Stem root rot in squash plants quickly destroys the plant by creating a condition that causes poor growth, a decrease in stored energy and limited water and nutrient flow. Root rot-causing pathogens are naturally occurring in most soils, becoming active when air and soil temperatures become warm and there is a dramatic increase in soil moisture.
Squash stem room rot is caused by two fungus types: Phytophthora and Fusarium. These fungus pathogens are soil borne and multiply when soil is oversaturated with water for extended periods and temperatures are warm. These fungus types over-winter in the soil, making them always present to cause an infection when the conditions are right.
Squash plants infected with stem root rot will show initial symptoms of wilted leaves. The leaves wilt during the heat of the day and may recover during the cool evening hours. Infected plants will die several days after the onset of wilting leaves. The base of the plant just below the soil line will be rotted and easily break off. Fusarium-infected plants develop light-colored areas of spore development on the stem near the soil level. Plants infected with Phytophthora fungus develop red-brown streaks on the inside of the stem.
Phytophthora fungus infections occur in a squash plant when the soil becomes saturated with water, causing the fungus to multiply and become mobile. The spores attach and enter the plant through the roots. Phytophthora fungus attaches to the inner walls of the stem and quickly breaks it down to limit water and nutrient flow. Fusarium fungus enters the plant through the roots and moves through the water-conducting vessels of the stem. The fungus growth plugs the vessels and prevents water and nutrients from moving through the plant.
There is no treatment to reverse plant damage caused by Phytophthora and Fusarium pathogens. Fungicide treatments will not effectively eliminate the presence of either Phytophthora or Fusarium fungus in soil. Infected plants will not recover and should be removed and discarded.
Stem root rot is prevented by planting resistant varieties of squash, since the fungus stays present in the soil for several years. Amend poorly draining soils by working organic compost to a depth of 10 inches. In areas with clay soil, plant squash in large raised beds filled with well-draining topsoil mixed with organic compost. Treat the planting area with a fungicide prior to planting to limit the risk of infection in new plants.