Involvement of carbohydrates and cytokinins in pathogenicity of Helminthosporium carbonum.
Appressoria formation, penetration and colonization by H. [Cochliobolus] carbonum on decolorized maize leaves were stimulated by addition of exogenous carbohydrates and leaf leachates. Germination and germ tube length, however, did not exhibit appreciable differences on decolorized or non-decolorized maize leaves. Lower germination was recorded by leached conidia on decolorized leaves, while appressoria, penetration and colonization were absent. Addition of exogenous nutrients (sucrose > leaf leachates > yeast extract > glucose) enabled conidia to produce appressoria, penetration and colonization. Opt. levels for various nutrients observed were 2% (w/v) sucrose/glucose or 0.1% (w/v) yeast extract. Higher concn inhibited the infection stages of the pathogen. Depletion of host carbohydrates from green islands/infection sites adversely affected appressoria formation, penetration and colonization, and the loss of carbohydrates from the spore affected germination. Cytokinin-like activity at the infection sites/green islands increased with the period of incubation of the host compared with activity in the surrounding tissue or in tissue under water drops. Cytokinin-like activity was also detected in filtrate extracts of C. carbonum and increased with growth of the fungus. TLC of cytokinin-like substances (tissue extract and culture filtrate) revealed that the major activity was confined to Rf zones 0.6 to 0.8 which co-chromatographed with zeatin and zeatin riboside.