Apple Disease Update: Tight Cluster and Pink Bud
The arrival of tight cluster signals the start of fungicide protection for diseases other than apple scab and frogeye leaf spot. Besides apple scab, protection against powdery mildew and cedar apple rust should be a focus for growers starting around the tight cluster stage.
Apple Scab Primary Infection: Beginning at the tight cluster/pink stage, ascopore maturity and discharge rapidly increases through bloom/petal fall. The release of these overwintering spores can result in a primary infection on any green tissue that is not protected with a fungicide. At MHCREC in Mills River, we initiated the model when approximately 50% of the buds reached green t\ip (February 28, 2018). As of April 2, 5 infection events (2 combined) were forecasted in Henderson Cty, NC: March 1, March 11-12, March 19-21, March 29-30, and April 2. Additional infection events are predicted for Edneyville on April 3, 4, and 7.
Powdery Mildew: In the southeastern United States, infection by the apple powdery mildew pathogen, Podosphaera leucotricha, is often most consequential to leaves rather than fruit. However, on more susceptible cultivars such as Rome Beauty, JonaGold, or HoneyCrisp, powdery mildew symptoms on fruit-net like russeting- can occur. Oftentimes such russeting can be mistaken for frost or chemical injury.
The powdery mildew pathogen overwinters in dormant buds that were infected during the previous growing season. The infected buds usually open a few days later than healthy buds which means that susceptible tissue is already present once powdery mildew spores (conidia) are produced. Infected primary shoots (shoots in which dormant bud was infected) will be stunted, malformed, and have a white-silver color.
Secondary powdery mildew infections on foliage will appear as white fuzzy tufts on either side of the leaf. Older mildew lesions may look red/purple in color. Curling of the leaves inward along the mid-vein is also characteristic of powdery mildew infections on apple. Preventative fungicide applications targeting foliar mildew should begin at tight cluster. Flower/fruit infections can occur as early as pink.
Apple Rusts: Fungicide applications targeting cedar, quince, or hawthorn rusts should also begin at tight cluster. Leaves are most susceptible at 4 to 8 days of age, and whereas fruit are most susceptible to infection between tight cluster and petal fall. Because these rust pathogens need two different hosts to complete their life cycle, infection may not be as severe in orchards that are several miles (at least 2) from the alternate host (e.g. Eastern Red Cedars or other susceptible junipers). Sterol Demethylation Inhibitor fungicides (FRAC 3) have historically been the most efficacious and continue to be the most efficacious against the apple rusts.