Incidence of Aphid-Transmitted Virus Infections Reduced by Whitewash Sprays on Plants

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Abstract
Weekly sprays of potato crops with 15% whitewashes of Loven and Yalbin reduced potato leaf roll virus (PLRV) incidnce in tubers from 0-61% of the level in the untreated plots in seven independent experiments. Similar sprays with 2% Virol oil, did not reduce PLRV in three experiments. Loven and Yalbin sprays reduced tomato virus Y (PVY) incidence in potato to 0-68% of the level in controls, which is roughly comparable to the reduction obtained by Virol oil (0-52%). Whitewash sprays were less efficient for PLRV and PVY control than coarse, white net covers. Weekly sprays with whitewash reduced total tuber yield by about 30%, about the same yield decrease caused by oil treatment in our conditions. However, the number and weight of tubers between 30 and 12 g (seed size) in white wash-treated plots was not affected. Two effects could explain the activity of whitewash. The whitewashes increased leaf reflectivity in the visible spectrum by 130-250%, whereas oil treatment caused no change. This change in reflectivity agrees with the reduced number (30-50%) of aphids landing in whitewash-treated plots, whereas no reduction in aphid landing occurred in Virol treatments. However, different aphid species responded differently; for example Aphis gossypii was attracted to whitewash-treated plots. In our experiments, whitewash and oil treatments did not affect PLVR acquisition by aphids. Loven and Yalbin plus a sticking agent Dabak reduced PLRV inoculation by aphids, although Virol plus Dabak and Dabak itself had no effect compared with untreated plots.