Evaluation of the susceptibility of apple genotypes in Queretaro, Mexico to powdery mildew (Podosphaera leucotricha)

Abstract

The aim of this study was to evaluate the susceptibility to powdery mildew of apple (Malus domestica Borkh) genotypes introduced in Queretaro, Mexico. For this study, a total of 38 apple genotypes were used: 33 for the field assay and 14 for the greenhouse assay (with nine overlapping varieties). During the field assay, fungus incidence and severity were evaluated in adult plants grafted on the clonal rootstock MM 106, and this test was performed on four consecutive evaluations from May to September 2012. On the last sample, the conidial density was also determined on leaves with visible patches of powdery mildew. For the greenhouse bioassay, apple genotypes, grafted in January 2012 on MM 106, were inoculated on their adaxial side with 200μL of 7×104 conidia/mL (14,000 conidia). The incidence and severity of the fungus on the new shoots were evaluated in August and in October. In addition, the conidial densities per cm2 of leaf area and per leaf were determined for the last sample, and the correlation between the parameters evaluated was examined in each assay and between assays. ‘Rayada’ and ‘411’ showed the highest incidence (59.4 and 40.6%, respectively) and severity (120.0 and 128.3mm2, respectively) to powdery mildew in the field bioassay, in which most genotypes showed no visible damage caused by the fungus. ‘Lourdes’ and ‘436’ were the most resistant genotypes to powdery mildew in the greenhouse bioassay (no incidence, or severity), which means that they are recommended for cultivation in humid environments, and/or where little agronomic management is undertaken. The situation was different for ‘424,’ ‘428’, and ‘Royal Gala’ (with incidence rates of 32.6, 27.5, and 25.5%, and severity rates of 86.6, 229.5 and 168.8mm2, respectively), which, given their agronomic and quality advantages, could be grown under a strict phytosanitary program, or be used as an inoculum for experimental purposes. Significant correlations (P≤0.05) between incidence and severity were obtained in most cases, but no significant correlation with respect to powdery mildew susceptibility under field and greenhouse conditions was found.