Līga Astra Kalniņa & Kerik Cox
Cornell University
Plant Pathology and Plant-Microbe Biology Section
Assessing the upcoming 2023 apple scab season.
Winter seemed to come late in early March, and only for a few weeks and with intermittent snow cover at best. Snow cover is effectively non-existent, and while it’s still cool in the evenings, we’re having considerable bouts of warm temperatures often approaching 60°F. In the Hudson Valley and Long Island, green tip is presently happening or imminent in the near future. The rest of the production areas in NY may still have a week or so before bud break. Fortunately, there are cooler days forecasted for the week for most of NY with temperatures remaining bellow 60°F, which could slow tree development. Overall, the season will be upon us shortly, and we need to consider early season management for apple scab. In 2022, there was little rainfall from early April to June, which was characterized by sporadic, but hot (>75°F) heavy thunderstorms in July and August. For 2021 and 2022, many regions received less than 5 inches of rainfall during the key periods of apple scab infection and pressure is at a historical low. Eerily, conditions seem to favor apple blotch (syn. Marssonina blotch), which comes in a later in the season and defoliates trees in late August and early September. In our orchards at Cornell AgriTech, unmanaged trees of numerous cultivars only had low incidences of apple scab. Unsurprisingly, there were no reports of apple scab control failures in commercial orchards anywhere in NY.
Inoculum reduction recommendations in 2023.
Despite the low levels of apple scab in 2021 and 2022, reduction of overwintering or “primary ascospore inoculum”, may allow for easy season long management even if there is considerable rain fall from green tip to petal fall. Reducing this initial inoculum will delay the epidemic, and in theory, if there is little rain early in the season, it could possibly delay the epidemic to a point in the season where it would be too dry and too hot for the apple scab fungus to cause infection. Since it’s hard to predict if there will be an early season drought, we should keep suppressing apple scab to keep orchard populations low. In addition to apple scab, reducing orchard floor leaf litter and fruit drops may greatly reduce the inoculum for other foliar diseases like apple blotch and numerous fruit rot diseases including bitter, black, and white rot. As soon as it is possible to safely get a tractor in the orchard, remove any remaining fruit drops and pruned shoots left on the floor from winter pruning as they may contain bitter rot or black rot inoculum. If orchard floor management was practiced in the fall with flail mowing or urea sprays, it won’t be necessary to repeat the practices this spring. Research out of the University of New Hampshire has demonstrated that there are diminishing returns for practicing inoculum reduction in the fall and spring. Even if the planting is in green tip, inoculum reduction may still provide considerable benefit by reducing inoculum pressure by tight cluster or pink, when tissues are at their greatest susceptibility to apple scab.
The two best options for inoculum reduction are to apply the urea to leaf litter or use a flail mower to shred the leaves. These practices hasten decomposition of the leaf litter. In the case of flail mowing, leaves should be first swept or raked from underneath the canopy into row middles as most of the apple scab inoculum is present on litter under the trees. Subsequently, go over the row middles with the flail mower set to scalp the sod. If urea is used, apply 40 lbs. of feed grade urea per acre in 100 gallons of water to the herbicide strip (5% solution). Dolomitic lime applied at a rate of 2.5 tons per acre can be used of in place of urea. Of the various options, applying urea is the simplest approach, but take care to flush the sprayer pumps with water afterwards since the urea is caustic and can corrode a pump over time. As suggested above, the use of orchard floor urea may also reduce inoculum of other diseases such as apple blotch, bitter rot, and black rot as it hastens decomposition of leaf litter, fruit drops and pruned shoots that harbor the pathogens causing foliar diseases, cankers, and summer fruit rots.
Delayed-Dormant copper for apple scab and fire blight inoculum reduction.
Excessive warm weather after petal fall in the last few years has led to some devastating fire blight epidemics in western NY. Orchards with fire blight in 2022, will likely have an excessive number of cankers this season. Presently, overwintering fire blight cankers are still dormant even in our high inoculum fire blight research orchard in Geneva. As the weather begins to warm (> 60°F) in the coming day/weeks, fire blight cankers could begin to ooze. Now is the time to scout for oozing cankers, especially in the eastern part of the state and New England. It’s important to note that cold weather will not kill fire blight bacteria overwintering in cankers; the bacteria will remain inactive, but viable at low (< 32F) temperatures. To mitigate the threat of oozing cankers and reduce both fire blight inoculum and early season apple scab inoculum, make one to two “delayed dormant” silver-tip applications of a high (>15%) metallic copper equivalent (MCE) copper fungicide (e.g. Badge, Kocide, Cuprofix). It may be hard to get into the orchards at silver tip due to wet fields, and the application can be delayed to ‘green tip’. Even at green tip, it is generally still safe to apply high MCE copper products. In the Geneva research orchards, a second application of a high MCE copper fungicide is often made at ¼” green with no consequences.
Early season apple scab management.
Once green tip is past, 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.
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.
As the season approaches bloom or if there is rain for several days after green tip, consider using products containing single-site fungicides (e.g. Cevya, Luna Tranquility, Luna Sensation, Flint, Fontelis, Merivon, Syllit, Rally, Rhyme, Inspire Super, Tesaris, Miravis, Aprovia). Many of the products containing single-site fungicides will provide a broader range of activity against other disease like powdery mildew, apple blotch, and fruit rots, which may cause latent infections at bloom, whereas the “Captozeb” combination does not. Given the concern for selection of fungicide resistance, it’s no longer recommended to apply products containing single-site fungicides for post-infection activity. Instead, think of making applications between infection periods focusing 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 shortly after infection of the previous week (Figure 1). For example, apply your selected product containing a single-site fungicide(s) (with 3 lb/A mancozeb – to protect against selection of resistance) 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, will likely be your strongest performer for applications between infection periods. However, Syllit may only be applied twice before pink. Another option would be to use Aprovia, Miravis, Tesaris, Luna Tranquility, Cevya, or Inspire Super. 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 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. 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.
Summary
A strong early disease management program should begin inoculum reduction with urea or flail mowing as soon as orchards can be entered. Prior to applying urea or flailing mowing to reduce leaf inoculum, make sure to sweep the orchard to remove prunings and any remaining apple drops, which may harbor inoculum from many other late season diseases. After orchard floor management, apply copper from bud break “silver tip” to early “green tip”. This application will help reduce overwintering apple scab and fire blight inoculum and protect against early-season apple scab infections. From green tip to bloom, implement a program of protectant fungicides (i.e. captan and mancozeb) to protect the developing fruit clusters. If there are prolonged wetting periods in the early season, an application of mancozeb (3 lb/A) along with either Aprovia, Cevya, Miravis, Sercadis, Syllit, Luna Tranquility, or Inspire Super may be helpful. As we proceed toward bloom, additional applications of products with single-site fungicides (e.g. Luna Sensation, Merivon) may be needed to manage powdery mildew and other fungal diseases that may begin as latent infections during bloom. Prior to making fungicide application carefully track of apple scab ascospore discharge and infection events predicted on the disease forecasting service of your choice using local weather.