This is from an article by Annemiek Schilder at Michigan State University and was published first in the Michigan Grape and Wine Newsletter vol. 2, issue 6. Even though it focuses on grapes primarily, there is very good advice here for all growers.
MSU Department of Plant Pathology
With the high cost of pesticide applications in terms of product cost, fuel and labor, as well as environmental impact, it is important to get as much beneﬁt from them as possible. There are several things to consider in improving fungicide spray efﬁcacy and efﬁciency:
1) Use an appropriate sprayer for the crop and calibrate your sprayer so you know you are applying the right amount of product per acre. This should be done at the beginning of the growing season. A good time to calibrate is in early spring. Check for worn disks, and be sure that all nozzle tips have the same angle and capacity rating. Use the right kind of nozzles for the intended application. Nozzles that produce very ﬁnd droplets may lead to more drift and less deposition on the target. The use of wettable powder sprays enlarges nozzle openings, so calibration of each nozzle is essential. Use only clean water when calibrating sprayers. Calibration instructions can be found in E-154 (Michigan Fruit Management Guide).
2) Adjust sprayer nozzles to aim at the intended target (e.g., fruit zone in grapes); reduce speed and/or airﬂow to get more fungicide on the target and less drift. Remember all product that drifts away is lost for the purposes of disease control and may contaminate non-target areas and crops. A “patternator” can help you understand where most of the spray is ending up.
3) Apply the fungicide in a sufﬁcient volume of water to obtain thorough coverage but not lead to run-off (any fungicide that runs off the plant is lost and is more likely to contaminate ground water). Coverage is especially important for protectant fungicides. Spray volume should be increased as a crop canopy expands; e.g., for an airblast sprayer in grapes, it is recommended to start out with 30 gpa early in the season then increase to 50 gpa after bloom and maybe as high as 75 gpa at veraison). Spray every row. For instance, with an airblast sprayer, insufﬁcient product is deposited on the second and third rows to get good disease control.
4) Check the pH of the spray solution, especially when using alkaline well water. While most fungicides are stable over a range of pH values, some fungicides (e.g., Captan, Dithane, Rovral) can degrade under alkaline conditions. For instance, the half-life of Captan is 32 hours at pH 5, 8 hours at pH 7, and 10 min at pH 8. The half-life of Dithane is 32 hours at pH 5, 17 hours at pH 7, and 34 hours at pH 9 (insecticides in general are more sensitive to pH than fungicides). For a list of pesticides and their sensitivity to pH, see the 2012 Michigan Fruit Management Guide, page 59-60. The pH can be adjusted with an acidifying/buffering agent. Avoid letting the spray sit overnight in the spray tank. Fungicides should, whenever possible, be mixed and sprayed as soon after mixing as possible.
5) Use an appropriate fungicide for the diseases you want to control. Regularly scout vineyards to know what diseases are present and if you are not sure what the problem is, send a sample to MSU Diagnostic Services. Use disease models if they are available (e.g. in the Enviroweather System) to estimate disease risk so as to optimize application timing. For best postinfection activity, apply fungicides as soon as possible after an infection period, weather permitting.
6) During rainy periods, systemic fungicides perform better than protectant/contact fungicides since they are less susceptible to wash-off by rain (although eventually, a portion of systemic fungicides also gets washed off by rain). To get the most curative (post-infection) action from a systemic fungicide, apply the highest labeled rate since activity is concentration dependent. The same goes for preventive action; usually a higher rate often extends the residual period. You may need to reapply protectant fungicides after 1-2 inches of rain. Systemic fungicides can last a bit longer.
7) Add an adjuvant (surfactant, sticker, penetrant) if this is recommended on the label. Some adjuvants have ultraviolet (UV) light blockers that slow UV degradation of pesticides. However, many modern fungicides are formulated in such a way that adjuvants are not necessary. In fact, some fungicide labels prohibit the use of adjuvants due to the increased potential for phytotoxicity. In our experience, most biocontrol agents (e.g., Serenade) do beneﬁt from the use of stickers/extenders, such as Nu-Film P. In some cases, adjuvants have a disease-suppressive effect because they are detrimental to pathogens; e.g., surfactants zoospores of downy mildew pathogens to explode.
8) Apply protectant fungicides (e.g., Captan, Ziram, Manzate) dur ing sunny, dry conditions to allow for quick drying onto the leaves. In fact, it appears that protectant fungicides become better adsorbed to the plant surface and more rainfast over several days after application. Any dew formation at night will help redistribute product over the plant surface. While it is better to have protectant fungicide applications on before a rain or heavy dew event which could represent an infection period, avoid putting on protectant fungicides within several hours before a rainstorm as you may lose much of it to wash-off.
9) Apply systemic fungicides (e.g., Elite, Abound, ProPhyt, etc.) under humid, cloudy conditions when the soil is moist. That way, the cuticle (waxy layer covering the plant surface) will be swelled up and allow the active ingredients to quickly pass through. Under extended hot , dry conditions, the cuticle becomes ﬂattened and less permeable; any product that is not absorbed may remain on the plant surface and break down due to UV light or microbes or get washed off by rain.
10) Spraying fungicides when there is a light breeze (e.g., 2-6 miles per hour at nozzle level) is actually better than spraying during still conditions, because even without wind there may be updrafts and eddies caused by the spray equipment leading to fungicide off-target movement. A little wind will aid deposition; you will know where the spray is going and can adjust your spray conﬁguration accordingly. If conditions are not good for spraying, it is better to wait an extra day for better conditions than to lose most of the product to drift. Another important timing consideration is the time of day, particularly as it relates to temperature and humidity relationships. Both temperature and humidity can affect fungicide drift. The higher the temperature and lower the relative humidity, the greater the opportunity for fungicide evaporation or volatilization. This can be avoided by spraying early in the morning when temperatures are lower and the relative humidity is higher. In addition to the reduced drift hazard from fungicide volatilization early in the morning, drift may also be minimized in the morning hours due of calmer winds and lower convective air turbulence.
11) If you are not getting the control you used to get with a particular product, do consider the possibility of fungicide resistance. This could be an issue with pathogens that have a high reproductive potential (e.g., powdery mildews, downy mildews and rusts) that have been exposed to repeated sprays of systemic fungicides, such as the strobilurins (Abound, Cabrio, Flint, Pristine), sterol inhibitors (Rally, Elite, Procure, Indar, etc.), benzimidazoles (Topsin M), phenylamides (Ridomil) and dicarboximides (iprodione). In grape powdery mildew in Michigan, strobilurin resistance has been demonstrated, and SI resistance is suspected. Furthermore, we also suspect strobilurin resistance in grape downy mildew. However, you must ﬁrst rule out poor spray timing and coverage as possible causes of poor fungicide performance. Ways to reduce the risk of fungicide resistance include alternating or tank-mixing fungicides with different modes of action and avoiding spraying systemic fungicides at below-label rates. Also try to avoid applying systemic fungicides on heavily sporulating colonies as the probability of selecting mutants increases with higher pathogen population densities. Use a contact fungicide (e.g., JMS Stylet Oil, Oxidate, Kaligreen) ﬁrst or in the tank-mix if appropriate to kill spores.
12) Tank-mixing certain fungicides can increase the risk of phytotoxicity. For instance, do not tank-mix sulfur and oil containing products or spray them within two weeks of each other. Do not apply sulfur to sensitive crop cultivars and/or at temperatures above 80°F. Do not apply copper under cool, slow-drying conditions. Also, do not tank-mix phosphorous acids (e.g., Prophyt, Phostrol) with copper, as copper can become more available and phytotoxic in acidic solutions. Also, do not apply phosphorous acids to plants that are stressed (e.g., due to heat or drought stress). Tank-mixing products containing difenoconazole (e.g., Revus Top, Inspire Super) with non-ionic surfactants or foliar fertilizers can increase the risk of phytotoxicity, particularly on succulent, fast-growing plant tissues. Some crop cultivars are sensitive to strobilurins, e.g., ‘Concord’ grapes are sensitive to Pristine and Flint. Captan can become phytotoxic when applied with oil or within four days of an oil spray.
13) Tank – mixing some fungicides can inactivate one or both. For instance, don’t tank-mix copper with the biofungicide Serenade (Bacillus subtilis – a bacterium), since copper is toxic to bacteria and can inactivate Serenade. While part of the activity of Serenade is due to antibiotics resulting from the fermentation process used to produce Serenade, some of the efﬁcacy is related to activity of live bacteria. Similarly, the biofungicide RootShield (Trichoderma harzianum – a fungus) is incompatible with the fungicides Orbit (propiconazole), Elite (tebuconazole), and Procure (triﬂumizole) which can kill the Trichoderma spores. Also don’t tank-mix bicarbonates with phosphorous acids or use lime with Captan as fungicide activity will be reduced. Fixed copper formulations and lime should not be used with Guthion, Imidan, Sevin, Thiodan, Bayleton, or Captan as efﬁcacy may be lessened and the risk of phytotoxicity increased.
14) Chemical compatibility also needs to be considered. For instance, fungicides in dissolvable are incompatible with spray oils and boron-containing fertilizers, because these materials prevent the bags from dissolving. In addition, chemical incompatibility may cause products to go out of solution/suspension and precipitate in the bottom of the tank. When tank mixing chemicals, read the label for indications of incompatibility with certain products. Also, add products in the following order unless otherwise directed on the product label: a) Small amount of water, and begin agitation; b) Water-soluble packets, c) Wettable powders as a slurry, d) Dry ﬂowables or water- dispersible granules; e) Liquid ﬂowables; f) Emulsiﬁable concentrates (oil concentrates); g) Remaining water.
15) Do not apply systemic fungicides to dead or partially dead leaves; this also applies to leaves in which the veins have been killed. Systemic products cannot move around in dead tissue or through dead leaf veins and therefore will not be distributed well in the plant.
16) To improve fungicide coverage of the fruit zone, timely leaf pulling and/or shoot thinning in the fruit zone will be helpful. In addition, shoot positioning will help open up the canopy and will improve fungicide coverage as well as sunlight penetration, which will help suppress fungal development. Overall, pruning/training grapevines to have a more exposed and open canopy will aid in disease control.”