DISEASE PROGRESSION AND YIELD LOSSES FROM ROOT DISEASES CAUSED BY SOILBORNE PATHOGENS OF SPINACH

Severity of spinach diseases caused by soilborne pathogens, mainly Aphanomyces cochlioides, but also Phytophthora cryptogea, Pythium spp., and Fusarium spp., was determined in a total of 23 commercial fields located in southern Sweden. Disease progress curves increased rapidly from emergence to when the cotyledons and the first pair of true leaves were fully developed. Thereafter, disease did not increase. A field disease severity index (ranging from 0 to 100), read after plant developmental stage 2 (one true leaf pair), was correlated with spinach yield. An incremental increase in field disease severity index corresponded to a yield loss of approximately 1% or 210 kg/ha. Soil treatment with dazomet, which breaks down to release methyl isothiocyanate in soil, resulted in yield increases of 100-150% in fields with established pathogen populations. However, only small yield increases were obtained in fields in which spinach was grown for the first time. Average field disease severity indices for spinach grown in monoculture, in rotation with other crops, and for the first time in a field were 39, 31, and 19, respectively.