In a 4‐year disease survey in commercial spinach fields, pathogens were isolated from spinach root pieces placed on selective agar media. Aphanomyces cladogamus was the most abundant pathogen, followed by Phytophthora. cryptogea and Fusarium oxysporum. Rhizoctonia solani was found only occasionally. Other pathogens isolated were F. redolens, F. sambucinum and Cylindrocarpon destructans. P. cryptogea was the most severe pathogen, causing death of most plants, but A. cladogamus also caused severe root damage. Isolates of F. oxysporum ranged from highly pathogenic, i.e. P. oxysporum f.sp. spinaciae race 1. to moderately pathogenic and non‐pathogenic, Rhizoctonia solani isolates also varied widely in their pathogenicity. Only a small number of the F. redotens and F. sambucinum isolates were pathogenic and most C. destructans isolates were weakly pathogenic. Isolation frequencies were relatively stable from year to year, but P. cryptogea was isolated more frequently in autumn than in spring. No clear relationships were found between pathogen prevalence and disease severity index of surveyed field plants, between pathogen prevalence and plant developmental stage, or between prevalence of the different pathogens isolated.
Several pathogenic Pythium spp, were isolated from spinach roots in a 4-year disease survey in commercial spinach fields in southern Sweden. Heterothallic and zoospore-producing isolates belonging to the filamentous, non-inflated zoosporangia (F) group of van der Plaats-Niterink were most abundant, followed by P. sylvaticum, P. ultimum var, ultimum and P. heterothallicum. The heterothallic hyphal swelling (HS) group and P. tracheiphilum were found in a few plants. Two types of pathogenicity tests were performed, to measure the effects of seedling infection and of root infection of older plants. These tests showed P. ultimum var, ultimum to be the most severe spinach pathogen inducing pre-and postemergence damping off as well as root rot of older plants. Also P. heterothallicum and P. tracheiphilum damaged both seedlings and older plants, Pythium sylvaticum, Pythium 'group HS" and Pythium 'group F' were pathogenic only to seedlings, P. ultimum var, ultimum and Pythium group F were isolated in significantly higher frequencies from spinach grown in the autumn season than in the spring. No clear relationships were found between Pythium prevalence and disease severity index of surveyed field plants, between Pythium prevalence and plant developmental stage, or between prevalence of Pythium and other pathogens isolated. This is the first report of P. heterothallicum and P. sylvaticum being pathogenic on spinach.
A fungus, of the genus Phyiophthora, frequently isolated from diseased spinach roots and also from field-grown wheat plants in an area in the south of Sweden, was identified as P. cryptogca on the basis of morphology, growth characters and cardinal temperatures. Mycelium or zoospores applied as inoculum in a series of pathogenicity tests induced symptoms in spmach, sugarbeet, wheat, cucumber, oil-seed rape, pea and oats. These ranged from death of all (spinach) or some inoculated plants (sugarbeet and wheat), to only slight root sj'mptoms (oats). Successful re-isolations from all plants tested, confirmed infectivity in all cases. This is the first report ot the occurrence of P. cryptogea in Sweden and, as far as we know, of pathogenidty of this fungus to wheat, oil-seed rape and oats.
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