Root Rots (Damping-Off)

Root rots can affect all of the cucurbits at any growth stage. However, Rhizoctonia and Thielaviopsis are generally limited to infecting cucurbit seedlings. In plants infected with Pythium, Phytophthora, or Rhizoctonia, the hypocotyl collapses and turns tan to brown, roots die, and cotyledons and/or leaves wilt. Infection by Acremonium or Fusarium occurs where the seed coat attaches to the hypocotyl. The hypocotyl and roots turn red-brown and the portion above swells slightly. Plants may not die, but become stunted with cotyledons and leaves turning dark green. Thielaviopsis basicola turns roots and lower hypocotyl black; cotyledons and leaves become dark green. Infections can lead to stunting, vine collapse, and death.

Pythium, Phytophthora, and Rhizoctonia require free moisture and cool soil temperatures for disease development. Planting early before soil temperatures are adequately warm or planting into poorly drained beds will likely increase disease. Melons following alfalfa generally have greater risk of infection by Pythium spp.

Acremonium spp. occurs in old melon fields in Stanislaus, Merced, Fresno, and San Joaquin counties. Disease incidence is severe when melons of any cultivar are planted without rotation. Rotation with wheat reduces incidence and severity. Planting depth also influences disease incidence. Deeply planted melons are more severely affected.

Fusarium equiseti root rot occurs in the central and south San Joaquin Valley and is more severe when melons are planted following cotton.

Fusarium solani is rarely seen but occurs occasionally on squash, pumpkin, and melons. The pathogen is most prevalent in the Colusa, Sutter, and Yuba county areas, but also has been observed on honeydew melons in Stanislaus County.

Thielaviopsis basicola does not usually infect cucurbits. Because the pathogen also attacks cotton, it is more likely to occur in the southern San Joaquin Valley. Chilling stress is necessary for disease to occur.

Prevent root rot damage to seedlings by planting appropriately. Fungicides also provide control.

• Planting beds should be high and well drained.
• Waiting to plant until soil temperatures are 65° to 70°F will generally prevent seedling infections by Pythium, Phytophthora, or Rhizoctonia.
• For Acremonium seedling root rot, shallow planting (0.5 inch) and irrigating up is the preferred method of managing this disease.
• For Thielaviopsis root rot, plant when soil is warm.
• Use treated seed in no-till, conservation-till, and when planting through cover crops to reduce the potential for seedling diseases.
• If Pythium, Phytophthora, or Acremonium spp., or Fusarium equiseti are present, an application of fungicides,depending on the species present, provides good control.
• Plants infected with Fusarium may recover if the weather is warm and the field is irrigated enough to wet the soil surface; this allows the roots to regenerate and the plants to produce a normal, but delayed, crop.

If root rots are observed during the vegetative growth stage, note infections to make management decisions for the next crop.