Relationship between the host specificity of pathogen and phytoalexin

Abstract

In order to know whether the inhibitory action of a phytoalexin (PA) obtained from a certain host-parasite combination is specific to the parasite species used or not, three kinds of PAs were assayed on several phytopathogenic fungi including congenial and non-congenial to the host plants. The following three PAs were used: (1) garden pea pods versus Ascochyta pisi (pea-PA), (2) rape pods versus Glomerella cingulata (rape-PA), and (3) broad bean leaves versus Glomerella cingulata (broad bean-PA). These pods were inoculated, on the inner epidermis, with spore suspensions, incubated for 24 hours at 20°C, then the spore suspensions were collected, and the resulting PAs were extracted with petroleum ether, the latter being evaporated under reduced pressure at 20°C, then dissolved in ethanol. Broad bean leaves were inoculated, after the epidermis of lower surface were peeled off, on this surface with spore suspension of Glomerella cingulata. The PA was prepared by the same procedure as above.
Series of PA agar media were made by adding varying amounts of PA ethanol solution to malt extract agar so as to prepare the media containing the PA at the concentrations of 8, 4, 2, 1, 1/2, 1/4, and 1/8 times of that of original PA-solution in the case of pea-PA, and 4, 2, 1, 1/2 and 1/4 times in the cases of rape- and broad bean-PAs, assuming that PAs were extracted throughly from the original PA-solutions with petroleum ether.
Each of these media, 0.7ml was poured into a watch glass, 30-32mm in diameter, and was inoculated with a disc of agar, 0.3mm in diameter, on which the fungus to be tested had been grown. After being incubated for 2-7 days at 20°C the diameter of the colony was measured. The results were presented in Figures 1, 2, and 3.
These results show that the pea-PA strongly inhibits the growth of Piricularia oryzae which can not infect the pea plant, inhibits to a medium extent the growth of Glomerella cingulata, which is a facultative parasite to the same plant, while inhibits very weakly that of Ascochyta pisi which is really a parasite to the plant.
The detoxication of pea-PA by A. pisi and also by Fusarium oxysporum, both being pathogenic to the plant, was certified by in-vitro tests. The disappearance of the opaqueness around the colonies of the fungi on the PA agar media indicated the existance of PA-decomposing ability of both fungi. Ethanol extraction of the transparent zone thus appeared on the agar media around the colony showed a very low inhibitory action on the conidia germination of Glomerella cingulata, and also a remarkable decrease in its optical density at 309mμ by spectrophotometer (Table 5).