Plant pathotoxins from Alternaria citri: The minor ACRL toxins

Five host-specific pathotoxins, ACRL toxins II, III, III′, IV and IV′, were isolated from the culture broth of Alternaria citri, the fungus causing brown spot disease of rough lemon. These toxins are related structurally to the major ACRL toxin, toxin I, and to its derivative compound A. Chemical and spectral studies indicated that the ACRL minor toxins were a group of analogous compounds of different chain lengths all of which have a α-pyrone group, in contrast to the dihydro-α-pyrone group in toxin I. Toxin II showed a very low biological activity (ED₅₀ greater than 10 μg/ml) whereas the other minor toxins had slightly higher activities ranging from 1 to 10 μg/ml. The dihydropyrone group in ACRL toxin I was correlated with high biological activity (ED₅₀ = 18–30 ng/ml).