As the UGA blueberry pathologist, I frequently get pictures of damage to blueberry fruit. Sometimes this damage is caused by a pathogen (fungus, bacteria, etc.) and sometimes this damage is caused by other factors (insects, environmental factors, or especially chemicals). The applications of blends of multiple chemicals, fertilizers, etc. are frequent in agriculture and can be critical for the effective simultaneous management of multiple issues (diseases, pests, nutrient issues, etc.). Unfortunately, the combination of some materials (which on their own are safe and effective) can result in a mixture that is incompatible for mixing and/or which can cause damage if applied to plant tissue. As such, it is critical that mixtures are only made according to the fungicide label* and that the compatibility and crop safety of these mixtures be assessed before applying them to a large acreage.
One method to test the compatibility of a potential tank mixture is to conduct the “Jar Test”. This method is recommended on the label of many pesticides. A brief protocol for the jar test is as follows:
Jar Test: Using a quart jar, add the proportionate amounts of the products to 1 qt. of water. Add wettable powders and water-dispersible granular products first, then liquids, and emulsifiable concentrates last. After thoroughly mixing, let stand for at least 5 minutes. If the combination remains mixed or can be remixed readily, it is physically compatible. Once compatibility has been proven, use the same procedure for adding required ingredients to the spray tank.
Before applying any tank mixture not specifically recommended on the fungicide label, the safety to the target crop should also be confirmed by applying the tank mixture to the target crop in a small area and in accordance with label instructions for the target crop. Crop plants (leaves, stems and fruit) to which the mixture is applied should be examined for several days following mixture application to look for any signs of phytotoxicity (damage) that may have resulted from the application. Some of the possible signs of damage resulting from incompatible fungicide mixes include spot development, tissue discoloration, fruit drop, and/or plant wilting and death.
Below, I have included information from a previous post that I made on this topic (in 2020) which was specifically focused on problems resulting from the use of tank mixes containing both the fungicide captan and EC formulations of other pesticides:
Fungicides containing captan are an important component of spray programs for disease control on blueberry. Solo and tank mix applications of captan products have an especially significant role in the management of fungicide resistance, since captan’s mode of action is not likely to lead to the development of fungal pathogens with resistance to this material. When used correctly according to label instructions, captan fungicides can be safe and effective; however, if captan is applied inappropriately or with incompatible tank mix partners, then it can result in damage to plant tissue.
When applied appropriately according to the label, the active ingredient in captan does not easily penetrate into the plant. Unfortunately, when applied in close proximity to oils, captan can penetrate plant tissues where it can be very toxic. For this reason, captan fungicide labels state that these materials should not be used in combination with or immediately before or closely following oil sprays. In addition, tank mixing an EC (emulsifiable concentrate) formulation and captan is NOT recommended and can result in phytotoxicity (i.e. a toxic effect on plant growth) on blueberry.
Previously, I received images from an agent with UGA Extension showing damaged blueberry fruit (see below). After investigating potential causes of this damage, it was determined that a tank mix of Captan, Sniper, and Malathion 57 EC had been applied prior to observations of this damage.
*Remember: the label is the law. Always follow the label directions. Consult the various labels for rates, other recommendations, and precautions.
