Fire Blight Visible in Central and North Virginia - Blight Removal Needed; Watch for SB&FS Accumulated Wetting Threshold in NEWA Model

1. Fire Blight. If 1 to 5% or more fire flight symptoms are visible, you have active infections taking place on actively growing shoots. If you see fire blight strikes (flags) in 2021, assume that in the next 3 years from 2021 the bacterium will be probably be present for a long time and your orchard will be at high risk from when the first flower open until terminal bud set on shoots, thus will require protection each spring when models like Maryblyt or EIP in NEWA predict infections risks. If you see fire blight symptoms do not apply streptomycin as the bacterium is in the tissue, so it cannot be reached by the antibiotic, and there is a risk that if you do you will promote streptomycin resistance acquisition in the fire blight bacterium Erwinia amylovora. This will reduce efficacy of this antibiotic, or even worse lead to the loss of its efficacy.  

At this moment, if symptoms are visible, apply plant growth regulator (PGR) prohexadione-calcium (Apogee, Kudos) to reduce susceptibility of not-yet-infected shoots and slow down the invasion shoots by the fire blight bacterium. 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 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. If symptoms are both 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. Management should also include spraying a copper material at 0.2 lb/A of metallic copper equivalent (if slow drying conditions were highly likely to occur after this application, as even this low dose could cause fruit russetting at slow drying conditions). After the copper spray work on fire blight symptom removal. 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 (Fig. 1). 

Figure 1. Orange droplets of bacterial ooze help spread Erwinia amylovora to new flowers and shoots (Photo by S. G. Acimovic, 2021)

 

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 ample time to extensively spread to the surrounding, uninfected 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 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 Cultivar 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.

If available, remove fire blight symptoms as soon as they appear. Cutting out the infected strikes at 18 to 24 inches below the visible margin of infection on dead shoots or flowers aims to remove bacteria that might have penetrated into the symptomless wood far below the infection margin. If these length requirements below symptom edge could not be met, pruning should be done into the closest perennial wood (at least 2-yr-old, and 4-6 inches into this wood if possible). Prune on a cool dry day and never before, during or soon after rain of heavy dew. These conditions will risk spreading fire blight more. All pruned out brush should be left on the ground close to the tree and flail-mowed or removed from the orchard after the branches are completely dry (cuts show no green cambium visible when bark is scraped with a knife). If there is enough space in the mid-rows to leave the brush on the ground, without preventing passage of air-blast sprays with a tractor, copper could also be applied after pruning. During symptom removal, pruning tools should be disinfected at least between each tree with 70% rubbing alcohol or 10% bleach solution. Even though the true benefit of using any specific disinfectant for tools is still debated or questioned, I would recommend using one of these disinfectant solutions. If pruning is done rapidly, as soon as first symptoms occur, continue daily scouting after the first pruning to remove any additional strikes that develop subsequently. 

2. Sooty Blotch and Flyspeck. Many locations in Virginia are close to or have reached the necessary threshold of accumulated leaf wetness hours (ALWH) for SB&FS infection: http://newa.cornell.edu/index.php?page=apple-diseases Please follow this SB&FS model on you nearest NEWA station to determine when will you reach 250 ALWH from petal fall and apply effective fungicides for SB&FS control. Just before you reach this thresholed, my recommendation at this time of the year would be to apply Captan 80 WDG 2.5 lbs/A + Inspire Super 12 fl oz/A, or Captan 80 WDG 3 lbs/A + Prophyte 64 fl oz/A, or Captan 80 WDG 2.5 lbs/A + Ziram 6 lb/A, or Captan 80 WDG 2.5 lbs/A+ Topsin 1 lb/A, or Captan 80 WDG 2.5 + Ferbam 4.6 lb/A. You can also apply the QoI (Group 11) fungicides as they are the most effective fungicides for SB&FS - Sovran (6.4) or Flint Extra (2.9) or Pristine (18.5) or Merivon (5.5) or Luna Sensation (5) oz or fl oz/A + Captan 80 WDG 2.5 lbs/A, but the use of Group 11 fungicides should be reserved more for control of apple bitter rot. From 10 June on, the infection pressure from rots increases. Ideally, Group 11 fungicide + captan should be applied just before a rain following a block of very warm/hot weather which favors bitter rot infections.