Evaluation of damping-off tolerance in spinach cultivars in field soils and in a standardized lab assay with Pythium ultimum

Spinach growers face increasing problems of damping off in the production of fresh market (baby-leaf) spinach due to increasing restrictions on chemical treatments. Damping-off tolerant cultivars are increasingly important, requiring proper evaluation methods. From three locations in the Netherlands with a history of spinach cultivation and from one location in France, potential damping-off pathogens were isolated from the soil, identified to species or genus, and tested for their pathogenicity. Pythium ultimum was the most prevalent pathogen in those fields, causing spinach pre- and post-emergence damping off. Eight spinach cultivars with two or three seed lots each, were evaluated at the same field locations and in a greenhouse with soil sampled from one of the Dutch field sites. Pre-emergence damping off was more discriminating for differences among the cultivars than post-emergence damping off. Variation in levels of infection among trials, replicates, and seed lots of same cultivars, emphasized the need for a more standardized phenotyping assay. For such an assay, a cornmeal/sand-based inoculum with a pathogenic P. ultimum isolate was added to a substrate mixture of sand, perlite, and vermiculite, moistened until 50% water holding capacity, in which spinach seeds were incubated for ten days in a dark climate cabinet at 15°C, including a control treatment without P. ultimum inoculum. The assay showed reproducible results for discriminating differences in pre-emergence damping-off tolerance levels among seed lots. However, cultivar differences in pre-emergence damping-off tolerance levels could not be confirmed due to a large variation among seed lots that needs further investigation.