APPLICATION OF FUZZY CLUSTER ANALYSIS TO TEMPORAL AND SPATIAL DYNAMICS OF CHINESE-SMALL-CABBAGE MOSAIC DISEASE AND ITS VECTORS

The temporal and Spatial dynamics of the aphid vectors (Myzzus persicae and Rhopalasiphum pseudobrassicae) of the Chinese-small-cabbage mosaic disease were outlined by means of fuzzy cluster analysis. At the early stage of the epidemic aphids mainly of alatae immigrating from the outside of the field were of low density and poisson distribution, but sometimes low aggregations were observed. After the colonization of the aphids in the field, the infestation increased greatly in a short time, but soon the rate of infestation began to decrease; the density of aphids, however, still continued to increase, resulting in a high aggregation in the season. When the aphid-infested plants reached 90%, the rate of infestation was greatly slow down; the aggregation became lower than before, being stable. The size of the aphid population continued to increase to a peak just before harvest of the crop.The dynamic of disease progress was also studied with a Fuzzy cluster analysis. At the early stage of the epidemic the inoculum (virus) was carried and spread by the immigrating alatae with a random distribution, which resulted in a random distribution of diseased plants in the field. Due to the rapid spatial spread of the disease, there was a rapid increase in number of diseased sampling units, which was paralleled by an increase in percentage of diseased plants in the field. However, at the beginning there was often only 1 diseased plant per sampling unit. Owing to the immigration and colonization of the vectors, inter-and intra-unit spread of disease occurred simultaneously and was accompanied with an increase of disease severity. The migration of vectors in the field facilitate the spread of diseased plants all over the field, resulting in a rapid increase of diseased sampling units to 100% and diseased plants as well. At the last stage of the epidemic, nearly all plants were infected and thus the disease progress was then reflected in the iacresease of severity.