Biological Characteristics and Molecular Mechanism of Procymidone-Resistance in Stemphylium eturmiunum from Garlic.

Garlic leaf blight (GLB) caused by Stemphylium eturmiunum is first reported in Jiangsu Province of China. The dicarboximide fungicide (DCF) procymidone is reported to possess broad-spectrum action in inhibiting filamentous fungi and widely used to control leaf disease of various plants. In current study, of 41 S. eturmiunum isolates collected from commercial garlic farms in Pizhou and Dafeng Counties of Jiangsu Province, eight isolates were resistant to procymidone. Three phenotypes were categorized according to the in vitro responses to dicarboximide fungicides: S (sensitive), S+ (LR to iprodione and procymidone), and R2 (HR to all iprodione and procymidone). The fitness of all the resistant isolates was decreased in accordance with the data of mycelial growth, conidiation and virulence. After treated with 10µg/ml procymidone for 4 h, mycelial intracellular glycerol concentrations of resistant isolates were significantly lower than those of sensitive isolates. Positive cross-resistance was observed between dicarboximides and phenylpyrroles, but no cross-resistance between dicarboximides and fluazinam or difenoconazole in the two resistant phenotypes. Nucleotide sequence alignment results of the two-component histidine kinase genes from the sensitive and resistant isolates indicated that amino acid mutations were located at the histidine kinase, adenylyl cyclase, methyl-accepting chemotaxis protein, and phosphatase (HAMP) domain of N-terminal region, and response regulator domain (Rec) of C-terminal region. To our knowledge, the DCF-resistance in S. eturmiunum is first reported, and the findings are helpful to establish the rational strategy to manage the DCF-resistant populations of S. eturmiunum in field.