Effect of dsRNA-containing and dsRNA-free hypovirulent isolates of Fusarium oxysporum on severity of Fusarium seedling disease of Essex soybean
Sixty-six isolates of F. oxysporum and F. solani were recovered from healthy and necrotic Essex soybean seedlings grown in naturally infested soil. These were tested for pathogenicity at 20 C and -0.01 MPa water potential in artificially infested, autoclaved field soil. Highly pathogenic, moderately pathogenic, and hypovirulent isolates of both species were identified. Fifty-seven F. oxysporum and nine F. solani isolates were tested for the presence of dsRNA. The presence of dsRNA was not associated with hypovirulence in F. oxysporum since some hypovirulent isolates contained dsRNA while other hypovirulent isolates did not. Furthermore, of six dsRNA-containing F. oxysporum isolates, three were hypovirulent, two were moderately pathogenic, and one isolate was highly pathogenic. Four segments of dsRNA, with sizes of 4.0, 3.1, 2.7, and 2.2 kb, were detected in extracts of all six F. oxysporum isolates. No morphological differences were found between dsRNA-containing and dsRNA-free F. oxysporum isolates. Attempts to cure dsRNA-containing hypovirulent F. oxysporum isolates, either by single-sporing of isolates or by using a range of concentrations of cycloheximide, were not successful. No dsRNA was found in any of the F. solani isolates tested. Pythium ultimum, an associate in Essex seedling disease, was isolated from water-soaked lesions and interfered with evaluations of disease caused by the Fusarium spp. Metalaxyl was used to control P. ultimum and had no apparent effect on symptoms associated with F. oxysporum and F. solani in field soil. Prior inoculation of Essex soybean seeds with conidia of dsRNA-free hypovirulent F. oxysporum isolates, plus metalaxyl seed treatment, significantly (p<0.05) reduced disease severity on both cotyledons and hypocotyls and increased the rate of seedling emergence in field soil, compared to the control plants treated with metalaxyl alone or not treated with metalaxyl. No significant (p>0.05) differences were found between dsRNA-containing and dsRNA-free hypovirulent F. oxysporum isolates in their effects on the reduction of disease severity. A mixture of two hypovirulent F. oxysporum isolates was significantly (p<0.05) more effective than single hypovirulent F. oxysporum isolates in increasing the rate of seedling emergence. Symptoms associated with P. ultimum were not affected by the prior inoculation of seeds with individual hypovirulent F. oxysporum isolates.
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