Greenhouse and field methods were developed to screen Allium spp. for resistance to botrytis leaf blight (causal agent Botrytis squamosa Walker). In greenhouse evaluations, plants were sprayed with laboratory-grown mycelial fragment inoculum and were incubated at 20C in a chamber with an atomizing fogger. For field inoculations, a portable fog system with windbreaks was erected around experimental plots, and the plants were sprayed with the inoculum on evenings when windless, temperate (18 to 22C) conditions were forecasted. The most effective mycelial fragment inoculum was <21 days old and had ≈45 to 50 colony-forming units/μl, resulting in an absorbance at 450 nm of 0.2 to 0.3. Rubbing the wax cuticle from leaves was essential to disease development in greenhouse but not in field experiments. Evaluations of eight Allium species, including 55 A. cepa L. accessions, were in agreement with previous studies.
Field and Greenhouse Procedures to Evaluate Onions for Botrytis Leaf Blight Resistance