April 10, 1997

IN THIS ISSUE:

Otherwise, send a check for $11.00, payable to University of Connecticut, C.E.S., and mail to Fruit Growers Newsletter; Department of Plant Science, U-67, 1376 Storrs Road, Storrs, CT 06269-4067.

Note that the label still requires a tank mix with horticultural spray oil (not a dormant oil) for use on pears.

Woodchucks are starting new families within their burrows now. April is a good time to stop their development, where unwanted in orchards. Like people, these animals prefer to remain indoors on cold nights, but when the evenings become hot, they may spend much of the night out of their den, making it difficult to be sure they are home when you want to treat them.

Smoke bombs containing sodium nitrate and sulfur are effective in killing woodchucks that are within the burrows when the bombs are lit. Giant Destroyer is the brand of bomb I find available locally in hardware stores.

Research in New Hampshire and New York has shown that, typically, 96-97% of the ascospores that eject from infected leaves during a given wetting event are ejected between 8 AM and 7 PM (D.S.T.). Consequently, the New England Apple Pest Management Guide (NEAPMG) no longer uses the original Mills Table and method for predicting scab infections. The original table and method were based upon the assumption that ascospore discharge was not influenced by time of day.

So you conclude there was no infection in your orchard. That will be a pretty safe assumption unless there was a lot of scab in your orchard last year. Going back to that 3 or 4% of ascospores that typically shot after 7 PM, the number of spores that became airborne would be low if there were few scabby leaves on the orchard floor. But 3 or 4% of a lot of spores might be a lot.

The bottom line: If you had more scab than usual last year, figure your hours wet and average temperature both the new and the old way. Then make your own choice. There is no guarantee with these methods.

Plan to begin a fungicide program at the Green Cluster stage where this fungal disease has been present in previous years. Benlate at 2-3 oz per 100 gallons, plus one of the other labeled fungicides at half rate (ferbam, ziram, or mancozeb) are usually effective at 10-14 day intervals through early June. Where Benlate has been used a lot in previous years, there is a possibility of resistance. Mancozeb alone at 1.5 lbs Manzate 200 per 100 gallons dilute equivalent, was as good as Benlate (4 oz.) and better than other labeled fungicides in 1991 trials by David Rosenberger in the Hudson Valley. A three week spray interval is usually adequate during the summer. Information in the Compendium of Apple and Pear Diseases indicates that primary infections are caused by spores that are carried from fallen leaves and twig cankers within pear trees by rain, rather than by air currents. Secondary infections can originate from primary or secondary infection during wet periods.

Under field conditions brown rot infection is unlikely before flower pistils are exposed. Pistils are highly susceptible to infection, and they often protrude from blossoms even before petals have opened. Infection requires rain or heavy dew, or near 100% relative humidity, for a time that varies with temperature. The table below, from work by Dr. Wayne Wilcox at Geneva, NY, illustrates the relationship of temperature and hours wetness to probability of infection.

This table shows that infection developed in 85-97% of blossoms treated with brown rot spores if blossoms were kept at 46F for 24 hours.

It is unlikely that any commercial peach orchard would develop 85-97% blossom blight after 24 hours of wetness, because infection incidence is related to inoculum density. Flowers directly beneath a brown rot mummy would have a 85-97% probability of being infected, but the relatively low inoculum density in an orchard with few mummies and brown rot twig cankers would result in little infection at temperatures in the 40's and 50's.

The source of brown rot spores is brown rot cankers on twigs infected the previous year, and brown rot fruit mummies in stone fruit trees or on the ground. Spores are carried by both wind and rain.

(From a 1994 article by Dr. David Rosenberger, in Scaffolds)

Following is a summary from a 1992-93 fungicide trial conducted at the Hudson Valley lab in Highland:

Black knot on plums can be controlled only by using a combination of sanitation (removing knots from trees annually) and effective fungicides applied from the popcorn flower stage through second cover. In 1992, several fungicides for black knot were evaluated at the Hudson Valley Lab. Infections occurring in 1992 developed during and after bloom in 1993. (Black knot usually requires two years to develop before knots become full grown and black.) As shown in the table below, Bravo is the most effective fungicide for controlling black knot. Note that in our trial we used Bravo at the highest label rate in five applications, including two applications after petal fall, so as to avoid confounding our test results by using two different fungicides within the same season. However, Bravo must not be applied to plums after petal fall in commercial orchards because applications after petal fall may cause fruit injury and are therefore prohibited on the label.

The best strategy for controlling black knot involves the following:

Other researchers have shown that none of the other fungicides labeled for brown rot on plums will control black knot. Thus, neither Funginex nor Roval is effective against black knot, not are any of the SI fungicides that might eventually be labeled for controlling brown rot on plums.

If you saw fireblight in your orchard last year, be sure to review the information on pages 21, 22 of the 1996-1997 New England Apple Pest Management Guide. For additional information, you may request a copy of the 91 page booklet Fire Blight - Its Nature, Prevention, and Control; a 1995 revised USDA publication by van der Zwet and Beer. Contact Agricultural Publications Bulletin Room, 8 AM - 4 PM, at 860-486-3336.

_________________________________

David A. Kollas

Extension Pomologist

Phone: 860-486-1944

FAX: 860-486-0682

dkollas@canr1.cag.uconn.edu