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Dodging an un-fun guy

Whether it’s Fusarium and Pythium lurking in your soil, or powdery mildew and late blight blowing in on the wind, pathogenic fungi can cause poor germination, stunted growth, reduced yields, inferior eating quality, cosmetic blemishes, poor storability, and melancholy.

Organic growers have a number of strategies for preventing or treating outbreaks of fungal disease. For economy, efficacy and environmental safety, it helps to understand your options.

Cultural Prevention

First try to manage fungal problems without reaching for pesticides:

  • Choose improved crop varieties that are resistant to disease.
  • Choose high-quality brands of potting soil and compost.
  • Do not put diseased plant material in your compost pile.
  • Give plants adequate spacing for good airflow through the vegetation. This includes proper weed control. For trees and a few horticultural crops (like tomatoes), pruning helps to maintain good airflow.
  • Choose drip irrigation instead of overhead irrigation to reduce moisture on the leaves. Or in smaller gardens, hand-water thoughtfully to prevent over-watering.
  • Clean your tools and greenhouse surfaces. High-quality brands of potting soil very rarely carry pathogens, but even clean potting soil is easily contaminated by dirty tools, trays or workbenches.

Preventive Treatments

If you’ve been hammered by Botrytis a couple years running, or if your extension office is reporting late blight in your area, it may be time for a sharper weapon. Many organic fungicides work best as preventive treatments—timely applications will prevent a problem, but by the time you see the first slimy leaves it’s too late. These products are more environmentally friendly than control treatments:

  • Biological controls (such as 8634 Actinovate, 8628 RootShield, 8637 Serenade and 8638 Companion) have two primary modes of action. They may colonize the leaf surface and outcompete pathogens; or they may produce exudates that are directly harmful to the pathogens.
  • Potassium silicate (8601 Sil-Matrix) strengthens epidermal cell walls, improving the plant’s resistance to fungal infection.
  • Giant knotweed extract (8625 Regalia) stimulates the plant’s immune system.
  • Elemental sulfur (8616 Micronized Sulfur, 8619 Safer Garden Fungicide or 8622 THAT Liquid Sulfur) prevents spore germination. (Some plants, including cucurbits, are sensitive to sulfur.)

Control Treatments—the last resort

The pumpkin leaves are crumpling and the dreaded “water-soaked lesions” of late blight have appeared on the potatoes—quick, do something! (Or throw in the towel and take a vacation.)

  • Potassium bicarbonate (8595 MilStop) kills pathogens and spores by a combination of osmotic pressure, pH and specific carbonate and bicarbonate ion effects. Most effective as a preventative, but may have curative control of powdery mildew.
  • Copper products, such as 8604 Bonide, are the only pesticides permitted in organic production that can cure an existing outbreak of most fungal diseases. Copper products should be used only as a last resort: they are mildly toxic to bees and should not be applied to flowering plants, and repeated use of these products can cause an unhealthy buildup of copper in your soil, which is difficult to reverse.