Prochloraz is widely used to control rice bakanae disease caused by Fusarium fujikuroi. The current study was aimed at monitoring the development of F. fujikuroi resistance to prochloraz in the Heilongjiang Province and analyzing the fitness of F. fujikuroi strains with different resistance levels. The results indicated that most of the 89 F. fujikuroi strains collected from the Heilongjiang Province were resistant to prochloraz, with resistance frequency reaching 92.1%. To assess the field resistance risk of prochloraz, 21 F. fujikuroi strains with different resistance levels were selected to investigate their biological characteristics and assess their fitness. Mycelial growth, sporulation, and germination rates were significantly different among the tested strains. However, when grouped into two subpopulations, no significant difference was tested between prochloraz-resistant and prochloraz-sensitive strains. Pathogenicity assays revealed that the disease severity index of prochloraz-resistant strains was higher than that of prochloraz-sensitive strains. Cross-resistance assays showed no cross-resistance between prochloraz and five other fungicides, namely phenamacril, ipconazole, tebuconazole, carbendazim, and fluopyram. Ffcyp51A gene overexpression was observed in the prochloraz-resistant F. fujikuroi strains after exposure to prochloraz. Collectively, these results indicated that F. fujikuroi resistance against prochloraz was severe. Furthermore, prochloraz-resistant strains were highly fit and could potentially become a dominant population in rice fields, consequently resulting in yield loss.
Keywords: Fusarium fujikuroi; cross-resistance; cyp51 gene; fitness; fungicide resistance; prochloraz.