Virus-Suppression and Aphid Resistance Effects on Spatial and Temporal Spread of Watermelon Mosaic Virus 2

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Abstract
The spatial and temporal characteristics of epidemics induced by watermelon mosaic virus 2 (WMV 2) were monitored in replicated planting of three genotypes of Cucumis melo. One genotype was resistant to Aphis gossypii and suppressively resistant to WMV 2, another to resistant to A. gossypii, and a third was a commercial variety susceptible to both WMV 2 and A. gossypii. Fourteen epidemics in two nonoverlapping plantings were analyzed separately. Final virus incidence in the aphid-resistant genotype and the aphid/virus resistant genotypes averaged 11 and 33% lower, respectively, than that in the susceptible genotypes during the spring planting. The nine epidemics in the different genotypes of the spring planting were statistically best described by various nonlinear models. The rate of disease progress also varied among genotypes. Infected plants of all three muskmelon genotypes were consistently observed in a clustered pattern, but the degree of clustering differed among genotypes. The five epidemics occurring during the summer planting were also described by different nonlinear models, but he best single-model, statistical fit of the disease progress data from those epidemics was provided by the linear model. The rate of disease progress and final disease incidence were not significantly different among genotypes and the infected plants were observed in a random pattern. The increased incidence of WMV 2 during the summer planting was attributed to an increase in the number of alighting aphids and sources of WMV 2 in the surrounding area. The effects of the seasonal abundance and species composition of alate aphid populations, the amount and proximity of virus sources, and the effectiveness of the different resistance components are discussed in relation to the field epidemics of WMV 2.