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.
Terry says
I just noticed what appears to be blight on some of my tomato plants. I’ll come in to the store tomorrow for some copper fungicide. I also noticed some blossom end rot in some tomatoes. Any suggestions? I have some lime with calcium but I already applied it when I planted? Thanks for posting.
ashley says
Hi Terry, Calcium and magnesium deficiencies can cause blossom end rot. Fortunately there are a lot of cal-mag products to feed your tomatoes with.
Madalyn says
Hey Terry,
Ashley’s right about calcium and magnesium, and a lot of times tomatoes need more than what is available in the lime applied at the beginning of the season. I’d recommend bone meal (also very high in phosphorus), crab shell, or a liquid cal-mag supplement. Luckily once you start adding calcium, the problem is resolved very quickly!
Jan Johnson says
I use Neem, Bt and Serenade for bug and disease control. Since Neem comtrols some baterial disease will it also kill the bateria in Serenade and Bt products if used together? The products are a little costly and I don’t want to waste time and money.
Jeff says
As I understand it, copper is a preventative, and potassium bicarbonate kills fungi but doesn’t prevent re-infection. Can they be mixed to both kill and prevent?
Prof. B. Bhattacharya says
I have an apple plant in my balcony. The climate in Kolkata is humid and hot. Leafs have converted to black. I applied Cu base fungicide three times with gap of five days. The condition has not improved. Please advice.
Prof. B. Bhattacharya says
Some corrections. Read all the leaf are gradually blackening. Blackening starting from leaf tips. Can I use ice in soil to cool down soil temperature.
ashley says
We believe the plant has what sounds like an over watering issue. Make sure there is plenty of drainage in the pot and the plant is in full sun. If it is a fungal issue we would suggest a sulphur based organic spray. Ice cubes will have no effect on cooling the tree down. If it is too hot there for apples, you might try another, more heat-tolerant tree.
Jo Ogbajie says
Hi everyone. I noticed yellowing of my cucumber plants last week. don’t know the kind of infections on the cucumbers. I need help on the best form of treatment. The cucumbers are fruiting now.
ashley says
Hi Jo, Yellowing could be caused by many different problems – pests, diseases, nutrient deficiencies, to name a few. Check your plants on the top and underside of leaves for pests and caterpillars, to either identify, or eliminate that possible cause. Downy mildew can cause yellowing – remove affected leaves and dispose of them. Cut back some foliage on the vines to let air circulate more freely. Good luck!
Melanie says
Hi…my daughter (14) received a cherry tomato plant for X-mas, and takes its care of living healthily, serious (and personal). The care instructions on the little label attached said, “Do not transplant. 6 hours of sunlight daily & water 1 in. wkly.” Every morning she puts it out in the sunlight, (highs avg. 70-75) in CA, and brings it in before the temps begin to cool. Unfortunately, (small) white fuzzy spots surfaced on the leaves, and the tomato’s remain green showing no signs of ripening, or growth. Which treatment and further care will have the best outcome? Thanks