Petal Fall Chemical Thinning

Petal fall is the first real opportunity to adjust crop load in apples. There are several factors that should be considered when deciding to use a petal fall thinning spray. Bloom is heavy in most areas of the state, due in large part to the light crop last year. There was little or no cold damage from low temperature this winter and early spring. Temperatures have been moderate for the past few weeks, assuring complete and full development of flowers parts. Flowers and spur leaves appear to be very healthy. Weather, in general, has been favorable for good bee activity and pollination. While memories of a light crop last year may cause you to adopt a conservative thinning strategy, this may not be appropriate, since all of the above factors favor a heavy initial set.

A petal fall application of carbaryl appears to be a very good thinning strategy now. Temperatures in the 70's are predicted by weeks end, and this is a prerequisite for good thinning. A carbaryl application now, given the weather forecast, will undoubtedly mean less aggressive thinning will be required later during the traditional thinning period. Generally, 0.5 lb/100 gal carbaryl adjust for the appropriate tree row volume will be sufficient.

More aggressive thinning at petal fall may be appropriate on some varieties and in some blocks that have a history of heavy set. Amid Thin is frequently used on Macoun and early maturing varieties. Experience at the Horticultural Research Center in Belchertown has shown that a petal fall spray of carbaryl combined with NAA at 6 to 8 ppm gives moderate thinning. Excellent thinning has been note in heavy set blocks where carbaryl plus NAA at 6 ppm was applied both at petal fall and at the 10 mm stage.

Scab as Seen from a Satellite and the Ground

Last week was the most significant scab infection period of the year. As we said in the email message sent out on Friday, spend your fungicide money on scab now. By our estimates, we have roughly 70% of the scab spores released. There's plenty of susceptible tissue for the fungus to infect. Any infection periods at this time are dangerous. Last weeks wetting, we recorded as 19 hrs. long at the HRC in Belchertown at 60 F. Given all factors, this presented a very high risk of scab infection. Lesions from this infection should show May 19 at the earliest.

By the end of this coming week, another 20% will be ready to go. Any new infection periods will continue to be a big threat. However we estimate, conservatively, that virtually all of the season's ascospores will have matured by the weekend of May 22. Then, we need to wait until the final wetting period, and then for any infections to develop, to figure out what damage scab has done this year.

Some of you may be checking Orchard Radar, Glen Koehler's site linking SkyBit weather predictions with models for insect and disease risk (on the web at: http://pmo.umext.maine.edu/apple/allmodels/Radardirectory.htm).

These predictions include a wealth of information, with estimates that subdivide infection periods into what Glen feels are significant subunits. We show you some of the data comparing the Orchard Radar with our on-site weather data from Belchertown. The tendency is for Orchard Radar to estimate more infections than the ground station and our own calculations estimate. They're both estimates, and only effective scab control, or lack of it, will tell which is more accurate.

No Threat Yet

As mentioned in last week's message, spotty warm weather (as we had on 5/5 and 5/9) will spark increases in tarnished plant bug activity. Over the past week, we have seen additional captures on white sticky monitoring traps positioned to monitor plant bugs, as well as substantial captures on traps positioned for European apple sawfly. Even with the increases, this season's crop of TPB has remained uniformly low, posing little threat in most monitored orchards. If population levels and activity build to an unacceptable level during bloom, a petal fall spray of Guthion or Imidan will certainly do the trick; applications targeting EAS or PC should offer good TPB control.

Sawfly Steady

Despite some early activity in a few hotspots, pressure from European apple sawfly has remained about average, according to captures on white sticky monitoring traps. Fairly low numbers of EAS have been captured in most monitored orchards, and the only blocks that have exceeded threshold levels are those that did so prior to bloom. As printed in last week's Healthy Fruit, the recommended treatment thresholds are 5 EAS per trap (if no pre-bloom insecticide was used) or 9 EAS per trap (if a pre-bloom insecticide application was made).

One important factor that can lead to effective control of EAS is the consistency of petal fall across cultivars. In a year when most varieties lose their petals at roughly the same time (as is projected for this season), damage from EAS is minimized. A petal fall application of either Guthion or Imidan is recommended if trap capture thresholds are exceeded, and solid control of EAS is usually achieved by the first application targeting plum curculio.

Blooming, Looming

Plum curculios began immigrating in low numbers into monitored experimental blocks last Wednesday (5/5). Even so, the weather so far this season has not been very favorable for widespread immigration from overwintering sites. From previous research, it seems clear that movement into orchards is most likely to peak during the afternoon and evening, roughly 5:00 to 9:00 PM. In order to occur in any threatening volume, weather conditions are critical. PC movement and activity is greater when it is warm, calm, and fairly humid--conditions which we haven't had during evening hours. It is forecasted that the remainder of the week (5/13-5/14) may offer more favorable weather to PC immigration into commercial orchards statewide.

If evening temperatures rise and humidity increases, there is a greater likelihood that a petal fall insecticide application will be beneficial. Guthion and Imidan remain as the materials of choice, though Guthion seems to offer longer residual control. Applications of Sevin for thinning purposes will provide substantial control of the in-orchard PC population for about 3 days, but control diminishes rapidly after that. If there is any doubt about the value of a whole-orchard application at petal fall, PC immigration can be greatly reduced by a border-row treatment, ideally covering the first 2 rows of trees. However, a border-row application will not carry the second-hand control of TPB and EAS.

We believe that the overwintering population of PC (and subsequent damage in commercial orchards) is considerably lighter than normal, given the lack of fruit on unmanaged hosts last year. However, from year to year, there is no substitute for each grower monitoring PC hotspots on a daily basis, examining fruit for evidence of feeding and egglaying. King fruit on early varieties are likely to receive the earliest injury, and growers should be prepared to move into action soon after petal fall.

Sap Suckers

Leafminer egglaying continues in earnest in all monitored blocks, though no sap-feeding mines have been observed. If no pre-bloom treatment was made targeting leafminer, then the best time to treat is within a week or so of petal fall. Timing of an application against the first summer generation is critical, as none of the threerecommended treatments (Provado, SpinTor, Agri-Mek) is effective against leafminer once they reach the tissue-feeding stage. In most years, there is about a 2 week window for application, and we recommend the following thresholds for treatment:

These thresholds are useful only if trap capture levels indicated a marginal population. This year, most growers should have a pretty clear indication of whether or not a post-bloom application against leafminer is warranted based on early-season trap captures.

Psylla Killa

Early nymphal stages of pear psylla are still dominant in most orchards, and no hardshells have been observed thus far. If the nymphal growth does not progress substantially in the next week or so, petal fall is a good time to treat the building population.

As mentioned last week and in the March Message, inspection of terminals offers the best indication as to whether treatment is necessary. If 10% or more of terminals are infested (at fruit set), then there are several options for treatment. Data from New York suggest that an application of Provado (20 oz. per acre) followed by a second application within 2-4 weeks will offer control of low to moderate populations, Agri-Mek in conjunction with horticultural oil applied 1-2 weeks after petal fall provides 4-6 weeks of protection, and back-to-back sprays of Mitac have shown an acceptable level of control, with the first application 2 weeks after petal fall and the second 7-10 days later. Pyramite (10 to 13.2 oz. per acre) is now an option with timing similar to Agri-Mek. A maximum of 2 applications of Pyramite can be used, and applications must be more than 30 days apart.

Healthy Fruit is written by Dan Cooley, Ron Prokopy, Starker Wright, Wes Autio, and Duane Greene 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. A text version can be e-mailed to you if you contact Doreen York. Please cite this source if reprinting information.