April 24, 1997

Hindinger Farm, 835 Dunbar Hill Road in Hamden grows tree fruits, vegetables and strawberries. A farm tour, beginning at 5:30 PM will be followed by a gathering in the barn to hear from researchers and others on topics of current interest. Philip Gordon will speak briefly about commercial chestnut growing opportunities. The meeting will be officiated by Donald Preli, president of the Connecticut Pomological Society. All interested persons are welcome. I will assess grower interest in a possible one-day tour in the lower Hudson River Valley in late July, or an earlier date.

DIRECTIONS: Dixwell Avenue to Benham Street. Right on Dunbar Hill Road.

Tent Caterpillar hatch also began about April 20; the black larvae emerging from their black, glassy egg-masses. They are very tiny now, but begin feeding on foliage immediately and produce protective webbing around their colony. Feeding will continue until Petal Fall or later, if no insecticide is used before then.

Merck, Technical Services representative, Dr. James Johnson is trying to get the message to orchardists that Agri-mycin 17 and Micoshield purchased before March 25, 1997 may be below minimum specifications due to natural degradation. If you have such product, contact your supplier before using it. If it is identified as in the lot that is below specifications, it will be replaced.

The following article, by Dr. Wayne Wilcox, Cornell, Geneva, was written 2 years ago, but it is included here for those who may want a brief practical review.

Remember that the serious blighting of twigs, branches, trunks, and rootstocks are entirely the result of secondary infections, which only occur after primary infections (blossom blight, mostly) develop. As with apple scab, the best way to control secondary infections is to control the primary ones to begin with.

Although there are exceptions to every rule, it is generally accepted that four separate events must occur in the proper order before blossom blight infections develop (which is why we don't see the disease every year). These are:

Once blossom blight develops, the bacteria present in infected tissue can be spread to shoots by the feeding of insects (aphids, leafhoppers) or by rainfall associated with "trauma" events (such as hail or severe wind-whipping) that create the wounds necessary for the bacteria to gain entry.

A few years ago, Dr. Paul Steiner at the University of Maryland developed a model (termed MARYBLYT) for predicting fire blight infections, based heavily on points #2 and #3 above. Specifically, the model predicts that blossom infections will not occur until (I) bacterial populations reach a threshold level associated with a MINIMUM accumulation of 200 DEGREE HOURS (above a base of 65F) after the first blossom in the orchard has opened; and (ii) a wetting event occurs after this threshold has been reached.

To determine degree hour accumulations, it is merely necessary to know the daily high and low temperatures in the orchard each day. Then, assume that the high and low each prevailed for 6 hr and that the average of the two prevailed for the remaining 12 hr of the day. [For example, on a day when the high and low temperatures were 77 and 57 (average 67), the accumulated number of degree hours above 65 would be 12 x 6 = 72 for the 6 hr at 77; 0 for the 6 hr at 57; and 2 x 12 = 24 for the 12 hr at the average temperature of 67. That is, there would be a total of (77 + 0 + 24) = 101 degree hours accumulated for that particular day, which would be added to those of all other days since first bloom, to give a running total].

There is also a "fine tuning" adjustment for cool days during bloom: the running total should be reduced by 33% after one day in which the daily high is less than 65; reduced by 50% after two consecutive days below 65; and reduced by 100% (start all over again) after three consecutive days below 65 or after any one day (or night) that goes below freezing. However, you reach a point of no return (no downward adjustments for cold weather) once the running total has gone above 400 degree hours.

We have always recommended that decisions about streptomycin use be influenced by both an orchard risk factor--particularly varietal susceptibility, rootstock susceptibility, and previous fire blight history--and an environmental risk factor. Blocks with a low orchard risk factor can tolerate relatively more Environ- mental risk, but even "low risk" blocks are in serious danger when a wetting event during bloom is preceded by enough warm weather to produce very high inoculum levels. For instance, the fire blight epidemic of 1991 in western and central N.Y. was caused by two blossom infection periods, each of which occurred after more than 2,000 (!) degree hours had accumulated since first open blossom.

Therefore, the MARYBLYT criteria should be used as a guide to timing sprays. Carefully determine the date of the first open blossom in the orchard, and keep a running total of accumulated degree HOURS above 65. Consider the weather risk to be high and rising once 200 degree hours have accumulated, especially if continued warm weather is forecast, and consider infection of susceptible varieties probable during the next wetting event. Ideally, strep should be applied before the rain occurs, but sprays applied within 24 hr after the start of the infection period will still be of significant benefit (remember that strep must be absorbed by the flowers, so wait for the rain to stop or it will be washed off). Whether to spray preventively, wait the forecast out and spray afterwards if the rain really does occur, or just ignore it depends on the usual factors: how far over the 200 hour threshold you are, value and characteristics of the block (e.g., danger to tree framework or rootstock on smaller trees), history of the block. Once a strep spray has been applied, wait 3-4 days and check the weather forecast, then reapply before the next wetting event if sufficient blossoms are still open.

In general, several people who have worked with the program find it useful but conservative in recommending strep sprays for our conditions. It hasn't missed any infection periods, but seems to call for "high risk" conditions more often than experience has shown to be necessary. Nevertheless, the concepts are very useful, and whether the appropriate threshold value is 200 degree hours or somewhat higher (probably depends heavily on the orchard), recognize that risk increases directly above this level.

If your non-russet pear varieties have had a history of russetting in recent years, consider a miticide for pear rust mite soon after Petal Fall. Effective materials include Kelthane, Vendex, Thiodan, Carzol, and Agri-Mek. Apollo and Savey are not effective on rust mites. If rust mite population is rather low, there may be only a little russet around the calyx of the mature fruit. The more mites, the greater the russetting. Injury occurs in the early weeks after Petal Fall, continuing to early July. No reason to make a treatment if these mites are not present, but it takes a good eye, using a 10X magnifying glass, to see rust mites.

Look for them on leaves or fruitlets.

Leafminer eggs, placed randomly on the underside of young leaves, are about twice the diameter of the winter eggs of European red mite, but are not so obvious because they have very little color. They are light yellow, oval in outline, flattened on the leaf, resembling a small blob of smooth gelatinous material. A magnifying glass of 5-10X magnification is needed to count them. The stop-sampling technique below was developed by Cornell University researchers for spotted tentiform leafminer.

If you find your monitored block exceeds the threshold, you may choose either to treat first generation with Agri-Mek or Lannate; or wait until second generation, when a Provado treatment might be the best choice.

_____________________________

David A. Kollas

Extension Pomologist

Phone: 860-486-1941

FAX: 860-486-0682