Disease

Root rot (Pythium and Phytophthora)

Pythium spp., Phytophthora spp.

Water molds whose spores swim to roots in saturated media and warm, poorly aerated nutrient solutions. They attack roots already damaged by low oxygen or high salts.

Signs

• Brown, soft roots whose outer layer slides off and leaves a thin white strand.
• Plants wilt even though the media is wet.
• In hydroponics: brown, soft roots with few root hairs; wilting or tip burn often shows first.

What favors it

Saturated media, poor drainage, warm nutrient solution with little oxygen, and high salts.

Prevention

• Use well-draining media and pots; let the root zone dry back between waterings.
• In hydroponics, aerate the solution toward saturation (about 8 ppm dissolved oxygen) and keep it at 68-72 °F (20-22 °C).
• Start with clean media, clean pots and clean water.

Often confused with

Sources

• Root Rots of Indoor PlantsUniversity of Maryland Extension
• Managing phytophthora on farmsUniversity of Minnesota Extension
• Phytophthora of VegetablesUniversity of Maryland Extension
• Pythium and Phytophthora associated with root disease of hydroponic lettuceUniversity of Technology Sydney (research)
• Root Disease Problems & Management in Hydroponic SystemsColorado State University Extension (PlantTalk)
• Root disease management in hydroponic systemsUniversity of Kentucky (e-GRO Alert)
• Summer Lettuce Production and Bitterness (solution chilled to 72 °F)UF/IFAS

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