THE GROWTH IN CULTURE AND MORPHOLOGY OF AN UNIDENTIFIED PHYTOPHTHORA SPECIES INCITING CROWN AND FOOT ROT OF RHUBARB IN SASKATCHEWAN

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Abstract
Crown and foot rot of rhubarb, Rheum rhaponticum L., caused by a species of Phytophthora, is reported in Canada for the first time.The fungus grew well on various media, but sporangia were produced profusely only on hemp seed extract agar and occasionally on cornmeal and malt extract agar. The length, the width, and the length/width ratio of 100 sporangia on three substrates are given. Intercalary and terminal hyphal swellings were common in water, hemp seed extract, malt extract, a complete medium, and sometimes oatmeal agar cultures. Knob-like hyphal swellings were formed on cornmeal. Sex organs were not observed in infected host tissues nor were they produced on any of the media even when the rhubarb isolate was paired with four species of Phytophthora. The fungus did not grow on Leonian's media to which had been added various concentrations of malachite green. Cardinal temperatures for mycelial growth were minimum, slightly above 0 °C; optimum, between 22 and 25 °C; and maximum, between 30 and 32 °C. At 32.5 °C no growth took place, but the mycelium was killed only after 144 hours.Pathogenicity to 2-month-old rhubarb seedlings, using two methods of inoculation, was proved. The fungus was pathogenic to mature rhubarb plants, wounded apple fruits, variety Delicious, and carrots. It was not pathogenic to wounded potato tubers, tomatoes, oranges, and McIntosh apples.The identification of the rhubarb isolate is discussed, and in spite of some similarities to described species the fungus is left unnamed until further work is done and the formation of oospores can be induced.

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