Current DD Accumulations

Growth Control Short Course: Part III

Our final approach to growth control is the use of ethephon (Ethrel®, Aventis CropScience). This approach is only appropriate where a significant crop loss has occurred, such as when there is frost or hail, because ethephon can be a strong (and unpredictable) thinner. Ethephon controls growth by releasing the naturally occurring plant hormone, ethylene. Ethylene has many side effects on tree growth too, so use it judiciously. Timing of ethephon application for growth control is when new shoot growth is 4 – 6 inches long, and at a rate of 1-2/3 pints per 100 gallons dilute spray. One month after application check your re-growth, and if more control is needed, a second application at 1 pint per 100 gallons is recommended.

Petal-fall Thinning Sprays

We have encouraged growers to started thinning early with a petal-fall spray. In blocks where bee activity was good and there is no visible frost injury, we still feel that this is a good approach that should be initiated now. However, many areas in New England experienced frost last week that resulted in fruit bud damage ranging from 5% to 95%. In these orchards, it is appropriate to assess the extent of injury thoroughly before deciding on any thinning action.

Many flowers should be cut to look for black or brown centers in the receptacle tissue. (See http://www.umass.edu/fruitadvisor/clements/frostfreeze.html for picture of bud freeze injury.) It is also necessary to determine if damage is equally distributed in all portions of the trees. Since the coldest air is near the ground, damage is frequently greater at the bottom of the tree. It is also appropriate to estimate the extent of leaf damage. If any leaf damage or excessive leaf curling is present, it is usually best to wait to determine initial set before deciding on a thinning approach. If 10% or less of the flowers are damaged and most of the damage occurs on the lower portion of the tree, a conservative approach is to apply the thinner only in the tops of the tree where frost damage is frequently less.

There are three thinning materials appropriate to use at petal fall: carbaryl, naphthaleneacetic acid (NAA) and naphthaleneacetamide (NAAm). Carbaryl is usually the chemical of choice at petal fall, because it is a mild thinner, and over-thinning is rarely observed. It frequently breaks up clusters and makes it easier to thin with lower rates of other chemicals. NAA is also an important thinning chemical at this time. At this timing it rarely over-thins. The more potent thinner NAA is very useful on varieties that generally are difficult to thin. NAAm is frequently used at petal fall to thin Macoun, early varieties, and varieties that shows severe epinasty (downward ‘wilting') following NAA application. The combination of carbaryl and 4 to 6 ppm NAA is an excellent combination spray where bloom was good and the block has a history of heavy set. The weather for the next few days is forecast to be cool. Therefore, response to petal-fall thinners will be modest, and over-thinning is quite unlikely.

Sawflies

In most orchards, bloom is over and we are into petal fall. Sawfly adults are distant cousins of honeybees (members of the same extended family). Like honeybees, sawflies pollinate apple blossoms. Afterward, however, sawflies move a half-inch or so toward the stem of a blossom and lay an egg in the ovary. They are present on apple trees at the same time of year as are honeybees. Their presence can be monitored with white sticky rectangles that closely mimic apple blossom reflectance.

Private consultants and growers who have used such traps for monitoring sawflies this year report below-average populations across the state. If sawflies had been abundant, then a whole-orchard petal-fall spray of an organophosphate would have been advisable. Because that's not the case this year, even in areas with traditionally high sawfly populations, then they are not part of the equation when deciding on whether or when to apply a petal-fall insecticide (other than Sevin as a thinner).

Plum Curculio

As indicated by captures of curculio adults in our odor-baited traps spread out across many commercial and unmanaged orchards, the pattern of curculio immigration this year is a virtual carbon copy of that in 2000. It began on the same day (May 1) and at the same stage of bud development (tight cluster). Numbers captured to date are almost identical to those last year. Like last year, the weather since petal fall has not been conducive to curculio movement into fruit tree canopies. So even though substantial numbers have moved into at least a few monitored blocks on warm sunny afternoons and evenings, they are hanging out under debris and laying low until the weather turns more humid or wet.

No egglaying scars have been detected yet in commercial orchards, though a few scars have been seen in unmanaged apple trees on fruitlets about 1/4-inch diameter.

Because threat from tarnished plant bugs is essentially over and sawflies are essentially a non-factor, the decision on whether to apply a whole-orchard treatment of insecticide (other than Sevin as thinner) at petal fall or soon after comes down to the probability of curculio injury. There is no simple answer here. If the weather stays cool, windy, and pretty dry as predicted, there's little chance of more curculio moving from overwintering sites into orchards or, if already there, of moving up into tree canopies. A spray applied to perimeter-row trees should be enough to give good whole-orchard protection, as curculio won't be moving from perimeter into interior trees under these conditions. If the weather forecasters misfire and it turns warmer, more calm, and wetter, then curculios may start crawling toward more interior trees and into canopies of perimeter-row trees, and a whole- orchard spray would be best.

If things progress as last year, excellent curculio control should be achieved through May. Last year, the major injury occurred during the last 3 weeks of June, when we got a flush of new immigrants that infested trees no longer protected by insecticide. Our new traps that monitor the course of immigration should tell us the extent of mid- and late-season threat of curculio this year.

Leafminers

At the Prokopy orchard mentioned last week, there was a flush of newly emerged leafminer adults captured on trunk and canopy traps this past Friday and Saturday, when the trees were in late bloom. Numbers captured were about fourfold greater than cumulative captures up to that time. If this same pattern is true for other orchards, then leafminer egglaying and appearance of sap-feeding miners will be much later than normal. So far, a few early sap-feeders have been seen in only a single orchard.

Provado, SpinTor, or Agri-Mek applied at petal fall could be too early to achieve maximum effect. On the other hand, if application is delayed until sap-feeding miners show up in numbers sufficient to evaluate need to spray, application may then be too late to be truly effective. As suggested in last week's message, this may be a year when it's best to hold off until counts of first-generation tissue-feeding miners can be made and then apply insecticide, if needed, in late June or early July against second-generation eggs and early sap-feeders.

Mites

In nearly 30 orchards sampled by ourselves and private consultants last week, fewer than 10% had any newly hatched red mite nymphs showing up on interior fruit cluster leaves. Most growers applied at least one oil spray, and many applied two sprays. So we're off to a very good start, state-wide.

If you monitor interior fruit cluster leaves and counts show an average of 3 or more motile red mites per leaf, then it's time to consider applying an insecticide soon. Several materials can effectively control red mite eggs or nymphs (but not necessarily adults): Apollo, Savey, Vendex, summer oils, and Pyramite. In our judgment, it's best to hold back on using Pyramite until later. Consider Pyramite as a summer rescue material and save it until it's really needed. Using it more than once per year can speed up resistance development and harm some mite predators.

Insecticide Program for Frost-injured Trees

If you were among the many unlucky growers who got hammered by frost and you don't expect to harvest any fruit from parts of your orchard, here's a thought on insecticide coverage on those trees for the rest of the year.

In a word, don't bother to apply any insecticide or miticide unless you see, by early June: (a) horribly high numbers of first-generation tissue-feeding leafminers--numbers so high (for example, 5 per leaf) that parasites are unlikely to achieve reasonable suppression; (b) horribly high numbers of red mites. If either of these two pests gets out of hand during fruit-bud formation in July and the soil is dry, then bud formation could be compromised by the combined stress of these pests and drought. The same applies to leafhoppers on young non-bearing trees but less so to leafhoppers on trees that have already achieved maximum size.

Pear Psylla

Some hard shells are starting to show up, so it's getting late for applying pesticide against soft-shell nymphs. There still is time to apply Argi-Mek plus oil or Mitac against a high population. May be best to save Provado, Pyramite, or Esteem for use against nymphs of the first summer generation, after a reliable assessment of psylla numbers can be made.

Scab in the Sahara

More and more, this year reminds me of the Pacific Northwest, the apple growing parts at least, where scab is a funny disease that happens in the East. The plain fact is that without rain, scab doesn't happen. I don't ever remember getting to petal fall without an infection period.

But don't let your guard down just yet. Remember that the scab infections last year left us with a lot of scab inoculum this year. To our knowledge, scab ascospores don't just fade away. This may be the year we test that, but so far, without any soaking rain, the ascospores are staying put, not releasing, not decaying, and not drying out. The short showers we have had in the state simply do not wet the old, dead leaf tissue that contains the pseudothecia. In order for ascospores in the pseudothecia to release, the pseudothecia have to absorb water, so the leaf has to absorb water. Assuming that eventually it rains, there are plenty of scab spores, and we can expect a potential for heavy infection. We have lots of spores and lots of new tissue.

The least expensive approach to dealing with scab involves the protectant fungicides, captan and the EBDCs. The situation this year may allow the use of the "full rate" (6 lbs. / acre for dry formulations) in a four-application program of the EBDCs. Most years, more than 4 applications of Dithane, Penncozeb, Manzate, Maneb, Polyram, or other EBDC would be required, making it necessary to use a 3 lb./acre rate (or equivalent). These higher rates (1.5 lbs/ 100 gal. dilute) would be preferable to the lower rates (3/4 lb. /100 gal. dilute) in advance of any intense infection period. Besides being relatively inexpensive, another point favors EBDC use at this time: they provide excellent summer- disease control for the early season.

Captan 80 WP at 1.25 lbs./100 gal. (or the equivalent of a different captan formulation) will also provide excellent scab protection. The only problem with it is oil phytotoxicity.

A case can be made for waiting to see whether an infection period actually comes along, and then applying a couple of SI/protectant applications. If an infection period doesn't come, then there's no need to put on any scab application. However, if it rains enough for an infection, it will require two applications of more expensive fungicides to manage the scab.

Wait and See on Blight

All that can be said at this time is to wait and see whether you have any fire blight. In orchards with a history of fire blight in the past two years, the blight-prediction programs showed high risk where rain fell on open blooms this past weekend. At this point, it's too late for streptomycin applications to have any effect.