March 6, 2023

THE NUTS AND BOLTS OF APPLE SCAB MANAGEMENT WITH INSIGHTS ON OTHER DISEASES

THE NUTS AND BOLTS OF APPLE SCAB MANAGEMENT WITH INSIGHTS ON OTHER DISEASES

Updated 3/7/2023 10:30 AM:

‘Gala’ trees at the VTech Lab (AREC) in Winchester experiment orchard are at green tip (GT). GT was reached  on Granny Smith in Nelson County in 1st March in Tyro, VA. GT was confirmed on Gala on the 1st March and 4th March in Red Delicious in Franklin County. In Botetourt County GT on Gala was 3rd March and on Red Delicious on 7th March. Green Tip (GT) is defined as the day at which on your earliest apple cultivar of interest, that is susceptible to scab, your trees reach 50% of fruit buds at green tip. The best way to determine this by randomly choosing 100 fruit buds across at least 10 or more trees, and tallying up the buds in dormant, silver tip, and green tip stage categories, and get that 50 out of 100 of all buds are at green tip. Same practice can help you determine if you are at QIG for last window to use copper QIG (50% of flower buds at QIG). The best way to predict apple scab infection using the weather forecast are the models like NEWA scab model is available here: https://newa.cornell.edu/apple-scab (free) and RIMpro model available here: https://www.rimpro.eu/ (subscription based).

If you had scab symptoms showing on leaves and/or fruit in 2022, both early or late in the season, you will need to apply fungicides starting at green tip (GT) and not skip the following applications at 5 to 7 days interval during the following stages: quarter inch green (QIG), half-inch green (HIG), tight cluster (TC), pink bud (PK), bloom (BL), petal fall (PF) and first cover (FC). The reason why is because you are starting the season with high infection pressure carried over from the previous season.  Your copper applied from GT to QIG is equal in efficacy to one mancozeb spray application.

In 2022, the first major scab infection was on 23 March which was at green tip in Winchester (Fig. 1), so early scab infections are possible. Apple Scab models for Winchester have not yet reported an infection is possible. However, at locations where GT started on 1st March (Tyro, VA), an infection was possible with the rain events 3rd and 4th March 2023 (NEWA model).  For this period, the delayed dormant copper spray applications I recommended via the blog post on 11 Feb 2023 available here: (A) Urea/Lime for Reducing Apple Scab and Apple Blotch Disease Overwintering Inoculum; (B). Delayed Dormant Copper for Apple, Pear & Peach, will be effective like a one application of mancozeb.

If you are interested in learning how to use and interpret RIMpro apple scab prediction model, e-mail me at: acimovic@vt.edu and I can help you subscribe and connect a closest NEWA weather station to it (you do NOT need to be the owner of a NEWA station to subscribe, you can use a neighbor’s station close to you). RIMpro is the best prediction model you can use to determine whether or not the early infection from GT to TC will occur or not. It can save you money in omitting fungicides when they do not need to be applied. If you did not have scab in your orchard last year, you might not need to apply fungicides for low infection severity events which RIMpro can indicate to you. This is the only model that shows you how severe the infection is for each wetting event. If you had visible scab symptoms last year in your orchard, you should spray for predicted, early-season infection events, that are at or above 100 RIM value (red curves on the graph, Fig. 1). Farms that had NO visible scab symptoms in the apple orchard last year, should spray only for infection periods at 300 RIM values or above. If you will use only NEWA’s apple scab prediction model, major infection events are when there is >15% ascospore discharge predicted. NEWA scab model is available here: https://newa.cornell.edu/apple-scab

 

Figure 1. RIMpro apple scab model output for 2022 in Winchester, VA (historical data from a weather station). White camel hump-like areas labelled “Germinating spores” show cumulative number of Venturia inaequalis ascospores that germinate over time and are read using the right-side vertical Y-axis scale that is labelled “Discharge”. The red curved lines are scab infections. Read each red curve’s peak RIM infection value(s) using the vertical Y-axis scale on the left-hand side of the graph labelled “RIM Infection Value”. Peak RIM value divided by 100 gives you the percentage of the total season’s ascospores that will cause infection for a given infection period (wetting event). Orange area labeled “Primary stroma” represents scab lesions that were initiated by infection from germinating spores and that are incubating in the leaf after which scab lesions will become visible. This is worth knowing because if no fungicide was applied before the infection started, some or all of the incubating infections can be eliminated by using fungicides with post-infection activity. The light red area in the middle graph labeled “Maturation”  is the proportion of mature ascospores that are ready for discharge with wetting events whereas the dark red area shows the proportion of immature ascospores still remaining in leaf litter on the orchard floor. The dark blue bars in the bottom graph showing wetting and dates, are the actual rain periods. The light blue bars are actual wetting periods when no rain is falling but trees are still wet after rain. Used by permission of RIMpro B.V., France.

If you had fire blight in your orchard in last two years, you should apply delayed-dormant copper spray anywhere from Silver to 1/4″ green (QIG) or 1/2″ green (HFG), if label permits HIG use for you copper product of choice. Many formulations limit use to QIG as the latest stage you can apply it to avoid phytotoxicity and russetting of future fruit. Copper can reduce fire blight bacterium inoculum which reaches bark surface from cankers during warm weather in spring in the form of exuded orange ooze of the bacterium or as ooze-less bacterial colonies. Copper will also affect any apple scab overwintering in buds. The usual rate is 6 to 8 lbs of some of the popular products like Cuprofix, COCS, NuCop (fixed coppers, 40 to 50% copper equivalent). However, if you had fire blight last year or heavy rains are predicted in spring, it is wise to use the highest labelled rate of copper for delayed-dormant use, so that some residue lasts on the tree through the rain. If fire blight was not an issue in your or any nearby orchards and you do not have fire blight cankers on your trees, you can apply mancozeb instead of copper. Keep in mind that maximum rate of copper allowed between silver-tip and green-tip stage is that of Basic Copper 53, limited to no more than 11.3 lbs of product/Acre (6.0 lbs copper/A), which might be something you can consider if you had fire blight and/or bitter rot in your orchard last year (if can help with reducing overwintering inoculum).

Mancozeb will reduce scab as well as bitter rot or black/white rot inoculum. Scab and bitter rot inoculum can overwinter under the bud scales. However these rots also overwinter in branch/trunk cankers or in fruit mummies left after thinning in the canopy and on the ground. You can mix mancozeb and copper, if you want. In some years, copper/mancozeb spray at GT to HFG may not be necessary if the weather conditions are cold, which prevents any small amount of released scab ascospores to establish infections. And RIMpro model can indicate this to you by showing only the white humps that are NOT followed by red curve line that rises above 100 RIM value (see an example in Fig. 1 of ascoposre germination of 31 March 2022, that did not lead to infection). Even though omitting this spray application can save you money, many still apply copper to cover for fire blight, scab inoculum in buds, and for any low infection events before tight cluster.

The next apple scab targeted spray application(s) are best determined by looking at the RIMpro’s apple scab model predictions (Fig. 2) for a significant infection period. In commercial orchards that did not have symptoms of apple scab last year, infection predictions with any red curved line peak at or above 300 RIM value based on the weather forecast, are significant infections. In an example in Figure 2, vertical light blue line separates current day and time within that day, and model outputs to the right of that line (in the example below it is May 6th to May 15th) are infection predictions based on weather forecast feeding into the model. The model outputs to the left side of the blue line are actual infections based on the historical weather data provided to the model by the weathers station.

Figure 2. Example of RIMpro apple scab model output showing the vertical light blue line which marks the current date and time within that day. To the right of this light-blue line are model outputs showing infection predictions based on weather forecast feeding into the model. The model outputs to the left side of the light-blue line are actual infections that took place based on the historical weather data fed into the model by the weathers station. Used by permission of RIMpro B.V., France.

 

For the first major scab infection in the spring (HIG to TC) my recommendation is to apply anilinopyrimidine (AP) fungicide Vangard (3 oz/A), Scala (5 fl oz/A), or Luna Tranquility (12 fl oz/A) in a tank mix with mancozeb (at least 3 lb/A). The AP fungicides work best during cool weather in spring and can be applied before or 72 hr after the time when rain event began and caused scab infection (kick-back activity), however, use kick-back mode of activity only in an emergency i.e. if you know you did not have any fungicide applied before the infection when you see it in the scab model like RIMpro or NEWA. You can use captan 2.5 lb/A plus mancozeb 3 lb/A instead, before the rain, keeping in mind that both of those fungicides do NOT have kick-back activity and that captan should not be applied anywhere near a past or planned future oil application (be cautions as captan uptake can cause tissue injury). Dodine (Syllit) is an option but is limited to two applications before pink bud (PK) and should not be used where resistance to dodine is present (active ingredient of Syllit). Vangard (3 oz/A), Scala (5 fl oz/A), or Luna Tranquility (12 fl oz/A) with mancozeb (3 lb/A) can be applied once more (at TC) and keep in mind that they are NOT effective against juniper rusts. That is why you are adding mancozeb as an early insurance against very weak rust infections that might surprise you at this time.

If weather is cold and the TC stage extends, gaps until PK bud can be filled with mancozeb applied alone (3 lb/A). At PK bud, the rust infections will dictate you to switch to the highly effective DMI fungicides that control rusts which should also be tank-mixed with mancozeb (3 lb/A).  If you had particularly bad powdery mildew last year in the orchard your Luna Tranquility (12 fl oz/A) at TC will be the best choice and should be used instead of Vangard or Scala, as it will control both powdery mildew and scab (it has two components: fluopyram, and pyrimethanil, which is the same active ingredient in Scala). However, at PK bud you will need to apply one of these: Rally, Sonoma, Rhyme, Topguard, Indar or Procure in a tank mix with mancozeb, which will be great choices and these fungicides will also work well against juniper rusts.

For the next major scab infection predicted by models, at early bloom and/or full bloom, my recommendation will be to use SDHI fungicides (group 7) i.e. either Aprovia, Fontelis, Sercadis, Miravis or Excalia plus mancozeb (3 lb/A). Due to limitation in use of ANY group 11 fungicides to only 4 applications per year (strobilurins or QoI-s), I would recommend you to NOT use Luna Sensation, Merivon, or Flint Extra for powdery mildew control in spring but to save group 11 fungicides for apple bitter rot control during summer (no you cannot use 4 spray of each of these fungicides per season; you can use 4 applications of any QoI-s per the whole growing season). The SDHI fungicides I listed above will be effective for powdery mildew, but are not effective for rust control, and that is why mancozeb is tank-mixed with them. If you must use Aprovia for scab, use either only one Aprovia application for scab or none, and save the rest of the three applications of Aprovia for apple bitter rot later in the summer as it is the only SDHI effective against bitter rot.

At petal fall, I would recommend using Rally, Topguard, Rhyme or Procure plus mancozeb (stop mancozeb at 77 days before harvest for early maturing cultivars), but if powdery mildew was not an issue last year, you can use Inspire Super plus mancozeb instead which is best option for scab and rust control. Another tank mix consideration is Fontelis, Sercadis, Miravis or Excalia plus mancozeb for scab and powdery mildew control at the same time. Juniper rusts can have extended infection periods up until 1st or 2nd cover. Warm wet weather at petal fall can allow Botrytis to infect the dying petals and move from petals into sepals where it can remain quiescent until fruit are moved into storage where the Botrytis can cause gray mold storage decay. Using Inspire Super at petal fall and/or first cover is be the best option because it contains cyprodinil, a fungicide that is effective against Botrytis (if Botrytis is not resistant to this fungicide). Powdery mildew can have an extended infection periods in highly susceptible cultivars until first or second cover, so reserving DMI fungicides for use by second cover in these cases is necessary. If powdery mildew was not an issue last year, Inspire Super plus mancozeb would also be my recommendation for the first cover (10 days after petal fall). If mildew was more a priority for you, use Aprovia, Fontelis, Sercadis, Miravis or Excalia plus mancozeb (3 lb/A). Depending on a year, right before or around first cover primary scab season usually ends, and emphasis shifts more to other diseases.

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Filed Under: 2023 Growing Season