PREVALENCE AND PATHOGENICITY OF SPINACH ROOT PATHOGENS OF THE GENERA APHANOMYCES, PHYTOPHTHORA, FUSARIUM, CYLINDROCARPON, AND RHIZOCTONIA IN SWEDEN

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. F. 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. I redolens 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.