We revealed that aerial ultrasound of 40.5 kHz could induce disease resistance in tomatoes and rice when the plants were irradiated with ultrasound of ca. 100 dB for 2 weeks during nursery season and reduced the incidence of Fusarium wilt and blast diseases, respectively, when plants were inoculated with pathogen 0 or 1 week after terminating irradiation. Disease control efficacy was also observed with ultrasound at frequencies of 19.8 and 28.9 kHz. However, cabbage yellows and powdery mildew on lettuce were not suppressed by ultrasound irradiation. No significant positive or negative effect on growth was observed in tomato and rice plants. RT-qPCR showed that the expression of PR1a involved in the salicylic acid (SA) signaling pathway was upregulated in the ultrasound-irradiated tomato.
Figure 1: Aerial ultrasonic conditions and an experimental flow. Seedlings of each plant were irradiated with ultrasound of 40.5 kHz frequency with ca. 100 dB sound pressure level. The oscillation pattern was composed of intermittent pulse waves. The pulse width is 7 msec, and the pulse frequency repeatedly shifts. The intermittent pulse waves are illustrated (a). Irradiation with ultrasound started when plants were 1 week old, continued for 1–2 weeks (24 hr a day), and plants were inoculated with pathogens at 0 or 1 week after terminating ultrasound irradiation (b). Tomato seedlings irradiated with aerial ultrasound in a limited irradiation area (50 cm diameter at 70 cm from the oscillator) in a sound-proof area (c).