Open Field and High Tunnel Screening for Powdery Mildew Resistance in Strawberry Species and Cultivars in Florida

Florida is the major fresh strawberry supplier to the eastern states and Canada from December through March, and usually there are several nights with frost each season when overhead irrigation is used to protect against freeze damage. The use of water for freeze protection may become challenging due to increasing water restrictions. Protected environments such as high tunnels and row covers may provide an alternative means of freeze mitigation. However, due to the lack of free water in these systems, conditions are favorable for development of powdery mildew disease. Many breeders have turned to wild relatives as sources of beneficial traits. In strawberry, the wild octoploid species Fragaria viginiana and F. chiloensis are fully interfertile with the cultivated species, F. × ananassa. Therefore, the objective of this study was to screen an elite group of wild strawberry accessions for resistance to powdery mildew. In 2010-11 and 2012-13, 27 wild accessions from the National Clonal Germplasm Repository in Corvallis, OR, and three cultivar checks were planted in a randomized compete block in both open field and high tunnel environments at the Gulf Coast Research and Education Center in Balm, FL. Plots were visually rated for percent coverage of mycelia on the abaxial surface of fully expanded trifoliate leaves using a modified Horsfall-Barratt scale of 0 to 6. Mean scores for the wild species and cultivars ranged from 0.00 to 5.44 and 0.63 to 1.18, respectively. There was a significant difference between genotypes, and the most resistant subspecies showed consistently high levels of resistance. With mean scores of 0.00, the most resistant subspecies were F. virginiana subsp. grayana which included accessions NC 95-21-1 (PI 612569) and JP 95-9-6 (PI 612320) and F. vigininia subsp. virginiana, which included accessions Eagle 14 (PI 612492), Fredrick 9 (PI 612493), Montreal River 10 (PI 612497), NC 96-48-1 (PI 612324) and NC 96-35-2 (PI 612323). Within F. chiloensis, BC Canada (PI 612488) and Darrow 72 (PI 236579) had mean scores below one. Ten accessions contributed to a significant year × location × genotype interaction, but there was a high correlation between environments for genotype mean scores with years combined (r=0.89, P = <0.001). This information may be useful for breeders, as sources of resistance to powdery mildew are available within the wild accessions. Many accessions are more susceptible than current cultivars, and this must be considered when using these accessions in breeding to access other important traits.