In this study, the occurrence of perilla mosaic disease in Aichi prefecture, Japan, was evaluated by conducting a questionnaire survey among perilla farmers. About 90% of the farmers reported the occurrence of this disease in their crop, while about 30% reported the disease to have affected over 10% of their cultivation facility. The survey results suggested that the risk of disease development was significantly increased when perilla was planted in the field around the cultivation facility. In the local cultivation facility, we performed cultivar control by removing infected perilla planted in the field that had been the source of this disease during the cultivation period in summer 2016. As a result, the rate of this disease decreased compared with that during the cultivation in summer 2015, when no control measures were implemented. In addition, it was revealed that disease in the cultivation facilities was more often reported near the side windows, with higher incidences occurring from July to November. During the cultivation in summer 2017, after removing the source of infection and properly applying pesticide spray from July to November, the rate of the occurrence of perilla mosaic disease decreased further compared with that during the cultivation in summer 2016.
To determine rust symptoms caused by shiso rust mites (Shevtchenkella sp.), perilla leaf color was quantified by image analysis, and symptomatic and healthy leaves were compared. On symptomatic leaves, the blue (B) value in the red, green, and blue (RGB) color system was lower on the back side of the leaf, whereas the red/green (a*) values in the L*a*b* color system were lower on both sides of the leaves, compared with those of healthy leaves.Complex color elements were investigated by principal component analysis to determine rust occurrence. Using the RGB system, less green (B) on the back indicated rust. Using the L*a*b* system, a* value on the front indicated rust and suggested that size may also contribute to the results.
Shiso mosaic disease is caused by the perilla mosaic virus (PerMV) and transmitted by shiso rust mites (Shevtchenkella sp.). Twenty PerMV-infected shiso rust mites were released in the center of a perilla plant field to investigate horizontal disease transmission. Disease in the surrounding plants started developing after 19 days; 47% of plants displayed symptoms after 40 days; and all plants displayed symptoms after 69 days. The diseased-plant density was the highest in the northeast, northwest, and southeast of the inoculated field, and was lowest in the southeast. The wind direction frequency during the one to two weeks before the field observation seemingly correlated with the density of diseasedeveloped plants in the field.
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