1. Major Scab and Juniper Rust Infections of 4-5 May; 2. Fire Blight Post-Infection Rescue Options

1. Yes, we had major apple scab and cedar-apple rust infections on apples May 4-5. If the rusts are your major problem in addition to scab, and you did not cover with a fungicide before these infections of 4-5 May, you can apply mancozeb plus a DMI fungicide today as DMIs can reach back 96 h and control these scab infections. However, be careful and reserve the this kick-back use mode of DMI-s to only urgent situations, for e.g. if you did not have a chance to spray before the rains 4-5 May due to tractor or sprayer breakdown. Otherwise always apply these materials before the predicted infection. Best choices are Inspire Super, Rally, Rhyme, Sonoma, Indar, Procure or Cevya plus mancozeb (3 lb/A). If the rust is not your primary concern,  and besides scab powdery mildew is, use Fontelis, Sercadis, Miravis or Excalia plus mancozeb (3 lb/A) and you will control both scab and powdery mildew.  Keep in mind that SDHI (FRAC 7) fungicides listed in the latter sentence will not have kick-back activity and should have been applied before the rains of 4-5 May. Adding LI700 to them wood have boosted the SDHI effect against the apple scab fungus.

2. We plan to report soon when the primary scab season will end. However, the fungicide coverage needs to be continued on all locations up to 2 weeks after the date of primary scab season end, when we report it. It will be difficult for us to report this date in 2024 for Central and Southern Virginia as we do not have any weather station subscribed and thus connected to a RIMpro Venturia model in these locations. The one we do have is not accurate as the leaf sensor is giving inaccurate data. Having a station connected to RIMpro is needed so as to be more accurate to call it a day wit the scab. The NEWA’s scab ascospore release model is underestimating the primary scab season end. Application of tight interval fungicide applications will be needed up to 2 weeks after the date of primary scab season end because you do not know how good your coverage was in the major, earlier spring scab infection events and the continued rust infections warrant fungicide applications ahead. If any scab lesions arise now (this is the time of the year when symptoms will become visible) due to previous  poor fungicide coverage issues or due to a missed fungicide applications before any rains during early spring, it will be much harder to have perfect looking fruit crop until harvest.

4. If any cider apple cultivars are still in bloom, fire blight infections were and will be highly possible from 6-9 May. Please look a the free EIP model to guide your streptomycin applications: https://newa.cornell.edu/fire-blight  Also, use the streptomycin date to type in the last application of this material so as to determine the next needed. If by any chance you have a small number of trees left with flowers on them, you can remove them with a hand to prevent infections. By an large, most commercial fresh apple farms are done with bloom so if your fire blight control during bloom was good, you should not expect problems on shoots. Just in case, if fire blight symptoms do occur, and you see flowers starting to blight, here is the only rescue option based on using EIP model historical infection data to which you should orient to apply these application up to 2-3 days after the infection event you did not protect against:

FIRE BLIGHT RESCUE TREATMENTS: if no streptomycin was used when major infection event has occurred on flowers (based on EIP model in NEWA’s website) spray apply 12 oz/100 Apogee or Kudos at 2 to 3 days after infection event to prevent shoot blight progression and the resulting canker formation on wood. This PGR works by inhibiting hormones of growth in the green tissues and thus reduces the vegetative tree growth i.e. shortens the shoot length. More importantly, prohexadione-calcium shortens the period of terminal shoot growth and reduces susceptibility of shoots to fire blight. Because the trees are no longer susceptible to fire blight infections when shoots stop its growth i.e. when terminal buds set on shoots, application of Apogee or Kudos will speed onset of terminal bud formation, thus reducing the period of shoot susceptibility to infection and reducing the chance for spread of fire blight on other shoots. Use Apogee or Kudos even if you see first symptoms. If symptoms are either few or numerous, meaning 1 to 20 or more blighted flower clusters per tree, mature bearing orchards should be sprayed with 12 oz of Apogee per 100 gal, preferably in a dilute spray, to get a good coverage of all shoots. This will help reduce the number of fire blight cankers developing on wood by invasion from infected flowers and shoots. The use of this PGR is especially warranted in high-density apple orchards (1-8 years old, any cultivar) and with susceptible cultivars, on trees older then year 8, where cankers can lead to tree death by girdling the central leader or rootstock. In the case you see symptoms uniformly spread in all orchard parts, which should indicate 30-40% incidence on shoots or more, this is a serious active infection and your priority must shift from growing fruit to saving trees from fire blight with the aforementioned PGR.

Management for shoot blight should also include spraying a copper material at 0.2 lb/A of metallic copper equivalent throughout summer, in combination with pruning removal of symptoms (if slow drying conditions were highly likely to occur after copper application, as even this low dose could cause fruit russetting at slow drying conditions). After the copper spray, work on fire blight symptom removal by pruning, which should be done only on cool and dry days, with no dew on the grass on the orchard floor or the trees. Copper will only kill fire blight bacteria present in bacterial white, yellow or orange ooze droplets visible on the surface of both blighted and freshly infected green clusters and shoots. Droplets of fresh bacterial ooze, which spread this bacterium to uninfected green tissues, emerge on infected green tissues much before the fire blight symptoms are visible. This means that bacteria usually have an ample time to extensively spread to the surrounding, uninfected late blooming flower clusters and shoots before the first shoot strikes are visible. Due to recommended low copper doses and the fact that copper does not kill bacteria inside the infected tissues, the effect of copper will be to slow down or prevent further spread of pathogen to new, uninfected shoots. More spray applications may be needed to achieve the desired effect. High rates of copper and copper applied just before wet and slow-drying conditions could injure fruit skin up through at least mid-June. Cultivars differ in their susceptibility to copper related russetting. You will need to take into account how prone is each of your cultivars to russetting by copper, watch the weather forecast to avoid applying copper before wet and slow drying conditions, and apply low-doses of copper probably multiple times with lower water volume (to promote quick drying after spray). When considering use of copper, a tank mix of Cueva and Double Nickel might be the best choice since there are some indications that this mix may be less prone to cause fruit russetting.