Field Resistance of California Strawberries to Verticillium dahliae at Three Conidial Inoculum Concentrations

in HortScience

Runner plants from strawberry (Fragaria ×ananassa Duch.) genotypes previously identified as relatively susceptible, intermediate, and resistant to wilt caused by Verticillium dahliae Kreb. were inoculated with a conidial suspension containing a mixture of five isolates of V. dahliae at 104, 105, and 106 conidia/mL. Disease symptoms were scored as the number of dead or severely stunted plants per plot, or on the basis of a subjective phenotypic resistance score assigned to each plot on eight dates during the spring after planting. Overall disease symptoms increased with inoculum concentration; resistance scores for all genotypes were 4.8, 3.7, and 3.2, and the percentages of plants stunted or dead were 6.8, 32.6, and 44.9 for the three conidial concentrations, respectively. The relative resistance categories were separable at concentrations of 106 and 105, whereas no separation was obtained at 104 conidia/mL. Genotypes originally classified as intermediate in resistance performed more like susceptible types at the highest conidial concentration. Significant resistance category × conidial concentration level interactions were detected for resistance score but not for the number of dead or severely stunted plants per plot. Regression coefficients for resistance score and percentage of stunting on conidial concentrations were statistically significant only for susceptible and intermediate genotypes. Some stunting was detected within all resistance categories at the highest conidial concentration, and the practical value of the resistance developed to date will depend ultimately on realistic field inoculum levels.

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