Morphogenesis of the powdery mildew fungus in water (4) The significance of conidium adhesion to the substratum for normal appressorium development in water

The surface of the barley coleoptile is relatively hydrophobic. When the co leoptile surface was treated with water for 20 min, the contact angle of wa ter droplets decreased and the surface became relatively hydrophilic. When conidia of Erysiphe graminis were inoculated on the hydrophobic surface of barley coleoptiles and immersed immediately in water, approximately 70 % of the appressoria were normal after 24 h. In contrast, when conidia of E. gr aminis were inoculated on the hydrophilic surface of pre-wetted coleoptiles , only about 30 % of the appressoria were normal. About 76 % of the conidia that were strongly adherent developed normal appressoria. In contrast, onl y 20 % of the conidia with weaker adherence developed normal appressoria. A pplication of a polycation(poly-L-ornithine : PLO) solution to the coleopti le surface prior to inoculation served to attach conidia to the surface of coleoptiles immersed in water. After PLO treatment, 70 % of the conidia wer e strongly adherent. Furthermore, in water, 72 % of the conidia developed n ormal appressoria on the surface of coleoptiles pretreated with PLO. These results suggest that normal appressorium development of E. graminis depends greatly on the intensity of adhesion of conidia to the substratum during t he process of morphogenesis in water. (C) 1999 Academic Press.