Abstract: Grape downy mildew is a major disease on grapes in Tianshui area. To clear the temporal dynamics of sporangium quantity and disease, the key factors to the disease initial occurrence, sporangia of Plasmopara viticola dispersal were observed using the spore-trapping machine and the disease state were investigated during the grape growing season in Tianshui area. At the same time, meteorological data were recorded in the field by automatic microclimate recording instrument. The results showed that the sporangium of P. viticola in Tianshui area was generally observed around the beginning of July. If conditions were suitable, it would be the quickly spreading period from late July to late August, and would enter the rapid decreasing period after September. The disease in the field initially occurred in the early or middle July, and reached its peak in the middle or late August when the conditions were suitable. Disease began to appear about 7 d after the sporangia of P. viticola were detected in the field, and there was a significant positive correlation between the disease index and the number of sporangium. The grape downy mildew is closely related to the meteorological factors of temperature, relative humidity, and rainfall, effective rainfall is the key factor to disease initial occurrence and spreading.

Key words: Vitis vinifera cv. Kyoho, Plasmopara viticola, disease index, sporangium, key factors