Temperature Fluctuations Influence Timing and Need of Early Season Sprays

Across the region tree phenology ranges widely.  Some orchards are still at early silver-tip on Honeycrisp, while other orchards are approaching tight cluster on Zestar, Ida Red and McIntosh. There are two warm nights ahead of us and high temperatures should reach 80F in northeastern Illinois Saturday.  This will result in significant growth across the area by Sunday night.  More storms predicted for Tuesday, April 18 and getting adequate protection on new growth may be a challenge.

Disease Updates
Apple scab
On April 9 and 10 some orchards recorded an infection period (IP) although some were not yet at green-tip on McIntosh.  The next possibility of an IP will be the thunderstorms April 14-15, especially with the higher temperatures Friday evening and Saturday.

The developmental threshold for the apple scab fungus is 32F. Our average daily high and low temperatures for this date are approximately 60F and 40F respectively. That translates roughly into an additional 125 DD per week (of the 900+ required for complete ascospore maturity). All areas of our region should now be beyond the 5% ascospore maturity used by some as a benchmark for the start of scab protection programs.

Fire blight
Several orchards experienced a short hailstorm last weekend on April 8 and 9. Although one and two year old wood could have suffered trauma, the probability of fire blight infections was low, since cankers were unlikely to be producing bacteria.  Applying streptomycin post-trauma was not advised after this hail since temperatures during the subsequent 24 hours were too cool, and insufficient green tissue available, for absorption of this product.

Powdery mildew
The first fungicide application for powdery mildew control is recommended between tight cluster and pink. Early cultivars are approaching tight cluster and growers that experienced problems with this disease in 2016 should be considering their options now. We will also discuss powdery mildew next week’s update prior to the April 25 call.


Nutrient Applications
Grower submitted questions this week included whether calcium should be included in pre-bloom fungicide applications.  Extension recommendations suggest foliar calcium sprays applied to address bitter-pit in fruit should begin during bloom.

On our introductory call March 31 Dr. Amaya Atucha discussed the 2016 fall weather was conducive for trees to maximize their sequestration of nutrients and that when trees come out of dormancy, they are drawing on these nutrient reserves until they begin pushing blossoms.  Once trees enter bloom, carbohydrate demands can out-strip leaf production.  Right now, whether cool or warm, cloudy or sunny, trees will not benefit from additional nutrients.  I think the speed at which buds have responded to the short windows of warmth we’ve experienced is a reflection of this capacity.


Insect Updates
Spring lepidoptera complex
The early lepidoptera species that overwinter as adults should now be flying on warm nights.  These include Spotted Tentiform Leaf-Miner (STLM) and Red-Banded Leaf-Roller (RBLR), two of the species we typically monitor with pheromone traps.  If you have begun trapping for these please let us know how the flights are progressing.  The pheromone for the Green Fruit-Worm (GFW) is also available commercially and has only been used at several orchards in our region.  If you are trapping for GFW we would be interested in receiving weekly pics of the trap-liners, sent either by text or email.  We are interested in confirming the species being caught – as there are three common GFW species that could be attracted to the pheromone.

Two species common early on apples are the Apple-Grain Aphid (AGA) and the Green Apple Aphid (GAA).  Both are minor pests that rarely cause economic damage except on non-bearing trees, where large populations in shoot terminals interfere with growth.  They can however illustrate the differences possible year to year between insect development and plant phenology.  This year, to date, I’ve observed neither species inhabiting fruit clusters or shoot terminals, however in 2012 when many orchards had bud-break in mid-March, numerous GAA were found in the opening buds within the first twenty-four hours.  Why the difference?  Many aphid species have developmental thresholds of 50F; trees can grow at temperatures 8-10 degrees cooler, depending on the levels of dormancy compounds that remain within the buds.  In 2012 between March 7 and March 21, many orchards recorded ten days of high temps in the upper 70F range, resulting in much larger base50 degree-day accumulations as buds opened than we have experienced this year, although we are four weeks later by the calendar.  This is a dramatic illustration of why scouting can be important before applying pesticides, particularly with insecticides.  Most spray guides will tell you when to control a pest by tying it to a specific plant growth stage, however just because a pest is likely to appear at, for example, Pink or Petal-Fall, does not necessarily mean that it’s out there.

European red mites (ERM) and San Jose scale (SJS)
These potentially devastating secondary pests are susceptible to control by pre-bloom oil sprays at 1-2% concentration.  We use the term “Secondary pest” to refer to their potential to be controlled by predators, parasites or pathogens.

Making best use of oil sprays
We have received a number of questions concerning oil applications pre-bloom.  For several years I have been recommending three pre-bloom oil applications, if possible.  The first application is applied as a tank mix with copper or EBDC at silver tip.  The second and third are applied with fungicide applications, at concentrations of 1-2% applied between green tip and bloom. Field evidence suggests we are getting better control of mites and scale with this methodology.

Temperature issue No. 1
Extension publications warn against applying oil applications when it is below 40F.  Why is this?  Oil applied at any temperature will disrupt the layer of waxy cuticle on leaves.  When temperatures are below 40F two problems can occur:

  1. The oil/water tank mix exists as an emulsion, i.e., micro droplets of oil suspended in a matrix of water molecules, which is maintained by agitation. If sprayed onto leaves when it is below 40F, this emulsion reportedly can break down into separate components.  Instead of a uniform layer of oil drying on the leaf, the micro droplets can coalesce, leaving some spots with heavier amounts of oil and less or none elsewhere.  This will result in uneven distribution of not only the oil but also the EBDC or copper fungicides suspended in the tank mix.
  2. There is little plant metabolism below 40F so the disturbed wax layer is not as quickly repaired allowing more desiccation to occur. Remember, any possible phytotoxicity from oil is exacerbated by a combination of the length of time required for the oil to dry and the speed at which the waxy layer is repaired.

Temperature issue No. 2
Oil can be safely applied above 40F, however even at temps below 50F there is less activity against ERMs or SJS, the primary targets.  Efficacy of our oil applications is dependent on the developmental thresholds for these pests.  If our target pests are not respiring, they are less likely to suffer mortality.  There will be some delayed mortality from the dissolution of these pests waxy cuticle at lower temperatures, but the higher the temperature, the more effective the oil is.  At temperatures between 40F and 50F, I think oil should primarily be considered a spreader-sticker, improving the distribution and adherence of whatever fungicide was applied.

Spray volume (GPA)
Extension publications typically recommend applying oil at a minimum of 100 Gallons per Acre (GPA) water.  The reality is that that few growers are able to manage this because of the increased time required to cover the orchard.  Most growers we work with are applying oil between 75-80 GPA, with some reducing that to 60 GPA in high-density blocks.  I also know growers that routinely apply oil at 40 GPA or less, and have done so for many years. While it seems common sense that greater water volumes are more likely to penetrate the nooks and crannies on bark, and thus do a better job of reducing ERM and SJS populations, there are orchards out there who apply oil at lower volumes and have not had problems managing these pests.  The take home is SJS and ERM and are like apple scab, they are quite tenacious.  Once an economic population has established in your orchard, it can take several years of “pulling out the stops”, to bring populations back to manageable levels.  Applying oil with more water per acre is always the first step.