Guido Schnabel, Brett Blaauw and Phil Brannen
Late frost has hurt many varieties in South Carolina and Georgia this year. The varieties that escaped the frost, the late bloomers with high chill requirements, still may have fruit and will probably go on a regular spray program, even if chill requirements were not met and a reduced crop load may occur. But what should be the plan for varieties that won’t have any fruit to pick? These blocks still need to be maintained in some fashion, or pests and diseases may get the upper hand. In large part, the goal is to maintain tree health and reduce buildup of inoculum for next year – at the cheapest possible price.
Producers will need to protect the trees from damaging insects (scale and borers for example), so that expense is nonnegotiable. Unsprayed trees might experience premature defoliation due to rust or bacterial spot, but this will depend on environmental conditions and geographic location. Middle and southern Georgia counties are more likely to see rust and subsequent premature defoliation. Bacterial spot will cause early defoliation on susceptible varieties if not managed. Bacterial canker might be an increased issue due to the cold damaged limbs and trunks, but there is not much one could do about it anyway.
The easiest approach is to go on a two-week spray program with 10 lbs/A or so of sulfur (90%) tank mixed with an insecticide (see below). When temperatures climb above 80F, sulfur may cause some phytotoxicity depending on cultivar and spray interval. Scout your trees, and when damage is getting to unacceptable levels, apply the lower labeled rate of captan for continued disease control without further damage. Where bacterial spot is anticipated, apply copper in tank mixes with sulfur. If copper phytotoxicity becomes an issue, after prolonged periods of no rainfall for example, discontinue applications. Knock pathogen-infected fruit to the ground.
As mentioned above, the two-week spray program needs to include a rotation of insecticides that primarily target scale, borers, and Oriental fruit worm. We recommend a rotation through the season with Assail (acetamiprid; neonicotinoid) at 6 fl oz, Mustang (zeta cypermethrin; pyrethroid) at 3.8 fl oz, Baythroid XL (beta cyfluthrin; pyrethroid) at 2 fl oz, and Diazinon (diazinon) at 2 pt [NOTE: if fruit is available and there is any intention of harvesting it, do not use Diazinon]. Additionally, even relatively small numbers of unmanaged fruit in an orchard can act as a potential reservoir for plum curculio, so incorporating Imidan (phosmet) at 2 lb into the rotation would be recommended in such a case.
Mating disruption for peachtree borer and lesser peachtree borer is still highly effective as a management strategy when the orchards within the areawide are under disruption. However, it is completely understandable if such an investment is difficult in a season like this or if neighboring orchards are unable to treat with mating disruption. If mating disruption is not used, continue with a fall, handgun application of an insecticide to targeting the trunks/scaffold limbs. Note that with chlorpyrifos banned, alternative options include: Cormoran (20 fl oz/acre), Rimon (20 fl oz/acre), or Asana XL (14.5 fl oz/acre).
Additionally, all blocks, bearing and non-bearing, should continue to receive two dormant oil applications every year. Very thorough spray coverage is essential for effective scale control, striving for 150-200 gal/acre with a 2-4% superior oil solution.
While the above recommendations are based on solid data and experience, there are other considerations we are not that certain about. A reduced spray program may allow for more inoculum (infectious fungus) to accumulate over the course of the season. We cannot be sure what the consequences of that would be.
During leaf drop in the fall and when/if conditions are right (prolonged periods of wetness), there might be an increased chance (more season-long inoculum buildup) that wood pathogens such as Phomopsis and bacterial canker get into the tree through the wounds created from leaf drop. It is possible (but data is scarce and not consistent) that applications of captan OR chlorothalonil OR lime sulfur mixed with high rates of copper applied PRIOR to any significant precipitation (at 20% and 80% leaf drop) will help to reduce these threats. Also, don’t forget that leaf curl is controlled through late fall/early winter fungicide applications.
The potential buildup of inoculum may require more efficacious fungicides for both scab and brown rot management next year.