2017 ASHS Annual Conference

Pepper (Capsicum annuum) is one of the most commonly cultivated horticultural crops worldwide. Infection of pepper plant by Phytophthora capsici generally causes root and crown rots and sudden wilts and occurs in very wet soil condition. To investigate the effects of soil volumetric water contents (θ, v/v) on growth and disease severity of pepper, an automated irrigation systems based on soil moisture sensor was used for 6-week old C. annuum ‘Nockwang’ grown in clay-loamy soil, after inoculating the pathogen zoospores into soil. The θs of each pot were maintained at 0.2 (≈3.5 kPa), 0.275 ( ≈3.1 kPa), and 0.35 ( ≈2.4 kPa) m³∙m^-3 for about 3 weeks using an automated drip irrigation system with 9 EC-5 soil moisture sensors (Decagon Devices, Pullman, WA) and 9 solenoid valves. During the experiment period, disease severity were evaluated on a scale of 0 (symptomless) to 5 (dead plant) every day and the general growth parameters were measured at 19 days after inoculation (DAI). The first disease symptoms were observed after 10, 6, and 4 DAI in the plants grown at θ thresholds of 0.2, 0.275, and 0.35 m³∙m^-3, respectively, indicating lower θ threshold delayed the appearance of disease symptom. At harvest, disease severity of the plants grown at a θ threshold of 0.20 m³∙m^-3 was the lowest at 1.67±0.80, and plants grown at a θ threshold of 0.35 m³∙m^-3 had the disease severity of 3.83±0.60, indicating low θ treatment alleviated the disease symptom. Most growth of the plants was not significantly different across the various θ thresholds. However, root fresh weight of the plants decreased with increasing θ threshold, the plants grown at a θ threshold of 0.2 m³∙m^-3 had thicker stems than the plants at θ threshold of 0.35 m³∙m^-3, and leaf relative water contents were higher with decreasing θ threshold, indicating the disease symptom was more severe at higher θ threshold. Overall, plants grown at a θ threshold of 0.2 m³∙m^-3 appeared to delay and alleviate the disease symptom of P. capsici on pepper, which might indicate that efficient irrigation control could alleviate the disease in a production field.