RESUMEN
Sixteen Meloidogyne isolates from tomato fields in California grown with resistant cultivars were multiplied on resistant tomato in a greenhouse. Of these resistance-breaking isolates, one was identified as M. javanica, and all others as M. incognita. The reproduction of the M. javanica isolate and four M. incognita isolates on six resistant tomato cultivars and on susceptible and resistant cultivars of pepper, sweetpotato, green bean, cotton, and cowpea was evaluated and compared to an avirulent M. incognita population in greenhouse pot trials. On resistant tomato cultivars, there were minor but significant differences between the resistance-breaking Meloidogyne isolates and between the different tomato cultivars. Of the other resistant crop cultivars, pepper was resistant to all isolates and green bean to all M. incognita isolates, while cotton and cowpea allowed reproduction of one of the resistance-breaking M. incognita isolates. The resistant sweetpotato cv. Bonita behaved like resistant tomato, allowing reproduction of all five resistance-breaking isolates but not of the avirulent M. incognita. Our results showed that variability exists among resistance-breaking Meloidogyne isolates, and that isolates overcoming resistance in tomato may also be virulent on resistant sweetpotato.
RESUMEN
ABSTRACT Thirty-nine isolates of Fusarium oxysporum were collected from tomato plants displaying wilt symptoms in a field in California 2 years after F. oxysporum f. sp. lycopersici race 3 was first observed at that location. These and other isolates of F. oxysporum f. sp. lycopersici were characterized by pathogenicity, race, and vegetative compatibility group (VCG). Of the 39 California isolates, 22 were in VCG 0030, 11 in VCG 0031, and six in the newly described VCG 0035. Among the isolates in VCG 0030, 13 were race 3, and nine were race 2. Of the isolates in VCG 0031, seven were race 2, one was race 1, and three were nonpathogenic to tomato. All six isolates in VCG 0035 were race 2. Restriction fragment length polymorphisms (RFLPs) and sequencing of the intergenic spacer (IGS) region of rDNA identified five IGS RFLP haplotypes, which coincided with VCGs, among 60 isolates of F. oxysporum from tomato. Five race 3 isolates from California were of the same genomic DNA RFLP haplotype as a race 2 isolate from the same location, and all 13 race 3 isolates clustered together into a subgroup in the neighbor joining tree. Collective evidence suggests that race 3 in California originated from the local race 2 population.
RESUMEN
Chlorothalonil was applied 2, 4, 6, 2 and 4, or 4 and 6 weeks prior to tomato harvest in fields in four counties in California over a 3-year period to assess the economic benefits of chlorothalonil applications, if any, on the reduction of black mold caused by Alternaria alternata. The percentage of visually infected fruit was significantly reduced across all four sites in 2 of the 3 years. At one location, the incidence of black mold was reduced over 50% in all 3 years of the study by a single application of the fungicide. Overall, the mean yield of marketable fruit was 79.8 metric tons per hectare. An average of 5.1% of the fruit harvested in all locations and years had visible symptoms of black mold. Generally, the most effective treatment was a single application of the fungicide 6 weeks before harvest, with no further benefit from a second application of the fungicide. Based on the cost of the fungicide and its application, a single dose of chlorothalonil applied to plots in Solano County, where the highest incidence of disease occurred, would net the grower $160.47 return per hectare. The application of fungicide resulted in significant economic benefits in tests in two of the counties, where the disease is historically a problem.