Investigations on Alternaria solani Infections: Detection of Alternaric Acid and a Susceptibility‐Inducing Factor in the Spore‐germination Fluid of A. solani

The spore‐germination fluid of Alternaria solani was examined for factors which are important in the early stages of infection and probably explain the selectivity of A. solani to tomato and potato as host plants. The following activities of the spore‐germination fluid were found: 1. Host‐selective toxicity at low concentrations. 2. Infection promotion and enhancement of A. solani infections on host plants when spore‐germination fluid was added to the inoculum. 3. An induction of susceptibility which enabled a non‐pathogenic A. alternata isolate to cause necrotic spots on the leaves of the host plants of A. solani when its inoculum contained spore‐germination fluid of A. solani. Properties 1 and 2 seemed to, be attributed to alternaric acid. This phytotoxin was detectable in effective concentrations in the spore‐germination fluid. A factor, responsible for the susceptibility‐induction for A. alternata on the host plants of A. solani was characterized as a water‐soluble fraction of the spore‐germination fluid which was free of alternaric acid and not toxic.