Breeding Strawberry Plants Resistant to Angular Leafspot Disease

The bacterium Xanthomonas fragariae causes the economically important angular leafspot disease to which a high level of resistance has not been found within the cultivated strawberry Fragariaxananassa. The 2002 introduction of resistance donors 'US 4808' and 'US 4809' by the Fruit Laboratory at the Beltsville Agricultural Research Centre provided sources of resistance at the octoploid level derived from F. virginiana. Moving this resistance through three generations of back crossing to different recurrent F. xananassa parents along with screening progeny for resistance with a spray inoculation method, followed by selection based on horticultural traits, has created improved resistant genotypes. Resistantxsusceptible crosses have produced from zero to 21% resistant progeny with a mean of 4.8% for 25 crosses. The presence of undesirable traits, such as small and soft fruit, variegation, pistillate flowers, and susceptibility to powdery mildew, has slowed progress, but this linkage drag can be overcome by increasing screened population size and additional backcross generations. Two resistantxresistant crosses were made in 2009, yielding 32.2 and 32.7% resistant seedlings.