Apple pest report

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The Extension apple scout Laura Miller checked 48 blocks for scab on August 8-12 and August 15-18. In each block, 100 shoots, fruit clusters, and water sprouts with approximately 1500 leaves were checked. Of the 48 blocks, 33 blocks (69%) had very low scab with 4 or fewer scabby leaves. An intermediate scab level of 5-18 scabby leaves was found in 14 of the 48 blocks (29%). One of the 48 blocks had epidemic scab.

After apple leaves begin to lose natural resistance to latent scab in mid-September, growers who check 100 shoots per block can get an estimate of the overwintering scab population that will be managed next spring. To check 100 shoots takes about 30 minutes. A scabby block will take less time because once you hit 18 scabby leaves, you can stop sampling.

If you find more than 18 scabby leaves on 100 shoots, that indicates that scab control should start with the first infection period after Green Tip next spring. If you find 5 or fewer scabby leaves and have been on a protectant fungicide program then that indicates a low potential for overwintering scab. If you used zero or only one sterol inhibitor or strobilurin fungicide application this summer, the low scab index value means you can delay the first fungicide spray next spring until the 4th infection period or Pink, whichever comes first.

If you find 5 or fewer scabby leaves and used 2 or more sterol inhibitor (Nova, Procure, Rubigan) or strobilurin (Sovran, Flint, Pristine) fungicide applications this season, the low scab count is good news but until we get a better understanding of the potential for unseen latent scab to persist in overwintering leaves, caution is advised in taking full advantage of a delayed spray approach next spring.

LATE SEASON SCAB INFECTION: The heavy rain this week raises some concern about the potential for late season scab infection. The first point in any scab discussion is that the most important factor is the inoculum level. If your block has little or no scab, then losing fungicide protection in the August 29 - September 1 rains does not create significant scab risk. Scab can only spread from active lesions. So the following points are directed at growers who do have enough scab around to create a hazard. How much scab is that? The best guideline is go out and check 100 shoots, using the 5 scabby leaf threshold described above.

Infection period duration: The required infection period duration for infection to take hold is longer for fruit than the for leaves. (The Mills table is designed for early season foliage.) And the required infection period duration for fruit gets longer as the season progresses.

Schwabe (1982) created a scab infection index for mature fruit shortly before harvest in an orchard with sporulating lesions and unprotected fruit. Index points are calculated as temperature (in centigrade) times hours of leaf wetness. For light-moderate-heavy potential for late season scab infections leading to scab development in storage, the index value minimums are 440, 601, and 1000 index points. This study was done in a high inoculum setting and Granny Smith fruit, but it still stands as the best guideline we have for interpreting late season fruit scab infection periods.

"Light" = conditions that created from "more than zero" up to 6% of the maximum percentage of storage scab.

"Intermediate" = conditions to create more than 6%, but less than 100% of the maximum scab level.

"Heavy" = conditions adequate to cause maximum infection percentage seen in the trials.

At Highmoor Farm, average temperatures on August 29-Sept 1 were near 70F, which is 21C. The wetting period was about 100 hours long. So the infection index value was over 2000, indicating that there was plenty of wetness for any scab conidia landing on unprotected fruit to have opportunity to initiate infection.

Time for lesions to develop: It can take as little as nine or ten days for lesions to appear on new foliage in spring. But mature fruit are a completely different situation.

Bratley (1937) found that fruit inoculated in late August or September did not result in lesions until fruit were in storage.

That study also concluded that apples infected within one to two weeks before storage are unlikely to cause visible lesions during the normal storage life of apples. That conclusion was later refuted by Bratley from observations in 1938 when apples in New England, New York , New Jersey and Pennsylvania that were picked after heavy rains in mid-September developed severe storage scab, while apples picked just before those rains did not develop storage scab (Bratley 1940).

Bratley (1937) found that storage scab develops faster at higher storage temperatures and higher humidity, and that the shortest incubation period for apples infected prior to storage was 23 days, the longest was at 195 days.

Schwabe (1982) found that on apples inoculated with scab going into storage, scab began appearing at 35-45 days at 20C (68F) and at 80 days at 1-2C (34-36F). Storage scab reached maximum at 100-120 days at 20C, and 180 days at 1-2 C.

Tomerlin and Jones (1983) found that the more scab on an apple going into storage, the greater the chance that the degree of scab will increase.

So for fresh sales, it appears that infections this late in the year are too close to harvest to be of major concern for causing visible scab lesions on this year's crop.

For apples to be stored, there is ample evidence that late season rains can lead to severe storage scab.

And of course, late season foliar infections would increase the overwintering scab inoculum on leaves for next spring.

So, given that fungicide coverage was depleted by the August 29-September 1 rains, is it worth the expense and effort to reapply fungicide for scab control as harvest nears?

1) For growers with essentially scab free blocks (but you only know if you've checked), my guess is the answer is "No". No inoculum means no scab. Keep in mind that nothing in scab management is absolute, there is no doubt at least one lesion out there, but spraying to prevent scab spreading from one lesion isn't worth the expense.

2) For growers with a 'questionable' level of scab (as in 5-18 scabby leaves per 100 shoots), who sell entirely for fresh market consumption with little or no storage, the answer depends on how much it is worth to you for reducing inoculum on overwintering leaves for next spring.

3) For growers who have scab at or above the questionable level, and who will be putting apples into storage, the answer is "Yes" unless you want to risk having late season infections between now and harvest showing up as storage scab a couple of months from now.

It's always scary pontificating on these topics knowing that nobody really knows the answer, especially me. So take my comments as just one opinion and cogitate on it yourself. You are the only person who can weigh the pros and cons. The point of this article is to inspire you to consider the question, not for me to pretend that I know enough to answer it for you.

"I have but one lamp by which my feet are guided, and that is the lamp of experience. I know no way of judging of the future but by the past". - Edward Gibbons

This is for growers who had flyspeck problems in 2003. It was only two years ago, but it's easy to forget four things that happened that year.

2003
1. We had two weeks of unbroken warm VERY humid weather in early August.

2. We had heavy rain that removed fungicide coverage on September 4.

3. Subsequent leaf wetness hour accumulation was relatively normal from September 5 - 15. But leaf wetness hour accumulation was much above normal for September 16 - 30. Starting from September 5, 270 leaf wetness accumulated by about September 28. September temperatures averaged 2.5 F above normal. In other words, September 2003 turned out to be warm and wet.

4. The early season McIntosh maturity forecast was for nonspur Macs at Highmoor Farm to reach starch index 4 at Highmoor on September 24. The actual date turned out to be around September 23-24. So harvest was far from done by the time flyspeck had time to develop enough to become visible in late September.

5. Growers started reporting flyspeck infections right around September 28. About a third of the growers at the winter meeting raised their hands when I asked who had flyspeck problems that fall. Packing line operators encountered higher flyspeck levels.

2005
That was then, this is now. So let's look at 2005 (again, using weather at Highmoor Farm as our reference point.)

1. As far as I can tell summer was not remarkable for or against flyspeck development. Unlike some spots in southern New England, we did have fairly regular rain.

3. Forecast leaf wetness accumulation for September 1-8 is below normal. But beyond that we have no information. Average temperature during that period is forecast to be about 3F above normal.

4. The early season forecast for nonspur McIntosh to reach starch index 4.0 September 20-25.

    5. Outcome? - too early to tell. What happens depends on weather and whether.
    a) September weather will determine how quickly leaf wetness hours accumulate for apples that lost fungicide protection on August 29.

    b) The outcome for individual orchards also depends on whether the trees are prone to flyspeck - thick canopies, poor air drainage, high grass; and

    c) whether growers who had flyspeck in 2003 decide to renew fungicide coverage in September if conditions suggest that renewed protection is warranted.

Nobody likes to spray past mid-late August as early cultivars mature and preparations for the main harvest require attention. Unfortunately, it isn't possible to see far enough ahead to forecast unusually heavy flyspeck pressure until that situation develops. One lesson learned from heavy flyspeck pressure in Hudson Valley in recent years is that renewing fungicide protection after heavy late-season rain removes protection can make the difference between having or not having flyspeck problems show up several weeks later.

I left 2004 out of the 2003 vs. 2005 comparison above. We had some gully washers on August 21 and September 9 in 2004, and as far as I know flyspeck was not a widespread problem. A key difference between 2003 and 2004 is 25% higher September leaf wetness hour accumulation in 2003, the year with widespread flyspeck problems. I suspect that many growers who were burned by flyspeck in 2003, did reapply fungicide after the August 21, 2004 wash-off. And the September 9, 2004 wash-off was two weeks closer to peak McIntosh harvest than this year's August 29 wash-off date will be to this year's harvest.

So here is a repeat of Dave Rosenberger's rule of thumb: Consider your expected harvest dates and make the final planned fungicide application late enough that protection should last until within 25-30 days of harvest. That is because it usually takes at least 25-30 days after protection wears off for enough subsequent leaf wetness hours to accumulate to have visible flyspeck.

August 29 + 25-30 days = September 24-28, i.e. the expected dates for peak harvest. If you didn't have flyspeck in 2003, then I don't know that there is too much to worry about this year (though the 2005 wash-off date is 5-6 days earlier than it was in 2003). But if you did have flyspeck problems in 2003 (and especially if you had flyspeck last year), then there is reason to consider reapplying fungicide protection after September 1.

Of course, any decision about fungicide protection has to consider both scab and flyspeck risk. And careful attention must be given to the preharvest interval for any material being considered for application.

In addition to fungicide timing, another factor is fungicide selection. The strobilurins Flint, Sovran, and Pristine, are the best products for prolonged protection from flyspeck. Pristine may be even better than Flint and Sovran, and if applied within a week or two of harvest also offers protection against postinfection storage rots.

Having started with a pithy quote, here is one to end on:

"History is merely a list of surprises. It can only prepare us to be surprised yet again." - Kurt Vonnegut

European red mites made somewhat of a comeback this year after being lower than what used to be normal for the previous 6 years. However, even this year's ERM populations were less troublesome than those of a decade ago. In any case, it is too late to bother with mite control unless the population is so heavy that their shear abundance is an intolerable nuisance for pickers who don't like to get red hands from all the mites they are crushing.

Codling moth second generation egg hatch is underway, but June - August insecticide applications for plum curculio and apple maggot also kill codling moth adults, eggs, and larvae and keep second generation codling moth from being a problem. The exception is organic blocks where earlier broad spectrum insecticide applications were not applied. In organic blocks where it might be worthwhile to apply a final Bt spray in early September.

Trapping and sprays for apple maggot usually stop August 31. An exception is for blocks of late cultivars such as Red Delicious and Golden Delicious where there are above threshold trap catches. As these are the last fruit available, they can be attacked quite heavily by apple maggot that have nowhere else to go late in the season. Trap catches were moderate to low in three monitored plots at Highmoor this summer.

White apple leafhoppers can be a problem at harvest if too numerous. Sevin (carbaryl) is an effective control with a relatively short preharvest interval.

Starch index values provided by Patti McManus, University of Maine Highmoor Farm Agricultural Experiment Station.

Note: SI values can progress differently than ethylene production.
Fruit firmness and other indicators should also be used to monitor fruit maturity and to identify optimum harvest date.
SI value of 4 -- 6 is optimum range for harvest of McIntosh bound for long term CA storage.

1. There is a very nice new tree fruit pest book coming out this fall. Here is a promotional blurb on it:

"The Tree Fruit Field Guide to Insect, Mite, and Disease Pests and Natural Enemies of Eastern North America, by A. Agnello, G. Chouinard, A. Firlej, W. Turecheck, F. Vanoosthuyse & C. Vincent, is intended for quick field identification and description of the major insect and mite pests, beneficial arthropods, and diseases associated with all the deciduous tree fruit crops in the eastern U.S. and Canada. A compilation of full-color fact sheets, this field ID guide will combine diagnostic keys, professional photos, physical descriptions and biological information on the distribution, damage caused, and management practices for 100+ species of insect and mite pests, 80+ diseases, and 30+ beneficial species, spiral-bound in a 5 x 8.5 inch booklet containing 230 pages. It is intended for use by growers, horticultural consultants, educators, scientists, hobbyist growers and students."

I've reviewed the book and seen the final layout. It looks to be a really nice resource, with multiple beautiful color pictures on every page. If I get permission to do so, I'll post a snapshot of one of the pages at the UMaine Apple IPM website to give you an idea of the format.

Normal price will be about $32, but we might be able to get it at a pre-order price of $18. Let me know if you would like to commit to buying a preorder copy (no money changing hands yet) so I can see if we have enough interest to qualify.

2. Scab fungicide resistance testing is being offered by Cornell University and by the Quebec Research and Development Institute for the Agri-Environment.

The Quebec test requires that you collect 100 scabby leaves that will be overwintered before conducting the test. The testing covers all five scab fungicide chemistries: DMI = sterol inhibitors, strobilurins, benzimidazoles (i.e. Topsin M and the late great Benlate), dodine (Syllit), and the more recent AP class (Vangard, Scala). The testing cost $250 US. You only pay when they get a conclusive result, i.e. positive or negative. But due to the text being new, they can't guarantee having results on time for the spring scab season.

To collect sample leaves for the Cornell test, a grower leaves a few corner trees unsprayed until first scab develops in spring. Leaves from those trees are collected and sent by overnight delivery to Cornell. The Cornell test costs more at $800, and is a bit more involved, but I don't know if the extra information it provides would impact grower decisions. The Cornell test covers the four major fungicide classes, though I'm not sure if it includes the AP class.

Please contact me if you are interested in either test.