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 t
he stems. One of the peppers (cv. Smith-5) showed resistance by total
inhibition of fungal growth. Capsidiol, a phytoalexin, which accumulat
es in the area of necrosis appears to be involved in this resistance.
Capsidiol accumulation was analyzed by gas chromatography and was corr
elated with the restricted growth of P. capsici, in vivo and in vitro,
confirming the former's fungistatic and fungitoxic properties. The ca
pacity to inhibit pathogenic growth was evident only when capsidiol pr
oduction exceeded 1 204 mu g 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 a
lso resulted in necrosis and in an accumulation of capsidiol, although
to a lesser extent than in the first inoculation. The greater accumul
ation of capsidiol in the stems of cv. Smith-5 is in accordance with t
he resistance shown by this cultivar to P. capsici, and confirms the i
mplication of capsidiol in the disease resistance of this cultivar to
fungal pathogens. Capsidiol has a fungistatic character at a mean conc
entration of 3.75 mM, and is fungitoxic at levels above 5 mM. This lev
el must be exceeded and all the growing hyphae must be affected for ca
psidiol to qualify from being fungistatic to being fungitoxic.