Prevention of Powdery Mildew Disease in Tomato Nursery by Improved Hot Water Spraying Device

In plant nurseries, reducing the frequency of chemical application is becoming a challenge owing to the appearance of hardly controllable pathogens, spread of diseases, and demand by farmers. This study was therefore conducted to develop a practical, alternative fungal control strategy against powdery mildew by using hot water spray in a tomato nursery. The expected effects of hot water spray treatment are induced resistance and disinfection. Gray mold was used as an experimental model to determine the conditions for practical application of hot water spray for inducing resistance to plant fungi by heat shock treatment. Hot water dipping of tomato seedlings at 50°C for 20 s induced resistance against gray mold and increased the expression of some pathogenesis-related genes, viz., pathogenesis-related protein 1a (PR1a), basic intracellular β-1,3-glucanase (GluB), and basic intracellular chitinase (Chi9). A prototype of a towable hot water sprayer was developed, and its performance was tested in the field. When hot water was sprayed at 57°C ± 2°C while moving at a speed of 0.5 m/min, the leaf temperature of certain parts of the seedlings reached approximately 50°C for 20 s, thereby inhibiting powdery mildew. Moreover, it was not necessary to heat the whole plant at the target condition because heat shock is known to induce systemic resistance. These results suggest that hot water spraying might be an effective technique to prevent powdery mildew in tomato nursery.