July 9, 2007

ENTOMOLOGY

Codling moth and oriental fruit moth estimated egg hatch (updated three times per week) and control recommendations may be found by going to: www.caf.wvu.edu/kearneysville/pheromon.html.

Japanese beetles pose a significant threat to all stone fruits as they reach maturity.  Beetles are often found in clusters on fruit that is within two weeks of harvest, and may be accompanied by Green June beetles.  Although, like Japanese beetles, Green June beetles prefer ripe fruit, they also may cause injury on green fruit.   Since feeding may be “clumped” or unevenly distributed, care should be taken in looking at a representative sample before making a spray decision.  Control is recommended if fruit feeding injury exceeds one percent.  Sevin is considered the most effective material available for control on all stone fruits.  Other options include Lannate, Provado and Surround.  Since beetles will continue to move into trees for another 2-3 weeks, multiple applications may be needed in higher pressure situations.

Western flower thrips (WFT) is a potential threat to peach and nectarine fruits near harvest, especially if weather conditions become dry.  WFT adults are slender, about 1/16 inch long, yellowish, and hold their wings over their backs.  Larvae are smaller and wingless, but otherwise resemble adults.  Several generations occur per year as populations buildup in white clover and other weeds in and around orchards, as well as in field crops such as alfalfa.  Flight activity of WFT peaks in July through September in stone fruit orchards.  If conditions are dry near harvest, ground cover hosts will become less attractive and make it more likely that thrips will move to stone fruit trees.  Feeding on fruit near harvest results in silvering injury, a benign surface blemish that can be quite extensive if thrips populations are unchecked.  Injury usually occurs in protected sites, such as in the stem end, the suture, under leaves and branches that contact the fruit, and between fruit.  Inspect the earliest maturing varieties during final swell for silvering injury.  Monitor thrips by counting adults on 10 fruit at 5 locations in the orchard.  Sample fruit from the ends of branches in the lower third of the tree canopy.  Five adult thrips per 50 fruits and the presence of silvering injury may justify control, depending upon the potential market, because extensive silvering can result in downgrading of the fruit.  Control options [preharvest interval] include SpinTor and Entrust [14 days on peach, 1 day on nectarine], and Lannate [4 days on peach, 1 day on nectarine].

PHEROMONE TRAP COUNTS
WEST VIRGINIA UNIVERSITY KTFREC

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.

PLANT PATHOLOGY

Infection periods.  We recorded four new infection periods since the last Orchard Monitor on June 25.  Please refer to the table below for details. Rainfall totals for June were about 2.8 inches in most locations in the Eastern Panhandle, according to our weather stations. The 65-year average rainfall for June is 3.4 inches. July average rainfall is 3.7 inches. So far we have avoided the long infection periods (48 to 72 hours) that are often accompanied by fungicide residue removal and poor spraying conditions. 

Table 1. Dates and conditions for infection periods at the WVU - KTFREC, 2007.

Accumulated wetting hours - Threshold Reached.  As of July 9, 2007, we have accumulated 250 wetting hours, for petal fall date of May 4 (last year at this time AWH = 355). Accumulated wetting hours are useful for predicting the appearance of sooty blotch on nonsprayed fruit. Symptom development for these diseases is highly dependent upon temperature and moisture conditions surrounding the fruit. The appearance of sooty blotch symptoms has been predicted with reasonable accuracy by using accumulated wetting hours (AWH). Visible signs of sooty blotch may appear following approximately 260 - 300 AWH (earlier in the season (260 AWH) if the disease was severe last year, later in the season (300 AWH) if not). The AWH threshold for making the decision to include Topsin-M in the spray program is 225 for high disease pressure and 275 for low disease pressure. Each of these threshold values presumes that 25 additional AWH will occur in the next 5 days after reaching the threshold. 

Managing flyspeck on apples – how to interpret the accumulated wetting hour threshold and how to get the best control with currently available fungicides (adapted from an article by David Rosenberger, Cornell University).

Here is an in-depth application of the AWH threshold applied to flyspeck management. First, some biology.  The fungi that cause flyspeck overwinter on and infect a wide range of host plants, growing on the external waxy cuticle of those plants.  What are “those plants”? For our discussion, we’ll say that “those plants” include most shrubs, bushes, vines, and trees that grow in orchard perimeters. Initially, these hosts produce ascospores, and then later, conidia, that blow into orchards. After a spore from one of the flyspeck fungi lands on an apple, the apple must be exposed to approximately 270 hr of accumulated wetting (AWH) before the flyspeck colonies become visible on the fruit.  Brown and Sutton in North Carolina were the first to identify the incubation period for flyspeck, and they found the best correlations when they ignored wetting periods of less than 3 hr duration.

Ascospores from the overwintering flyspeck fungi are released at around petal fall stage of apple fruit development. Therefore, flyspeck colonies initiated by ascospores may begin appearing on unsprayed fruit at 270 AWH after petal fall. Because sprays for scab, mildew and rust usually prevent infection of apple fruit by the flyspeck ascospores, ascospore infections are not usually seen in commercial orchards. However, a lot of activity is taking place on those wild hosts we mentioned earlier. Infections initiated by ascospores on those wild hosts begin releasing conidia coincident with their becoming visible (after 270 AWH).  When those infections produce conidia, orchards are exposed to a continuous supply of conidia blowing around and around throughout the remainder of the summer and fall.  If fungicide residues on fruit drop below effective levels, then the conidia will initiate flyspeck infections on fruit.

In experiments where trees are left unsprayed after 2nd cover (i.e., they are protected from flyspeck ascospores but not from conidia), flyspeck incidence and severity on fruit increases dramatically around 540 AWH.  Thus, flyspeck requires 270 AWH to produce conidia on wild hosts and another 270 AWH to infect and produce visible colonies on apples.

In New York, the results of two recent trials suggest some limitations to our currently available fungicides:

1 - Two inches of heavy rain may be enough to eliminate fungicide residues. Pristine has the best residual activity, but none of the fungicides had adequate residue to completely protect against flyspeck after about 2 inches rain.

2 - Fungicides applied after flyspeck infections have been initiated can arrest growth of the flyspeck fungus temporarily, but they do NOT eradicate the infections.  Sovran is better than Pristine or Flint for suppressing infections but it is not better than Topsin-M + Captan. So, Pristine provides the best residual protection, but Sovran and Topsin M provide the best post-infection activity.

Based on the above, what we know about the biology and the activity of our fungicides can be summarized in statement form to help us formulate our management options: 

1. The period of least risk for significant flyspeck infection occurs between petal fall and 270 AWH (for the reasons noted above).
2. After 270 AWH, fruit should be continuously protected with fungicides. Any gaps in protection after 270 AWH may allow flyspeck infections to be initiated.
3. Two inches of rain can remove virtually all fungicide protection.
4. Fungicides applied after infections are initiated do not eradicate all infections. Post-infection sprays will arrest incubating infections for varying (and at this point, unpredictable) periods of time. When the fungicide residues drop below inhibitory levels, the surviving flyspeck infections begin growing again. Predicting when suppressed lesions resume growth is difficult because we can't accurately predict when fungicide residues are exhausted.
5. The 270 AWH incubation period for flyspeck can perhaps be viewed as a "grace period" for lapses in fungicide coverage.  If apples are consistently protected from infection during summer and fungicide residues are removed by heavy rains on September 1st, then flyspeck will not appear on fruit so long as fruit are harvested AND COOLED before they are exposed to 270 hr of wetting.  However, if apples are left unprotected through 90 hr of wetting in July and/or August after conidia are being released, then part of the grace period will have been used in July-August and flyspeck may appear on fruit more quickly than otherwise expected in September.
6. In real life, the total grace period for lack of fungicide protection during the growing season is probably less than 270 AWH because flyspeck can continue to grow on wet fruit surfaces after harvest until fruit are cooled below about 45 F.  Fluctuations in air temperatures as storage rooms are filled can cause condensation on surfaces of cold fruit already in the room, and that moisture can allow continued growth of flyspeck.  I don't know how much of a 270 hr incubation period can be completed after harvest, but I suspect that up to 70 hr of the required 270 hr incubation period could occur after harvest if fruit a are not cooled rapidly.  Application of a postharvest fungicide drench might suppress growth during the cool-down period after harvest, but I am not aware of any data that address this question.
7. Given all of the above, the safest approach for controlling flyspeck will be to maintain fungicide coverage throughout summer after the 270 AWH threshold has been reached.  If extended rainy periods preclude timely re-spraying of blocks after heavy rains, then that lapse in coverage may use up part of the preharvest "grace period."
8. Wet autumn weather such as we have had in recent years may be contributing to elevated inoculum levels in hedgerows and woodlots.  Thus, extra caution (i.e., extra sprays in September and perhaps even in early October for late varieties) may be warranted until we get a dry summer-fall combination to break the current high inoculum cycle.
9. Late summer sprays for flyspeck can be compromised by incomplete coverage of fruit surfaces.  Including a surfactant with the fungicide during late summer may be helpful, but an excess of surfactant will only cause excessive run-off, thereby leaving less residue on fruit than a spray applied with no surfactant.  Probably the best way to improve coverage in late summer sprays is to reduce tractor speed and increase the volume of water applied per acre.

Conclusions:  Control failures with flyspeck usually occur either because of poor spray coverage during the latter part of the growing season or because trees were left unprotected through more than 270 hr of wetting during the preharvest interval.  Fungicide protection on fruit is exhausted after 2 inches of rain, so fungicide sprays may be needed in September if heavy rains occur with more than 25 days remaining before fruit will be harvested. 

See our “Current Conditions” Web page for details that are updated at least three times weekly.

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