In 1999, during an evaluation of pumpkin (Cucurbita pepo) fruit for susceptibility to naturally occurring postharvest diseases, a soft rot of unknown etiology was noted. No fungal growth or sporulation was seen on the fruit surface and no root or crown rot was observed in the field. When fruit were cross-sectioned, masses of white, floccose mycelium covering large sections of the seed cavity were observed. Rot was observed in 21 fruit (6.4% of the total). The fungus was isolated from symptomatic fruit on a modified P10ARPH agar medium, semi-selective for Phytophthora spp. (2). Isolates from eight fruit formed papillate, ovoid sporangia, abundant chlamydospores, and colonies characteristic of P. nicotianae (1). No oospores were produced. Four sound pumpkin fruit (cv. Early Autumn) were inoculated with four isolates (one isolate per fruit). Each isolate was recovered from a different fruit. Pumpkins were surface sterilized at the point of inoculation by wetting with 70% ethanol. Inoculation was done by removing a small amount of mycelium from pure culture using a sterile, wooden toothpick and inserting it 2 cm deep into opposite sides of the mid section of sound fruit (two inoculations per fruit). Control fruit were punctured with sterile toothpicks (once per fruit). First symptoms appeared 4 days after inoculation at room temperature (22 to 24°C). Symptoms consisted of circular, water-soaked areas originating from the point of inoculation. Average diameter (based on four measurements on two fruit) of the water-soaked lesions were 3 cm at first appearance (i.e., 4 days) and 11 cm 10 days after inoculation. No symptoms developed on controls. When symptomatic fruit were cross-sectioned, masses of white, floccose mycelium were noted. Reisolation of this mycelium onto selective medium yielded P. nicotianae, thus fulfilling Koch's postulates. This is the first report of P. nicotianae causing fruit rot of pumpkins.
References: (1) D. C. Erwin and O. K. Ribeiro. 1996. Phytophthora Diseases Worldwide. The American Phytopathological Society, St. Paul, MN. (2) H. D. Shew. Phytopathology 77:1090, 1987.
