May 23, 1997

In This Issue:

*Pearleaf Blister Mite (PBM)

*Black Rot Causes Fruit Rots On Pears As Well As Apples

*New Orchards to Be Seen on Hudson Valley Tour - July 29

*Woodchuck Follow-Up

*Boron Nutrition

*Zinc Tree Sprays

Pear trees that have not had a prebloom oil spray for several years may now be showing small green or reddish blisters on a few or many leaves. Within the blisters are tiny white or lightly colored, elongated mites, looking more like small worms than mites. PBM is similar in size to pear rust mite or apple rust mite, but can be very difficult to see, even with a 15X magnifying glass because it remains most of the time within the blisters. Viewed from the undersurface of the leaf, a mite or two is sometimes visible through the small opening in the middle of the gall. Adults overwinter at the base of buds and beneath outer bud scales, becoming active as the buds begin to swell in early springs. They feed upon both leaf and floral tissues. Galls are formed as a result of mite feeding, and eggs are deposited within the galls. Eggs hatch and the new generation remains within the galls until mature, one to two weeks later. Depending upon what publication is read, the mites either continue to reproduce and form new blisters throughout the growing season, or pass through June, July and August in a non-reproductive stage, producing only 2-3 generations per year. In September or October they migrate from leaves back to protected positions beneath outer scales of flower buds or vegetative buds.

Most recent publications on tree fruit pests attribute a rather serious fruit injury to early PBM feeding. Before the flower clusters have fully opened, some mites feed upon tissue that will later become the pear fruit. These injured areas become russetted as the fruitlet develops, and normal growth can be disrupted. The mature injured fruit exhibits brown russet spots, oval in shape, often sunken, 1/4 to 2 inch in diameter, frequently merged with other spots to form larger, sometimes deformed areas.

Effective control requires that treatments be made during the period between harvest and the green cluster stage of flower buds. Treatments applied later than green cluster may not prevent damage to fruit and foliage.

Cornell spray recommendations indicate that 2-3% oil applied before swollen bud is effective - the same timing used to kill European red mite eggs, and to repel pear psylla female adults. But for improved control, a mixture of 1% oil plus Diazinon 50WP (1 lb per 100 gallons dilute) or plus thiodan 50WP (2 - 1 lb) is recommended. This spray should be applied in September or October after harvest, and again just before green tissue becomes visible in the spring. In California, a postharvest spray of liquid lime sulfur is recommended (2 gallons in 100 gallons of water per acre).

(Source: David Rosenberger, Cornell, Hudson Valley Lab, May 19, 1997 Scaffolds)

Last year, black rot appeared in some blocks of Bosc pears. The control for back rot on pears is similar to that for apples, but the choice of fungicides is more limited. Captan and Topsin are not registered on pears. Therefore, Benlate is the only really effective fungicide for controlling black rot on pears. The mancozeb fungicides will be adequate under light pressure, but not where inoculum levels or weather conditions favor development of black rot.

Black rot infections can occur throughout the season. Therefore, in blocks where rot is a concern, Benlate should be added to mancozeb sprays beginning at petal fall and should be included with ziram sprays applied during summer.

If you want to see how some of the best newer orchards are being planted and managed, mark your calendar for Tuesdsay, July 29 (rain date, July 30). Ulster County (New York) Fruit Specialist Warren Smith will lead the Connecticut group, making two or three orchard stops in the morning, and then several more in the afternoon. All sites are within short distances of each other, so little time will be wasted on the road. In the afternoon, Dr. Terence Robinson, Cornell, Geneva, will accompany the group to describe and report on trials he has set up with growers to test Vertical Axe, Freestanding Central Leader, Slender Spindle, and possibly others. We will see some of these trials first hand. A stop at a new packinghouse facility, and an orchard emphasizing direct farm marketing will likely be included either in the morning or afternoon.

There is no plan to hire a bus for this trip, but a count of persons interested in making the trip will be needed to assist our hosts in accomodating the group. If you would like to participate, please let me know by postal-mail, e-mail, telephone, or FAX, indicating the number from your farm who likely would go. Those I hear from will receive further details of the trip.

In an earlier issue of this newsletter (April 10) I wrote that I was looking for a faster way to seal smoking smoke-bombs in groundhog dens. I found 40 lb bags of pelletized limestone at The Home Depot. The bags were of a heavy plastic, and the pelletized granules provided a reasonably malleable unit that I thought would mold itself to fit any shape den opening. Hoping the easiest possible method would work, I just dropped a bag over escape holes without having first placed a board over the hole. That made a nice fit, but left the bag exposed to any alarmed resident. So far, none of the bags have been ripped open. I have been pleased with the speed and minimal labor involved with this technique.

I have been recommending annual application of boron to apple and pear orchards in Connecticut at the rate of one pound actual boron per acre per year. Apple leaves I collected from my orchard in early August, 1996 showed between 31 and 34 ppm boron in the four samples. That is enough to prevent deficiency symptoms in the fruit, but is lower than Warren Stiles= (Cornell) recommed range of 35-50 ppm. Because the orchard has been getting one pound of boron per acre, annually, I conclude that rate is a little low. Therefore, I will now suggest two pounds per acre per year.

Boron can be applied either as a tree spray, or spread on the ground. Dr. Stiles believes more boron gets to the roots if application is made directly to the ground. Roots need boron to aid in calcium uptake. If the soil supply is adequate, fruits and shoots will receive adequate boron via the tree=s plumbing system.

Excess boron is toxic to plants, and can produce early ripening and fruit drop in apples. Therefore, accuracy is important in the application rate. An accidental doubling of the 2 lbs per acre rate would probably not produce undesirable results but a tripling of rate probably would.

Because such small amounts are needed per acre, it is likely that spraying (the ground) will be more convenient than spreading dry fertilizer borax. Sprayable forms of boron, such as Solubor^TM can be dissolved in water and applied with a weed sprayer. Five pounds of Solubor contain one pound of actual boron.

The 2 lb boron per acre rate (10 lbs Solubor) is the broadcast rate, assuming it is applied uniformly over an acre of ground. If it is applied in strips, you will need to adjust the rate so trees will not get more than they would get in a broadcast application. Thus, in a young orchard, where roots extend only, say, half as far as in a mature orchard, the boron could be applied in a narrow band (2-4 feet wide) at half the mature orchard rate; that is, one pound rather than 2 pounds of boron per acre. Use leaf analysis in late July or early August to determine how well your application rate is meeting the intended goal of 35-50 ppm.

Another way to estimate the boron requirement in a strip application is this: An orchard in which the dilute spray requirement is 300 gallons would need 2 pounds of actual boron per acre of orchard, regardless of how wide a strip is treated. Therefore, an orchard that requires only 150 gallons per acre as a dilute tree spray would need only half as much boron per acre in a strip application over the root zone.

I don=t see symptoms of zinc deficiency in my own orchard, yet leaf analysis showed all sampled varieties were well below the 35-50 ppm levels recommended by Warren Stiles, Cornell. His recommended treatment to combat the condition (8-10 ppm) is to apply zinc chelate (EDTA) sprays at the rate of one pound dry formulation, or one quart liquid formulation per 100 gallons dilute equivalent at tight cluster to pink bud stage, and again 2-3 weeks after petal fall, and a third spray 4-5 weeks after petal fall.

When the zinc-containing fungicides (mancozeb, Polyram, and Ziram) are used throughout the growing season, leaf analysis usually shows adequate zinc. Growers who use captan, a non-zinc fungicide, may benefit by inclusion of one or more of the zinc chelate sprays in the early post-bloom season.

_________________________

David A. Kollas

Extension Pomologist

Phone: 860-486-1944

FAX: 860-486-0682

email: dkollas@canr1.cag.uconn.edu