The severe yellowing disease (amarelão) on melon plants is a serious problem in Brazil, although the causative agent remained unknown for a long time. Recently, recombinant isolates of cucurbit aphid‐borne yellows virus (CABYV) were reported as the possible causative agents of this disease on melon plants. Although aphids are known to be the vectors of the common type of CABYV isolates, almost no aphid colony was observed in the major melon fields in Brazil with high incidence of the severe yellowing disease. In contrast, whiteflies are often abundant. Based on this observation, the hypothesis of the transmission of recombinant CABYV by whiteflies was evaluated. After thorough transmission experiments, we found that this recombinant CABYV isolate was transmitted by the whitefly Bemisia tabaci MEAM1, but not by Aphis gossipii. Furthermore, the host response by whitefly‐based inoculation in cucurbits and other indicator plants showed differences in host range when compared to the common type of CABYV. Due to its transmissibility by the whitefly and the distant relationship of the P3/P5 protein to CABYV, the name “cucurbit whitefly‐borne yellows virus” is proposed for this recombinant CABYV. This is the second report of polerovirus transmission by the whitefly B. tabaci, following the report of pepper whitefly‐borne vein yellows virus.
Over the last two decades, begomovirus diseases have increased in importance in Brazilian tomato crops. The major management strategy in Brazil is the application of insecticides to control the whitefly vector, but this is often unsuccessful. The objective of this work was to study the spatial and temporal progression of the disease in two processing tomato production areas in Central Brazil. A total of 24 plots (225 plants each) in six fields were evaluated weekly by visual inspection for plants with begomovirus symptoms. The predominant begomovirus in tomato plants with symptoms was Tomato severe rugose virus, and it was also detected in weeds and other crop plants. No correlation between incidence of begomovirus disease and whitefly population was found. The disease progression was rapid, with a slightly aggregated distribution of plants with symptoms. No relevant differences were observed in the temporal and spatial analyses, although an important difference was detected between plots located at the centre (PC) and the edge (PE) of the fields. In the temporal analysis, the begomovirus incidence and area under disease progress curve values were lower in PC than in PE. In the spatial analysis, plants with begomovirus symptoms were more aggregated in PC than in PE. These results suggest that the distribution of plants with symptoms in PC and PE could be a result of three dissemination mechanisms: one random (primary) and two aggregated, a real secondary spread, and a false secondary spread. The implications of these differences on disease management are discussed.
Melon plants with severe yellowing symptoms from in Brazil were analyzed by high-throughput sequencing. Sequences homologous to the genome of the polerovirus cucurbit aphid-borne yellows virus (CABYV) were frequently retrieved. Two draft CABYV genomes were assembled from two pooled melon samples that contained an identical putative recombinant fragment in the 3' region with an unknown polerovirus. The complete genomes of these isolates revealed by Sanger sequencing share 96.8% nucleotide identity, while both sequences share 73.7% nucleotide identity with a CABYV-N isolate from France. A molecular-clock analysis suggested that CABYV was introduced into Brazil ~ 68 years ago.
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