Volume 6, No. 9
May 27, 1998
Prepared by the University of Massachusetts Apple Team in cooperation with New England Fruit Consultants and Polaris Orchard Management

The Public and the Missing Macs

We aren't sure exactly why, but the press has picked up on the poor apple set issue, even to the point of its being the cover story on the 5/26 issue of the Boston Globe. Both Springfield television stations also have carried stories, with the coverage on one station quite positive. And the apparent truth is that the McIntosh crop is quite variable by area of the state. It appears that the crop in Eastern/Central Massachusetts is greatly reduced. I've heard anywhere from 5% to 40% of a "full" crop.

There are choices on how to respond to questions by the press or the general public as to the potential for an apple crop in the fall. One is negative - "no crop; it's a disaster!". The other is positive -"Well, there won't be as many Macs, but what we have will be big. And we have Empires and Macouns. The stand will be open. You can expect the same high quality you always find here." For the sake of retail growers, a positive answer might be more useful.

For those who primarily wholesale McIntosh, the bottom line could be vastly different: no McIntosh, no income. In this situation it is much harder to be positive. But, there is still reason not to be totally negative. The "no crop" answer gives consumers the impression that supermarket apples will be all that will be available this fall. That will make it necessary to spend lots of money on advertising to sell the crop that's there.

As we stressed last week, don't panic. The crop may not be as bad as you think. Wait another week or so before taking any drastic measures. Be sure that you have taken a representative sample in your whole orchard, and have not just focused on the trees that "look sparse." Then look at the numbers and review the chart from last week's Healthy Fruit. Remember that increased size may add a lot.

If the Mac situation is not optimal, think about ways to reduce expenses:

 

Stopping Scab Important for the Long Term

Massive scab infections have broken out on abandoned trees. So far, the only reports of scab in commercial orchards are that there are some spotty outbreaks that are big enough to deserve special treatment. Given the disastrous fruit set, some growers are asking, "Why bother?"

First and most important, poor control this year will make scab management much more difficult for the next couple of years. Tree health may suffer. Given the sheet scab on non-sprayed trees, it won't be surprising to see them defoliated by midsummer. Letting scab go will have long-term effects on production.

Last week we recommended that growers apply either captan alone (2 lbs. 50W or 1.5 lbs. 80W), or a combination of captan with a benzimidazole (Benlate or Topsin), Syllit, or an SI fungicide (Nova, Procure, Rubigan). Since the deluge that caused the scab infections, there have been several days of dry, warm weather. Using captan in combination with hot weather, where the temperature is over 80 F, is a very effective way to stop the spread of apple scab. Captan really should be at the center of any effort to arrest new scab infections.

The next key question is, of course, how do you know if you've been successful? There is, unfortunately, no clear answer. It does help to understand how scab lesions function if they are not sprayed. Generally, after the first appearance of the fuzzy lesions, there is a burst of spore production. This is one of most dangerous times in a scab epidemic, because each of the thousands of spores in a lesion may cause new infections. This peak spore production continues for a week or two, then drops off to relatively low amounts after about 4 weeks. By then, all or part of the lesion will usually appear much darker than it did initially, and may be brown or even black. Natural resistance in the tree increases form June through the summer, especially if weather is hot and dry. So, epidemics slow down naturally. But they don't stop. Lesions may appear smooth, and reddish, brown or black, making them look "burned out". But they generally continue to produce a few spores through the summer. At any point, wet, cool weather can bring on new infections.

So, where scab has become established, fungicide treatments and monitoring will have to continue through the summer and even into the fall. Last year, pinpoint scab developed in more than one orchard where scab had established itself, and fungicide applications stopped by mid August. Use two eradicant applications at approximately 7 day intervals right after scab is discovered, applied when temperatures exceed 80 F if possible. Continue with a full rate of captan (2 lbs. 50W) applied every 10 to 14 days for the next 3 weeks (a couple of applications). After that, captan rates can be cut in half, and used at the more normal summer intervals of approximately 3 weeks. If weather gets wet and cool, and new scab reappears, then increase rates and shorten the interval again.

 

Controlling Apple Tree Growth

Over the last ten years, we have discussed, on a number of occasions, ways of reducing apple tree growth and of encouraging flower bud formation. These techniques are particularly useful in settings where trees are planted too close or where trees are relatively young but appear uninterested in fruiting. But these techniques are also helpful in years where the crop is lost, such as this one for some growers.

Girdling is one of the most reliable approaches. It can take the form of a single cut made with a knife completely encircling the trunk at the base of the tree (referred to as scoring) or a similar cut but with the removal of some bark tissue (referred to as ringing). Scoring is easily performed with a curved linoleum knife, and ringing can be done with a saw (for 1/16 inch cuts) or with a specially designed ringing tool (for 1/8 inch to 1/4 inch cuts). This is best done when terminal growth is between 4 and 6 inches, but it will be effective somewhat later. Generally, we recommend not girdling weak trees. It is best to use scoring primarily and ringing only sparingly.

For situations where the crop is lost, ethephon can be an easily applied, excellent treatment for reducing vegetative growth. Apply ethephon at 1.6 pints per 100 gallons dilute (500 ppm) when growth is 4-6 inches and again a month later if needed at 1 pint per 100 gallons (300 ppm). Do not use ethephon on fruiting trees, since it will cause some thinning and early fruit drop.

Leafhopper Lurking

In the past several years, populations of both white apple and rose leafhopper have been kept well in check. This has been largely as a secondary benefit of early-season applications of Provado against leafminer and Sevin XLR as a chemical thinner. This year, a good number of growers have opted to use Agrimek against leafminer in order to capitalize on its effectiveness against red mites. With the light crop load in most areas, Sevin has been used sparingly; few if any growers have applied 2 sprays of Sevin as a thinning agent. Coupled with the relatively mild winter, the stage may be set for a banner year for leafhoppers.

If only a petal fall application of Sevin XLR was used, or if neither Provado nor Sevin was applied, growers should keep an eye out for immigrating rose leafhoppers and developing white apple leafhopper nymphs. Like leafminer, it is the third generation that causes the greatest problems, with substantial excrement on fruit and nuisance to pickers.

In orchards prone to buildup of large populations of LH, it is most important to nip the population early in the season. In order to prevent injury caused by feeding of first-generation WALH and RLH during June, we recommend a treatment threshold of 3 nymphs per leaf. Growers whose orchards annually experience troublesome LH populations at harvest should use a provisional threshold of 25 WALH/RLH nymphs per 100 leaves in June. If these thresholds are exceeded, a half rate of Thiodan should go far toward control of the developing population.

 

European Red Mites Building

The warm, dry weather of the past ten days is very conducive to rapid multiplication of red mites. Fortunately, orchards which received either a prebloom or petal fall mite treatment remain quite free of egglaying females. However, in blocks that received only 1 oil application, or even 2 oil applications under sub-optimal weather conditions, egglaying females are definitely in evidence.

We have now reached the point where mite control by use of either Agrimek with oil or Vendex with oil is questionable. Neither of these chemicals works to its maximum potential when applied this late because the hardening of leaf tissue partially bars the absorption of the material into the foliage. Based upon information from New York, the action threshold for treatment against ERM during the first two weeks of June is 1 or more motile mites present on 45% of middle-aged fruit cluster leaves. If this threshold is exceeded, an application of Agrimek or Vendex is possible, but the effectiveness will certainly be less than optimal. The best alternatives available are Pyramite or Kelthane. Carzol is also available for use, but it is well known as being extremely harsh on mite predators.

Plum Curculio Lingering

As mentioned in last week's issue of Healthy Fruit, the current orchard management challenge is determining when to apply the final treatment against plum curculio. Though there is no substitute to regular monitoring of orchard fruit for evidence of fresh feeding and oviposition, a degree-day model was developed in Geneva, New York to assist in determination of when the PC season is nearing its end. According to this model, the last treatment against PC should be applied such that the residual effectiveness of the material (usually 10-14 days) will protect the orchard through the accumulation of 340 degree days (DD). To calculate DD, take the average of the day's maximum and minimum temperatures, then subtract the base level of 50 to determine the day's DD total. For example, in one monitored orchard in Conway, 165 DD have been accumulated to date since May 11. Thus, if 20 DD are accumulated daily on average, the model would indicate that 8-9 days worth of coverage are still needed.

Leafminer Mines

In orchards where Provado or Agrimek were used at petal fall or shortly thereafter, very few mines have progressed beyond the sap-feeding stage. In orchards where populations have remained low this spring and have not yet warranted treatment, a good proportion of the mines have developed to the tissue-feeding stage. This year, as was the case last year, the majority of scouted orchards did receive a post-bloom application against LM because trap captures and subsequent levels of mines indicated a need for treatment.

We are at the end of the period during which action can be effectively taken against the first-generation mines. Thus, for mid-season treatment, we recommend the following thresholds of first-generation tissue-feeding mines: 7 mines per 100 leaves for McIntosh and 14 mines per 100 leaves for all other varieties. If these thresholds are exceeded, growers should consider an application of either Provado or Lannate.

 

Healthy Fruit is written by Dan Cooley, Ron Prokopy, Starker Wright, Wes Autio, and Karen Hauschild except where other contributors are noted. Edited by Dan Cooley. Publication is funded in part by the UMass Extension Agroecology Program, grower subscriptions, and the University of Massachusetts IPM Program. Healthy Fruit is available on the Web at The Massachusetts Tree Fruit Advisor, (click here). A text version can be e-mailed to you if you contact me, at dcooley@microbio.umass.edu. Please cite this source if reprinting information.