Cloudy, hot, humid summer days are hard on everyone, but flowering plants, trees and fruit & vegetable plants can be devastated during this weather by the appearance of some common garden diseases. One day your garden is blooming and lovely, the next your tomatoes are turning brown and your hosta is turning yellow! Fungal diseases in particular are a huge problem for mid-summer gardens where hot and humid weather is common. Fungal diseases love the constant moist air, little rain or sun, and cool nights of summer. Annoyingly, once one plant becomes infected, you’re likely to see the problem spread if you don’t take action.
If you know your garden is prone to certain garden diseases, like powdery mildew or black spot, or if you suspect a fungal disease is attacking your favorite plants, there are a number of products available, both conventional and organic that you can use for prevention. I like copper fungicides like Espoma Earth-Tone or Bonide Liquid Copper Fungicide. Some organic gardeners also report good luck with Neem Oil (which is both an insecticide and fungicide), or Serenade Garden Disease Control.
Before you fork over your life savings for the latest fungicides or organic microbial treatments, be sure you are doing the most to first prevent moldy conditions in your garden:
– Use a layer of mulch to keep your your ground covered and your leaves away from the soil where most fungal diseases hang out.
– Thin out, divide, or prune back crowded plants so that they get good air flow and a little bit of sunlight.
– Water and feed your plants to keep them healthy. Plants that are stressed for water or nutrients are more susceptible to common garden diseases.
– Try to water deeply at the base of your plants. Avoid watering in late afternoon or evenings!
– Disinfect your pruners with a bleach or a vinegar formula so that you don’t spread microbes from diseased plants to healthy plants.
– Don’t compost diseased plant materials that you have cut back or cleared away. Most home compost piles won’t be hot enough to kill off infectious organisms. Bag that stuff and send it to the town dump or town composting operation. If you have nowhere to send the diseased plants, some people recommend burying the diseased plant materials in a part of your yard away from vegetable or flower gardens.
– Plant resistant varieties of flowers and vegetables. There are many newer introductions of flowering plants and vegetable seeds that have been bred to resist common garden diseases. Ask your local garden center about these varieties.
– Rotate your crops. Three years is the recommended minimum for waiting to replant a crop in a site that was previously infected with Late Blight, for example.
Blight can refer to several common diseases of tomato, potato, and other fruiting plants (squash, melon, eggplants, etc). Early Blight and Late Blight are actually caused by different pathogens, but can occur throughout the growing season. Late Blight is especially difficult to manage. You may want to get in touch with your local cooperative extension for advice if you suspect it in your garden.
Rose Black Spot
I imaging black spot has been bothering gardeners since the first roses were cultivated thousands of years ago; it is probably the most common rose disease and plagues the species in hot summer weather. If left untreated, black spot is unattractive and can weaken a the plant, as well as spread to neighboring roses.
Horticulturists recommend treating black spot with fungicides or a diluted solution of Neem Oil. Weekly treatments are recommended once the temperature climbs over the 70s. You should also remove spotted leaves from the affected plants, and clean-up any fallen leaves or rose buds that could act as a source for future outbreaks. Lastly, never water roses from above as wet leaves encourage the development of black spot. Instead, always water your roses early in the day, at the bases of their stems.
Powdery Mildew on Flowering Plants
Many plants can be infected with powdery mildew, a white or fluffy-looking fungal disease common to many flowers, trees, and vegetable plants. It lives in soil and once you have it in your garden, you’re likley to see it spread to susceptible plants. Plants that seem to be particularly hard hit by this class of fungal diseases are lilacs, aster, and phlox. I’ve also seen it on my purple coneflower and peony. Unfortunately, if you have it on one plant, it’s likely to spread to others in your garden, especially when the weather conditions are right.
Luckily, powdery mildew doesn’t really hurt your perennial plants. Once they have finished blooming, it’s a good idea to cut the plant to the ground and dispose of the diseased plant materials far away from your garden (don’t compost it!). The disease can be more serious if it affects your trees or vegetable crops, but it can be treated with many common fungicides (weekly spraying when the weather is hot).
If you suspect you have it in your vegetable garden, you may want to check out our discussion of powdery mildew versus blight. It’s important to understand the difference and how to treat them effectively.
Blossom End Rot
Blossom end rot isn’t a fungal disease, but it is a very common disease for summer vegetable growers–related to drought and nutrient deficiency. If fruiting vegetables (like tomatoes, squash, eggplants, and cucumbers) are unable to absorb enough calcium they may start to rot at their “blossom-end” (the end opposite the stem). Blossom end rot can be caused by lack of water, pH problems in the soil, or an imbalance of soil nutrients (too much nitrogen or not enough calcium or both).
If you suspect you have blossom end rot, it’s a good idea to get your soil pH level tested. You may also want to start using a more balanced vegetable fertilizer, and be sure that your plants are getting regular and deep waterings.
Do your hostas struggle with heat and humidity? Start the summer looking lovely, but by July or August are turning brown or developing ragged holes? In fact, while hosta can become stressed from heat, nematodes, slugs, or sudden frost, they also can suffer from a fungal disease, which can be mistaken for other pests or problems. Anthracnose can be treated with common garden fungicides, but it is a good idea to take some leaves or a picture of the damaged plant to a plant expert or a garden center specializing in hostas to help you identify and treat the problem.
Summer garden diseases certainly take some of the fun and enjoyment out of gardening, but it’s important to understand why they happen and how to manage them when they make an appearance. Different climates offer different challenges for gardeners, and if your plants or lawns seem to be suffering from a mysterious disease, it’s a good idea to take a picture and bring it to your local garden center or master gardener for advice on how to treat the problem and stop the spread to other plants. Help is out there; the bigger challenge is properly identifying the problem so you can find the right solution.
If you don’t have a local resource, please share your garden disease questions with us here in the comments, or on the GGS Facebook page where are horticultural experts can weigh in. We love a good mystery and we love to help!