate by regressing the relative growth against the log10 of fungicide concentration. The A. alternata isolates were sensitive or slightly reduced in sensitivity to boscalid. One A. solani isolate was highly resistant to boscalid. More than 90% of the A. alternata isolates were resistant to some degree to azoxystrobin; both A. solani isolates were highly resistant. Overuse of azoxystrobin may have resulted in a shift in Alternaria populations to higher resistance to this fungicide. Resistance to boscalid was rare, but fungicide resistance management programs should be followed to reduce the risks of resistance development in Alternaria species.