Apple Scab management from Green Tip to Bloom.
Now that nearly everywhere and every variety is past green tip, it’s advisable to start applying protectant fungicides for apple scab, timed according to infection events predicted by weather conditions. For any fungicide application, it is advisable to use an apple scab forecasting system such as the one in the NEWA system (http://newa.cornell.edu/). This will identify predicted ascospore releases and potential infection events to improve application timing and cost-effectiveness of fungicide investments. Regardless the disease forecasting simulation, one should target fungicides applications just prior to predicted large releases of ascospores (> 15% discharge) during weather conditions conducive to infection. It fine to maintain protection a discharge event less than < 15% ascospore discharge. If you want to try biopesticides, these are the type of events to target for those materials safely.
One of the most popular protectant fungicide programs for early season disease management consists of a tank mix of Captan with Mancozeb at half maximal rates for each product (e.g. Captan 80 at 2.5lbs/A & Mancozeb 3lbs/A). The combination is referred as “Captozeb” in the vernacular sense and has excellent residual (Mancozeb) and redistribution (Captan) properties, but has little to no post-infection activity, and must be applied before rains. Re-application is warranted when unprotected tissues emerge 7 days later or when considerable rainfall (> 1”) occurs.
Prior to bloom, it’s best to stick to protectants, captan, mancozeb, and copper as the single site fungicides should be saved for post-bloom and beyond petal when disease pressure for powdery mildew, rusts, apple blotch, bitter rot, and fly speck / sooty blotch pressure increase. The protectant fungicides aren’t effective against these diseases. However, there are some single-site fungicides that are only really affective against apple scab, and would include: Scala (Bayer), Vangard (Syngenta), and Sylliy FL (dodine, UPL). If there are rains for several days as we approach bloom, we should consider using products containing single-site fungicides with post-infection activity. Given the concerns for selection of fungicide resistance and weaker curative activity, it’s no longer recommended to apply products containing single-site fungicides for post-infection activity. Instead, think of making applications between infection periods with a focus on protecting against the infection period with the largest discharge. Use disease forecasting to identify periods where substantial ascospore release (> 15% discharge) is predicted to occur after infection of the previous week (Figure 1). Considering the example in figure 1, apply your selected product containing a single-site fungicide(s) (with 3 lb/A mancozeb) for “next week’s” infection within 24-48 hours after the last infection period. Such an application should protect against the next predicted infection and perhaps afford some curative activity if any germinating spores slipped through the fungicide coverage from the previous week. Of the products with single-site fungicides, dodine, sold as Syllit FL, will likely be your best option for (curative activity) or applications between infection periods. However, Syllit may only be applied twice before pink. Another option would be to use Cevya, Luna Tranquility, Luna Sensation, Flint Extra, Fontelis, Merivon, Syllit, Rally, Rhyme, Inspire Super, Tesaris, Miravis, Aprovia). These have some curative activity because are less soluble than their predecessors. This makes them better protectants and keeps them out of our fruit at harvest. As the season progresses into bloom, Luna Sensation or Merivon, which contains quinone outside inhibitor (QoI) fungicides. QoI fungicides are highly effective against mildew and apple blotch, have excellent post infection activity against these populations. and would be better choices for orchards of mildew susceptible cultivars as well as plantings along the lakes where apple powdery mildew pressure can be high.
Figure 1. Output from the NEWA apple scab disease forecasting tool in mid-April in the Hudson Valley in 2019 (4-16-19). An ideal time for protecting the crop with a fungicide would be before the predicted ascospore discharge on 4/20. The date of 4/16 would also be an ideal time for selecting a product containing a single-site fungicide, which has post-infection activity. Such an application would also protect against the minor infection on 4/15 (4% ascospore discharge) and the subsequent infection a few days later on 4/20.
Figure 2. Generalized summary of key fungicides and their properties with respect to stopping infections on the surfaces of leaves. Even within groups fungicides differ in their properties: Red: redistribution protectant; light blue: sticky protectant; blue: systemic; yellow: translaminarl; violet: vapor phase. Fungicide names mentioned: trifloxystrobin – Flint Extra and Luna Sensation (Bayer CropScience); Difenoconazole – Inspire Super (Syngenta); Mefentrifluconazole – Cevya (BASF); Dodine – Syllit FL (UPL). *Note Kasugamycin shown as a rice blast fungicide – do not use as a fungicide in apples. (Created by Isabella Yannuzzi, 2020)