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 postemergence 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. Preemergence damping-off was more discriminating for differences among the cultivars than postemergence damping-off. Variation in levels of infection among trials, replicates, and seed lots of the 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 and moistened until 50% water-holding capacity, in which spinach seeds were incubated for 10 days in a dark climate cabinet at 15 degrees C, including a control treatment without P. ultimum inoculum. The assay showed reproducible results for discriminating differences in preemergence damping-off tolerance levels among seed lots. However, cultivar differences in preemergence damping-off tolerance levels could not be confirmed due to a large variation among seed lots that needs further investigation.