MARKET DISEASES OF FRUITS AND VEGETABLES 29 



decayed tissues when broken, will, under slight pressure, yield a yellow- 

 ish-brown liquid and a rather pleasant fermentation or yeasty odor. 

 With age the decaying tissues become cinnamon to light chocolate 

 brown in color but are never black. As water is lost by evaporation 

 the decayed areas become withered and firm, and under very dry con- 

 ditions the sweetpotato may become a hard, brown mummy. 



Although infected sweetpotatoes usually decay completely, under 

 some circumstances only a part will rot and become dry. Side infec- 

 tions through wounds sometimes progress around the sweetpotato, 

 causing the so-called ring rot stage of rhizopus soft rot. Frequently 

 these rings of broken-down tissue shrivel and sink, forming a dry, 

 groovelike ring around the sweetpotato. Dry rot in rings or at the 

 ends of sweetpotatoes may again start active decay under favorable 

 moisture and temperature conditions. 



The coarse, white, stringy mold growth, or "whiskers," with glisten- 

 ing white and black spore balls (sporangia), usually serves to identify 

 rhizopus soft rot. This growth of the fungus is conspicuous at the 

 ends and through breaks in the skin of sweetpotatoes held in a humid 

 atmosphere (pi. 16, A). However, under dry conditions the fungus 

 may cause extensive decay without any surface mold (mycelium) 

 being visible (pi. 16, i?). 



CAUSAL FACTORS 



There are many species of Rhizopus that may cause decay of sweet- 

 potatoes, but those that usually cause most serious damage are R. 

 nigricans and R. tritiei. These species are ever present in the soil, 

 water, and air. Consequently, it is safe to assume that all sweet- 

 potatoes are contaminated with one or more of these fungi during 

 harvesting, storage, and marketing. Whether or not infection and 

 decay follow depends upon the moisture and temperature conditions 

 and the presence of fresh wounds on the sweetpotatoes. Surface 

 moisture or fresh moist wounds offer the most favorable conditions 

 for infection by contacting mycelium or germinating spores of 

 Rhizopus. For this reason the best protection against rhizopus soft 

 rot are careful handling to avoid wounds and drying and curing the 

 sweetpotatoes as promptly as possible after harvesting. 



The various species of Rhizopus that cause rot of sweetpotatoes have 

 different reactions to temperature, but they can be divided roughly 

 into two groups. On this basis R. nigricans represents the relatively 

 low-temperature group and probably causes most decay. It develops 

 slowly between 38° and 53° F., rapidly between 65° and 75°, and is 

 retarded at temperatures between 86° to 94°. R. tritiei is the most 

 common species of the high-temperature group. Its minimum tem- 

 perature for growth is 40° to 48°, optimum 90° to 95°, and maximum 

 near 107°. At temperatures between 65° and 85° either or both of 

 these species of Rhizopus may be found causing decay of sweet- 

 potatoes. The rots produced are so similar that it is impossible to 

 distinguish them unless cultural and microscopic studies are made. 



CONTROL MEASURES 



Since all species of Rhizopus commonly invade sweetpotatoes 

 through wounds, it is of greatest importance that harvesting and 



