Chemical Thinning
Fruit size on McIntosh is or soon will be in the 7 to 10 mm (1/4 to 3/8 inch) stage of fruit development in most orchards in Massachusetts. All chemical thinners are effective during this stage. This is your best opportunity to start thinning or finish the thinning job started earlier.
Observe blocks treated with carbaryl at petal fall. Favorable weather for thinning occurred in most places last week. Effects of petal fall (PF) sprays of carbaryl at the Horticultural Research Center made at PF + 2 days (May 16) were apparent one week later (May 24). Differential growth of the fruit in a cluster is obvious with some fruit abscising, pedicels yellowing, or growth being substantially slowed. If NAA was used alone or in combination with carbaryl at PF, it is unlikely that any fruit drop will be seen, nor is it anticipated to occur within the next week. Slowing of fruit growth is the only reliable method to assess thinning effectiveness of NAA at this early fruit development stage. If you have a heavy initial set, do not be lulled into thinking that your chemical thinning job was completed with a PF spray. What you have done will make the rest of the thinning job much easier, and a lot safer. Follow up treatments are appropriate on heavily set trees.
Thinning Chemicals
Carbaryl continues to be a useful chemical and a somewhat conservative choice at this thinning time. Petal fall application of the XLR formulation have caused some spur leaf chlorosis this year. If this is of concern to you, the 80WP formulation would be a better choice. Making a second follow-up application of carbaryl alone will probably cause less thinning than switching to Accel, NAA, or Amid Thin because all of these chemical thin by a different mode of action.
This is the time to use Accel. It is particularly temperature sensitive and works best when applied when two or three days are predicted to be in the mid to upper 70's following application. It is a mild thinner when used alone. In this year when more aggressive thinning appears appropriate, we generally recommend application of Accel with carbaryl. This has proved to be a very good and effective combination. Carbaryl and Accel do not need to be applied at the same time, but where aggressive thinning is appropriate, Accel plus carbaryl may be used on trees previously receiving a PF spray of carbaryl.
NAA is also effective when applied at this time. Rates between 3 and 10 ppm are common when used alone and modest to moderate thinning is desired. Where more aggressive thinning is required, we generally recommend combining NAA with carbaryl rather than using higher rates of NAA. We do not recommend application of NAA on trees that previously received Promalin or combining NAA with Accel on Delicious or Fuji. Less thinning is likely to occur and small seedless fruit may be produced that persist to harvest. Trees treated with Promalin should be thinned with carbaryl and Accel combinations to avoid small fruit.
Weather and Thinning
Temperature in the three to four days following thinner application is more important than the stage of fruit development at the time of application. When fruit size reaches 7 mm we suggest that you apply thinners at the first good opportunity when at least 3 days in the 70's are predicted. The weather forecast for the balance of the week is marginally favorable for thinning with the possibility of showers each afternoon. Do not let the threat of rain deter you from applying thinners. Experience has shown that if a thinner is applied and the spray has been dry for at least 30 minutes, you can expect 80% or more of the response from the thinner. The extended forecast for Memorial Day weekend is favorable for thinning. Have your thinners on in place when this warm weather arrives. We do not want to cry wolf, wolf, but this may be your best and last chance to thin fruit that have already reached the 7 mm fruit size stage.
See No Weevil
In order to track curculio immigration and activity in the absence of chemical controls, we are conducting several projects in blocks of unmanaged apple trees. These projects are designed to test prototype monitoring traps and attractants, and to catalog PC trap captures and activity alongside fruit development and weather patterns. One of these sites (in South Deerfield) has the highest PC population that we know of in the state, and offers fairly clear peaks in immigration and activity. At this site, immigration (measured by large sticky panels at the outside of the tree canopy) increased dramatically this weekend. In fact, nearly half of all immigrants so far were captured between 5/20 and 5/24, with the bulk of those arriving on 5/22. Increases in fruit injury have logically followed increases in immigration, from 4% (5/17, 7 mm fruit) to 11% (5/20, 9 mm fruit) to 31% (5/24, 12 mm fruit).
In contrast to substantial increases in damage in unmanaged blocks, little PC egglaying activity has been observed in commercial orchards. Over the past 3 days, 5000 fruit were sampled in 8 orchards across the state, and a total of 4 injured fruit were recorded. However, the PC season has been consistently cool (not conducive to heavy immigration), and we have no indication of what proportion of the total population has arrived. Judging from the past 2 years, we expect that the greatest amount of PC damage will occur between insecticide applications, either between 1st cover and 2nd or 2nd cover and 3rd. Spikes in damage may occur over the next week or two, but we still do not expect substantial problems from PC. This pattern was expected; as discussed in earlier issues of Healthy Fruit, we believe that the general lack of fruit on wild trees last year has depleted the overwintering population.
As we have discussed over the past several weeks, management of heavy leafminer populations in commercial orchards is not going to be easy this year. Statewide, LM growth stages are widely variable. In some later-developing orchards, eggs have yet to hatch, while early tissue-feeding mines are evident in other areas.
We can only surmise that the cool weather has delayed LM growth in some orchards, and that others have remained warm enough for sustained growth. Because of the wide gap in growth stages, timing a spray against the first generation (or subsequent generations) becomes much more difficult. It is critical that the treatment against first-generation LM is applied when sap-feeding mines are present; for some orchards it is getting late, for others it is still too early. Regular visual inspection of leaves for the abundance and development of LM is the best tool for timing treatment—refer to last week’s issue for recommended treatment thresholds.
Even in areas with annual mite control problems, there is very little evidence of first-generation red mite adults or nymphs. The lack of a building ERM threat could be a result of highly effective pre-bloom management practices, or ERM absence could be due to the occasional driving rains of the past week. Light early-season populations are always welcome, but with warmer, drier weather, the population can increase rapidly.
In many orchards last season, we saw a large accumulation of white apple and rose leafhoppers present at harvest. However, few growers used Sevin as a thinner last year, opening the door for season-long LH population expansion. This year, most growers have used Sevin, and control of WALH nymphal populations has been excellent. In orchards that did not receive a thinning treatment and do not intend to use Provado for LM control, growers are advised to keep an eye on the developing LH populations; a mid-June treatment with a half-rate of Thiodan should clear up any potential problems.
