The 2018 peach bloom period in New Jersey, about 10 days late this season, is now coming to a close. Shucks are splitting and detaching, exposing the newly formed fruit to the outside world for the first time. And the bacterial spot pathogen, Xanthomonas arboricola pv pruni, has been patiently waiting in the overwintering spring cankers and black tip cankers for its chance to “be fruitful and multiply”. Warm, wet, and windy conditions will allow these bacteria to increase in numbers and be transported to the fruit and leaves to form this season’s first (primary) infections.
Along with oxytetracycline antibiotic (FireLine, Mycoshield), fixed copper-based bactericides (Kocide 3000, Badge X2, Nordox, Champ, COC, etc…) constitute an important component of disease control measures for peach bacterial spot. Sprays of these two types of materials, particularly during the 3-4 week period following early shuck-split, are important to prevent significant infection and crop loss. Unfortunately, copper is also phytotoxic to peach foliage, so significant defoliation can occur if the rate and frequency of application are too high. However, copper is quite toxic to bacteria, so only 0.5 oz of actual (metallic) copper per acre is needed to obtain control during the post-bloom period.
Below are some of the do’s and don’ts for successful use of copper in your peach bacterial spot management program.
- Do use sufficient spray volume to obtain best coverage possible. The recommended volume for air-blast sprayers is 100 gal per acre.
- Don’t applying copper under cool, slow drying conditions, as this promotes movement into plant tissue, thereby augmenting phytotoxicity
- Do select copper materials that have small particle size (~ 1-2 microns). In addition to improved coverage, smaller particles tend to adhere better to plant surfaces.
- Don’t tank-mix coppers with products that are acidic.
- Do adjust spray water pH to be ≥ 6.0. Acidity (lower pH) will solubilize the copper, causing greater foliar injury and less residual activity.
- Don’t use aqueous liquid formulations of copper. These have soluble copper that readily injures plant tissue. Note the copper sulfate in Bordeaux mix is such a material, but addition of lime in the mix “fixes” the copper, making it less soluble.
- Do apply copper materials before an infection (wet) period. Copper acts as a protectant, so it must be present on the tissue to intercept and kill the bacteria before they enter the plant.
- Don’t expect copper to re-distribute to new tissue during subsequent rains. Copper materials tend to stick where they hit and not spread much across a leaf or fruit.
- Do alternate copper with oxytetracycline applications. This approach reduces copper phytotoxicity, has a lower cost than an all oxytetracycline program, and provides for resistance management.
- Do apply copper and oxytetracycline bactericides on a weekly basis, ideally timing the spray to occur before a rain. Past research has shown that a 7-day interval provides the most consistent and effective control. Fewer applications at extended intervals and higher rates has not been as successful.
