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Healthy Fruit 1997 Vol. 5:10

Jun 11, 1997

Dry and Getting Hotter

It has gotten dry, but in spite of the recent warm weather, overall the season from March 1 remains about 4_F below normal. The rainfall over the past month is about 2 in., about half of the normal amount, and since the beginning of the month about a tenth of an inch has fallen at Belchertown, compared to the 1 inch average.

In the earlier developing areas of the state, fruit have developed to 3/4". In the later areas, primarily in the western portion of the state, fruit size ranges from 1/4"-1/2".

Summertime, and Eradication is Easier

Following the end of primary scab season, reports around the state indicate that most conscietious spray programs, whether based on Sis with a protectant, or on protectants alone, have stood up well. Even with the wind, most growers have gotten good control. Scab is generally absent in commercial orchards. A few notable exceptions have occurred where early applications were not made.

If your orchard is one of these exceptions, take advantage of the hot weather to apply an eradicant. As mentioned a month ago, captan will suppress spore development, that is, burn out existing scab. This eradicant action is best at temperatures above 80 F. The equivalent of 1 1/4 lb. per 100 gal. of Captan 80W (it can be 80W, 50W or Captec, take your pick) will do the job. Since we do not have obvious cases of Benlate, Topsin-M or Syllit resistance in Massachusetts, either of these materials may be combined with captan to improve burn out. Check out the May 14 Healthy Fruit for details.

'Curc Beaming Into Orchards

In the 48 commercial orchard blocks which we are sampling on a 3-day interval, the cumulative curculio injury levels to date range from zero injury to 4% injured fruit, with most blocks remaining below .5% cumulative damage. With the very recent warm days, we have seen a noticeable increase in PC egglaying in unmanaged apple trees. In most commercial orchards, the sprays applied have thwarted any significant increase in PC egglaying. Temperatures in the mid-80s to low 90s over the past several days (6/8-6/11) is good fortune for management of the immigrating PC population. The consistently warm weather will bring out the bulk of the invading population, effectively shortening the period of PC invasion and subsequent egglaying.

At this point, the orchard management challenge is determining when to apply the final treatment against PC. With no effective monitoring trap as yet, we must rely on two other approaches. First, there is no substitute for continuing to monitor orchard trees for evidence of ongoing PC feeding and egglaying. This allows for a reasonable estimate of the active PC populations present in the orchard; any secondary PC invasions will be evidenced by increases in fresh egglaying and feeding injury between sprays. All told, regular monitoring of fruit for fresh PC injury during the interval between applications offers a fairly reliable determination of whether or not an additional application against PC is warranted.

The second approach aimed at determining when the last application against PC should occur is use of the degree-day model developed in Geneva, NY. According to this model, the last treatment against PC should be applied when 340 degree days (DD) have been accumulated since petal fall. To calculate DD, take the average of the day's maximum and minimum temperatures (For example, 80o max. and 60o min.=70o daily average.), then subtract the base level of 50o to determine the day's DD total (70o average minus base-level 50o=20 DD). In an orchard very near Amherst the accumulation has reached 175 DD as of today. In some later areas in the Berkshires, the accumulation to date is approximately 100 DD. Thus, if 20 DD are accumulated daily on average, the model would indicate that 7-9 days remain before the last PC spray should be applied. Refinements of this system are presently being made by Glen Koehler in Maine to validate the model for New England growing conditions, and we should know at the close of this PC season what modifications need to be made.

Leafminer Treatment Timing Approaching Critical

In orchards where Provado was used at petal fall or shortly thereafter, very few mines have progressed beyond the sap-feeding stage. In orchards where populations have remained low this spring and have not yet received a treatment against leafminer, a good proportion of the mines have developed into the tissue-feeding stage. This year, the majority of scouted orchards did receive a post-bloom application against LM because trap captures and subsequent levels of mines indicated a need for treatment.

Currently, we are approaching the end of the period during which action can be taken against the first generation mines. As stated in last week's issue of Healthy Fruit, our treatment thresholds for first-generation sap-feeding mines are 7 mines per 100 leaves for McIntosh and 14 mines per 100 leaves for non-McIntosh varieties. If these thresholds are exceeded, we recommend a treatment with Lannate within the next couple of days. Agrimek (with hort. spray oil) can still be used, but its effectiveness will be reduced due to increased hardening of the leaf tissue which does not allow for maximum absorption of the chemical into the foliage.

Mites Like the Heat

This hot, dry weather is very conducive to rapid multiplication of European red mites. Fortunately, orchards which received either a pre-bloom or petal fall mite treatment remain quite free of egglaying females. However, in blocks which received only 1 oil application, or even 2 oil applications with less than optimal weather conditions, egglaying females and eggs are definitely in evidence.

We have now reached the point where mite control by use of either Agrimek with oil or Vendex with oil is questionable. Neither of these chemicals works to its maximum potential when applied this late (two weeks after petal fall) because the hardening of leaf tissue bars the proper absorption of the chemical into the foliage. Based upon information from New York state, the action threshold for treatment against ERM during the first 2 weeks of June is 1 or more motile mites present on 45% of middle-aged fruit cluster leaves. If this threshold is exceeded, an application of Agrimek or Vendex with oil is possible, but the effectiveness of such an application will certainly be less than optimal. The best available alternative is Kelthane. Carzol is also available for use, but it is well known as being extremely harsh on predators.

Regarding Pyramite, the Pesticide Bureau will meet on June 25th to consider registration of this chemical for use in Massachusetts, and Ron has been asked to be in attendance. We are hopeful that the outcome will be positive and the material will be available for use by early July.

Pear Psylla Progress

Thus far, treatments of either Provado or Agrimek against pear psylla have worked extremely well, with few psylla remaining in treated areas. In areas which did not receive a treatment of Provado or Agrimek, moderate populations of psylla adults are present, but no second generation egglaying has been observed as yet.