New North American Isolates of <i>Venturia inaequalis</i> Can Overcome Apple Scab Resistance of <i>Malus floribunda</i> 821

Abstract

Apple scab, caused by <i>Venturia inaequalis</i>, is a destructive fungal disease of major apple cultivars worldwide, most of which are moderately to highly susceptible. Thus, development of scab resistant cultivars is one of the highest priorities of apple breeding programs. The principal source of resistance for breeding programs has been the scab resistance gene <i>Rvi6</i> that originated from the Japanese crabapple <i>Malus floribunda</i> (Sieb.) sel. 821. Isolates of <i>V. inaequalis</i> able to overcome <i>Rvi6</i> have been identified in Europe, but have not yet been reported on the American continents. We recently discovered scab infection on <i>M. floribunda</i> 821 trees in a research orchard at Geneva, NY, U.S.A., where approximately 10% of the leaves bore profusely sporulating apple scab lesions, many of which had coalesced to cover entire leaves. We observed both chlorosis, typical to <i>Rvi6</i>, and pinpoint pitting symptoms typical to failed infections by <i>V. inaequalis</i> on hosts bearing the <i>Rvi7</i> gene. We assessed genetic diversity and population genetic structure of 11 <i>V. inaequalis</i> isolates in total, of North American and European origin, isolated from <i>M. floribunda</i> 821, 'Nova Easygro', 'Golden Delicious', TSR33T239, 'Schone van Boskoop', and 'Prima', using 16,321 genome-wide SNPs. Population genetic structure and PCA separated the isolates into distinct European and U.S. groups. The forgoing suggests that the new <i>Rvi6</i> virulent isolates emerged within U.S. populations, rather than being transported from Europe. The complete resistance breakdown in <i>M. floribunda</i> 821 but not in descendant cultivars, which kept their field resistance, suggests that durable resistance to apple scab will require a more comprehensive understanding of <i>Rvi6</i> mediated resistance in diverse genetic backgrounds.