With the recent and quick transition to warmer spring temperatures, disease prevention measures are soon to be a top priority in apple orchards. If this warming trend continues, I anticipate bud development progressing beyond bud swell and silver tips in the upcoming weeks. To minimize the impact of diseases in your orchard, there are proactive steps you can take now to reduce inoculum. Due to the early start of the growing season, it will be important to get the most out of your fungicide and bactericide sprays this season, ensuring you have full protection during the most critical periods of plant susceptibility and pathogen activity. These susceptibility windows may vary in duration depending on environmental conditions and progression of apple phenological stages. Focus on targeting your preventative sprays during the most critical periods of infection for each of the diseases you are trying to control.

Preventative sprays

• Dormant copper sprays are recommended to reduce pathogen inoculum for fire blight and apple scab.
  • A full rate of a fixed copper should be applied no later than green tip for fire blight and apple scab inoculum reduction.
    • Read more about copper formulations at the link at the end of this article.
    • *Examples of fixed copper include: Cuprofix Ultra Disperss, Kocide 3000, Champ, and the list goes on.
  • Copper may also help reduce overwintering inoculum of other fungal diseases such as apple leaf blotch.
  • Copper can cause fruit finish issues if applied after green tip. If you do apply copper after this stage, lower rates of copper should be used to protect against phytotoxicity.
  • Applications of copper under cool, slow drying conditions can worsen phytotoxicity.
  • Do not tank mix copper with foliar fertilizers or phosphorus acid products. Check the pH of your spray solution, as low pH will allow copper to become more soluble, increasing its residual activity and therefore increasing the potential for phytotoxicity.
  • Many growers will also use this early application timing to apply oils for insect control. Oil and copper can be mixed as the risk for phytotoxicity is low when little to no green tissue is present. Discontinue this mixture as green tissues begin to emerge to avoid phytotoxicity.
• The application of a 5% solution of agricultural grade urea (46-0-0) to the orchard floor (40 lb. per acre in 100 gal. of water) can help reduce apple scab inoculum by providing nitrogen to help microorganisms decompose leaves.
  • Urea can be applied to leaf litter in late autumn and/or early spring.
  • Note: Urea provides nitrogen so adjust your fertilization program appropriately.

Sanitation checklist

• Prune out dead, damaged, and diseased tissue, including fire blight “holdover” cankers. Prune during dry periods.
• Remove mummies from the tree canopy – this will reduce the inoculum of several summer rots including bitter rot and black rot. If you can’t remove the mummies from the orchard and destroy them, throwing them in the row middles will still cut down on fungal inoculum in the tree canopy.
• Dispose of pruning’s away from the orchard perimeter by chopping, burning (where permitted), or burying.
• Remove dead stumps and brush piles from the orchard to prevent fungal inoculum buildup.

Silver leaf prevention

• The silver leaf pathogen releases spores during rain events in the fall and early spring (spore discharge can take place at temperatures 39-70°F).  Refrain from pruning during wet periods to minimize pathogen spread and creation of new wounds (i.e., points of infections) during active spore dispersal periods.
• Eliminate branches with conks (shelf fungi) by cutting at least 4 inches below areas where bark staining is visible. This prevents spore production and limits fungal spread.
• Simply removing the conks themselves won’t prevent fungal growth or save the affected tree, as extensive wood infection and damage often precedes conk appearance.
• In plants heavily affected by silver leaf symptoms, removing all symptomatic branches may not be feasible, as the loss of numerous branches could potentially exacerbate the disease.
• Trees may exhibit symptoms in one year but show signs of recovery in subsequent years. Rather than pruning symptomatic branches, consider flagging diseased trees and monitoring them. If trees do not recover and/or numerous conks are observed, those trees should be removed.
• Trees affected by severe silver leaf infections leading to mortality should be promptly removed from the orchard.

*Mention of a product is not an endorsement.

Resources:

• Copper formulations for fruit crops: https://www.canr.msu.edu/news/copper_formulations_for_fruit_crops
• Midwest Fruit Pest Management Guide 2023-2024: https://ag.purdue.edu/department/hla/extension/_docs/id-465.pdf

This article was posted in Apples, Disease and tagged Apples, disease, disease prevention, Leslie Holland.