7 habits lead to highly effective fungicide use

Proper fungicide use is one of the most misunderstood aspects of plant-health management and results in poor outcomes. One of the most common misperceptions is that if a chemical is used to control a disease-causing agent, it will cure the plant, no matter when and how they use it. Wrong!

Unlike aspirin, which often cures headaches and eases hangovers, fungicides do not cure diseases. They act more like a vaccine than an antibiotic or other drug. Unfortunately, most people (understandably) approach plant-health problems like human health problems and they expect a set treatment to “cure” the disease after the problem has occurred. This doesn’t mean that fungicides don’t work -- far from it. A properly chosen, and properly applied, fungicide is an important component of any disease-management program. To use them most effectively, a fungicide should be integrated with the proper cultural practices and an understanding of pathogen and disease biology.

Most fungicides, particularly natural or organic fungicides, protect plants by covering their surface and preventing infection from occurring as long as that protective coating isn’t washed off or outgrown. Other fungicides are systemic and are active within the plant, preventing disease from establishing. Using a fungicide effectively requires a solid strategy of integrated plant management. As opposed to focusing on the pest or pathogen in a traditional integrated pest management (IPM) program, integrated plant management focuses on identifying the right plant for the site and management strategies aimed at keeping that plant healthy. The backbone of integrated plant management includes carefully matching the plant to the soil type, sunlight levels and watering conditions; proper sanitation; and appropriate fertilization and pruning, when necessary. These strategies work together to prevent disease problems from developing in the first place.

7 habits for highly effective fungicide use

Prevention is the key word here and it is essential to successful plant management. Even in the best situations (early diagnosis, low disease levels, resistant cultivars), it is important to remember that fungicides are only effective when properly used. Here are the 7 steps.

1. Have the correct diagnosis.

This is not the time for guessing. In addition to your state’s plant diagnostic clinic, several books and Web sites diagnose the most common diseases. Before applying any chemical, make certain of your diagnosis. Fungicides are only effective when the disease-causing agent is fungal.

Fungicides work best by preventing fungal disease from becoming established in the first place. If you are certain of your diagnosis, proceed to the next step of identifying which pesticide is best. The most effective fungicides are often fairly specific in their use. “Spray and pray” has never been an effective management strategy.

2. Select the appropriate chemical and formulation.

This means reading the label. Even after a fungicide has been recommended by a reputable source, you should always read the label. This isn’t just a good idea, it’s the law.

Although many older chemicals (e.g., chlorothalonil, mancozeb, copper, etc.) and the strobilurins have activity against a wide range of pathogens, most do not. Often the best control is obtained with a specific fungicide, instead of one with a broad range of activity. This may require some research to identify which fungicide works best. An excellent Web site to consult is the Turf and Ornamental BlueBook: www.greenbook.net.

When examining the label, determine if the plant you wish to treat is labeled and that no contraindications exist. Certain plants develop phytotoxic reactions to some fungicides, and these reactions are almost always noted on the label. Make sure you can follow the recommended instructions as to the number of applications or the addition of adjuvants. Spending a few hours is not going to change the outcome of your disease problem (not spending that time can cost you your entire crop)!

After identifying which chemical to use, think about how you should deliver it. Fungicides are formulated in different ways. Some of the most common formulations are:

* G(R) -- Granular.

* DF -- Dry Flowable.

* WG -- Wettable Granule.

* WP -- Wettable Powder.

* EC -- Emulsifiable Concentrate.

3. Apply the chemical at the right time.

By the time symptoms are severe, it is too late to spray. Fungicides, in general, protect new growth. No fungicide in the world is going to cure or heal any lesions present, although the DMI fungicides (FRAC code 3, containing the active ingredients triademefon, myclobutanil, tebuconazole and propiconazole to name a few) can “burn out” existing infections. Ideally, the first fungicide application should be made just before the pathogen contacts the plant surface.

Unfortunately, this requires an understanding of the pathogen life cycle and the environmental conditions that trigger the release and spread of the pathogen. Fungicides protect new growth and stop the spread of the disease -- nothing more, nothing less. When applying a fungicide, recognize temperature limitations and apply in acceptable weather conditions. Be sure to protect yourself appropriately.

4. Apply the appropriate amount.

Many people believe if a little is good, a lot is better. Nothing could be further from the truth, or more toxic to your plant. Most fungicides have gone through numerous trials to identify optimal amounts. Excessive amounts can result in phytotoxicity. This is really no different from an overdose in a human: Two aspirins can control a headache; 50 aspirins can kill you. Making up the right amount in the beginning is helpful, so you don’t feel you’re wasting fungicide -- or money.

Overdosing isn’t the only problem. A much larger problem lies in underdosing or cutting labeled rates. Less than labeled amounts usually fail to control the problem and can create fungicide-resistant pathogens.

5. Cover plants adequately.

Fungicides applied to the foliage act as a chemical barrier on the surface, preventing the pathogen from infecting the treated plant. To be effective, coverage must be complete. The effectiveness of the barrier depends on how well the plant surface was covered and how well the spray spreads to the plant. Some plants, particularly those with very hairy or waxy foliage, are difficult to cover properly.

To improve coverage and adherence, many fungicides have adjuvants or spreader-stickers. Usually, the label alerts the user to these problems. Some fungicides, like Daconil, leave a residue that makes observing coverage simpler. In deciding a spray program, it is often difficult to combine good coverage and no residue. Ultimately, it is your decision: black spots or white residue.

6. Apply chemicals at the appropriate frequency.

Your work isn’t done after applying the fungicide. The first fungicide application establishes a barrier on plant surfaces. Continued application of fungicides in rotation is needed to keep the barrier active and effective against the disease-causing agent. The fungicide label usually states the frequency of reapplication. A key difference between synthetic and natural fungicides is apparent: Organic fungicides require more applications as they are quickly washed off or break down, usually within several days to one week. Synthetic fungicides usually have greater than 10-day reapplication (assuming plants haven’t received more than 1 inch of rain). These fungicides can be oxidized or break down when exposed to sunlight. Regardless of which fungicide you chose, reapplication or rotation with another fungicide is needed to keep the barrier active.

Two other factors impact application frequency: Plant growth and rainfall. The rate of plant growth affects barrier completeness. As the plant continues to grow, new leaves and shoots appear. This tissue is unprotected and vulnerable to infection. Repeated application of fungicides protects new growth. Rainfall also impacts the frequency of reapplication. Repeated or frequent rainfall (or overhead watering) can remove the barrier. If excessive rainfall or rapid growth of the plant occurs, use a shorter interval between sprays. This protects new growth and any growth with the fungicide washed off. If plant growth is slower, or less watering has occurred, the longer suggested interval should be used as a guideline.

7. Rotate your chemicals.

Every time certain fungicides are used, there is a chance that the target organism may develop resistance. This is stated on most fungicide labels. This is less true for natural or organic fungicides, which are usually toxic by a variety of mechanisms. With newer synthetic fungicides, which kill in a very specialized fashion, the risk of resistance is much higher. For this reason, it is important to develop and implement a strategy to prevent resistance from developing. Over time, the resistant strain replaces all other strains and the disease becomes increasingly difficult to control. To prevent this, many fungicide labels provide information to assist you in developing good rotations.

By rotating, or rotating and tank-mixing your fungicides, you reduce the risk of fungicide-resistant fungal pathogens developing and extend the useful life of favorite fungicides.

- Janna Berkerman

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Janna Beckerman  is an extension plant pathologist at Purdue University, jbeckerm@purdue.edu.

June 2008