The use of copper for disease control on apples has always been viewed from two directions as the very positive disease-control benefits of coppers are counterbalanced with the risk of phytotoxicity to trees, most notably through russeting of apple fruit. Two major apple pathogens, the fire blight pathogen Erwinia amylovora and the apple scab pathogen Venturia inaequalis, are highly sensitive to copper, and we have no reason to believe that copper is a material that is at risk for resistance development in either of these pathogens. Thus, the potential benefit in using copper early for disease control is high. The only limitation is application timing as coppers applied after 0.25- to 0.5-inch green tip could result in phytotoxicity including fruit russeting.
Primary inoculum refers to the bacteria that cause the initial infections in a growing season. After colonization of flower stigmas, bacterial populations in orchards can skyrocket, and blossom blight infections can occur with significant losses sure to follow. Therefore, limiting primary infection by starting control practices early is a critical first step in a season-long control program.
The predominant location of overwintering bacterial inoculum for fire blight is in cankers. These cankers are initiated from shoot blight infections occurring in the previous season and represent internal populations of the pathogen. As temperatures warm up in a growing season, cankers begin to ooze bacteria that can then be transmitted to and colonize flowers. Fire blight infections leading to blossom blight are initiated during bloom. Bacterial colonization and infection of open flowers lowers yield and initiates internal, systemic infections of trees that can lead to rootstock blight and death of younger trees planted on susceptible rootstocks. The occurrence of shoot blight (wilting and dieback of actively growing shoots) is also typically higher in orchards where blossom infections have occurred.
Besides pruning and removal of fire blight cankers during the winter, the best method for reducing initial inoculum populations of plant pathogenic bacteria in orchards is to use an early application of copper to cover trees with a "blanket" of copper. This tactic should be reserved for orchard blocks where fire blight has occurred in one of the previous two years, or blocks of highly-susceptible cultivars adjacent to blocks with recent fire blight. Entire trees should be sprayed, not just alternate rows. High rates of copper can be used (about 2.0 metallic copper per acre), with timings immediately prior to the trees breaking dormancy or up to about 0.5-inch green tip. Be sure that the correct rate of copper is used and that sprayers are properly calibrated.
The goal of this management practice is to have copper available to protect the plant tissue from bacterial colonization over time as the tissue develops. Thus, the copper being applied to trees at 0.5-inch green tip will actually be needed during or immediately prior to bloom, when cankers traditionally are oozing. Because the copper must be sprayed much earlier in the season than the dates of its targeted effectiveness, this management strategy can be defeated by rain amounts of 2 to 3 inches between green tip and bloom that wash the copper residues off trees.
In theory, any formulation of copper should be effective in disease control (copper sulfate, cupric hydroxide, copper oxychloride, etc.) in that each delivers what is needed for disease control, namely, free copper ions. These copper ions are taken up by cells and cause toxicity by non-selectively denaturing proteins in cells. Dr. David Rosenberger of Cornell University recently published an excellent article on copper formulations. In the first full paragraph on page 8 of this issue, Rosenberger gives a thorough review of the advantages of more finely-ground copper formulations as compared to coarser-ground coppers for this delayed dormant application for fire blight control.
First apple scab spray
You can double-dip in disease prevention with this early-season copper application as copper compounds, although not quite as effective as Captan or EBDCs for apple scab control, can play a role in scab management as the first spray of the season at 0.25- to 0.5-inch green tip. Copper, at 2 pounds metallic equivalent per acre, represents a separate mode of action that is not at risk of resistance development.
The green tip timing is absolutely critical for apple scab management. Although the amount of tissue available for infection is small and the scab spore load is typically small at this early timing, any infection that occurs early will result in severe consequences later in the primary scab season. This is because lesions initiated at green tip will be producing secondary spores (conidia) at a timing between pink and petal fall that coincides with what is typically the period of highest primary spore concentration from overwintering leaves. Thus, early infections can be a killer because they compound the spore load in an orchard, which can lead to significant fruit infection. It is true there is not a lot of green tissue present in orchards at green tip. But, be certain that spores of the scab fungus can find that tissue; for any spores released at this timing, that is their primary function. And once they land on that susceptible tissue, infection, producing a lesion and conidia becomes the primary function.
Finally, beware of phytotoxicity!
Copper phytotoxicity can occur on apples, with the predominant copper problem on apples being increased fruit russeting. Although phytotoxicity is a potential problem, if used wisely, copper bactericides applied early will effectively begin the 2012 disease management season and lower primary fire blight disease inoculum and control early apple scab.