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.

Chemical compatibility also needs to be considered. For instance, fungicides in dissolvable bags 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 or 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:

  1. Small amount of water, and begin agitation
  2. Water-soluble packets
  3. Wettable powders as a slurry
  4. Dry flowables or water-dispersible granules
  5. Liquid flowables
  6. Emulsifiable concentrates (oil concentrates)
  7. Remaining water

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.

To improve fungicide coverage of the fruit zone, timely leaf pulling and 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 and training grapevines to have a more exposed and open canopy will aid in disease control.

Dr. Schilder’s work is funded in part by MSU’s AgBioResearch.