Organic Treatments for Fungal Infections

The hot and humid days of July and August are prime time for fungal infections to start popping up on your trees, vegetables, and landscaping plants. Some of the most common infections are powdery mildew, rust, leaf spot, and blight. Preventative treatments, and acting quickly to treat fungal infections when you first notice them are crucial to keeping your plants healthy and prevent annually recurring infections in trees and perennials. Luckily, myriad organic products exist to prevent and treat fungal infections. We know that the options can sometimes be confusing or seem overwhelming, so here’s a helpful primer:

Sulfur Fungicide

Products include: Bonide Sulfur Plant Fungicide, Safer Garden Fungicide

Sulfur comes in either a powder or a concentrated liquid. It works better as a preventative than a treatment or cure. This is because the sulfur prevents fungal spores from germinating. It can be effective in slowing down an infection by keeping the fungus from multiplying rapidly, but it won’t have much of an impact on existing infections. It is most effective when applied at the beginning of the season, and on a regular basis before you notice signs of a fungal disease. Sulfur should never be applied within 4-6 weeks of using an oil spray. It will have a phytotoxic reaction with the oil, and result in plant death. Some plants are especially sensitive to sulfur products, so definitely avoid using it on gooseberries, apricots, raspberries, currants, and anything in the cucurbit family (cucumbers, squash, melons). It’s also important to consider that with continued application, sulfur can lower the pH of the soil and start to affect plant health.

Copper Fungicide

Products include: Bonide Copper Fungicide

Copper is a good choice of fungicide because it essentially kills the fungus. It is very successful for treating and preventing tomato blight and can be applied once a week. It’s important to follow the dilution instructions very carefully, since too strong a mixture can burn leaves. Young plants, and new growth are often very sensitive to copper, so applying an extra diluted mix is a good idea. It’s also important to apply copper during cooler times of the day when the sun is not too bright, so evening applications are preferred. Azaleas and Rhododendron are sensitive to copper. With long term use, copper can accumulate in the soil and eventually make it to streams as runoff. Large accumulations of copper can reduce the diversity of naturally occurring plant life. It’s unlikely that home gardeners will contribute to this problem, but it is a factor to consider.

Oil Based Fungicide

Products include: Organocide, Neem, PureSpray Green, Monterey Horticultural Oil

Neem oil is going to be the most effective oil for controlling fungal infections. It is a good choice for mild to moderate powdery mildew infections, but doesn’t do much good for blight, leaf spot, or rust. Oils are mostly used to control insects, but since many fungal infections are spread by insects, keeping pests at bay is a good practice for maintaining plant health.

Bacterial Fungicide

Products include: Serenade, Actinovate

Bacterial fungicides, also called biofungicides are an exciting new development! They contain bacteria and other microbes as the active ingredient and work in a variety of ways. Some bacteria actually prey on and consume the fungus infecting the plant, some trigger a defensive response in the plant that helps it prevent the fungus from further invasion, and some bacteria produce a toxin or antibiotic that acts against the fungus. Biofungicides work best as a preventative spray, but an application after disease is noticed can be very effective at preventing the spread of the pathogen. Unfortunately, spraying a biofungicide after you notice an infection will not likely cure the plant of the disease, except in the case of mild to moderate powdery mildew infections. Since these sprays contain living organisms, it’s important to store them at a constant temperature. Room temperature is usually fine, but leaving them outside or in a shed or garage where temperatures fluctuate daily and seasonally, can reduce the number of live microbes and decrease the efficacy.

Potassium Bicarbonate

Products include: GreenCure

Potassium bicarbonate is safer for the environment, and less harsh on plants than some of the other fungicides like copper and sulfur, so it is a good choice for sensitive plants. It works by killing the mature fungus as well as the spores, so future infections are less likely. With prolonged use, potassium bicarbonate can raise the pH of soil, so testing the pH regularly is recommended.