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Healthy Fruit 1997 Vol. 5:11

Jun 18, 1997

Dry and Getting Hotter

Most growers are wondering whether the lack of rain is beginning to be a problem. In most soils, fruit trees are still getting enough moisture. However, in sandy or shallow soils, lack of moisture is getting to be a real problem.

Diseases Down

The dry weather has all but stopped disease progress in commercial orchards. With primary scab done, and very little brown rot pressure on stone fruit, growers are looking at summer disease management as the only disease issue of any concern. In fact, this is of minor concern until humidity and/or rainfall increases. Low rates of fungicide (no more than 3/4 lb. Captan 80 or equivalent) will be enough to protect fruit for a 2 to 3 week period. In the table below, Dave Rosenberger of Cornell has listed protection intervals and rainfall needed to wash off a number of fungicides useful against summer diseases.

Rate per 100 gal. dilute spray
spray interval days
maximum rainfall (in.)
Benlate +
3 oz
or Mancozeb
1 lb
or Ziram/sulfur
1+1 lb
Topsin M +
3 oz
or Ziram 76W
1.5 lb
or Captan 50W
2.0 lb
Ziram 76W
1 lb
Captan 80W
3/4 lb

PC Prosper

With the very warm and humid weather of last week, we saw the largest numbers of curculio captures of the year on sticky, clear panels placed just outside the tree canopies. These traps are designed to intercept PCs approaching the tree canopy by flight. The number of PCs captured on these traps has since dropped significantly, leading us to conclude that last week's immigration was likely one of the largest PC influxes of the year, which was followed by a large amount of egglaying activity late last week and into this week. In a monitored block of unmanaged apple trees, PC damage increased from 15% to 80% over the past week, and fresh PC egglaying scars have been observed in many sampled commercial orchards as well.

Thus, the question remains as to when the last PC spray should be applied. In last week's Healthy Fruit, we discussed the temperature driven model from New York state that aims to predict the timing of the last PC treatment based on a degree-day (DD) model. Apparently we misinterpreted the implications of the NYS model; a point which waskindly brought to our attention by Glen Koehler of Maine earlier this week. We had indicated that the last PC spray should occur when 340 DD have been accumulated after petal fall. In truth, the NYS model simply suggests that the final spray should be timed such that the residual activity of the chemical is still effective when 340 DD are accumulated.

To date in an orchard near Amherst, 295 DD have been accumulated since petal fall. Given the dry weather pattern which we have experienced thus far this season, coupled with the residual effectiveness of Guthion and Imidan (approx. 10 days under dry weather conditions), we conclude that a PC application made last weekend or early this week should be enough to carry through to the end of the PC season in early- and middle-developing areas. In Conway, as well as other later-developing areas, the final PC spray will likely occur toward this weekend. If there are any PC which have yet to immigrate into the orchards, they should surely do so over the weekend, and the residual effects of sprays this week should handle the stragglers.

Leafminer Languish

Provado, which was used this year in many monitored blocks as a treatment against leafminer, seems to have arrested LM development at the sap-feeding stage. Blocks which did receive such a treatment are showing little or no evidence of mines progressing to tissue-feeders. In blocks which received no LM treatment, tissue-feeding mines are readily apparent, and the first pupae have been seen. Adult parasites have been seen in blocks which were not treated for LM this year; these parasites will contribute to suppression of LM if they can endure the sprays directed at plum curculio.

Red Mites Resurgent

Eggs of the first summer generation of European red mites are just beginning to hatch. The very warm weather conditions of the past week favor rapid ERM development, particularly in blocks which received little or no early treatments against ERM. Growers who did not apply Savey, Apollo or Agrimek should keep a careful eye on the presence and developing population density of ERM nymphs.

Onset of Leafhoppers

Growers who used Provado this year for leafminer control should have relatively little to worry about with leafhoppers. Also, growers who have applied 2 sprays of Sevin XLR as a thinning agent should experience reduced LH problems. If only a petal fall application of Sevin XLR was used, or if neither Provado or Sevin was applied, growers should keep a sharp eye out for immigrating rose leafhoppers and developing nymphs of white apple leafhoppers. A good number of RLH adults was seen earlier this week at the Horticultural Research Center in Belchertown, indicating that RLH immigration is now in progress. To achieve maximum effectiveness of sprays against RLH, we suggest holding off on the treatment until RLH nymphs hatch later this month.

With leafhoppers, it is the 3rd generation which causes the greatest problems; substantial excrement on fruit and nuisance to pickers. However, it will soon be the time to nip LH populations before they can build numbers in their 2nd and 3rd generations. A half-rate of Thiodan should go far toward control of the developing population.

Stopping Stubborn San Jose Scale

In recent years, we have not seen much San Jose scale on fruit at harvest in most orchards. However, some problem blocks do remain, especially those comprised of full standard-sized trees. Oil at 1/2" green would have been a good first step toward control of SJS, but this may need to be followed up by a pesticide treatment in the next week or two. If such a treatment is necessary, a spray of Lorsban against the crawlers in the next couple of weeks is the next step in dealing with this pest.