WVU Extension Service: The Orchard Monitor: Committed to the Integration of Orchard Management Practices
April 5, 2004

Upcoming Events


Pheromone Trap Counts Plant Pathology Horticulture



April 9 - West Virginia University Holiday.  The WVU KTFREC will be closed.

April 20, 7:00 p.m. - Spring In-depth Fruit Meeting at the Alson Smith Research and Extension Center, Winchester, Va.  Dr. Jay Norelli, USDA plant pathologist, will discuss "Fire Blight".  Dr. Ross Byers, VA Tech Professor Emeritus in Horticulture, will discuss "Thinning and Return Bloom of Apples".  For more information contact Cyndi Marston at 540-665-5699 or at cmarston@vt.edu.

April 22, 6:00 p.m. - Tree Fruit Grower Twilight Dinner and Meeting at Gourmet Central (in Hampshire Industrial Park), Romney, W. Va.  Extension Specialists will discuss early-season insect and disease management strategies and horticultural issues.  For more information contact the Hampshire County Extension Office at 304-822-5013.


Pear psylla adults overwinter in or near pear orchards.  When daytime temperatures exceed 50F, adults return to pear trees, mate and begin laying eggs (pale cream to yellow-orange colored) in crevices on fruit spurs.  The use of oil during the dormant to white bud stage delays egg-laying because females do not like to lay eggs on oily surfaces.  Oil application shortens the length of the egg-laying period, resulting in a population with a more uniform age structure which makes management easier.  Oil can be used from dormant to the white bud stage, but the rate should be gradually reduced from 3% (dormant) to 2% (green cluster bud) to 1% (white bud).  An effective strategy is to make two applications of oil at 2% each, the first at dormant to bud swell and the second at the green cluster bud stage.  An insecticide to kill adults should be combined with oil, especially with the second of two oil sprays.  Pyrethroids (Asana, Ambush, Danitol, Pounce, Warrior) are a good option to combine with oil.  Actara, Assail, or Calypso at green cluster bud, or Esteem, Assail, or Calypso at white bud are newer products that are excellent options for prebloom psylla control. Three prebloom applications (dormant-green tip, green cluster bud and white bud) of Surround have also provided very good control of overwintering adults.  Surround should not be tank-mixed with oil.

Pear psylla adult

Pear psylla eggs

Rosy apple aphids that hatched from overwintering eggs are all wingless females called stem mothers that become adults by the pink stage of apple bud development. Whereas the newly hatched nymphs have a dark green color, adults are purplish or dusty gray.  A single stem mother is capable of curling a leaf, and it is from within this protected shelter that numerous live young are produced.  Rosy apple aphid must be controlled before petal fall in order to prevent fruit injury.  To monitor aphid abundance, make a 3-minute examination of 5-10 trees per block at the prepink stage and count the number of fruit spurs showing curled leaves with live aphids.  An insecticide application is recommended by pink if an average of one or more infested fruit clusters per tree are found.  Options include a pyrethroid (Ambush, Asana, Danitol, Pounce, Warrior), Actara, Assail, Calypso, Dimethoate or Thionex.  Although Danitol also has activity against mites, all pyrethroids increase the potential for higher mite populations because of their higher toxicity to predators.

Rosy apple aphid colony

Tarnished plant bug adults overwinter under bark and leaves in woodlots, fence rows and rock breaks, and around alfalfa, other legumes or weeds that are seeded late in the summer or early fall.  They become active and fly to apple trees near the tight cluster stage.  Feeding on apple buds before bloom usually results in early bud abscission and is rarely a problem, whereas feeding from bloom to shortly after fruit set results in a deeply sunken dimple in the side or calyx end of the fruit.

A white visual, non-UV reflecting sticky trap has been useful for monitoring tarnished plant bug populations and for making decisions regarding management in apple orchards.  Where these traps are installed at green tip, prebloom control is recommended if the cumulative capture averages 2.4 per trap by tight cluster or 4.1 per trap through late pink.  Control options include pyrethroids (Ambush, Asana, Danitol, Pounce, Warrior), Actara, Assail, Calypso, Dimethoate, Thionex, or Carzol.

Tarnished plant bug adultTarnished plant bug apple injuryWhite visual trap

Dimethoate registration on apples has been cancelled by EPA at the request of the registrant.  Existing material may be used through the end of the 2004 season.


March 22 0
March 29 54 0
April 5 12 11

RBLR = Redbanded leafroller; STLM = Spotted tentiform leafminer; OFM = Oriental fruit moth; CM = Codling moth; TABM = Tufted apple bud moth; DWB = Dogwood borer; LPTB = Lesser peach tree borer; PTB = Peach tree borer; AM = Apple maggot.
1In a commercial orchard adjacent to an abandoned orchard near Kearneysville.


Apple scab: We recorded two separate wetting periods last week, the first on March 29 - 30 for 14 hours at 44 F and the second on March 31 - April 3 for 62 hours at 44 F. The second of these will be recorded as our first apple scab infection period for the 2004 growing season.

Table 1. Dates and conditions for apple scab infection periods at the WVU - KTFREC, 2004.

No. Date 2004 Hours/degrees F
1. March 31 - April 3 62 hr/44 F

It is important to avoid early infections on sepals, as these are difficult to detect and can provide conidial inoculum throughout the early part of the growing season. Copper applied last week (March 25th through 30th) would have provided adequate protection during these rain events. Copper sprays, even at the low label rate, will provide scab protection similar to that provided by a mancozeb fungicide applied at 3 lb/acre. However, Cu does not provide back action against scab.

Powdery mildew spores are first available at about the tight cluster stage of bud development. For best mildew control, fungicide applications should begin at tight cluster and continue until terminal growth stops in midsummer. The spray interval is generally 10 days from tight cluster through petal fall, when leaf tissue is developing rapidly, and is lengthened to 14 days after petal fall. Excellent powdery mildew control can be expected when Nova, Rubigan, or Procure are used on a 7 to 10 day interval for scab control. The strobilurin fungicides, Sovran and Flint, also provide good control of powdery mildew. (Deciding how to configure sprays of SI's and strobilurins should depend upon other disease concerns, particularly the rust diseases - more on this in the next newsletter). Severity of powdery mildew is directly related to the amount of overwintering inoculum in shoot and blossom buds and the length of the spray interval. Check blocks of highly susceptible cultivars (Jonathan, Ginger Gold, Rome Beauty, Stayman Winesap, Idared, Paulared, Granny Smith) to determine the amount of overwintering inoculum. Where mildew is a problem, maintaining shorter spray intervals (not over 7 days) more effectively reduces mildew infection than increasing fungicide rates. On highly susceptible cultivars, special mildew sprays applied between the regular sprays from pink through the cover sprays is the most economical way to effectively manage the disease and prevent a repeated buildup of mildew for the following year. These extra spray applications will easily pay for themselves with increases in yield and quality.

Current Conditions: Fruit producers with access to the World Wide Web can access a web page called "Current Conditions" that will provide daily (or "as-needed") updates on current disease and insect development issues.  The page should help to bridge the information gap between issues of The Orchard Monitor and provide the grower with the timely information that is needed for making good pest control decisions.

To view the "Current Conditions" page, click here, or go to the WVU - KTFREC Home Page at: http://www.caf.wvu.edu/kearneysville/wvufarm1.html and select "Current Conditions" from the menu.

Kearneysville Weather. An electronic weather station at WVU-KTFREC is collecting hourly weather data on numerous items of interest to orchardists, including temperature maxima and minima, relative humidity, total rainfall and rainfall rate, wind speed and direction, and solar radiation. For your use, the Current Conditions page on our Web site has links to a current summary that is updated every 15 minutes. There also is a station in Arden that is updated every 6 hours, with links to those data on the web page mentioned above. As in past years, we will continue to provide the output from our Maryblyt model, as well as degree-day information for the development of various insect pests.


Weather Factors That Affect Thinning Response.

Frost. Frost before application of thinners can greatly increase the amount of thinning obtained from chemical thinners.  Frost at bloom can damage fruitlets and reduce seed set, which can result in increased natural drop and greater chemical thinning response. Frost can also damage spur leaves, resulting in greater uptake and thus greater thinning response. Wherever flowers and leaves have been damaged by frost, extreme caution should be used with chemical thinners. Typically, lower rates would be used in such cases.

Sunlight Levels before Application.  The amount of sunlight for the 3-5 days preceding application of chemical thinners has an important effect on chemical uptake and response. Intense cloudy weather before application of thinners can result in increased chemical uptake and greater thinning response, due to greater succulence of the leaves and a thin waxy cuticle. In addition, intense cloudy weather results in reduced carbohydrate supply for fruit growth and reduced fruit growth rate.  This results in increased natural drop and greater chemical thinning response.  Growers should make adjustments in the rate of thinner used if intense cloudy weather precedes application.

Temperature at Time of Application.  The uptake of chemical thinners is greater at higher temperatures than at lower temperatures.  The optimum is between 70-80 degrees Fahrenheit. However, applications made in the morning or evening when it is cool have a longer drying time on the leaf, resulting in a slow but sustained uptake of chemical, while at  higher temperatures during mid-day, drying times are shorter, resulting in a short but rapid uptake of chemical.  Thus, the total amount of chemical taken into the plant appears to be very similar at lower and higher temperatures in the field. Recent research results indicate that similar thinning is achieved regardless of the time of day applications are made; thus, growers should not be too concerned about the temperature or the time of day chemical thinners are applied.

Weather After Application.  Temperature and sunlight levels for the 3-5 day period after application of thinners are predominant weather factors affecting chemical thinning response.  The interaction of temperature and sunlight with the chemical thinner creates stress in the tree, which is necessary to make some fruit drop off. Warmer temperatures increase thinning response, while greater light levels decrease thinning response.

Night temperatures are also an important factor to consider. Warm night temperatures (<60 degrees Fahrenheit) give greater thinning response. With high night temperatures, fruits use up the carbohydrates that were produced during the day at a fast rate, resulting in a deficit of resources for fruit growth and causing the weakest fruits to drop. The greatest thinning can result if warm night temperatures are combined with intense cloudy/warm daytime weather. Under these conditions, the tree produces little reserves during the day and at night the fruits use up all of the reserves produced during the day, making the fruits very susceptible to the stress caused by chemical thinners.  Under these conditions, excessive fruit drop can result.

The least effective thinning will be achieved when bright, warm daytime weather is accompanied by low night temperatures. Under these conditions, the tree produces large amounts of carbohydrates during the day and fruits use them up at a slow rate during the night.  Under these conditions, there is little stress created by chemical thinners and the thinning response is poor.

With low day and night temperatures, regardless of light levels, little stress is created with the addition of chemical thinners, resulting in poor thinning.

Growers should critically examine the weather forecast for the 3-7 day period following application of thinners and adjust rates of chemical thinners up or down 50% based on forecasted temperatures and sunlight levels.

Tree Factors that Affect Thinning Response.

Pollination.  Poor cross-pollination results in low viable seed number per fruit, greater natural fruit drop and greater sensitivity to chemical thinners. In contrast, high seed numbers per fruit result in more difficult-to-thin conditions.  In general, if seed numbers are less than 5, thinning rates should be reduced.

Initial Cropload (Fruit Set).  A high initial cropload usually results in a relatively high final cropload, regardless of chemical thinning program. Therefore, to achieve a given cropload each year, the initial cropload must be considered when determining the aggressiveness of the thinning program. Growers should use a more aggressive thinning program when initial fruit set is high and a less aggressive thinning program when initial fruit set is lower.

Fruit Size at Time of Application.  Fruitlets are more sensitive to NAA and Accel at 10-12 mm fruit diameter than at smaller or larger sizes.  In warm years, when fruit growth rate is rapid, chemical thinners should be applied slightly before fruits reach 10 mm diameter (7-10 mm). In cool years, when fruit growth rate is slow, the application of chemical thinners should be delayed until fruits are 12-15 mm in diameter. Growers should attempt to time chemical thinner application according to a suitable weather window within the preferred fruit size window.

Sensitivity of the Tree.  The internal physiological status of the tree determines it's sensitivity to chemical thinners.  This is the least understood aspect of chemical thinning and is often the reason chemical thinning results are variable.  In general, growers should use a less aggressive thinning program under conditions where tree carbohydrate supply for the fruitlets is expected to be low, and a more aggressive thinning program when tree carbohydrate supply is expected to be high. The carbohydrate supply available to the fruitlets is affected by:

1.    Cropload the preceding year (heavy croploads the previous year not only result in lower return bloom, but also in lower
 carbohydrate and nitrogen reserves and lower fruit set).

2.    Previous fall weather (cool, cloudy weather the previous fall results in lower carbohydrate reserves for the winter).

3.    Previous season foliage condition (heavy insect and disease damage to foliage during the previous season limits the ability
 of the tree to store adequate reserves for the winter).

4.   Winter damage (severe winter temperatures can damage vascular tissues necessary for the transport of reserves from the root to the top in the spring).

5.   Warm temperatures in late winter and early spring (February 15-April 15 results in the tree using it's carbohydrate reserves, which results in lower fruit set and lower yield).

 Source:  Cornell Cooperative Extension Pest Management Guide 2004.


Trade and brand names are used only for the purpose of information, and the West Virginia University Extension Service does not guarantee nor warrant the standard of the product, nor does it imply approval of the product to the exclusion of others which may also be suitable. The West Virginia University Extension service assumes no responsibility in the use of hazardous chemicals.

Individuals requesting an accommodation at a meeting because of a disability should contact one of the Extension Specialists at the WVU Kearneysville Tree Fruit Research and Education Center at 304-876-6353 at least five days prior to the event.

Helping you put knowledge to work

P. O. BOX 609
PHONE:  304-876-6353
FAX:  304-876-6034
WEB:  www.caf.wvu.edu/kearneysville

The West Virginia University Cooperative Extension Service, U.S. Department of Agriculture, West Virginia County
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