Accumulation of phytoalexins in Ulmus americana in response to infection by a nonaggressive and an aggressive strain of Ophiostoma ulmi

Alcohol extraction was made from seedlings of Ulmus americana L. first inoculated with either a nonaggressive or an aggressive strain of Ophiostoma ulmi (Buisman) Nannf. and cross-inoculated 2 weeks later. The content of mansonones A,C,E,F, and G, phytoalexins in U. americana, was determined weekly for 1–6 weeks after the first inoculation. Inoculation with the nonaggressive strain led to greater levels of mansonones than did inoculation with the aggressive strain. The aggressive strain counteracted the effect of the nonaggressive strain in cross-inoculated seedlings. Mansonones F and E were the two quantitatively most important mansonones, representing 40.0 and 32.4%, respectively, whereas mansonones G,C, and A represented 15.2, 8.7, and 3.7%, respectively, of the total quantities of mansonones A,C,E,F, and G present in each sample.