July 17, 1997

The recent hot and humid weather can cause a dramatic explosion in populations of pest mites in the orchard -- even in blocks that were well below threshold just 10 days ago. Please refer to Cornell's July mite-monitoring charts, which were included in the last issue of the Vermont Apple IPM Update. Especially at this time of year, it is important to be monitoring the orchard on a regular basis for mites to make sure that you are able to catch a potential problem before extensive damage occurs.

Please refer to the June 25th issue of the VT Apple IPM Newsletter for information on the newly registered miticide, Pyramite.

We have started to catch Apple Maggot flies on our baited Ladd traps at the HRC in S. Burlington. Growers throughout the state have also reported AM fly trap captures. The recent heavy rainfall that we have seen in Vermont can trigger further emergence of AM fly.

Sphere-type traps baited with a lure that emits apple volatiles attract AM flies so efficiently that an insecticide cover spray is not required until a threshold of 5 flies per trap (cumulative average) is reached. The threshold recommended for unbaited sticky-coated red spheres is a cumulative average of 2 AM flies per trap.

At the Horticultural Research Center in S. Burlington, VT we are just starting to see sap-feeding mines from this season's second generation of leafminers. The 1997 Massachusetts March Message recommends treatment when cumulative second-generation mines reach 50 mines/ 100 leaves for McIntosh or 100 mines/ 100 leaves for non-McIntosh trees. Please see page 31 of the New England Apple Pest Management Guide for treatment options if your orchard is above threshold.

"Unlike other leafminers of apple, L. speculella is characterized by frass (small black pellets) that is constantly expelled on a silken thread from the mine by the feeding larvae. Just prior to pupation, larvae spin cocoons, which are suspended by threads and resemble a hammock. Apple leafminer probably has 4 to 6 generations per year in southeastern New York.

"Moreover, unlike our other leafminers, larval damage is confined to the youngest foliage, particularly terminal leaves of vigorously growing shoots. Root initials or water sprouts that are partially shaded are the preferred sites for feeding and pupation. Severely mined leaves turn brown and die; most such leaves drop off prematurely, thereby decreasing the number of the most photosynthetically active leaves. The potential for damage is greater in young orchards than in mature ones, and vigorous trees usually sustain higher infestations than do less vigorous trees.

"Populations normally do not achieve high abundance or cause critical damage until the beginning of the harvest period of our earliest cultivars. Insecticidal control of larvae or adults at this time may not be a reasonable tactic because of the pre-harvest interval of most materials; and just as importantly, because infestations do not damage fruit or cause premature drop of fruit. Broad-spectrum insecticides typically used in cover sprays (OP's) are unlikely to provide significant control of adults or larvae. The optimum control tactic would be 1 or 2 sprays of either methomyl, oxamyl, endosulfan or a pyrethroid at petal fall or 1st cover. Undoubtedly, imidacloprid at the same timing would also do some good. We consider that sprays are necessary only on non-bearing trees where vigor is essential, or on bearing trees that had high infestations the previous season." (From Scaffolds, No. 16, July 7, 1997)

New plantings are most prone to damage by this insect. The following is a description of the damage this insect can cause, written by Dr. Art Agnello of Cornell University (Excerpted from Scaffolds No. 16, July 7, 1997):

"PLH does not overwinter in the Northeast but instead migrates on thermals (warm air masses) from the South. Because PLH migrate constantly during the season, there are no distinct broods or generations and the pest may be present continuously in orchards from June through harvest.

"PLH feeds on tender young terminal leaves. Initially, injured leaves turn yellow around the edges, then become chlorotic and deformed (cupping upward) and later turn brown or scorched. Damage is caused by a toxin injected by PLH while feeding. PLH also occasionally causes symptoms similar to the effects of growth regulators, such as excessive branching preceding or beyond the point of extensive feeding. PLH damage is often mistaken for injury caused by herbicides, nutrient deficiency, or overfertilization. PLH injury may not be serious on mature trees but can severely stunt the growth of young trees.

"Nymphs and adults should be counted on 50-100 randomly selected terminal leaves in an orchard. Older trees should be sampled approximately every three weeks during the summer. Young trees should be sampled weekly through July. PLH nymphs are often described as moving sideways like crabs, whereas WALH generally move forward and back. No formal studies have been conducted in N.Y. to determine the economic injury level for PLH on apples, so we suggest a tentative threshold of an average of one nymph or adult PLH per leaf. Little is known about the natural enemies of PLH, but it is assumed that they cannot control this pest in commercial New York orchards."

Please see page 109 of the Apple Pest Management Guide for pesticides that are effective against this insect, should treatment become necessary for your orchard.

July and August are the months to focus on summer diseases, sooty blotch and flyspeck. There are a number of cultural practices, aimed at improving air circulation and drying, which can greatly reduce disease development. These include summer pruning, thinning to break up fruit clusters, mowing, and cutting dense hedgerows. The following article by Dr. Dave Rosenberger of the Hudson Valley Lab, outlines a model that can help in management of summer diseases (From Scaffolds No. 17, July 14, 1997):

"In northeastern United States, fungicides are applied to apples from mid-June through August primarily to control sooty blotch and flyspeck. Four or five summer fungicide applications may be needed to control these diseases in wet years, whereas only two or three well-timed applications are needed in dry years. Omitting summer fungicide sprays is risky, however, because gaps in fungicide protection during critical periods in summer can result in the sudden appearance of numerous flyspeck infections just before harvest.

"Field research conducted in the Hudson Valley over the past 10 years has been used to develop a model for timing apple fungicide sprays during the summer. The model targets flyspeck because fungicide programs that control flyspeck will nearly always control sooty blotch under N.Y. conditions. The N.Y. Flyspeck Model has worked well in small-plot tests where fungicides were applied with a high-pressure handgun. The model is currently being tested in commercial orchards with funding supplied by the New York State IPM program.

"The concepts used to develop the N.Y. Flyspeck Model are outlined below. Because the model has not yet been validated with airblast sprayers in commercial orchards, we are not yet recommending this approach for timing summer fungicides. Omitting fungicides is always risky because potential losses from disease on fruit can quickly obliterate any savings that accrue from withholding sprays. Nevertheless, the information and concepts used to develop the N.Y. Flyspeck Model may be useful in deciding how to time summer fungicides even if the ultimate decisions on fungicide timing end up being considerably more conservative than those suggested by the model.

"The first step in constructing the N.Y. Flyspeck Model was the development of a table of estimated residual activities for various summer fungicides (Table 1). This table was developed using data from small-plot field trials conducted in the Hudson Valley from 1987-1996. Residual activities shown in the table are shorter during summer than for the last spray before harvest because cooler conditions in the fall slow development of sooty blotch and flyspeck, and also because late infections will fail to develop symptoms before harvest and therefore are of no concern.

"In addition to the residual activity of fungicides shown in Table 1, research has shown that Benlate and Topsin M provide limited eradicant activity against sooty blotch and flyspeck. Their eradicant activity decreases as the time between infection and fungicide application increases. Benlate has reasonable eradicant or suppressive activity against flyspeck infections that have accumulated fewer than 100 hours of wetting after infections occurred. (The eradicant activity for Topsin M has not yet been defined, but Topsin M probably has less eradicant activity than does Benlate.) Working in North Carolina, Brown and Sutton showed that sooty blotch and flyspeck appear on fruit only after fruit are exposed to 275-300 hours of accumulated wetting following infection. Thus, it appears that Benlate will provide eradication of flyspeck and sooty blotch provided the infections are less than one-third of the way through the incubation period, with "incubation period" defined as 275-300 hours of accumulated wetting after infection.

"By taking advantage of both the residual activity of fungicides and the eradicant activity of Benlate, it may be possible to eliminate one or two summer fungicide sprays after the last scab spray is applied in early to mid-June. The logic is as follows:

"CAUTION: Omitting fungicide sprays during July is not recommended if tree canopies are dense (as in trees left unpruned last winter) or if fruit are clustered. In orchards with dense canopies or clustered fruit, complete fungicide coverage will almost certainly be impossible during late summer when the canopy reaches maximum density and the clustered fruit prevent fungicide from reaching the center of clusters. In such orchards, Benlate should be applied during July when the likelihood of decent coverage is better than it would be in August. Even a very tight fungicide program may fail to control flyspeck during wet seasons in orchards with dense canopies."

Table 1. Estimated residual activities (under New York conditions) of fungicides used for controlling sooty blotch and flyspeck.

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