Aetiology and integrated management of Phytophthora wilt of chilli (Capsicum annuum L.) in Bhutan

Chilli (Capsicum annuum L.) is one of the most important crops in Bhutan but its production is seriously reduced by wilt disease caused by Phytophthora capsici. Although this disease was first reported in 1995 in Bhutan, it was not studied in detail and the causal pathogen was not identified prior to this study. The effective management of Phytophthora capsici requires a multifaceted approach including both cultural and chemical control methods. A nationwide survey of Phytophthora wilt of chilli in Bhutan was conducted in 2018 to collect 54 samples from symptomatic plants for identification of the causative pathogen. Phytophthora capsici was consistently isolated from all sample plants. The results from both morphological and molecular studies revealed that P. capsici was the causative pathogen of wilt of chilli. A single root drench with metalaxyl (10 µg a. i./mL) and potassium phosphonate (0.1% a. i. w/v) were effective in controlling Phytophthora wilt of chilli when they were applied one week before or on the day of inoculation. Evaluation of the effects of bed height on Phytophthora wilt of chilli in two naturally infested fields in Bhutan showed that plant survival differed significantly (p = 0.0105) between the raised and flat beds in the wetter site at Bhur, but not in the drier site at Darachu. Raised beds significantly increased the marketable yield of chilli at both sites. All seven Bhutanese chilli cultivars were highly susceptible to all 18 isolates when screened in seedling stage. This study has demonstrated the effectiveness of most components of an integrated disease management program for Phytophthora wilt of chilli. Chilli farmers in Bhutan should combine cultural practices and fungicide application to manage this disease until resistant cultivars are available