Fruit rot caused by Pythium species can be confused for that caused by Phytophthora capsici, another oomycete pathogen, because they both produce white growth on affected fruit. With Pythium the growth is mostly mycelium and resembles cotton or shaving cream whereas with Phytophthora it resembles yeast and is mostly sporangia (spores). Both pathogens survive in soil and wet soil provides favorable conditions for their development. Species of Pythium that cause fruit rot are considered weak pathogens. They can also cause root rot. Pythium fruit rot has been observed rarely, and when seen it is often unmarketable squash and cucumber fruits discarded in the field, and it has not been causing enough loss to be of concern; in contrast, Phytophthora fruit rot is one of the most common diseases and can cause total crop loss.
Below: Cucumber fruit affected by Pythium fruit rot.
Below: Pumpkin fruit affected by both Pythium fruit rot (on left) and Phytophthora fruit rot (on right).
Below: Pythium fruit rot was seen affecting one fruit on a zucchini plant plus a few discarded fruit in a commercial crop that was at the end of the harvest period.
