The common root rots of cucurbits are caused by three species of fungus. Pythium root rot develops from either Pythium aphanidermatum, Pythium irregulars and/or Pythium ultimum. The effects of these fungi on different cucurbit crops varies by season, soil temperature and the availability of moisture. Pumpkin and squash root rot usually results from Fusarium fungus in the soil.
Pythium root rot can occur at any growth stage of a curcubit according to the Library4Science.com. Initial symptoms include stunted growth and the production of small, yellowish leaves. Older plants may also develop wilting, failure of fruit to set and mature, and sometimes collapse and death. The roots of affected plants may appear water-soaked and flaccid, with some sunken, darkened lesions on the larger, fleshy roots. Occasionally, the crown of the plant becomes girdled and the above-ground stem breaks off and dies.
Cucumber, squash and watermelon are primarily harmed by Pythium irregulars and Pythium ultimum during the spring and fall months. These annual vines prosper when soil and air temperatures are warm, while their growth slows in spring and autumn's cooler temps and moist soils. This is when Pythium prospers and inflicts most damage. Plant these crops in well-draining soils, and never in areas that were previously cropped with other cucurbits, which tends to create a soil with higher spores of the fungi lingering. Also, don't plan them where peas or spinach grew the previous year.
Pythium aphanidermatum grows when soil and air temperatures are warmest in summer--at the same time muskmelon and cantaloupe vines grow madly. Root rot occurs on the out-competed cucurbit vines. Interestingly, alfalfa and beets grown the previous year in the same area of the garden increase the chances of root rot on muskmelons and cantaloupes, according to Library4Science.com.