Controlling flyspeck on apples

Editor’s note: This article was originally published in Scaffolds Fruit Journal Volume 15. No. 15 on June 26, 2007. This publication is made possible by Cornell Univerisity – NYS Agricultural Experiment Station and Cornell Cooperative Extension. Check the label of any pesticide referenced to ensure your use is included.  

Flyspeck has caused more commercial losses in New York and New England over the past few years than during most of the previous decade. What has contributed to those losses? How can we prevent them from re-occurring this year? Let’s start by enumerating what we know about flyspeck biology:

Flyspeck infects a wide range of host plants. That means that the flyspeck fungi can grow on the waxy cuticle of most bushes and trees in orchard perimeters, and these hosts can produce inoculum that blows into orchards. (Recent work in Iowa suggests multiple species may be involved in causing flyspeck, so in this article I will refer to the causal organisms as flyspeck “fungi.”)

After a spore from one of the flyspeck fungi lands on an apple, the apple must be exposed to 270 hours of accumulated wetting before the flyspeck colonies become visible on the fruit. Brown and Sutton in North Carolina were the first to identify the incubation period for flyspeck, and they found the best correlations when they ignored wetting periods of less than a three hour duration. I have since found good correlations with the 270 hours of accumulated wetting over several years in the Hudson Valley when we included all measurable wetting periods. Variability among types and locations of wetness sensors is so great that quibbling about the details of whether to include or dismiss short wetting periods is probably meaningless.

In Massachusetts, Cooley and Lerner showed that ascospores for flyspeck are released around petal fall. Therefore, flyspeck colonies initiated by ascospores may begin appearing on unsprayed fruit at 270 hours of accumulated wetting after petal fall. However, scab sprays usually control the ascospores, so ascosporic infections are not common in commercial orchards.

Infections initiated by ascospores in wild hosts begin releasing conidia as soon as those infections become visible (after 270 hours of accumulated wetting after petal fall). Once those infections produce conidia, orchards in the northeast are exposed to a continuous supply of conidia blowing throughout the remainder of the summer and fall. If fungicide residues on fruit drop below effective levels, then the conidia will initiate flyspeck infections on fruit.

Where trees are left unsprayed after second cover (i.e., they are protected from flyspeck ascospores but not from conidia), flyspeck incidence and severity on fruit increases dramatically around 540 hours of accumulated wetting after petal fall. Thus, flyspeck requires 270 hours of accumulated wetting after petal fall to produce conidia on wild hosts and another 270 hours of accumulated wetting after petal fall to infect and produce visible colonies on apples.

Results of two recent trials that indicate limitations of current fungicides

Two inches of heavy rain may be enough to eliminate fungicide residues. In an experiment at the Hudson Valley Lab in 2004, we applied all of the common summer fungicides to test plots on August 17. We received 2.15 inches of rain on August 20-22. We had 270 hours of accumulated wetting between August 22 and September 26. Incidence of flyspeck on Golden Delicious fruit on September 27 was 64, 50, 31, 27 and 8 percent, respectively, for plots treated with Captan alone (30 oz of 80W/A), Flint, Sovran, Topsin-plus-Captan and Pristine. Pristine had the best residual activity, but none of the fungicides had adequate residue to completely protect against flyspeck after 2.15 inches rain.

Fungicides applied after flyspeck infections have been initiated can arrest growth of the flyspeck fungus temporarily, but they do NOT eradicate the infections. In a 2005 experiment, summer fungicide sprays were initiated at either 337 or 450 hours of accumulated wetting after petal fall to determine if these fungicides could provide post-infection activity that would reach back through either 67 hours of accumulated wetting (i.e., 337 hours minus the 270 hour threshold for conidial infections) or through 180 hours of accumulated wetting from the start of infections. None of the postinfection treatments provided satisfactory disease control. By September 26, flyspeck incidence exceeded 19 percent in all treatments, even though we maintained fungicide coverage up until harvest (i.e., less than 2 inches of rain between sprays and between the last spray and harvest). Sovran was significantly better than Pristine or Flint for suppressing infections, but it was not significantly better than the Topsin-plus-Captan standard. Thus, Pristine provides the best residual protection, but Sovran and Topsin M provide the best post-infection activity, even though the post-infection suppression is less than we had hoped for.

Results from these recent trials have caused me to re-evaluate earlier hypotheses. The long incubation period required for flyspeck coupled with our inability to accurately predict or monitor fungicide residues on fruits makes it difficult to interpret results of fungicide trials in dry years. Did fungicides applied in July or August really eradicate earlier infections, or did they just slow fungal growth enough to allow fruit to be harvested before flyspeck appeared on fruit? Wet years such as we have had in the Hudson Valley in 2004 and 2005, provided more definitive evidence concerning the limitations of our fungicides.

Combining all that we know about flyspeck along with some working hypotheses, I’ve compiled the following statements to help formulate options for controlling flyspeck.

The period of least risk for significant flyspeck infection occurs between petal fall and 270 hours of accumulated wetting after petal fall for reasons noted above.

After 270 hours of accumulated wetting after petal fall, fruit should be continuously protected with fungicides. Any gaps in protection after 270 hours of accumulated wetting after petal fall may allow flyspeck infections to be initiated.

Two inches of rain can remove virtually all fungicide protection.

Fungicides applied after infections are initiated do not eradicate all infections. Post-infection sprays will arrest incubating infections for varying (and at this point, unpredictable) periods of time. When the fungicide residues drop below inhibitory levels, the surviving flyspeck infections begin growing again. Predicting when suppressed lesions resume growth is difficult because we can’t accurately predict when fungicide residues are exhausted.

The 270 hours of accumulated wetting after petal fall incubation period for flyspeck can perhaps be viewed as a “grace period” for lapses in fungicide coverage. If apples are consistently protected from infection during summer and fungicide residues are removed by heavy rains on September 1, then flyspeck will not appear on fruit so long as fruit are harvested and cooled before they are exposed to 270 hours of accumulated wettingl. However, if apples are left unprotected through 90 hours of wetting in July or August after conidia are being released, then part of the grace period will have been used in July-August and flyspeck may appear on fruit more quickly than otherwise expected in September.

In real life, the total grace period for lack of fungicide protection during the growing season is probably less than 270 hours of accumulated wetting because flyspeck can continue to grow on wet fruit surfaces after harvest until fruit are cooled below roughly 45°F. Fluctuations in air temperatures as storage rooms are filled can cause condensation on surfaces of cold fruit already in the room, and that moisture can allow continued growth of flyspeck. I don’t know how much of a 270 hour incubation period can be completed after harvest, but I suspect that up to 70 hours of the required 270 hour incubation period could occur after harvest if fruit are not cooled rapidly. Application of a postharvest fungicide drench might suppress growth during the cool-down period after harvest, but I am not aware of any data that addresses this question.

Given all of the above, the safest approach for controlling flyspeck will be to maintain fungicide coverage throughout summer after the 270- hours of accumulated wetting after petal fall threshold has been reached. If extended rainy periods preclude timely respraying of blocks after heavy rains, then that lapse in coverage may use up part of the preharvest “grace period.”

Wet autumn weather, such as we have had in recent years, may be contributing to elevated inoculum levels in hedgerows and woodlots. Thus, extra caution (i.e., extra sprays in September and perhaps even in early October for late varieties) may be warranted until we get a dry summer-fall combination to break the current high inoculum cycle.

Late summer sprays for flyspeck can be compromised by incomplete coverage of fruit surfaces. Including a surfactant with the fungicide during late summer may be helpful, but an excess of surfactant will only cause excessive run-off, thereby leaving less residue on fruit than a spray applied with no surfactant. Probably the best way to improve coverage in late summer sprays is to reduce tractor speed and increase the volume of water applied per acre.

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