Why fungicide applications fail

Remember last summer when you found some diseased turf on your grounds that looked similar to the problem you had the year before? After you spotted the problem, you loaded your sprayer with your favorite fungicide and applied the product to your turf. But, to your dismay, the disease persisted, and your turf continued to die. What happened? You've used the right product and applied it according to label directions, but your application failed to do the job that it always has done. As you prepared to make another application, the reason your previous fungicide application failed to manage the disease remained a mystery. On certain occasions, fungicide applications fail to provide the desired disease management. It is important to identify the reasons for these failures to prevent them from occurring in the future. This article will help you identify the causes of fungicide-application failures.

Diagnosis and fungicide selection Inaccurate disease diagnosis is the first item you should consider. Your principle concern is whether the decline in turf quality is due to a fungal pathogen or some other cause. Problems commonly misidentified as fungal diseases include insect damage, black layer, chemical injury (such as spills, herbicide damage and fungicide misapplications), nematodes and environmental damage (such as excessive shade, compaction, over-watering and drought). Fungicide applications have little or no effect on non-fungal turf diseases.

Incorrect disease diagnosis also prevents you from selecting an appropriate fungicide. Although any given fungicide will manage a broad spectrum of diseases, it will not manage all diseases. For example, most broad-spectrum fungicides do not control Pythium blight, which requires specific fungicides. Without proper diagnosis, you might select a fungicide that does not control this disease resulting in a quick and permanent demise of the turf. To successfully control a disease with a fungicide, you first have to accurately diagnose it.

Even if your diagnosis is correct, you have to be sure that you select the appropriate fungicide for the job. Be sure to check the label for the specific disease you have identified. In addition to the label, you also can check other sources, such as university field reports, extension bulletins, trade journals, turfgrass-management books and university personnel.

Be cautious about using old material you have in your storeroom. Fungicides stored over 2 years lose their activity and may fail to work when you apply them.

Fungicide loading By improperly loading a fungicide in your sprayer you also can experience treatment failure. First, consider the water you use for your tank mix. Extreme water pH (too acid or alkaline) can reduce fungicidal activity. This is especially true of high pH (greater than 8.0) with fungicides. Optimally, you should use water with a pH of near 7.0 for mixing pesticides. Fortunately, if your water pH is not optimal, you easily can correct it with pH buffers that you add to the water before mixing fungicides in your tank.

For fungicides to effectively manage diseases, you must use them at recommended rates. You also have to be careful that your calculations are accurate. When applicators do make calculation errors, they typically make them in their attempts to determine the correct rate, treatment area or amount of product to add to the tank. Before deciding on a fungicide application rate, be sure to double-check the rate on the label. It's easy to make simple mistakes such as misreading units (pints per acre, fluid ounces per 1,000 square feet, pounds per acre or dry ounces per 1,000 square feet) or confusing preventive and curative rates. You have to know the treatment area to determine how much fungicide you need to put in the spray tank. The workplace can be hectic, and you easily can be distracted. When this happens while loading fungicides into the sprayer, you may forget how much fungicide you have put into the sprayer. An error in any one of these areas can result in a misapplication. So do not hesitate to check the label before loading the sprayer, and double-check your work.

Mixing multiple fungicides in your spray tank can save you time, but you must be sure they are compatible. Incompatibility can result in the formation of insoluble precipitates in your tank that will prevent you from spraying accurately. Fungicide labels often contain information on mixing compatibility. If the label does not address compatibility, you should test a small volume of the spray mix in a glass jar and allow it to stand for 30 minutes. Look for separation or settling of fungicides in the jar. The order that you add fungicides of different formulations to the tank also may affect compatibility. You should add different formulations of fungicides to the tank in this order: wettable powders, flowables, solubles, powders, surfactants and then emulsifiable concentrates.

Fungicides begin to lose their activity if you allow them to sit too long in a spray tank. After you fill your tank, you might run into an unforeseen emergency, and your application may be delayed. Weather may also delay you from applying. Whatever the cause, you must realize that fungicide activity declines the longer you let your tank-mix sit. The loss of fungicide activity may begin within 12 hours after mixing and is accelerated by poor water quality (such as pond water or high or low pH).

Sprayer calibration and application Perhaps the most common cause of fungicide-application failure is from incorrect sprayer calibration. Using a miscalibrated sprayer is the same as a doctor giving you medication without telling you how much to take. If you fail to calibrate your sprayer you may be applying too much or too little fungicide, which can result in loss of turf from fungicide toxicity or uncontrolled disease. To avoid these problems, remember to recalibrate your sprayer whenever you make modifications to your nozzles, pressure or speed.

In addition to properly calibrating your sprayer, you also should apply the material in the recommended volume of water, at a constant speed and at the recommended pressure. Coverage is especially important for contact fungicides that depend on foliar coverage to provide plant protection. To ensure adequate coverage, use a minimum of 1 to 3 gallons of water per 1,000 square feet. This dilution rate will provide the coverage you need for effective control. It also is important to maintain a constant speed while you spray. Speeding up to "stretch" an application to cover that last half green at the end of the day will only lower the rate of the fungicide you apply. Also, be sure you adjust your spray pressure for the nozzles you use. Excessively high sprayer pressures result in small droplets that are susceptible to drift. After all, the intention is to treat the turf not the air or non-target turf, shrubs and trees. Calibrating and adjusting your sprayer takes time, effort and involves math, but it can save you money, your turf and your job.

The success or failure of a fungicide application is determined by what you do during-and after-the application. To control patch diseases, you often must drench the treated turf with water to move the active ingredient into the crown and root zone where it can protect the plant. Failure to do so typically results in inadequate control. Similarly, you do not want to irrigate turf treated with a contact fungicide until the fungicide has had time to dry on the leaf surface.

Environmental considerations It pays to be aware of weather forecasts when you plan to spray. Unless you are going to drench your application for patch-disease management, you want to avoid spraying when rain is expected. You do not want rain to wash off a contact fungicide you have just applied. The thatch itself is a barrier to an effective fungicide application. The organic matter in thatch acts as a sponge for fungicides. An excessively thick thatch can absorb fungicides you intend to wash into the crown and soil.

Curative applications pose special problems when considering the effectiveness of fungicide applications. By definition, you make these applications after the onset of disease. While the application may have stopped the infection process, the turf is already damaged. Do not expect a curative application to immediately provide healthy turf. If conditions are not favorable for turf growth, do not expect rapid recovery. Curative applications can save turf that is already infected but it is necessary to accept that the turf often needs time to heal after a successful fungicide application.

Resistance Resistance is one of the first things that may come to mind when your fungicide fails to manage disease. It is also one of the least-likely explanations. Fungicide resistance occurs when fungal populations develop that are not sensitive to certain fungicides. Resistance arises when you use the same class of fungicide at high rates over extended periods. The only way to be certain if you have fungicide-resistant pathogens is to have them examined in a lab. Don't immediately assume that the cause of any fungicide failure is due to fungicide resistance.

Conclusion When your fungicide application fails to provide the disease management you desire, it is important that you examine your fungicide-application practices and identify why the application failed. It often is difficult to recognize your own mistakes, and it may be helpful to ask a colleague or manufacturer's representative to review your application procedures. The most important thing is to correct the mistake responsible for fungicide-application failure to prevent it from occurring in the future.

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