EFFECT OF VOLATILE AND UNSTABLE EXUDATES FROM UNDERGROUND POTATO PLANT-PARTS ON SCLEROTIUM FORMATION BY RHIZOCTONIA-SOLANI AG-3 BEFORE AND AFTER HAULM DESTRUCTION

The acceleration of black scurf development after haulm destruction was mainly due to changes in the exudation of volatiles from tubers. Volatile products from decomposing potato roots and stolons and, probably, unstable substances in the tuber exudate as well, further promoted sclerotium formation. Sclerotium production by R. solani AG-3 was investigated on agar media, periderm strips and harvested tubers, which were exposed to the volatile exudates from growing subterranean potato plant parts. The volatile exudate from growing tubers contained both inhibitory and stimulatory substances which were not identified definitely. Inhibition dominated during tuber growth, decreased when plants were yellowing and disappeared after the shoots were excised. When the inhibitory components were trapped by KOH, the non-trapped volatile tuber exudates from young growing plants were as stimulatory as those from plants after haulm killing. CO2 might be an inhibitor as tuber respiration was negatively correlated to black scurf formation. Tests in vitro suggested that inhibition of sclerotium formation by CO2 can be overcome by stimulatory nutrients. Sclerotium production on agar media was not stimulated by ethylene, although volatiles from harvested ripe apples were very stimulatory. The results imply that after haulm killing, the increase in black scurf development may be prevented by loosening the soil and quick separation of tubers from plant residues thus preventing accumulation of the stimuli. © 1990 Koninklijke Nederlandse Planteziektenkundige Vereniging.