Ditch the diseases

Use this guide to help combat the most common diseases of hydroponic leafy greens and herbs.

Hydroponic leafy greens and herbs are susceptible to several diseases that reduce plant quality and marketability. We’ve outlined the symptoms, cultural management and preventive fungicide controls for five of the most common diseases: Pythium root rot, powdery mildew, Botrytis blight, basil downy mildew and Sclerotinia blight. None of the available treatments are curative, so apply treatments preventively. As always, we recommend consulting with a diagnostic laboratory to positively identify diseases, which can save you much guesswork in terms of prevention and control.

Pythium root rot

Commonly affected crops

  • spinach
  • arugula
  • basil
  • lettuce

Signs and symptoms

  • Brown discolored roots, poor overall rooting
  • The outer part of the root sloughs away while the inner portion remains behind (aka “rat tails”)
  • Leaf wilting or yellowing (due to poor water and nutrient uptake by the roots)

Sanitation and cultural control measures

  • Inspect new plant material prior to transplanting
  • Use foot baths with active sanitizing agent for all workers entering a greenhouse * Sanitize tools
  • Test if water source harbors Pythium species and install a water treatment system. Pythium aphanidermatum and P. myriotylum proliferate under warmer temperatures; use a chiller to decrease water temperature (to 72° F, e.g.)

Preventive fungicides

  • Use preventive drenches with biofungicides at the seedling stage. Several are labeled for Pythium root rot management in greenhouses. For more information, go to bit.ly/greenhouse-root-diseases

Downy mildew

Commonly affected crops

  • basil
  • arugula (these crops are affected by different species of downy mildew)

Signs and symptoms

  • Yellowing on upper leaf surfaces in bands delimited by major veins
  • Irregular black spots may eventually appear on upper leave surfaces
  • Underside of leaf may show gray sporulation

Sanitation and cultural control measures

  • Start with pathogen-free seed
  • Select resistant varieties
  • Reduce leaf wetness (maintain low humidity, air circulation, and good plant spacing)
  • Keep plants lit during the night or extend the day length: the pathogen needs at least seven hours of darkness to sporulate
  • Remove infected plants

Preventive fungicides (not exhaustive list)

  • Conventional: potassium salts of phosphorous acid (Fosphite) (Group 33); cyazofamid (Ranman)(Group 21); and mandipropamid (Revus) (Group 40)
  • Organic: Bacillus amyloliquefaciens (Triathlon), hydrogen dioxide (Oxidate 2.0), neem oil (Triact 70), potassium bicarbonate (Milstop), and Steptomyces (Actinovate)

Botrytis (gray mold)

Commonly affected crops

  • lettuce
  • rosemary
  • many other herbs
  • Signs and symptoms

  • Fuzzy grey/brown growth (i.e. spores) when humid)
  • Soft brown rot on stem and older leaves
  • Plant may eventually wilt and die
  • Sanitation and cultural control measures

    • Remove dead plant material
    • Control humidity and avoid moisture on plant surfaces
    • Ensure good air movement with horizontal or vertical airflow fans

    Preventive fungicides (not an exhaustive list)

    • Bacillus amyloliquefaciens (e.g. Triathlon BA)
    • Bacillus subtilis (e.g. Cease, Companion)
    • Potassium bicarbonate (e.g. Milstop)
    • Streptomyces (e.g. Actinovate SP, Mycostop)

    Powdery mildew

    Commonly affected crops

    • lettuce
    • rosemary
    • sage
    • mint

    Signs and symptoms

    • Gray-white powdery growth on upper leaf surfaces and in some cases on stems or flowers

    Sanitation and cultural control measures

    • Remove infected plants (bag up and toss)
    • Favored by humid nights and dry days, use venting and heating to control humidity
    • Ensure good air movement with horizontal airflow fans
    • Select more resistant cultivars

    Preventive fungicides (not an exhaustive list)

    • Bacillus subtilis (e.g. Cease, Companion)
    • Bacillus amyloliquefaciens (e.g. Triathlon BA)
    • Potassium bicarbonate (ex: Milstop)
    • Streptomyces (ex: Actinovate SP)
    • Hydrogen dioxide (Oxidate 2.0)

    Sclerotinia blight (white mold)

    Commonly affected crops

    • lettuce
    • cilantro (less common)

    Signs and symptoms

    • Wilting of outer leaves
    • Light brown lesions (water soaked)
    • Watery soft rot
    • White cottony mycelium (under humid/moist conditions)

    Sanitation and cultural control measures

    • Inspect incoming plants/liners
    • Avoid growing outdoor/soil lettuce
    • Avoid cool/damp conditions
    • Maintain good airflow and avoid tight plant spacing
    • Remove infected plants

    Preventive fungicides (not an exhaustive list)

    • Bacillus amyloliquefaciens (Triathlon BA, Double Nickel)
    • Bacillus subtilis (e.g. Cease, Companion)
    • Streptomyces (Actinovate)

    Reprinted with permission from the authors. Find the entire e-gro alert here: bit.ly/common-hydroponic-diseases

    Author’s note: The mention of trademarks or brand names is for informational purposes only and does not imply its approval to the exclusion of other products that may be suitable. Always follow the pesticide label.

    December 2022
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