Cross-resistance in Alternaria brassicicola from naturally infested broccoli seeds against two succinate dehydrogenase inhibitor fungicides

Abstract

Seeds can serve as a vehicle for the dissemination of pests and pathogens around the world. We recently demonstrated the association of pathogenic Alternaria brassicicola isolates with reduced sensitivity to azoxystrobin (quinone-outside inhibitor [QoI]) in naturally infested commercial broccoli seeds. In this study, we further demonstrate that these isolates were also resistant to two succinate dehydrogenase inhibitor (SDHI) fungicides. Sensitivity of representative A. brassicicola isolates (n = 58) from naturally infested broccoli seedlots to QoI and SDHI fungicides was evaluated under in vitro conditions. Interestingly, 15% (n = 9/58) of the A. brassicicola isolates with reduced sensitivity to azoxystrobin also displayed reduced sensitivity to two commonly used SDHI fungicides (boscalid and penthiopyrad) in broccoli, indicating a potential case of cross-resistance to SDHI fungicides. Ninety-three percent of the isolates (n = 54/58) were resistant to both boscalid and penthiopyrad, while 100% isolates displayed sensitivity to fluopyram. Sequence analysis of sdh genes revealed the presence of only one point mutation (H134R) in the sdhC gene in isolates that displayed resistance to boscalid and penthiopyrad. We also developed and validated allele-specific primers targeting the H134R mutation for rapid screening of SDHI resistance in A. brassicicola. We also found that boscalid-resistant isolates exhibited significantly reduced mycelial growth. However, spore germination rates among different resistant isolates were not different, suggesting that resistant isolates remain competitive in natural populations. Overall, this study provides the first evidence of fungicide resistance to SDHI fungicides in A. brassicicola isolated from naturally infested broccoli seeds and underscores the importance of seeds as a potential source for introducing fungicide resistance across geographical locations.