Induction of Resistance in Cocoyam (Xanthosoma sagittifolium) to Pythium myriotylum by Corm Treatments with Benzothiadiazole and its Effect on Vegetative Growth

Abstract

Root rot disease of cocoyam (Xanthosoma sagittifolium) caused by Pythium myriotylum (Oomycete) is a major constraint in cocoyam production in many countries in the world. Benzothiadiazole (BTH), a systemic acquired resistance inducer, was used in two concentrations (15 and 50 mg/L) as corm soaking treatment for 24 h duration to test for its effect on root rot disease resistance and vegetative growth of cocoyam under greenhouse conditions. The fungicide Metalm 72 WP (metalaxyl + cuprous oxide) was used as standard chemical control. Corm treatment with BTH significantly reduced cocoyam root rot disease (CRRD) severity compared with the pathogen-inoculated control. This was expressed by a disease index percentage of 37 and 20 for 15 and 50 mg/L BTH respectively after 28 days post-inoculation as opposed to the control with a disease index of 60. BTH at 50 mg/L was found to be more effective than the fungicide in inducing significant levels of resistance against P. myriotylum. In the absence of the pathogen, BTH and the fungicide applied as corm soaking treatment did not significantly influence the chlorophyll content. BTH significantly reduced shoot and corm/root dry weights compared to the control and fungicide respectively. However no visible phytotoxic effect was observed. BTH was tested for possible effects on             P. myriotylum in vitro. There was no inhibition of mycelial growth with 15mg/L of BTH in the growth media in comparison with the control. No remarkable antifungal activity was observed with 50 mg/L. The promising effect of BTH against CRRD indicates that this compound may be future alternatives to traditional chemicals for disease control.