PHYTOTOXIN PRODUCTION BY ALTERNARIA SPECIES.

Abstract

Alternaria tagetica is capable of producing phytotoxins in vitro. One toxin was identified as zinniol with: bioassays; thin-layer and gas chromatography; staining properties; ultraviolet and mass spectrometry, and nuclear magnetic resonance spectroscopy. The identity of the other toxin(s) has not been established. Symptoms similar to those caused by infection developed on detached marigold leaves treated with either of the toxins or synthetic zinniol. The toxins are not host selective. The distribution of zinniol among Alternaria species was studied. Thirty-one isolates of 10 pathogenic Alternaria spp. were tested for their ability to produce zinniol. Analyses were performed by gas-liquid and thin-layer chromatography. Of the seven pathogenic large-spored, long-beaked species tested A. carthami, A. macrospora, A. porri, A. solani, A. tagetica, and an unnamed isolate from Phaseolus vulgaris pods produced zinniol. A. brassicae, a non-pathogenic isolate of A. zinniae, and three pathogenic species lacking large-spores and long-beaks (A. alternata, A. citri, and A. raphani) did not produce zinniol. The quantity of zinniol produced varied greatly among species, among isolates of a single species, and between trials of the same isolate. All hosts of the Alternaria spp. tested were sensitive to zinniol at 50 to 200 micrograms/ml. Conservation of zinniol in pathogenic large-spored Alternaria spp. may be indicative of its importance in pathogenesis. Light affects the behavior of Alternaria tagetica in vitro and in vivo. In vitro zinniol production occurred only during active fungus growth in the light; in the dark zinniol production occurred primarily after growth stopped. In all filtrates, the quantity of zinniol rapidly declined once zinniol production ceased. Fungus growth was inhibited by both continuous and alternating light and sporulation occurred on one of three test media and only under alternating light. More lesions were produced on inoculated plants kept in dark humidity chambers than in illuminated humidity chambers. Low illuminance was more conducive to lesion development than high illuminance and more lesions developed on plants exposed to low illuminance for 48 hr prior to inoculation than on those exposed to high illuminance. The limitations of studies in which the effect of light has been overlooked are discussed.