Pumpkins (Cucurbita spp.) and other plants cannot grow healthy and strong without having a strong, healthy root system to transpire nutrients and water throughout the plant. Root rot, a condition in which the roots literally rot in the soil, is caused by excessive moisture in the soil. Root rot occurs when fungi present in contaminated soil spreads to the roots.
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The most common pathogens that lead to root rot are Phytophthora spp., Fusarium equiseti, Fusarium solani, Acremonium spp., Thielaviopsis basicola, Pythium spp., and Rhizoctonia solani. While all these pathogens can lead to root rot, they might occur at different growth stages. Rhizoctonia solani and Thielaviopsis basicola, for example, mostly occur in pumpkin seedlings, especially when planted in cool soil. These fungal spores grow rapidly in wet soil. Root rot is particularly common in heavy soils such as clay and clay loam that hold water around the roots. Phytophthora spp. and Fusarium equiseti are among the most common root rot pathogens, but the others occur more frequently in old fields without crop rotation each year.
The above-ground symptoms might not appear until the rot has progressed beyond treatment, but the first sign for all pathogens is usually wilting. The leaves of plants affected by Pythium, Phytophthora and Rhizoctonia fungi fade to yellow or white and the leaf edges turn brown, but usually after the roots have already died. The crown of the plant from which stems emerge turns brown in the case of Fusarium and Acremonium fungi. Roots affected by Thielaviopsis basicola turn black. Squash vine borers can cause some of the same discoloration symptoms, so look closely for the presence of bugs or cracks in the stems to rule out bugs as the cause.
You might be able to save the pumpkins if you catch the problem early. If you suspect root rot, pull up the pumpkin plants immediately to check for brown, water-soaked, soft and mushy roots. You can clip away the affected roots if there are only a few damaged roots, but discard the pumpkin plant if the majority of roots are damaged. Rather than return a pumpkin plant to the same infected garden plot, plant your pumpkins in a sterile garden plot or in pots with pasteurized soil. Some gardeners have found success with taking a cutting from a pumpkin stem, rooting the cutting and starting an entirely new disease-free pumpkin plant. Fungicidal sprays can sometimes help improve early signs of Pythium, Phytophthora, Acremonium and Fusarium fungi.
Prevention is the best way to ensure you don't lose a pumpkin crop to root rot. Pumpkins and other cucurbits should not be planted until the soil temperature reaches 65 to 70 degrees Fahrenheit. Pumpkins need plenty of water to develop fruit, but allow the top inch or two of soil to dry out before watering the crop. Water drains downhill, so if you have heavy soil with poor drainage, work the soil into mounds about 12 inches high so the excess water drains through the mound and away from roots. Other cucurbit plants such as watermelons (Citrullus lanatus) are commonly grown in mounds for the drainage benefits. Mix in coarse sand to lighten heavy soils and improve drainage before you plant the pumpkins. Shallow planting also reduces the risk of root rot.