Scaffolds 00 index

APPLE DISEASE CONTROL WITH NEW FUNGICIDES

(Wayne Wilcox wfw1@nysaes.cornell.edu and Dave Rosenberger dar22@cornell.edu, Plant Pathology, Geneva & Highland)

Apple growers have two new options for controlling diseases this season. "Sovran" from BASF and "Flint" from Novartis are broad-spectrum fungicides from the new chemistry class commonly known as strobilurins. Both materials received federal registrations for pome fruits (apple, pear, and quince) in 1999, and New York State registrations are now in place as well. They are welcome additions to the "tool chest", not only because of their inherent activities, but because they should take some pressure off the SI fungicides. This is important not only where the SIs have begun to "slip" due to resistance development, but also where they haven’t (i.e., let’s keep it that way). In an ideal world, each fungicide group would help keep the other alive.

Origin and mode of action of strobilurin fungicides:

Strobilurin chemistry was derived from a natural anti-fungal compound that occurs in a small mushroom, Strobilurus tenacellus, which grows on fallen pine cones in Europe. Chemists in several companies modified the original compound to make it more stable and more effective as a fungicide.

The strobilurins are very active against a wide array of plant pathogenic fungi, generally at rates of only one to three ounces of active ingredient per acre. They have very low toxicity to birds, earthworms, beneficial insects, predaceous mites, and mammals (including humans). They break down quickly in soil but have good residual activity on foliage and fruit. Because of their broad spectra of activity and favorable environmental profiles, they are the most significant new group of fungicides to be developed since the sterol inhibitors.

Unlike the SI fungicides, the strobilurins are excellent inhibitors of spore germination; thus, they are excellent protectant fungicides. These materials are retained primarily within the waxy cuticle of leaves and fruit, which means that they are more rainfast than traditional protectants. This also means that they don’t redistribute very well from leaf to leaf in rainwater, although they do redistribute well within the waxy layers of a given leaf (or fruit). Furthermore, a small portion of the total dose does diffuse from the surface of a sprayed leaf and, after a few days, enough accumulates on the other side so that it offers fungicidal protection on that unsprayed side (termed "translaminar" activity). This general pattern of fungicide movement is unique to the strobilurins, and different manufacturers have made up their own trademarked names to describe it, e.g., "surface systemic" for Sovran and "mesosystemic" for Flint. You’ll be hearing these terms in the advertisements.

In addition to being excellent protectant fungicides, the strobilurins are powerful antisporulants. That is, when applied beyond their period of true "kickback" activity, they allow lesions to develop but few secondary spores form on these lesions. This is particularly significant for a disease like apple scab, where economic damage (fruit scab) is usually caused by the secondary spores that develop on infected leaves. For instance, in trials conducted in Geneva in both 1996 and 1998, early infection periods were missed (unintentionally) and significantly less fruit scab developed when the first two sprays consisted of a strobilurin (Sovran in 1996, Sovran or Flint in 1998) rather than an SI plus mancozeb. This reduction in fruit scab was directly related to the reduced number of sporulating lesions produced on cluster leaves treated with strobilurins in the early sprays versus those treated with other materials.

The strong protectant and antisporulant activities of these materials are functions of their retention in the cuticle on the surface of the leaves and fruit. Conversely, good curative or kickback activity usually requires a fungicide to penetrate the cuticle and get inside the leaf, i.e., to get down where the fungus is doing its business after it’s established an infection. Thus, the strobilurins generally are not as effective in a kickback mode as are compounds with a higher degree of systemic activity, such as the sterol inhibitors. However, apple scab may provide an exception to this general rule. That is, the apple scab fungus grows just beneath the cuticle, so enough fungicide to provide true postinfection control may actually "leak through" the underside of the cuticle and do the job. Both Flint and Sovran are labeled to provide approximately 4 days of postinfection control for apple scab. At this point, however, the trials that have led to these claims are extremely difficult to evaluate, and it’s not clear whether postinfection sprays truly kill the incipient infections or merely keep them from sporulating (in which case, they could potentially reactivate without additional applications of the fungicide). There is no question that Sovran and Flint provide scab control when applied postinfection, but it seems risky to deliberately design postinfection control programs with these materials until more is known about the details just discussed.

Used alone without contact fungicides, Sovran and Flint will perform in the orchard similarly to SI-protectant tank mixes in their heyday. It is important to recognize that these strobilurin fungicides control scab (and many other diseases) on apple fruit at least as effectively as mancozeb and captan, and much more effectively than SI fungicides ever did when the SI’s were applied alone. Sovran is labeled for use at 10-14 day intervals, whereas Flint is labeled for use at 7-10 day intervals. Based upon limited comparisons of the two (Flint has only been available to the university community since 1998), it would appear that this difference in recommended spray intervals is a result of "product positioning" by the respective manufacturers rather than differences in product activity. Spray intervals of greater than 10 or 11 days are not recommended during the primary scab season, due in part to the need to cover new tissues as they emerge.

What about fungicide resistance?

Strobilurin fungicides work by inhibiting a single biochemical pathway involved in mitochondrial respiration in fungal cells. Mitochondria are the energy-producing units within cells, so disrupting mitochondrial function results in death of the fungal cells as they "run out of gas". Because strobilurins inhibit a single biochemical step, resistant strains of various pathogens will develop if these fungi can utilize an alternative biochemical pathway that bypasses the step blocked by strobilurins. Resistance to strobilurins has already appeared in powdery mildews of cereal grains and cucurbit crops in Europe and Asia, as well as in Botrytis of greenhouse crops. Thus, resistance is a very real concern, and resistance management must be incorporated into plans for using strobilurin fungicides from Day 1 of their introduction.

To date, strobilurin resistance appears to follow the "Benlate model"; that is, resistant isolates are virtually immune to the fungicides and multiply rapidly if they are not controlled by some other material. Furthermore, a fungal strain that is resistant to Sovran will be resistant to Flint and vice versa. Therefore, both companies have agreed on identical labeling which requires use patterns that incorporate resistance-management principles: (i) No more than four sprays of any strobilurin may be used per season; and (ii) A strobilurin fungicide can be used no more than three times in a row; if two or three sequential applications are made, an effective unrelated fungicide must be used in the next two applications before strobilurin use can resume.

Note that tank-mix combinations are NOT a part of this strategy, and that both fungicides are priced to be used alone. Thus, this strategy is to (i) minimize the selection of resistant strains by limiting the number of selection events (sprays); and (ii) limit the opportunity for resistant strains to multiply, by using unrelated fungicides in rotation. Restricting the number of sequential strobilurin sprays to two might be an even more effective anti-resistance strategy, although three is legal. Economics will help enforce the limited-spray strategy, but it is important that growers and advisors not deliberately short-circuit the intent to limit the buildup of resistant fungus strains; e.g., by failing to rotate with effective unrelated materials. For instance, rotating Flint or Sovran with only benzimidazole/captan sprays would not be a good resistance-management strategy with respect to the powdery mildew fungus, because mildew is already resistant to the benzimidazoles in many orchards.

Strobilurin fungicides can be phytotoxic to some crops:

Sovran is phytotoxic to a few sweet cherry varieties and therefore will not be registered for cherries. All of the foliage on Somerset, Sweetheart, Valera, Van, and Vandalay can be killed if trees are sprayed directly with Sovran. Less severe phytotoxicity (mostly leaf spotting) has been observed on Cavalier, Emperor Francis, Royalton, Schmidt, Summit, and Viva trees that have been sprayed directly. Drift (other than direct blow-through) and the concentrations resulting from residue remaining on spray tank walls are believed to pose relatively little danger. Tart cherries and other sweet cherry varieties show little or no phytotoxicity from Sovran, even when sprayed directly.

Flint is phytotoxic to Concord grapes when applied directly, and is specifically not labeled for use on that variety. Azoxystrobin (Abound, Quadris), another strobilurin fungicide which is registered for use on grapes and some vegetable crops, is extremely phytotoxic to certain apple varieties (e.g., Macs, those with Mac parentage, and Gala), even at very low concentrations resulting from drift or spray tank residue. Thus, each of these strobilurins has a problem with phytotoxicity to a few varieties of one specific crop. Fruit growers producing apples and stone fruits or apples and grapes may wish to consider the potential for phytotoxicity when selecting which of these fungicides they will use on their farm.

Sovran and Flint provide excellent control of apple diseases:

In university trials, Sovran and Flint have provided excellent control of apple scab, powdery mildew, sooty blotch, flyspeck, and black rot. Both Sovran and Flint provide only marginal control of cedar apple rust and quince rust, especially when used at the lower end of labeled rates.

Which product is better, Sovran or Flint? That depends primarily on the rates of the respective products that are used in comparisons. Evidence to date suggests that Sovran and Flint provide comparable control of apple diseases when the rate of Sovran is double the rate of Flint. Thus, Sovran at 4 oz/A will provide the same level of control as Flint at 2 oz/A, and early indications are that the products will be priced accordingly. The Sovran label gives recommendations in rates per 100 gallons (1.0–1.6 oz) as well as rates per acre (4.0–6.4 oz) whereas the Flint label lists only rates per acre (2.0–2.5 oz). The minimum labeled Sovran rate of 1.0 oz/100 gallons may prove marginal under high apple scab pressure, based upon experience in our high-inoculum test orchards at both Highland and Geneva. Experience with Flint is more limited, but a rate response was also seen in the one comparative study conducted in Geneva. That is, a rate of 0.5 oz/100 gallons — presumably equivalent to the 1.0 oz/100 rate of Sovran — was less effective than a higher, unlabeled rate (1.0 oz/100).

Therefore, we suggest that apple growers in New York use Sovran at a minimum rate of 1.33 oz/100 gallons of dilute spray for tree-row-volume applications. Technical support personnel from BASF recommend a minimum of 2.0 oz/acre even on the smallest trees, a recommendation that prudently recognizes the potential inefficiency of spray capture in small trees and the relatively high crop value in high-density plantings. We recognize that a rate of 1.0 oz/100 for Sovran may be adequate for purely protective sprays under modest pressure, but both the label and our personal experiences indicate the need for higher rates if postinfection activity is required. The minimum rate for Flint is 0.67 oz/100 gallons dilute basis. The latter is based not only on the 2:1 formula for Sovran:Flint, but also is derived by dividing the lowest label rate (2 oz/A) by 3, using the increasingly standard assumption that per-acre rates must be divided by 3 to arrive at rates per 100 gallons for apples in New York State. A common "fudge factor" (at least 150 gallons/A dilute basis, even on the smallest trees) yields a minimum per-acre rate of 1.0 oz for Flint.

Suggested use patterns for Sovran and Flint:

What is the best timing for strobilurin fungicides? There is no single "correct" answer. However, we believe that the best timing in many locations is around the tight cluster plus pink bud stages, except in areas where quince rust is a concern. Applications at tight cluster and pink target the period of peak apple scab ascospore discharge and the beginning of the mildew season, and they will also suppress secondary spore production on lesions that managed to sneak through in earlier infection periods. Sovran or Flint applied at these early stages, before SI fungicides are used, will help "break the cycle" and limit the ability of SI-resistant scab and mildew strains to re-establish on new foliage. Using Sovran or Flint to control early-season infections will result in less selection pressure for scab and mildew strains resistant to the strobilurins because inoculum levels are lowest early in the season (an example of the old concept that X % resistant strains in a small population provides fewer problem individuals than X % resistant strains in a large population). Combinations of contact and SI fungicides can then be used at petal fall and first cover to provide continued protection against scab and mildew. Having a contact fungicide present at petal fall should reduce potential risks from a myriad of minor diseases for which we still have limited data on activity of strobilurin fungicides.

A typical apple fungicide program might therefore involve an initial application of a protectant fungicide (maybe two in very wet years), followed by two sprays of Sovran or Flint at approximately 10-day intervals starting about tight cluster. A single mancozeb spray may be needed during an extended bloom period to bridge the gap between the second strobilurin spray and the petal fall application. At petal fall and first cover, an SI-contact combination could be used to round out the scab and mildew season. If desired, a final strobilurin spray could be used to extend the period of optimal scab and mildew control and provide excellent flyspeck control, before switching to routine summer fungicide programs; or, summer programs could be initiated now. In blocks where scab pressure is low and no mildewcide is needed, the SI’s might be omitted and contact fungicides used alone to provide protection through petal fall and early summer.

In blocks where quince rust is a concern, the SI-contact combination should probably be used at tight cluster and pink, the period when quince rust infections occur on fruit. Sovran or Flint could then be used at petal fall and first cover when fruit are no longer susceptible to rust. Sovran and Flint will provide acceptable (but not perfect) protection of leaves against cedar apple rust, but they will not provide adequate protection against quince rust on fruit during the tight cluster and pink stages.

Sovran and Flint are very effective for controlling flyspeck (and sooty blotch) and could be used as substitutes for benzimidazole-captan sprays during summer, especially where the four-day re-entry interval for captan creates management problems. The best timing for Sovran and Flint in summer sprays remains to be determined, as does their cost effectiveness at this time in much of the Northeast. If Sovran or Flint scab sprays are applied at tight cluster and pink, then they should not be used again until second cover. During early summer, good spray coverage is still possible whereas dense foliage, fruit clustering, and limbs drooping under heavy crop loads often compromise spray coverage in late summer. However, if Sovran or Flint scab sprays are applied at petal fall and first cover, then additional summer applications would need to be delayed until July or August because of the requirement for intervening applications with some other class of fungicides.

Sovran has a 30-day preharvest interval and the label indicates that it should not be used as the last spray of the season. This prohibition was based on the assumption that growers might apply Sovran for scab control starting at green-tip, and using it both to end the season and begin the following season would compromise resistance management. Flint has a 14-day preharvest interval, and Novartis has actively investigated the value of late-summer sprays. Both Flint and Sovran appear to have residual activity against flyspeck that is equivalent to that provided by Benlate and Topsin M.

APPLE SCAB UPDATE

(Dave Rosenberger dar22@cornell.edu, Plant Pathology, Highland)

Squash mount assessments of scab ascospore maturity were made March 16 and again March 23 with leaves collected from an abandoned orchard in Highland, NY. No mature ascospores were found in either evaluation. These results suggest that ascospore maturity for this spring is lagging tree phenology. Early scab infection periods should pose relatively little risk for infections in commercial orchards as compared with 1998 when ascospore maturity was very early or 1999 when maturity was "on time" in relation to tree bud stages.

Reasons for the delayed scab ascospore maturity this year are not known with any certainty. However, the relatively dry spring to date may have left the overwintering leaf litter too dry for optimal development of scab ascospores. At the same time, drier-than-usual conditions may be allowing more rapid soil warming. Warm soils and recent warm night temperatures may be promoting tree development at the same time that ascospore maturation is being delayed by dry conditions.

3.27 General Information