Disease development and enhanced susceptibility of wounded pepper roots to Phytophthora capsici

Abstract

Greenhouse experiments were conducted to determine disease progression of phytophthora root rot on nonwounded and wounded pepper plants (Capsicum annuum) and to determine whether susceptibility to Phytophthora capsici decreases with wound aging. Two isolates of P. capsici were used in this study, one less aggressive than the other. Trimming the roots immediately prior to inoculation with either isolate increased susceptibility significantly (P ≤ 0·05) compared with plant roots that were not trimmed. Both isolates caused a higher level of disease severity on disturbed/trimmed than on disturbed/nontrimmed roots. Disease also occurred earlier with the more aggressive isolate on both wounded and nonwounded roots. Disease severity was three to four times more severe on plants treated with the aggressive isolate (NM6011) than on those inoculated with the less aggressive isolate (NM6040), regardless of root treatment. In separate experiments, pepper roots were wounded and allowed to age for up to 5 days before inoculation. Resistance to P. capsici increased as the wounds aged, resulting in significantly (P ≤ 0·001) lower disease severity on plants with 3‐ and 5‐day‐old wounds than on those inoculated at the time of wounding and the nondisturbed/nontrimmed controls. Wounding of the roots followed by immediate inoculation with zoospores resulted in significantly higher levels of attachment than when roots were inoculated with zoospores 48 h after wounding. The 48‐h postwounding inoculation treatment showed the same amount of zoospore attachment as nonwounded roots. Increase in plant resistance correlated (P ≤ 0·01) with an increase in total peroxidase activity. Isoelectric focusing‐polyacrylamide gel electrophoresis (IEF‐PAGE) indicated increased band intensity of three acidic and one basic isozyme as wounds aged. These data suggest that wound repair plays a role in decreasing infection and resultant disease symptoms of pepper to P. capsici.