Reasons to fungicides fail

One thing to consider is inaccurate disease diagnosis. There are common problems that may be misidentified as fungal diseases including insect damage, chemical injury, bacterial diseases, nematodes, and environmental damage  Fungicide applications do not affect these other problems.
do not use dated fungicide material. The general rule is that fungicides stored over two years begin to lose their activity and may fail to work.

Loading fungicide in the sprayer

Mixing fungicides with too acidic or alkaline of water can reduce fungicidal activity, especially for water with a pH greater than 8.0. Optimally, water with a pH near 7.0 should be used for mixing pesticides. If water pH is not optimal, it easily can be corrected with pH buffers that are added to the water before mixing in fungicides.

For fungicides to effectively manage diseases, you must use them at recommended rates. In addition, care needs to be made when calculating treatment area and the amount of product to add to the tank. Be sure to carefully check the label before loading the sprayer and double check your calculations.

Mixing multiple pesticides in a spray tank can save time, but be sure they are compatible. Incompatibility can result in the formation of insoluble precipitates in your tank. Fungicide labels often contain information on mixing compatibility. If the label does not address compatibility, test a small volume of the spray mix in a glass jar for 30 minutes, and then look for separation or settling of pesticides in the jar. The order that pesticides of different formulations are added to the tank also may affect compatibility. Add different formulations of pesticides to the tank in this order: wettable powders, flowables, solubles, powders, surfactants, and then emulsifiable concentrates.

Fungicides begin to lose their activity if they sit too long in the spray tank. Fungicide activity declines within 12 hours after mixing and is accelerated by poor water quality (high or low pH).

Sprayer calibration and application

Perhaps the most common cause of fungicide-application failure is from incorrect sprayer calibration. If a sprayer is not properly calibrated, too much or too little fungicide can be applied, which can result in fungicide toxicity or unmanaged disease. To avoid these problems, recalibrate the sprayer after any modifications to nozzles, pressure, or speed are made. Also, the material should be applied in the recommended volume of water, at a constant speed, and at the recommended pressure. Be sure to adjust the spray pressure for the nozzles used. Excessively high sprayer pressures result in small droplets that may drift. Calibrating and adjusting a sprayer takes time, effort, and involves math, but it can save money and make fungicide applications more effective