2022 Volume 64 Pages 89-97
We investigated the preventive effects of granular phosphorous acid fertilizer (GPAF) on basil downy mildew (BDM) caused by the oomycete Peronospora belbahrii Thines. In the pot experiments inoculated by a spray of BDM spores, 4 g or 8 g of GPAF suppressed BDM significantly by 100%. Furthermore, in the pot experiments inoculated with BDM-infected basil plants, 4 g and 8 g GPAF applications significantly suppressed BDM by 60% and 100%, respectively. In the field trials conducted in 2019, after the application of 2–16 g GPAF treatments three times every other month, the percentage of diseased leaves was significantly lower than that in the control, reaching approximately 50%. In contrast, in the field trials conducted in 2020, in which GPAF was applied only once before the occurrence of BDM, the percentage of diseased leaves with the 4 g GPAF treatment was significantly lower (by 55%) than that in the control. However, with the 8 g GPAF treatment, the percentage of diseased leaves was almost the same as that in the control. These results indicate that a single application of GPAF during the rainy season, when the infection pressure is high in the field, is not very effective. This may be due to the low phosphorus acid concentration in basil. Thus, we concluded that adequate absorption of phosphorous acid from the early growth stage effectively suppresses BDM. Consequently, GPAF is expected to contribute to the integrated pest management of BDM.
