Apple Disease Update: Half-Inch Green
Even with the cold snap this past week, most apple cultivars in Western NC are now at or have surpassed the green tip phenology stage. For cultivars that have made it to half-inch green or beyond, high rate copper applications should be discontinued (see previous green tip apple disease update post). During the HIG growth stage, protection of green tissue from apple scab should be the primary focus in regards to disease management for apple growers. Despite the cooler weather, Venturia inaequalis ascospores are continuing to mature. As of 3/23/18, ascospore maturity had reached 16% in Edneyville, NC (Gala green-tip: 2/28/18), and 9% of total ascopores are predicted to have been discharged. To date, there have been four apple scab infection events since green tip, as predicted by the Revised Mills Table (newa.cornell.edu). Ascospore maturity will usually progress slowly through tight cluster and then tends to most rapidly progress between pink bud and bloom.
Fungicide applications during HIG should be targeted at protecting emerging tissue from primary (ascospore) infection. For best control, multi-site protectant fungicide should be applied in tank mixture with a single-site, locally systemic fungicide. If a dormant oil was or will be applied for scale, do not apply captan at HIG, and instead use a 1/2 rate of an EBDC such as mancozeb. Syllit FL (1.5 pt) + Mancozeb (1/2 rate) is a good option for this growth stage because the active ingredient (dodine) is highly effective against apple scab but fails to appreciably control other apple diseases that occur later in the season. Due to the history of resistant scab populations to dodine, make sure to tank mix the product with mancozeb. Other warnings and restrictions on the Syllit FL label include limiting applications of the product to two applications/year, discontinuing applications at pink bud, and avoiding applying the the product with some coppers, sulfurs, and oil. Products containing an AP fungicide (FRAC 9) should also be considered at HIG as AP fungicides have been observed to be more effective in colder weather. Specific fungicides containing an AP include Scala, Vangard, and Luna Tranquility. Additional fungicide options can be found in the 2018 Integrated Orchard Management Guide for Commercial Apples in the Southeast.