Heritable tolerance of ash yellows phytoplasmas in green ash

Abstract

Variation in response of green ash (Fraxinus pennsylvanica Marsh.) to infection by ash yellows (AshY) phytoplasmas was assessed. Twenty seedlings of each of 35 half-sib families from 15 sources (37n50∞N, 70n98∞W) were planted in pairs (two-tree plots) in a randomized complete block design at Ithaca, N.Y. One plant of each pair was inoculated at age 2 years, and some of these plants again at age 3 years, with New York strains of AshY phytoplasmas by bark-patch grafting. DAPI tests revealed 99% of inoculated trees and 2% of controls were diseased 4 years after first inoculation. Incidence of dieback and mortality associated with winter damage and transplanting shock was significantly higher in diseased trees than in noninfected controls. Foliage of many diseased trees appeared lighter green than foliage of noninfected controls. Foliar color, measured with a Minolta SPAD-502 chlorophyll meter 4 years after first inoculation, varied significantly (p < 0.005) between diseased trees and controls, among families, and in response to family × treatment interaction. Analyses 5 years after first inoculation revealed significant (p < 0.01) phytoplasma-associated suppression of growth in height, stem diameter, and volume index (diam. 2 × height × 10 n2 ). Average increase in volume index of diseased trees in years 4 + 5, as a proportion of the increase in noninfected controls, was 0.44 and varied among families from 0.13 to 0.84. Family × treatment interaction on volume growth was significant (p < 0.001). Heritability of volume growth response to phytoplasmal infection was 0.60 ± 0.17 (mean ± SE) for families and 0.52 for individual trees. These findings indicate potential for selection or breeding of AshY-tolerant green ash.