FIELD-RESISTANCE OF CALIFORNIA STRAWBERRIES TO VERTICILLIUM-DAHLIAE AT 3 CONIDIAL INOCULUM CONCENTRATIONS

Runner plants from strawberry (Fragaria xananassa Duch.) genotypes pre viously identified as relatively susceptible, intermediate, and resist ant to wilt caused by Verticillium dahliae Kreb. were inoculated with a conidial suspension containing a mixture of five isolates of V. dahl iae at 10(4), 10(5), and 10(6) conidia/mL. Disease symptoms mere score d as the number of dead or severely stunted plants per plot, or on the basis of a subjective phenotypic resistance score assigned to each pl ot on eight dates during the spring after planting. Overall disease sy mptoms increased with inoculum concentration; resistance scores for al l genotypes were 4.8, 3.7, and 3.2, and the percentages of plants stun ted or dead mere 6.8, 32.6, and 34.9 for the three conidial concentrat ions, respectively. The relative resistance categories mere separable at concentrations of 10(6) and 10(5), whereas no separation was obtain ed at 10(4) conidia/mL. Genotypes originally classified as intermediat e in resistance performed more like susceptible types at the highest c onidial concentration. Significant resistance category x conidial conc entration level interactions were detected for resistance score but no t for the number of dead or severely stunted plants per plot. Regressi on coefficients for resistance score and percentage of stunting on con idial concentrations were statistically significant only for susceptib le and intermediate genotypes. Some stunting was detected within all r esistance categories at the highest conidial concentration, and the pr actical value of the resistance developed to date will depend ultimate ly on realistic field inoculum levels.