August 21, 2006 Volume 15 No. 23 Update on Pest Management and Crop Development
Insect model degree day accumulations:
Flyspeck disease on apples continued to cause problems for some New York and New England apple growers in 2005. Last summer I published an article reviewing our current understanding of flyspeck development in northeastern United States (Scaffolds Fruit Journal 14, 20 June 2005; http://www.nysaes.cornell.edu/ent/scaffolds/2005/050620.html). To summarize, flyspeck ascospore release begins about the time that apple trees reach petal fall. Ascospores are relatively unimportant in sprayed orchards because scab fungicides prevent infections, but the ascospores initiate new infections on wild hosts in the orchard perimeters. After 270 hr of accumulated wetting, counting from petal fall (hr-awpf), primary infections initiated by ascospores on wild hosts begin releasing conidia that blow into orchards, where they can initiate infections on apple fruit. Another 270 hr of accumulated wetting (hr-aw) is required before the flyspeck becomes visible on fruit. We have documented repeatedly over the past five years that flyspeck incidence on fruit left unprotected after early June increases rapidly soon after we reach 540 hr-awpf. Dew periods must be included in calculations of hr-aw, and in some years dews contribute a significant number of wetting hours during August and September.
In the article cited above and in previous extension articles, I have suggested that Topsin M, Sovran, Flint, and Pristine would eradicate flyspeck infections on apple fruit that had not yet accumulated more than 100 of the 270 hr-aw required for lesion development on fruit. That recommendation was based on my interpretation of previous field trials, and it meant that adequate flyspeck control should be expected if the first summer fungicide spray was applied anytime between 270 and 370 hr-awpf. Unfortunately, research conducted in 2005 suggests that our model for spray timing needs some revisions. The details of the 2005 trial have been published (see citation at the end of this article) and will not be repeated here, but the resulting changes in our understanding of flyspeck are outlined below. Those who want only the practical recommendations without the confusing details can skip directly to the last section of this article.
Revised understanding of fungicide activity against flyspeck
Our 2005 field trial provided evidence that none of our apple fungicides truly eradicate pre-existing flyspeck infections. Instead, the fungicides appear to arrest fungal growth, thereby extending the incubation period for flyspeck. Infections initiated in late June or early July before the first summer fungicide is applied may become “quiescent” until fungicide residues drop below the levels required to suppress fungal growth. As fungicide residues dissipate during the interval between the last spray and harvest, some of these suppressed colonies can begin growing again and then appear on fruit prior to harvest, even when there is less than 270 hr-aw between the presumed end of fungicide protection and the time that fruit are harvested.
As an example, consider a hypothetical case where the first summer fungicide was applied at 370 hr-awpf. Some of the earliest flyspeck infections in that orchard would have a 100 hr “head start” toward the 270 hr-aw required for disease appearance. If fungicide coverage was maintained throughout the rest of the summer, those early infections would remain suppressed right up through harvest. However, if a heavy rain (more than 2 inches rainfall) occurred on 1 Sept. and removed all fungicide residues, then the suppressed flyspeck infections could resume growth and might appear after only 170 hr of additional wetting, counting from the end of the 1 Sept. rain event in this example.
If fungicides suppress but do not eradicate pre-existing flyspeck infections, then we must grapple with other unresolved issues. Do suppressed lesions really take off again where they left off as I’ve suggested in the example in the previous paragraph, or does post-infection application of fungicides set back lesion growth so that more than 270 hr-aw (pre-spray hours plus end-of-season hours) are required before the disease appears on fruit? Do weather conditions following post-infection fungicide sprays affect survival of flyspeck infections? How can we estimate when suppressed flyspeck colonies resume growth, since we have no accurate means of assessing when fungicide residues on fruit surfaces drop below suppressive levels? I don’t have answers to these questions, but the answers may not be essential for designing control programs on a practical level.
Revised spray-timing recommendations for controlling flyspeck
Given all of the evidence available to date, a conservative approach to controlling flyspeck can be outlined as follows:
Practical suggestions for late-summer control of flyspeck
The bottom line is that in years favorable for flyspeck development, we can take fewer risks (i.e., we need tighter summer fungicide programs) than we had previously supposed. In hot, dry seasons, one or two fungicide sprays will still provide complete control of flyspeck, but in wet years a final fungicide spray will be needed in September for varieties that will be harvested after September 20.
Severity of flyspeck in any given year may be influenced by the weather conditions the previous year because wet summer/fall seasons will give rise to more over-wintering inoculum in wild hosts. The higher inoculum levels will result in greater disease pressure the following year. Wet summers also give rise to situations during the growing season where fruit are unprotected due to fungicide wash-offs from thunderstorms or other rain events. Those intermittent periods without fungicide protection are of little consequence in years with dry September weather, because they generally will not exceed the 270-hr-aw grace period required for signs of flyspeck to appear on fruit. In a wet September, however, the cumulative effect of intermittent lapses during the growing season plus preharvest wash-off of fungicides can result in significant losses to flyspeck if no sprays are applied in September.
Decisions on if and when to apply a September spray should be made after careful consideration of numerous factors:
Even if fungicides are applied at the correct times, control failures can occur due to poor spray coverage. Using more water per acre and reducing travel speeds can provide improved coverage, especially for heavily cropped trees where fruit are clustered on limbs.
There has been little research on the value of spray adjuvants for improving fungicide activity against flyspeck. However, a good spreader/sticker might improve fungicide coverage and retention for sprays applied in late summer. Be aware, however, that using too much spreader can actually increase run-off from the fruit, thereby reducing fungicide residues. A really effective sticker might prevent fungicides from redistributing to the back sides of fruit during subsequent wettings. Thus, I am reluctant to recommend any specific adjuvants for late summer sprays because I’m not certain how they will affect flyspeck control.
Rosenberger, D.A. and Meyer, F.W. 2006. Post-infection control of flyspeck with new fungicides, 2005. Fungicide and Nematicide Tests (online.) Report 61:PF024. DOI:10.1094/FN61. The American Phytopathological Society, St. Paul, MN. http://www.apsnet.org/online/FNTests/reports/2006/PF024.pdf