378 PHYSIOLOGY OF THE FUNGI 



to rot it. The inhibition in this case was traced to the mahc acid in 

 the apple. The pectinase produced by Pythium was most active in an 

 alkahne medium, near pH 8.0, while that of B. cinerea was more active 

 in an acid medium, at pH 5.0 to 5.5. 



In contrast with the destructive fungi which rot the host tissue are 

 those which cause wilting and certain types of necrosis without disintegra- 

 tion of the host cells. These fungi produce little or no pectinase. Some 

 common fungi which cause wilting of mature plants are species of 

 Fusarium, Verticillium, Cephalosporium, and Ceratostomella. It is now 

 generally believed that in most cases wilting caused by fungi is due to 

 toxins or to the plugging of the vessels by polysaccharides or other similar 

 metabolic products of the fungus, rather than to plugging by the excessive 

 mycelial growth in the vessels. Extracts of the mycelium or the culture 

 filtrate of a number of these fungi cause effects that are the same as or 

 similar to those caused by the fungi themselves in their respective hosts. 



A definite correlation between the pathogenecity of two strains of 

 Fusarium lycopersici and the toxicity of their metabolic products was 

 demonstrated by Haymaker (1928). There was similarity of symptoms 

 and of the effect of temperature on wilting. The culture filtrate was more 

 toxic when the fungus was grown at 28°C. than that obtained at any other 

 temperature. The toxic substance was not identified. Other workers 

 (Plattner and Clausson-Kaas, 1945; Woolley, 1946) have reported that 

 the wilt-inducing compound produced by F. lycopersici is lycomarasmin, 

 a peptide of aspartic acid. Gaumann and Jaag (1947) reported that 

 clavacin exerted a wilting effect on detached tomato shoots similar to 

 that of lycomarasmin. But, whereas lycomarasmin acted chiefly on the 

 cells of the leaf blade, clavacin is toxic mainly to the phloem and paren- 

 chyma of wood and cortex of the stem and petiole. The action of both 

 compounds is believed to be similar, destroying the semipermeability of 

 the plasma membranes, thereby decreasing the water-holding capacity of 

 the cells and inducing wilting. 



Various polysaccharides have been shown to produce wilting in tomato 

 cuttings (Hodgson et al., 1949). Since there was a direct relationship 

 between molecular weight and wilt-inducing action of these compounds, 

 it was concluded that their action was mainly by mechanically interfering 

 with the transportation of water. Dimond (1947) also reported wilting 

 of elm leaves due in part to a polysaccharide produced by Ceratostomella 

 ulmi in culture. Its action is believed to be similar in naturally infected 

 elm trees. 



More recently, Feldman et al. (1950) have presented evidence to show 

 that the primary wilt-inducing agent produced by C. ulmi is not the 

 polysaccharide, but a toxin. The production of toxin in liquid culture 

 filtrate was greatly influenced by the pH of the medium, being greater in 



