Lorraine P. Berkett, Gwendolyn Neff, Jon Clements, and Roger Brouillette

June 5, 1997

Stage of Bud Development: A few days past Petal Fall at the UVM HRC

Apple Scab: According to the degree day model that tracks maturation of ascospores, we are in the "Final Phase" of the primary scab season at the UVM HRC. As of this morning, we had accumulated 867 degree days since green tip (base 32F). Note that your orchard may be farther along or behind the HRC given differences in temperatures over the past month. Based in this model, the next soaking daytime rains could bring the end of the primary scab season. At this point you should thoroughly check your orchard for primary scab lesions. At the HRC, we started observing lesions this past week on non-sprayed older fruit cluster leaves. You should continue to check for lesions at least two weeks after the last infection period before you ease off on your scab management program. Please refer to page 15 of the 1996-1997 New England Apple Pest Management Guide for options if you do find lesions.

Fire Blight (FB): At the HRC, we have been using MARYBLYT -- which is a predictive computer program for forecasting fire blight disease. The program rates the risk of fire blight blossom blight. It assumes the bacterial pathogen is present and identifies an infection period when four requirements are met: (1) flowers are open; (2) at least 198 degree hours greater than 65F have accumulated after first bloom; (3) at least 0.01"rain or dew that day or at least 0.10" of rain fell the day before; and (4) an aver. daily temp. equal to or greater than 60F. When all of these conditions are met in sequence, the first symptoms of blossom blight is predicted to occur when an additional 103 degree days have accumulated (base 55F). During bloom, the risk of infection is considered ‘low’ when one factor is present; ‘moderate’ when two factors are present; ‘high’ with 3 factors; and infection occurs when all 4 factors are present. The program has a predictive feature where you can put in the temperature and rainfall predicted for the next few days and it will predict the risk of infection for those days. When we put in the predictive temperatures and rainfall during bloom the program did predict infection would occur on a number of those days. However, when the actual weather conditions were subsequently inputted, the program stated that conditions were not met for infection on any day during bloom. Actual temperatures were cooler than those predicted and we did not get many of the predicted showers.

On May 30 and June 2, there was a high risk (i.e., 3 of the 4 factors were present) at the HRC. However, it appeared that the cool temperatures at night were low enough to negatively impact the infection potential. On June 3 and June 4 there was again a high risk (3 of the 4 factors present) at the HRC. With these two dates, the only missing variable was rain. As a check, we will continue to monitor conditions until 103 degree days (base 55F) from May 30, June 2, 3, and 4 have accumulated to see if any blossom blight symptoms appear.

Plum Curculio: (CLICK HERE FOR AN IMAGE OF PLUM CURCULIO) If this insect has not already ventured into your orchard, it will be there soon. Peak migration of this insect into orchards usually occurs during the two weeks following petal fall. One has to wonder whether the cool temperatures this spring will cause a drawn-out PC season. Drs. Reissig and Nyrop at Cornell University have developed a degree day model to predict the timing of the last PC spray. Starting at petal fall, the number of degree days (Base 50F) are added for each day until 340 Degree Days have accumulated. At that point the last PC spray should be applied. To calculate the degree days for each day, average the high and low temperature for the day and subtract 50, the remaining number is the degree days for that day.

If you are planning to put on a border spray for this insect, please review "Border Row Sprays" on page 83 of the 1996-1997 New England Apple Pest Management Guide for a review of factors that you should consider. 

Leafminers: At this point in the growing season, LM populations should be in the sap-feeding stage. Check the undersurface of fruit cluster leaves on the interior portion of the tree for mines Thresholds: 7 mines per 100 leaves for McIntosh; 14 mines per 100 leaves for Non-McIntosh.

Click here for an image of a Spotted Tentiform Leafminer Larvae

Options for managing populations above thresholds include Provado and Agri-mek. However, but please note that optimal timing of Agri-mek for leafminers and mites is at petal fall. 

Jon Clements

In the afternoon of May 30, 1997, I attended a field meeting sponsored by Kevin Iungerman of Cornell Cooperative Extension, at Forrence Orchards in Peru, NY. Present were Cornell Fruit Specialists Drs. Richard Straub and David Rosenberger from the Hudson Valley Lab, and Drs. Warren Stiles and Terence Robinson from the Geneva Experiment Station, as well as approximately 20 apple growers from the northern Champlain Valley of New York. The topic was post-bloom thinning, and various insect and disease observations. Below are my impressions gleaned from short presentations by each of the Cornell specialists, and the ensuing discussion with growers.

Entomologist Richard Straub got the ball rolling by pulling a rootstock/scion/graft-union segment from a dwarf apple tree out of a bag and pointed to an area where American Dogwood Borer had done its business. He explained the borers prefer to inhabit an area just under the graft union in the rootstock, and will bore into this segment year after year, eventually resulting in weakening or death of the tree. It appeared the tree had tried to heal itself where borers had entered and exited, as exhibited by rough callous tissue surrounding an indented area. Although not necessarily fatal, Straub said it is definitely not good for the trees, and recommended growers check their dwarf rootstocks for the pest. He said it was present only where mouse-guards (both wire and plastic) were present, and especially in non-herbicided orchards where grass was growing up in the mouse guard, but he wasn't 100% sure these guys are prevalent this far north (his sample was from the Hudson Valley). Lorsban directed at the trunks during July and August should prevent borers from further attacking infested trees.

Straub was obviously pleased that European Red Mite control options are numerous this year. Growers are experiencing good luck with Apollo and Savey, both likely giving season-long control during light pressure years. But he reminded us that annual use of these products could lead to resistance. He mentioned Agri-Mek is a superb post-bloom miticide, while making it clear that it needs to be used with oil or another spreader-adjuvant (per the label) to be absorbed. And he brought up the recent federal label for Pyramite, although wasn't sure if it would be available in time in New York for 1997. It is an excellent miticide that growers will be able to use late in the season where mites have gotten out of hand.

Finally, Straub mentioned seeing signs of Green Fruitworm damage in several orchards. Now is the time to treat this pest he warned, as they are quite small (1/4"), therefore easy to kill. You don't want them to get larger when they become more difficult to kill and do more damage. Unfortunately Straub said, the commonly used post-bloom insecticides Imidan and Guthion are not particularly effective against Fruitworm. He advised if you have a consistent Fruitworm problem then application of a pyrethroid insecticide such as Lannate or Lorsban will do the trick.

Plant pathologist Dave Rosenberger said the Apple Scab situation looks promising this year, mostly because of the limited occurrence of scab infection periods during the relatively dry and cool month of May. He's tempted to speculate that the potential for primary infection now is quite low because research has shown that spores released post-bloom become rather "lazy," i.e., apparently have a more difficult time becoming airborne and infecting trees. Specifically, at 800-850 Degree Days, base 32 from Green-Tip, you can probably forget about the likelihood of primary infection. He suspects growers had such a hard time last year with scab because primary infection occurred early (during Green-Tip through Half-Inch Green), and then secondary infections ran rampant throughout the summer under favorable conditions. Rosenberger questioned if it's worthwhile including Sterol-Inhibiting fungicides in post-bloom sprays, and concluded "probably not," that well-timed contact fungicides ought to do the job now, and the next rainfall will likely mark the end of primary scab season.

Horticulturists Terence Robinson and Warren Stiles talked about fruit thinning-certainly the current topic on everybody's mind. Because trees were largely still in Full Bloom in the area, Robinson said it's difficult to assess the need for thinning now. Stiles mentioned it's the strongest bloom he's seen in seventeen years of coming to the area, and much better than what was observed both in the Hudson Valley and western New York. Robinson said it would have been an excellent year to attempt some blossom thinning. Wilthin is now registered for blossom thinning, and he thinks growers need to consider blossom thinning (a common strategy out west) more seriously-it has the advantage of giving a couple stages to do thinning and has been quite effective in some of his thinning experiments.

So, what did Stiles and Robinson recommend for an OVERALL THINNING STRATEGY for 1997? At this point, and in one word, "aggressively." Then, they discussed several important points that growers need to remember when thinning:

• earlier thinning applications (i.e., Petal Fall to 8 mm fruitlet size) have a greater likelihood of increasing fruit size and promoting return bloom compared to later timings (8 - 15 mm)
• cloudy weather during the one or two weeks preceding thinning applications has a tendency to make foliage 'softer,' hence better uptake resulting in more effective thinning
• temperature during time of application is not as important as the day and night-time temperatures following application. Growers need to get the best weather forecast possible when planning their application timing-daytime temperatures at least in the 70's with relatively warm nights are what's needed to result in maximum thinning efficacy
• tank-mix combinations of NAA and Sevin, or Accel and Sevin (it's not recommended you combine NAA and Accel applications in the same year) exhibit a synergistic effect and are usually most potent. The surfactant properties of both XLR and WP Sevin formulations (but especially XLR) are a possible explanation for this effect.
• Keep in mind that from an economic standpoint, Macs are easily over-thinned, so it was suggested either NAA (3-5 ppm) or Sevin (one pint per 100 gallons) or Accel (20-30 g./acre) be used alone. Empires, however, almost always benefit from combinations of NAA and Sevin or Accel and Sevin. Using Accel instead of NAA with Sevin has consistently resulted in substantially greater Empire crop values, because of enhanced fruit size above and beyond a simple thinning response.

Summing up the thinning discussion, Stiles was clear-start with Sevin as a base, then consider the above factors and overall tree condition (including bloom, foliage, vigor) in determining the need for "spiking" the tank with NAA. This year in particular, he warns "don't get too chicken and back-off!"

On a final note, Stiles mentioned a couple other important horticultural issues. First, it's important to keep weeds down, especially at this time of the year when they are competing for soil water and nutrients. Stiles has been impressed with an herbicide program that many growers are adopting using fairly low rates of Roundup (one quart in 35-50 gallons per acre) twice-once in May, then again in late July.This one-two punch of Roundup seems to give good, economical season-long weed control. And lastly, Stiles pushed the foliar nutrient sprays, including pre-bloom Boron and Zinc (too late for this year); several applications of epsom salts (for Magnesium, 15 lbs. per 100 gallons dilute equivalent) beginning at Petal Fall; and urea (for Nitrogen, 5 lbs. per 100 gallons) at Petal Fall or 1st Cover, particularly where trees appear weak. One gets the impression Stiles thinks the value of these foliar nutrient sprays is commonly under-estimated.

All in all, many of the important post-bloom questions and issues that were on these growers minds were answered or discussed by the Cornell fruit specialists. Certainly, the mood was optimistic at this early stage, considering the 'snow-ball' bloom and lack of major problems evident in the New York Champlain Valley orchards.

As part of AIM (Apple Information Manager Project), we are now offering the Vermont IPM Newsletters and Pest Updates via e-mail. Just send us your e-mail address and we'll put you on the list. Send a message to either Gwen Neff gneff@zoo.uvm.edu or Jon Clements at jon.clements@uvm.edu with your request and e-mail address. You can also sign yourself up. Just send a message to vt-apple-ipm@orchard.uvm.edu with the word help in the SUBJECT line. You will get a return message telling you how to subscribe.

AIM also now features an e-mail discussion list for apple growers, called "GROWER-AIM." The list open to anyone who wishes to communicate with others on the list about New England apple production (pest management, horticulture, marketing, etc.) and the New England Apple Information Manager. When you join an e-mail discussion list you can receive and send messages to the entire group of people subscribed to the list. It's a great way to spark an interesting discussion, get the word out about something that interests you, or get a wide range of answers to a question (and probably more questions, too).

You can subscribe to GROWER-AIM by visiting the "Communications" section of the AIM website:

http://orchard.uvm.edu/aim/communication/default.html

Follow the instructions at the bottom of the web page -- and be sure that you check the box marked GROWER-AIM.

You can also subscribe to GROWER-AIM by sending e-mail to: grower-AIM@orchard.uvm.edu

with the word "subscribe" (don't include the quotation marks) in the subject line.

You'll receive a help file once you sign up which will give more information about using the e-mail discussion list -- and tell you how to unsubscribe should you need to.

The International Dwarf Fruit Tree Association is spreading its wings this year and holding two summer tours with one in the east and one in the northwest.

The eastern tour will be held in Massachusetts on Sunday through Tuesday, June 22-24. The northwest tour will be held Monday and Tuesday, July 21-22, in the Wenatchee district and northern Columbia Basin. Enclosed is a two-page description of the Massachusetts tour

. Alumni UVM Apple Team member, Terry Bradshaw, is now employed at the Tougas Family Farm, featured on Monday's tour.

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