Heritabilities of Foliar Powdery Mildew (Podosphera aphanis) Resistance in the University of Florida Strawberry Germplasm

The Florida strawberry industry supplies the eastern United States and Canada with strawberries from November through March.  An annual, highly intensified growing system gives rise to cultural challenges including fungal disease control.  One such ubiquitous, wind-spread disease is powdery mildew (PM) caused by the obligate parasite Podosphera aphanis.  To control PM, multiple applications of fungicides are applied each season.  Planting resistant cultivars would reduce the need for chemical controls; however there are no commercial cultivars with PM resistance for Florida.  To date, there have been no published reports on powdery mildew resistance in the Florida strawberry breeding population.  In 2010, clonally replicated individuals from seven bi-parental crosses arising from eleven parents were included in a field trial designed as a modified Latin square with four replications.  Seedlings were evaluated visually for percentage of coverage of PM mycelium over the leaf surface on a scale of 0 to 6.  The overall population mean was 2.8 with individual family means ranging from 1.8 to 4.0. Broad and narrow sense heritabilities for PM resistance were estimated as 0.51 and 0.50, respectively.  In 2011, crosses were made from parents selected from the 2010 population.  From six out of seven families, one resistant and one susceptible genotype were selected as parents from within the top and bottom 10% in order to perform divergent selection.  A mating design consisting of a partial, circular half diallel was constructed for both classes of parents.  Seedlings were field planted in Fall 2011 in a randomized complete-block design with four replications and rated for PM.  The means for the seedling populations arising from the resistant and susceptible classes of parents were 1.8 and 3.7, respectively, which equates to 36% and 32% changes over the base population. Heritability estimates in both the broad and narrow sense for the resistant subpopulation were 0.68, and for the susceptible subpopulation the estimates were again the same and were 0.61.  In 2012, a second generation of parents were divergently selected and subjected to the same mating design for further analysis of response to selection; seedlings will be evaluated during the 2012–13 season.  Considering the moderate to high heritability estimates and clear response to selection, we conclude that resistance to PM is genetically controlled through mostly additive effects.  Through field selection based on natural infection and traditional breeding techniques, good progress can be made toward developing strawberry cultivars with resistance to PM for Florida.