1. Apple Scab and Juniper Rust Infections Predicted 19 – 20 April and 26 – 27 April; 2. Fire Blight Infections Extended into 15-19 April After The 12 – 14 April Initial Infection Predictions

1. APPLE SCAB, CEDAR APPLE RUST, POWDERY MILDEW. With new oncoming rains predicted in weather forecast as of this moment, will lead to apple scab and cedar-apple rust infections on 19-20 April and on 26 – 27 April. The latter rain prediction is too far ahead in time and we do not rely on any prediction beyond 48 h into the future. So this 26-27 April predicted rain can change i.e. vanish from forecast, but if it doesn’t it is still useful for tentative planning on your next fungicide spray. Keep looking at the apple scab prediction models to see if this rain will eventually lead to infection or not. With these oncoming rains we believe that cedar apple rust galls will emerge and allow inoculum to spread from red cedars to apple so protection with DMI fungicides in mix with mancozeb will be critical (Inspire Super, Procure, Cevya, Rally, Rhyme, Sonoma, Topguard, etc.). Make sure to stop mancozeb at 77 days before harvest for early maturing cultivars. The DMI fungicides will cover you for scab rust and powdery mildew.

In regions where you think Rally is not that effective due to potential resistance, add Microthiol Disperss to your spray tank in addition to DMI fungicide and mancozeb. Another tank mix consideration is either Fontelis, Sercadis, Miravis or Excalia plus mancozeb for scab, rust and powdery mildew control at the same time (most weight for rust control will be falling on mancozeb, so use these mix in areas where rust was not a historical problem).

Juniper-apple rusts can have extended infection periods up until 1st or 2nd cover in Virginia. We are in a year with dry and warm spring when cedar galls have been delayed from emerging out, at least in Winchester area. But with oncoming rains 19-20 and 26-27 April they will emerge and infect.

Warm wet weather at petal fall can also allow Botrytis cinerea 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 stem end rot and 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). 

Conditions for powdery mildew will be very conducive this year as it likes dry and warm weather. Powdery mildew can have an extended infection periods in highly susceptible cultivars until the first or second cover too, so reserving DMI fungicides for use from pink bud until second cover in these cases is necessary. If powdery mildew was not an issue last year, Inspire Super or other DMI plus mancozeb would also be my recommendation for the first cover (10 days after petal fall).

If powdery mildew was more of a problem last year for you, use SDHIs: either Fontelis, Sercadis, Miravis or Excalia plus mancozeb (3 lb/A). Add a surfactant to SDHIs as it allows their longer residual activity. For scab, consider using Axios this year – it is a new fungicide in FRAC 52 group and is very effective. Only three applications per year are allowed. Incorporating Axios into spray program will help prevent fungicide resistance to FRAC 3 (DMIs) and 7 (SDHIs). At this time of the year systemic fungicides here are much needed for severe scab events like we expect 19-20 and 26-27 April. With warmer temperatures the green tissue will increase its surface area intensely and the DMIs and SDHIs (if you add surfactant) will enter your leaves while they expand and protect them through the multiple rains. The idea is to have enough fungicide residue cover to stay on the leaves through the oncoming infection periods.

If rust is not of a concern for your location, as maybe no cedars are close to you, mancozeb (3 lb/A) plus Fontelis, or Sercadis, or Miravis, or Excalia, would be a better choice for scab control (all these single-site SDHI i.e. group 7 fungicides will not be effective to control juniper-apple rusts).  This blog is based on the outputs from the NEWA apple scab model available at https://newa.cornell.edu/apple-scab

2. FIRE BLIGHT. As predicted in our previous blog: Fire Blight Infections 12-14 April in All Virginia as Announced Last Week we had infections on 12-14 APril but we also went into an extended block of continuous fire blight infections from beyond 14 April, i.e. all the way from 12 – 19 or 13-18 April (varies depending on a location in Virginia). In these conditions it was crucial that you used the function “Streptomycin Spray Date” in the NEWA model, by typing in the date of you latest streptomycin spray, to help you decide when to spray your next streptomycin application. Adding the date of your streptomycin into this box of the NEWA fire blight model recalibrates the EIP value and box color and shows you when the infection will occur next (EIP 100 or above, and dark red color of the box). If your apple trees were still in bloom during this period 12 – 19 April or 13-18 April, multiple streptomycin applications were needed to protect your crop. This is because high temperatures triggered new flower opening every day, and these new flowers were not protected with streptomycin you applied 2 days ago. Now, I know that frost has severely affected some apple cultivars and significant amount of crop will be lost, but as we have seen at the last extension meeting in Jenkins Orchard, a lot of lateral flowers kept on opening on these varieties and needed protection from fire blight. Also, not all cultivars were in full bloom during 8 April frost. Thus, using fire blight models to plan your multiple streptomycin applications was critical this year as fire blight infections were predicted in many locations from 12 – 19 April or 13 – 18 April. The only way to go through periods with continuous infections like this is by using the function “Streptomycin Spray Date” in the NEWA model. Keep in mind that if you have a young orchard, with long-lasting or so called rat tail bloom, using the NEWA fire blight model to time streptomycin spray applications is necessary until no flowers are visible anymore on trees.