The effect of chemical haulm destruction and haulm pulling on potato black scurf caused by Rhizoctonia solani AG-3

Factors influencing black scurf formation in untreated crops and after haulm destruction were investigated. As potato tubers mature they may gradually become covered with black scurf, the sclerotia of the fungus Rhizoctonia solani AG-3. After haulm destruction, black scurf formation is stimulated by changes at the tuber surface due to accelerated tuber maturation. These changes probably start within 3 to 6 days.The final amount of black scurf at harvesting depended on the method of haulm destruction and not on the cultivars tested or on the degree of tuber infestation. Mechanical haulm destruction by 'haulm pulling' (HP) stimulates black scurf formation less than chemical haulm destruction (CHD) or cutting of the shoots (COS). This is probably because it breaks the stolons, disturbs the soil and removes most plant residues from the vicinity of the tuber. After CHD or COS, the sap flow from roots to tubers continued for a week, and more liquid and precipitable substances exuded from the tubers after HP. This increased exudation may promote black scurf formation: also on agar media sclerotial growth was increased within 4 days after a nutrient supply and sclerotial growth continued over a longer period of time when the medium was liquid. In field and pot trials, severing the roots in addition to CHD or COS moderated black scurf formation and extended the period of time in which tubers could be harvested with a firm skin and limited black scurf.The initiation of sclerotium formation on tubers is probably promoted by non-water-soluble components of the tuber periderm. Sclerotial maturation was promoted by precipitated water-soluble tuber exudates. Volatile and unstable components which become available on the surface of intact tubers promote sclerotium formation. The final mass of sclerotia, however, is probably influenced most by stimulatory and inhibitory volatile tuber exudates. When the inhibitors were trapped by KOH, sclerotium formation on exposed agar plates was stimulated just as much by tuber volatiles from green intact plants as by tubers after haulm destruction. Inhibition dominated during tuber growth, decreased only gradually during tuber maturation and disappeared rapidly after haulm killing. Carbon dioxide may be an inhibitor because tuber respiration was halved within 3 days after haulm killing. Volatile products from decomposing roots and stolons had no effect of their own, but they increased the slight stimulation of sclerotium formation by naturally maturing tubers. The effect of a gaseous product on sclerotium production may depend on other volatiles in the vicinity as well as on the food base available. Perspectives for black scurf control were discussed