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Healthy Fruit 1998 Vol. 6:6

May 6, 1998


Time Critical!

  • Maintain scab protection
  • Evaluate bloom and set for early thinning decisions

It's Wet and Bloom

The rain and fog have kept at it for going on five days. After today (May 6, 1998) there may be a little relief for most of state tomorrow, and for the next few days, though showers are still in the forecast.

Bloom has progressed quickly. It hasn't been especially cool (high 50's and 60's) so tree growth has kept going during bloom. Apple and other fruit development is generally about 2 weeks earlier than usual at present.

McIntosh growth stages for various

  • Hawley tight cluster
  • Ashfield bloom
  • Belchertown early petal fall
  • Northboro late bloom
  • Sterling late bloom
  • Wilbraham petal fall

Scab Summary

There have been very few infection periods at the HRC this year. Two, both rated as "low risk", to be exact. Looking at abandoned trees, a few scab lesions can be found, but certainly not many. It appears that we have largely escaped the early-season pressure seen further south. That is, unless "mistakes were made". There is one report of scab in a commercial orchard with a history of scab.

This past week the situation got a lot worse. The scab pressure has been extremely high. First of all, inoculum levels have peaked. There's plenty of "soft", susceptible foliage. And of course, it has been rainy and foggy.

Fungicide use gets extremely complicated right now, with extended wetting and bloom at the same time. Here are some of the issues.

The prolonged rain releases scab spores, and gives them plenty of time to infect. The relative warm temperatures, in the 50's and 60's, keep the spores maturing and releasing. At the same time, the rain is washing fungicide from leaves. And the trees keep right on growing, outstripping the protective layer of fungicide. So, by mid-bloom, 5 days or so from the last fungicide application, fungicide protection is weak to gone.

One option is to hope that an SI fungicide can be applied at petal fall, and that petal fall will come within 10 days of the last fungicide application. With this approach, remember to use at least TWO SI applications after petal fall. Also apply a contact fungicide with the SI to maintain fruit protection.

A second option is to apply a protectant fungicide of some sort during bloom. Putting on anything during bloom isn't very popular, particularly since the fungicide can't be mixed with an insecticide. Most years a bloom fungicide is not necessary. This year may be an exception. This year, consider it if you are not going to use two SI applications following petal fall.

Captan offers the most effective broad-spectrum management of scab right now. (Take a look at last week's article on scab.) But captan use is complicated by oil, which may go on with Agri-Mek. Is there a phytotoxicity problem? The general safety guidelines say "do not use captan within 10 days of an oil application". However, there are several reasons to consider shortening that interval to 7 days or even a little less. First, the amount of oil applied with Agri-Mek at petal fall is only a quart, compared to the 1 to 2 gal. applied earlier in the season. Temperatures are more moderate than they are earlier in the year (hopefully). And good scab control can be gotten even if the rate of captan 50 is cut back to 1 lb./100 gal.

The EBDC fungicides offer good scab protection, though they should be used at full rates to get the most benefit. They don't have oil problems, but they may interfere with mite biocontrol. This year, with all the wet weather, if you are using oil, an application of EBDC at petal fall, or even 2 applications at bloom and petal, will probably have minimal effect on mite predators.

An SI fungicide alone is not a good idea. At petal fall, it becomes a worse idea. Issues of resistance and activity spectrum aside, SIs don't protect fruit as well as foliage, and may lead to a situation where leaves don't get infected but fruit do.

This is a high-risk period for scab. If captan isn't an option, EBDCs are the next best one. If you can't see spending all that time on a single fungicide application, but are still worried that protection isn't adequate, consider an alternate row application.

Fire Blight Problems?

In the past, the only real option for predicting fire blight was the Univ. of Maryland/USDA model, Maryblyt. This has generally worked well. It has been used almost exclusively as a computer program, and a number of people who might find it useful have not tried to use it. More recently Tim Smith in Washington state has been working on a model for fire blight in the Northwest. It uses a combination of inoculum risk, and potential for bacterial build-up and infection in bloom. Last year, this model showed promise in Maine. It doesn't require a computer, but simply a record of high and low daily temperatures during bloom.

By Maryblyt's estimation, we have had moderate risk of fireblight, starting over the last weekend in many areas. On the other hand, the Washington model rates the risk of fire blight as low. Maryblyt assumes a relatively uniform population of bacteria at the beginning of the season, and estimates the inoculum build-up before bloom. This year, Maryblyt estimated that the bacterial population was high going into bloom. The Washington model estimates inoculum using the history of fire blight in the vicinity of the orchard being evaluated, and ignores build-up prior to bloom. It says that temperatures during bloom so far have been too low to cause fire blight problems. Take your choice.

Streptomycin applications are most effective if applied just before or within 24 hours of high to extreme infection risk. That's why we have models to estimate risk.

European Red Mites

Hatch of overwintering red mite eggs has been seen in all areas of the state. However, nymphal levels remain fairly low, suggesting that early-season oil and prebloom miticide routines have been fairly successful thus far.

The availability of Pyramite as a mid-season rescue material should not carry with it a false sense of security. Maintaining early-season mite control is critical in keeping summer populations at bay, as numbers of mites can increase exponentially through the growing season. In general, a single oil application will kill 60-70% of overwintering eggs and a double oil program will kill 90-95%.

Without regard to the early-season management program used, we encourage growers to monitor for mite population buildup on a weekly basis. With such monitoring, we recommend the following treatment thresholds, developed in New York and intended for use in deciding when treatment is warranted, beginning at petal fall.



Time Frame



Action threshold based on % leaves with motile mites



May 15-June 1*






June 1-June 15*






June 15-July 1*






July 1-July 15**






July 15-August 15**




* Take middle-age fruit cluster leaves
** Take middle-age leaves from anywhere 

Plum Curculio Efforts

This year is the second of a major effort across the state to monitor the buildup of plum curculio populations. The study involves the use of black pyramid traps intended to intercept PC moving towards tree trunks and cylindrical twig-mimic traps intended to monitor PC population levels within the tree canopy. At each monitored site, trap captures and fruit damage will be assessed every 3-4 days.

Thus far, in three monitored orchards at 25-50% petal fall (5/5), low numbers of PC have been captured in the black pyramid traps, as immigration has likely been inspired by the relatively warm, calm, humid evenings of this past week (5/1-5/5). Given that the curculios are stirring in the hedgerows, movement into orchards will begin in earnest when the temperature exceeds 70o for a couple of consecutive days.

From year to year, there is no substitute for each grower monitoring PC hotspots on a daily basis: examining fruit for signs of feeding and egglaying. King fruit are likely to receive the earliest injury, and growers should be prepared to move into action soon after petal fall.

Petal Fall OP Sprays

There are two basic approaches to the first application of pesticide against plum curculio, border row and whole-orchard spraying. Unfortunately, there is no sure-fire way to decide which will be the most efficient. In general, if the weather is conducive to high levels of PC activity (2-3 days above 70o or 2 days above 80o) then a whole orchard spray is probably in order. If the weather after the first detection of PC feeding or egglaying in the orchard is cooler, then the invasion is slower and a border row spray is probably sufficient. In deciding which strategy to employ, another important consideration is the presence and activity of tarnished plant bug and European apple sawfly.

Although TPB have been a no-show in most orchards this year, other parts of New England have seen moderate populations developing over the past couple of weeks. As reported in last week's issue of Healthy Fruit, TPB can stay active in the orchard through bloom, and damage during this time can be significant. Plant bug monitoring traps are not a good indicator of activity during bloom, so growers who traditionally have high TPB populations should inspect blossom clusters through bloom to monitor for increases in TPB damage. The window of opportunity is currently open for sawfly activity as well, as most varieties in most areas of the state are between early bloom and early petal fall. Thus far, no monitored orchards have exceeded thresholds for a treatment specific to EAS, and the petal fall PC application will likely take care of the EAS that are present.


Leafminer adults have continued to build on red sticky trunk traps. Despite the wet weather during bloom, first-generation egglaying is apparent on blossom cluster leaves. Research over the past 20 years has shown that LM populations can increase 7 times from first to second generation. Given this, growers who are seeing trap captures exceeding threshold levels would do well to consider a petal fall application of either Provado or Agrimek.