How to Identify and Fight Garden Fungal Disease

Whether you're nurturing your first tomato plants or consider yourself a garden pro, plant disease can hit unexpectedly. The most common garden offender is fungal disease. Michigan State University Extension confirms that fungal pathogens are behind 85 percent of all plant disease.1

Fungal pathogens wait in soil, sneak up on new plants, and even bide their time on pruning shears before seizing opportunities to strike. Once active, fungal diseases exploit plant weaknesses, leaving plants prone to more disease and insect pests. Protecting your garden with savvy culture practices and effective fungal treatment helps keep the beauty and the bounty flowing.

Understand the Opposition

For a look at the different ways fungal pathogens operate, consider these common fungal diseases:

Black spot: Dark spots on the upper sides of leaves reveal black spot in action. Never on leaf undersides, the spots expand until the leaf is yellow and dotted with black. Like many fungal diseases, black spot must have water freely available on the plant surfaces, in droplets or as a film of water, before it can reproduce and spread. Crowded, wet conditions and overhead watering help black spot flourish.

Rust: This fungal disease earned its name from the rust-orange pustules that form on the undersides of leaves. The fungus grows and spreads, upper leaf surfaces discolor, and leaves eventually fall from the plant. Cool, moist weather and wet foliage fuels rust as it spreads with the help of wind, water and unwitting insects.

Botrytis blight: Once beautiful and vigorous flower petals and buds decay and rot, and show signs of fuzzy, gray mold with botrytis blight. Pathogens behind this airborne disease attack during cool, damp spring and fall days. High humidity, poor air circulation and overcrowding create prime botrytis blight conditions.

Powdery mildew: White, powdery growth on leaves, new shoots and other plant parts often signal powdery mildew has arrived. Unlike many fungal diseases, powdery mildew does not need free water to develop and spread; it stays active even in dry, warm weather. High humidity and poor air circulation encourage this wind-borne disease, which targets succulent new growth.

Get Culture on Your Side

In keeping with the principles of Integrated Pest Management, an effective challenge to fungal disease involves balancing proper plant culture with an appropriate response. Putting the following practices into action helps protect your garden and limit its vulnerability:

  • Choose plant varieties with proven disease resistance, and match your planting site to the plant's requirements. Poor matches predispose plants to stress and disease.
  • Irrigate wisely. Overhead watering can disrupt powdery mildew spores, but it also encourages water-spread pathogens. Water close to the ground to reduce wet leaves, and water early in the day so excess moisture dries by nightfall.
  • Improve air circulation and increase light penetration in and around plants through judicious pruning and proper spacing. Thinning plants or rearranging surroundings can help.
  • Prune infected plant parts promptly and dispose of the debris — don't compost it. Always cut back into healthy tissue, so no disease remains.
  • Sterilize your pruning implements by wiping them with a common household disinfectant. When you suspect disease, wipe before and after each cut or well-intentioned snips may spread the problem.

Fight Fungal Disease From the Start

A proven fungicide that prevents disease from becoming active and treats it quickly is a crucial part of your protection plan. GardenTech Daconil Fungicide, with the active ingredient chlorothalonil, provides protection against a broad spectrum of fungal pathogens and simplifies controlling fungal disease from your garden.

Prevention is key to protection, particularly with regard to susceptible plants or plants that have experienced fungal problems in the past. Most roses, for example, are especially at risk for blackspot and other fungal diseases. The University of California Statewide Integrated Pest Management Program recommends a fungicide containing chlorothalonil, the active ingredient in Daconil Fungicide, to protect healthy rose tissue and prevent botrytis blight and black spot.2

Used as a preventive and an active treatment, Daconil Fungicide can prevent, control or stop more than 65 types of fungal disease on flowers, vegetables, shrubs, fruit and shade trees. Follow label directions for the plant you're treating and the suspected threat. For example, on ornamentals such as roses and azaleas, use Daconil Fungicide as a preventive, before disease appears, as recommended by North Carolina State University's Plant Pathology Extension3 and Clemson Cooperative Extension.4 Protect hollyhocks, known for their vulnerability to rust, from the early seedling stage on. For flowering annuals, such as zinnias, which are prone to powdery mildew, treat at the first sign of disease.

Manage Edible Harvests

When protecting vegetables and other edible crops, always follow what's known as the PHI or pre-harvest interval. The amount of time recommended between treatment and harvest, PHI varies depending on your crop and the disease. For example, when using Daconil Fungicide to treat squash or tomatoes, as recommended by the University of Wisconsin-Extension,5 you can treat right up to harvest day. But allow beans, which are often plagued by rust and botrytis blight, seven days between treatment and harvest. Simply follow label instructions for the crops involved.

When it comes to protecting your garden, it's not your years of experience that matter. With good garden practices and GardenTech Daconil Fungicide, you can stop fungal disease in its tracks and get back to enjoying your garden's bounty.

Always read the product label and follow the instructions carefully.

GardenTech is a registered trademark of Gulfstream Home & Garden, Inc. Daconil is a registered trademark of GB Biosciences Corp.


1. Isleib, Jim, “Signs and symptoms of plant disease; Is it fungal, viral or bacterial?," Michigan State University Extension, 2012.

2. Koike, S.T., et al, “UC Pest Management Guidelines Rose (Rosa spp.)," University of California Statewide Integrated Pest Management Program, updated 2009.

3. Jones, R.K. and D.M. Benson, “Rose Diseases and Their Control in the Home Garden," North Carolina State University, 1999.

4. Williamson, Joey, James Blake and Nancy Doubrava, “Azalea & Rhododendron Diseases," Clemson Cooperative Extension, updated 2015.

5. Gevens, Amanda, Ken Cleveland and Lauren Thomas, “Home Garden Fungicides," University of Wisconsin-Extension, revised 2012.