Testing-Wetness Hour Models to Predict Need for Summer Fungicide in New England
The usual way of controlling sooty blotch and flyspeck diseases of apples is by spraying with fungicide every two or three weeks for most of the summer. Research indicates however that since the fungal pathogens responsible for these diseases thrive in dampness, control could be effected just as well, and at lower cost, by scheduling spraying as some function of the accumulated hours of wetness over the course of the season. Researchers in North Carolina have suggested commencing the collection of wetness data ten days following petal fall. An application of fungicide is normally made in early June anyway, for control of apple scab, but these researchers suggest that further application of fungicide be delayed until approximately 200 hours of wetness have accumulated.
Other researchers in New York have suggested commencing the collection of wetness hours with the apple scab spray, then making the next spray after 150 hours of wetness have accumulated. The purpose of the research described here was to test these models to see whether either might be applicable for central New England.
Messrs. Clark and Goodband laid out several ½-acre blocks in their respective orchards. One block in each orchard was designated to receive applications of fungicide according to the usual practice; applications would be withheld in another pair of blocks until the New York threshold was reached, and from a third pair until the North Carolina threshold was reached. Mr. Goodband designated yet another block in his orchard where fungicide spraying was held back even longer, until 336 wetness-hours after petal fall, or 262 wetness-hours following the apple scab spray. This is referred to as the “extended” treatment. Fungicides used were Captan and Benlate at Clark’s, and Captan and Topsin at Goodband’s. The apple varieties grown were Golden Delicious and McIntosh at Clark’s, and Empire and McIntosh at Goodband’s.
At Clark’s orchard petal fall occurred on May 10, and Captan was sprayed June 9 for apple scab control. As 1998 was an exceptionally rainy year, the New York threshold occurred too early to be of any use, so this model was abandoned. The North Carolina threshold occurred July 2; this block was sprayed July 9, and again July 26. The block receiving the standard treatment was sprayed June 22, July 9, and July 26. The difference between the North Carolina treatment and the standard, then, was that the standard block was sprayed June 22, and the North Carolina treatment was not.
At Goodband’s petal fall occurred May 11. He also sprayed Captan for apple scab on June 9. Like Mr. Clark, he too abandoned the NY model. The NC threshold occurred July 2 at Goodband’s, and he sprayed the NC block that day. The extended treatment was not sprayed until July 16. The spraying schedules of the two orchards are summarized below. An “x” indicates the treatment was sprayed on that date:
Treatment 6/9 6/22 7/9 7/26 6/9 6/24 7/2 7/16 8/17
Standard x x x x x x x x
NC x x x x x x x
Extended x x x
Results: Little sooty blotch was observed in any block at either orchard. At Clark’s orchard little or no significant difference was observed in the incidence of flyspeck between treatments, indicating that use of the NC model to eliminate a spray can give acceptable results.
Mr. Goodband reported similar results; however, since at his orchard the NC treatment delayed an application, rather than eliminating it, his results only show that there is some flexibility in the scheduling of the fungicide spraying. Apples subjected to the extended treatment displayed considerable flyspeck. Forty-three percent of McIntosh apples from this block were afflicted by harvest time, which is a commercially unacceptable level. Mr. Goodband also reports that the Empire appeared to suffer less from flyspeck than did the McIntosh apples. He attributes this to the more open manner of growth of the Empire trees.