Accumulation of pyrenocines in cultures of the onion pink root pathogen Pyrenochaeta terrestris and some observations on their phytotoxicities

Abstract

Pyrenochaeta terrestris (Hansen) Gorenz, Walker, and Larson, causal agent of pink root of onion (Allium cepa L) produces three pyrones (pyrenocines A, B and C) showing varying degrees of phytotoxicity. These metabolites accumulate only in the growth medium. No pyrenocines are produced during the first four days of growth. Subsequently, pyrenocine levels sharply increase and reach maxima after 10–12 days and then steadily decline and are barely detectable beyond 22 days of growth. Initially, pyrenocine A predominates and ultimately pyrenocine B predominates. The proportion of pyrenocine C remains essentially constant from day 6 to 18 and subsequently increases to 30% of the total pyrenocines. Pyrenocines A and B spontaneously interconvert in control growth medium and reach equilibria consisting of 40–45% A and 55–60% B. The accumulation of the pyrenocines varies considerably from one isolate to another, Pyrenocine A is the most inhibitory to onion and radish seedling elongation with an effective concentration for 50% inhibition of 5 and 40 μg ml−1 respectively, Pyrenocine A is toxic to onion protoplasts with a concentration of 0·4 mg ml−1, causing 50% reduction in protoplast viability. Similar concentrations of pyrenocines B and C are without effect.