The 2012 ASHS Annual Conference

Cotoneaster is a diverse genus in the family Rosaceae, with approximately 400 species of highly variable plants from ground covers to trees.  A limited number of species are available in the nursery trade.  The common species commercially available are valued for their ability to flourish in difficult sites.  Although cotoneasters are well adapted for abiotic stresses, many species are highly susceptible to fire blight (Erwinia amylovora, Burrill, Winslow et al.).  Fire blight is a bacterial disease endemic to North America and has a wide host range in the sub-family Maloideae.  The disease is vectored by floral and vascular feeding insects and also is spread by rain splash and mechanical injury.  Symptoms result in flower and shoot necrosis and in severe infection the entire plant may be killed.  Previous reports on disease resistance Cotoneaster spp. are conflicting and comprise a limited range of the genus.  Our objectives were to clarify previous results and expand knowledge of disease resistance of unreported taxa.  In this experiment, 31 species of cotoneaster were arranged in a glasshouse in a randomized complete-block design.  Susceptibility to fire blight was determined by foliar assay with isolate EA153 at a concentration of 1.53 × 109 CFU·mL^-1.  Lesion length was recorded over eight weeks and the percentage of shoot infection was calculated.  Disease susceptibility varied among species.  Susceptibility ranged from 0% to 100% shoot necrosis with a mean of 36% among species.  Ten species fell below the 5% shoot infection that was used as the acceptable disease threshold.  Cotoneaster arbusculus, C. atropurpureus, C. dielsianus, and C. splendens had no symptoms (0% shoot infection).  Additionally, there were six species that exhibited symptoms below the disease threshold.  Our results agree with previous reports that C. franchetti and C. simsonii exhibit fire blight resistance.  We also report fire blight susceptibility level for eight other species, which have not previously been screened for fire blight susceptibility.