How to Use Fungicides

Fungicides are powerful chemicals that, when used properly, can prevent and combat the fungal diseases that cause plant damage. Misuse, however, can result in poor disease control, damage to the plant and even ecological harm.

Different fungicides are used for different diseases, so proper identification of the disease pathogen is critical to treating the plant. You may need assistance from your local Cooperative Extension Service office or plant disease diagnostic clinic to help you diagnose fungal symptoms. Prevention of fungal disease through proper plant care is always preferable to treatment.

Most fungicides are preventative and sit atop plant tissue to act as a barrier that prevents fungus from entering the plant. Also commonly called topical fungicides, contact fungicides or protectants, preventative fungicides are applied when risk for pathogen development is high, such as during long periods of wet or humid weather. And certain fungicides are best applied when plants are flowering. In the home garden setting, fungicide is often applied using a hand-held, air-compressed sprayer.

Curative fungicides help fight diseases that have already infected the plant. Also known as penetrant-type fungicides, they infiltrate the plant's tissue and can move inside the plant to kill or slow down the fungus.

Some species of plants have a reaction, known as phytotoxicty, to certain chemicals. Phytotoxicity, which can be worsened by hot weather, can result in plant damage or death. At-risk plants are listed on the label as contraindicated plants. Always read the fungicide label carefully to make sure you can treat the disease with that particular chemical. Labels also include important dosage, timing and health information. Follow label instructions exactly.

If dilution is required, mix fungicide with water according to the rates recommended by the fungicide label.

Using a hose-end or hand-held sprayer, apply fungicide evenly to the tops and bottoms of plant foliage. It is especially crucial to coat the lower leaves, as fungal diseases often start at the bottom of the plant.

Re-apply as needed. Follow label instructions for exact timing requirements; usually you must wait seven to 14 days between applications, depending on rainfall and growth of the plant.

If the fungicide label says to "spray the fungicide until run off," this does not mean spray until the chemicals drip off the plant and onto the ground. It means that you must spray until the solution covers the surface of the plant just to the point when the droplets begin to run. 

Re-application is often necessary because factors such as rain, sun and oxidation reduce the effectiveness of the chemicals over time. Re-application also protects vulnerable new shoots and leaves as they appear. Apply more frequently during periods of rapid plant growth and rainy weather -- less frequently during drier periods with slower plant growth. Organic fungicides often break down faster than synthetic fungicides and may have to be applied more frequently. 

Follow label instructions for safety and storage information. Protect yourself during application by wearing long sleeves and pants, socks and shoes and rubber gloves. You may also need protective eye wear. Store fungicides in a secure location away from children and pets. Do not wash equipment or dump fungicide solution in or near bodies of water, as some fungicides are toxic to aquatic invertebrates.

For more information on choices of fungicides labeled as organic, see "Organic Fungicide Options."