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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. 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 compost are less likely to be attacked by fungal pathogens.

Choose plants with proven disease resistancePlant 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 wiselyOverhead 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 circulationAvoid 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 areasAt 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, but they’re in short supply at the moment. So consider mixing a solution of 10% bleach with water and dipping your pruners in it between cuts.

But even if you do all those things, your plants may still succumb to fungal diseases. Enter fungicides.

Two of my favorite broad-spectrum fungicides also happen to be organic products. One is called Serenade, and the other Revitalize. The active ingredient in both is a bacterium. And neither will harm honey bees or beneficial insects.

Two other all-natural 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 and sulfur are also great fungicides, and they’ve been used for decades to control some of the worst pathogens. Keep in mind, however, that copper can be toxic to plants, so follow the label instructions to the letter.

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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6 responses to “Fighting Fungal Diseases”

  1. William McClure says:

    Paul, I want to go slightly off topic. What are your recommendations for blueberry plants? I live in Central Ohio, zone 6a I believe. Also, do you need 2 different types for pollination? Thanks. Also, will you ever release a DVD set of Gardening by the Yard? Thanks!!!

  2. donwood1 says:

    I have a problem with four large oak trees and a red maple. They are all surface rooted and some large ones (wrist sized) now have smaller ones (1″ to 2″) crossing over them which interferes with mowing. Will it harm the trees if I cut the smaller ones off?

    • Paul James says:

      Cutting roots larger than 1″ in diameter can be risky, so if you decide to do that, don’t get carried away. You might consider applying a light layer of bark mulch to cover the roots so you don’t have to look at them.

  3. how do you get rid of the wild violets? they have a spade shape leaf. I dont have flowers just the leaves and nothing I have found kills them.. help

    • dont know what the moderation is but I m still waiting for some help

    • Paul James says:

      Wild violets are extremely difficult to control without resorting to post-emergent herbicides, especially those containing 2,4-D. If you routinely remove the leaves with a hoe you’ll eventually starve the plants, but that can take weeks, if not months.