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Fighting Fungal Diseases

By Paul James

Anthracnose. Black spot. Rust. Downy and powdery mildew. Those are just a few of the fungal diseases that can ravage plants. But there are thousands more. Dealing with them can be a drag. Not dealing with them can be disastrous. Here’s what you need to know.

Fortunately, not all plants are prone to fungal invasion. But many popular plants – Coreopsis, crabapples, lilacs, Photinia, roses, and tomatoes, just to name a few — are highly susceptible, especially when spring weather is cool and wet, or humidity is high in summer (sound familiar?). However, there are things you can do to minimize the likelihood of attack regardless of weather conditions.

Maintain good soil fertility — Plants grown in healthy soil enriched with composted products (including those that contain Mycorrhizae, a good fungus) are less likely to be attacked by nasty fungal pathogens.

Choose plants with proven disease resistance — Plant breeders are constantly working to develop new varieties that are resistant to common fungal diseases. The information is typically found on the plant tag. Most new varieties of crabapples, for example, are far more resistant to mildew.

Water wisely — Overhead watering encourages fungal diseases, especially if leaf surfaces remain moist overnight. So try to water early in the morning, and water the base of plants rather than the leaves.

Improve air circulation — Avoid overcrowding plants so that air can flow around and through them. Stagnant air creates a breeding ground for fungi. Consider pruning the interior of plants to increase air flow within them.

Prune infected areas — At the first sign of disease, prune infected areas and dispose of the debris. Cut back to healthy tissue, sterilizing your pruners between each cut with a common household disinfectant. I use wipes containing bleach, or a solution of 10% bleach with water.

But even if you do all those things, your plants may still succumb to fungal diseases. Enter fungicides, which can be either organic or synthetic. Organic products such as Revitalize contain a bacterium that won’t harm honey bees or beneficial insects. Two other organic options are Horticultural oil and Neem oil. And in addition to being very effective fungicides, both also control various insects that prey on plants and are often responsible for the spread of fungal diseases. Spray early in the morning or late in the day when bees are less active.

Copper, sulfur, and zinc are also key ingredients in a number of fungicides, and they’ve been used for decades to control some of the worst pathogens. Keep in mind, however, that copper and zinc can be toxic to plants, so follow the label instructions to the letter. There are also systemic fungicides, which are absorbed by the plant and provide lasting treatment.

Apply fungicides weekly once foliage begins to emerge as a preventive, or begin spraying at the first sign of infection.

Happy gardening, ya’ll.

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2 responses to “Fighting Fungal Diseases”

  1. Greg Rosta says:

    Hi Paul,

    Not a fungus question, but a pest one…Last year my foil was aphids. This year, striped cucumber beetles! They struck while I was away for the weekend…one of your favorites (and now mine) is the Dead Bug Brew. Do you know if that stuff will work on striped cuke beetles? Not surprisingly, I’ve got mixed signals on the web. And, how much damage can a squash or cuke plant take from those things before its fatal?


    • Paul James says:

      Spinosad will work, but you’ve got to spray or dust when they’re young. As they mature, they’re more difficult to control. You might consider a product that contains pyrethrum. The damage question is hard to answer. Usually plants can lose up to 40% of their leaf mass and still survive, but that varies from plant to plant.

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