Capsidiol: Its role in the resistance of Capsicum annuum to Phytophthora capsici

Abstract

Inoculation of the stems of three Capsicum annuum L. cultivars showing different degrees of sensitivity to the fungal pathogen Phytophthora capsici, resulted in a hypersensitive reaction being expressed along the stems. One of the peppers (cv. Smith‐5) showed resistance by total inhibition of fungal growth. Capsidiol, a phytoalexin, which accumulates in the area of necrosis appears to be involved in this resistance. Capsidiol accumulation was analyzed by gas chromatography and was correlated with the restricted growth of P. capsici, in vivo and in vitro, confirming the former's fungistatic and fungitoxic properties. The capacity to inhibit pathogenic growth was evident only when capsidiol production exceeded 1 204 μg ml‐1, a level reached in the resistant variety after 6 days of incubation. Experiments on induced resistance showed that a second inoculation of the stems of the three cultivars also resulted in necrosis and in an accumulation of capsidiol, although to a lesser extent than in the first inoculation. The greater accumulation of capsidiol in the stems of cv. Smith‐5 is in accordance with the resistance shown by this cultivar to P. capsici, and confirms the implication of capsidiol in the disease resistance of this cultivar to fungal pathogens. Capsidiol has a fungistatic character at a mean concentration of 3.75 mM, and is fungitoxic at levels above 5 mM. This level must be exceeded and all the growing hyphae must be affected for capsidiol to qualify from being fungistatic to being fungitoxic.