INHERITANCE OF POWDERY MILDEW RESISTANCE IN GREENHOUSE-GROWN VERSUS FIELD-GROWN CALIFORNIA STRAWBERRY PROGENIES

Seedlings from 17 strawberry (Fragaria X ananassa) progenies were eval uated for resistance to powdery mildew (Sphaerotheca macularis f. sp. fragariae) using controlled greenhouse inoculations and field trials w ith differing natural infection levels. Genetic differences contribute d a large fraction of the phenotypic variance among individuals for bo th disease incidence (H-2 = 0.44-0.71) and disease severity (H-2 = 0.7 0-0.94) in all cases, but the variance attributable to breeding value for these traits varied substantially with infection level(h(2) = 0.12 -0.90). Likewise, genotypic and breeding value correlations for a sing le trait scored in different infection environments suggest that diffe rent genes may confer resistance with different levels of disease pres sure. Greenhouse evaluations corresponded well with rankings obtained under high levels of field infection. However, evaluation of genetic p otential only under conditions of extreme infection may ignore valuabl e components of partial resistance, and should not be used in isolatio n.