OCCURRENCE, DEVELOPMENT, AND LOSSES ASSOCIATED WITH SILVER SCURF AND BLACK DOT ON COLORADO POTATOES

Silver scurf, caused by Helminthosporium solani, and black dot, caused by Colletotrichum atramentarium, are pathogens of tuber periderm whose presence in Colorado was only recently reported. A field survey conducted in September 1977 revealed that C. atramentarium was more prevalent (21.8% tuber infection) and had a wider distribution than H. solani (5.4% tuber infection). A greater incidence of both pathogens was observed on thin skinned tubers of chipping cultivars (49.0% infection) than on thicker skinned tubers of table stock cultivars (9.1% infection). Conidial development of H. solani is tretic, pleurogenous, and requires 17-21 hours per conidium. Conidial septations appear while conidial elongate, and require 3-5 hours per septum. Light microscopy revealed that at least 11 conidia per conidiophore are produced in culture in 54 hours at 20-25 C (68-77 F) and humidity >90%. Scanning electron microscopy showed that fructifications of H. solani (conidiophores-conidia) arise from beneath infected tuber periderm. Histological studies indicate some peridermal loosening and sloughing. Heavy deposition of unidentified compounds was observed in infected periderm, and hyphae were restricted to periderm cells. Fresh weight loss of tubers naturally infected with C. atramentarium was significantly greater than fresh weight loss of nearly noninfected (< 1% surface area infected) control tubers. Periderm infected with either H. solani or C. atramentarium appeared similar, i.e. shriveled, suggesting infections from either pathogen may result in increased fresh weight loss through alteration of the periderm. © 1979 Springer.