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.
Recently, 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.