Selecting a Biofix date for apple scab disease forecasting

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. The first step when using any apple scab disease forecasting system is to determine the biofix, which is the date at which to start the model. Biofixes are typically based on a characteristic of the host or pathogen. Determining the biofix for apple scab can be frustrating as the forecasting system will provide them by default, regional extension specialist may release information regarding shooting towers and squash mounts, and finally there are different cultivars on the farm will be at different stages of maturity. Historically, shooting towers and squash mounts were first used to determine maturity and ejection of ascospores and have highest level of accuracy. However, these technics are labor intensive and require a high level of skill and are only applicable to the orchard from which the leaves were collected. Fortunately, a considerable amount of peer reviewed research has demonstrated that ascospore maturity could be reliably estimated based on temperature accumulation (degree-days) after 50% flower bud break on ‘McIntosh’. This research led to a ‘Maturity Model’ that would allow any grower to estimate ascospore maturity without the need for labor intensive and highly technical squash mounts and shooting towers. ‘McIntosh’ was chosen since most of the eastern United States planted ‘McIntosh’ at the time of the model development, and local populations of the pathogen were well adapted (evolved) to mature and release when ‘McIntosh’ had green tissue. It is important to note that estimations based on the biofix of 50% flower bud break are considered to only capture the peak and tail end of ascospore maturity and release. Despite the intense scientific validation of this model for estimation, researchers routinely find mature ascospores in squash mounts and shooting towers long before bud break. Indeed, in 2019 and 2020, Dr. Acimovic (Virginia Tech) often captured mature ascospores in eastern NY 1-3 weeks before the 50% McIntosh budbreak biofix. To complicate matters, local populations have likely now begun to mature and release in accordance with the development of newer local varieties as ‘McIntosh’ is not as widely planted anymore. Perhaps, ‘Gala’ budbreak should now be used as the biofix?

Since we know that ascospores are present before green tip, one solution might be to choose a biofix 1-2 weeks prior to budbreak. Unfortunately, starting ascospore maturity and release simulations early may cause the end simulation early and any potential threat of ascospore release at the end of the epidemic may be missed. Fortunately, ascospores released prior to green tissues won’t be able to infect, and if they could, the recommended copper application as silver tip (see above) will afford excellent protection. With all this information, what should one do about simulations and biofixes? We’d like the to recommend the following solution. As green tip approaches, leave your simulation model with the 50% bud break biofix, but if the weather warns of potential infection or considerable rainfall, temporarily set your biofix back to 10 days earlier and see if the amount of ascospores potentially ejecting increases dramatically. If the simulation increases dramatically, perhaps consider a single-site fungicide or dodine (see the next section) application. Once you begin to approach tight cluster, set your biofix back to 50% bud break to best capture the peak releases and ensure that the simulation doesn’t end too early.

Inoculum reduction recommendations in 2024.

With the late season rains in 2023, there could be high levels of apple scab or apple blotch in the coming spring. Reduction of overwintering, or “primary ascospore inoculum”, will allow for easier season long management especially if there is considerable rain fall from green tip to petal fall. Any efforts to reduce 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 high levels of disease. 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 early on. 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 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 both 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. Although fire blight was devastating statewide due to a cooler bloom in 2023, orchards with late season shoot blight could still have overwintering cankers. Presently, with the cool weather following budbreak last week, overwintering fire blight cankers may still be dormant in commercial orchards. As the weather begins to warm (> 60°F) in the coming day/weeks, fire blight cankers could begin to ooze. While we are having this cool spell, 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 (< 32°F) temperatures. To mitigate the threat of oozing cankers and reduce both fire blight inoculum and early season apple scab inoculum, make one or 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, but the application can be delayed to ‘green tip’. Even at green tip, it is generally still safe to apply high MCE copper products but may be advisable to leave out oil at these late development stages. In the Geneva research orchards, we often make a second application of a high MCE copper fungicide at ¼” green with no consequences.

Assessing the upcoming 2024 apple scab season in this changing climate.

Once again, we’ve had a mild winter with many of days of temperatures well over 50 °F and little to no snow cover. Moreover, we’ve had several days in February and March where temperatures often exceeding 60°F. Not only did we have green tip early, around March 12th in the Hudson Valley and Long Island, but we also had green tip in the Finger Lakes and Western NY at nearly the same time. When I first started, green tip in the Finger Lakes and WNY was nearly 2-3 weeks behind the Hudson Valley. With these exceptionally warm winters or early springs, we’re beginning to break dormancy on a more similar schedule. Unfortunately, the season is nearly upon us all and we should be poised to take action against diseases like apple scab. Fortunately, we are having a week of extremely cold weather that has completely halted the maturity and ejection of ascospores.

In 2023, there was a moderate level of rainfall from early April to June, but was easily managed. Interestingly, many areas of the state experienced many heavier rains (> 0.75 inches) after petal fall through June, July, and August. These rains may have led to problems in orchards if any early season infections slipped through. The 2021 and 2022 seasons were much drier and some regions received less than 7 inches of rainfall during the key periods of apple scab infection. Statewide, apple scab disease pressure had been at a historical low. Like last year, the later season rains seem to favor apple blotch (syn. Marssonina blotch; Diplocarpon mali), which starts a little later in the season than apple scab and can defoliates trees in late August and early September. Indeed, trees of susceptible cultivars in our orchards at Cornell AgriTech, that did not receive applications best timed to address apple blotch infections, developed symptoms on leaves and fruit and defoliated at or just before harvest. Fortunately, despite the erratic weather, there were no reports of apple scab or apple blotch control failures in commercial orchards.

Apple Scab management from Green Tip to Bloom.

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.