Evaluation of delphinium cultivars for resistance to powdery mildew

Powdery mildew caused by Erysiphe aquilegiae var. ranunculi is one of the major diseases affecting field-grown delphiniums (Delphinium spp.). Two lath house and two field experiments were conducted in 2003 and 2004 to evaluate nine delphinium cultivars for resistance to the disease. 'Blue Bird', 'King Arthur', 'Cameliard', and 'Galahad' were consistently more resistant [0-1.1 disease severity (ds) on a 0-5 scale] than 'Casa Bianca', 'Blue Shadow', 'Belladona', and 'Bellamosum' (2.9-5.0 ds). Oriental Blue' was moderately resistant (0-3.3 ds). Spearman's coefficients of rank correlation were significant for all pairs of experiments (0.70 ≤ r_s ≤ 0.98; P ≤ 0.0354) except for the 2003-04 field experiments (r_s = 0.66; P = 0.0525). Pearson correlation coefficients between experiments (0.86 ≤ r ≤ 0.99) were highly significant (P ≤ 0.0027). Disease severity values from lath house evaluations were almost identical to those from field evaluations.