Studies on the mechanism of action of β-methylaspartic acid in the suppression of Aphanomyces root rot of pea

β-Methylaspartic acid (MAA) effectively reduced root rot of pea, caused by Aphanomyces euteiches, at concentrations as low as 4 p.p.m., and prevented disease at 15–20 p.p.m. The degree of methylation of pectic materials in healthy pea plants was not increased by MAA, which thus could not account for induced resistance to root rot. Further studies revealed that MAA in the absence of glutamic acid affected zoospore germination and inhibited growth of A. euteiches in vitro. Inhibition of growth and of root rot development was almost completely reversed by glutamic and aspartic acids, and partly reversed by several other amino acids. Studies of the mechanism of inhibition of growth of A. euteiches by MAA indicated that MAA did not affect glutamine synthetase or aspartate oxoglutarate transaminase, two amino acid metabolizing enzymes produced by A. euteiches, nor did it affect the uptake of 14C-labeled glutamic acid by mycelium of the fungus and the incorporation of glutamate into alcohol-insoluble material. It was concluded that MAA directly affects growth of A. euteiches and in this way controls root rot of pea.