Sensitivity of Didymella bryoniae to DMI and carboxamide fungicides

• Published in: Phytopathology 2010-06, Vol.100 (6), p.S126-S126
• Main Authors: Thomas, A, Stevenson, K L, Langston, D B
• Format: Article
• Language: English
• Subjects: Citrullus lanatus; Didymella bryoniae
• ISSN: 0031-949X

Didymella bryoniae, the causal agent of gummy stem blight (GSB) of watermelon, has a history of developing resistance to fungicides, most recently the carboxamide fungicide boscalid. To facilitate fungicide resistance monitoring, baseline sensitivity distributions were established for DMI fungicides tebuconazole and difenoconazole and the carboxamide fungicide penthiopyrad that were recently introduced or are being evaluated for GSB control. In all, 77 isolates with no prior exposure to carboxamides or DMIs were tested using a mycelial growth assay to determine the effective concentration at which mycelial growth was inhibited by 50% (EC sub(50)). EC sub(50) values for boscalid, penthiopyrad, tebuconazole and difenoconazole ranged from 0.007 to 0.127, 0.009 to 0.189, 0.073 to 0.388 and 0.012 to 0.135 kg/ml with median values of 0.037, 0.030, 0.135 and 0.046 kg/ml. There was a significant positive correlation between the sensitivity to penthiopyrad and boscalid (P < 0.0001, r = 0.63), indicating a significant potential for cross-resistance between these compounds. In 2009, 104 isolates collected from fungicide-treated watermelon fields were tested for resistance to boscalid and penthiopyrad using a discriminatory concentration of 3.0 kg/ml. Of the isolates tested, 86 were resistant and 12 were sensitive to both fungicides. Because of the significant potential for cross-resistance, growers will be advised not to use boscalid and penthiopyrad in the same fungicide spray program.