Effect of substances associated with leaf surfaces on appressorium formation by Colletotrichum acutatum

Abstract

In six species of Colletotrichum, appressoria developed more readily on leaves than on glass. Colletotrichum acutatum Simmonds germinated equally well on glass and leaves but produced very few appressoria on glass. A chloroform extract from the surface of sugar-beet leaves at all concentrations increased numbers of proto-appressoria on glass. At low concentrations the water-soluble component of the chloroform extract promoted formation of appressoria whilst at higher concentrations it inhibited germination. Formation of mature appressoria on chloroform extract was enhanced in the presence of a phylloplane bacterium, Pseudomonas sp. isolate 14. A steroid, which was the major single component of the chloroform extract, had no effect on formation of appressoria. A chloroform extract from lower leaf surfaces was more effective than that from upper surfaces in stimulating formation of appressoria when used in amounts corresponding to unit area of leaf. A similar extract from chrysanthemum leaves prevented germination of conidia but when diluted 5 times stimulated formation of appressoria. A chloroform extract from the surface of tomato leaves was divided into 14 fractions by column chromatography. The first fractions (containing mainly alkanes) had little effect on formation of appressoria; the middle fractions, which inhibited fungal growth at higher concentrations, enhanced formation of appressoria at lower concentrations. The last three fractions from the column greatly stimulated growth of germ-tubes but prevented development of appressoria. A pure sample of a 28 carbon alkane had no effect on formation of appressoria.