Scaffolds 00 index

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

Apple scab ascospore counts as determined from squash mounts:

Apple scab squash mount assessments completed with leaves collected on Monday, April 17 showed considerable variation among various locations in eastern N.Y. Ascospore maturity in the Hudson Valley was still a relatively low 11%, but there was considerable variation among the pseudothecia examined. Some pseudothecia had 25% mature spores and 3% empty asci. In Saratoga, ascospore maturity was running ahead of the lower Hudson Valley, probably because the Saratoga region had less dry weather this spring to slow ascospore development. The tower discharge counts showed that the Saratoga region had many ascospores ready to discharge with the rains that occurred during the week since the counts were mande.

In the Champlain Valley, ascospore maturity on April 17 was still below the threshold of 15% mature spores generally required for commercially important discharges, but that threshold was presumably reached during the latter part of last week as spores continued to mature.

(Wayne Wilcox wfw1@nysaes.cornell.edu, Plant Pathology, Geneva)

This is shaping up to be a spring that is "made" for the advantageous use of strobilurin fungicides ("strobies") to control apple scab. Early green tip infection periods may or may not have been important in orchards that may or may not have received appropriate fungicidal protection. Growers who applied a half rate of mancozeb in alternate rows before the long and wet infection period that we just finished, now wonder whether or not they retained enough fungicide to do the job. Undoubtedly, many growers are doing just fine at this point. But a betting man would be wise to wager that, if they don't do something about it, other growers will be looking at cluster leaf scab as fruit buds become exposed and start to develop into apples. And we all know that the most important source (by far) of fruit scab is spores of the scab fungus that develop on cluster leaf lesions. Cluster leaf scab is just plain bad news.

As we've explained before, one of the primary strengths of the strobies is their ability to suppress the formation of spores from recent infections. In our Geneva trials in years with similarities to this one, applications of Sovran (1996, 1998) or Flint (1998) provided outstanding control of cluster leaf scab when they were made more than a week after one or more important early infection periods. At the end of the year, there was significantly less FRUIT scab in these treatments than in any other of the other treatments in the trial, primarily because we controlled the critical source of inoculum coming from early cluster leaf infections.

I have heard of a lot of skepticism about the strobies out in the industry. Some of it is based on price (OK, this is a judgement call). Some of it is based on misinformation or lack of experience. Some of it is based on bad experiences with other "new" products that failed to live up to their marketing hype. All I can say to these latter concerns is, this is why public institutions do disease control trials. I have gotten poor results with some newer recent introductions (e.g., Procure and Vangard) and have said so (and have heard the unhappiness from interested parties afterwards). Similarly, I have gotten very good to excellent results with Sovran and Flint, and am saying so. I don't care what fungicide a grower does or doesn't use. I do care that they have the best information available to make an informed decision.

Thus, if it were my orchard and I had any doubt about my scab control up to this point, I'd apply a strobie sometime this week before the next rain comes. This should suppress sporulation from any previous infections and provide good protective activity for another week or more. When such suppressive activity is needed, full rates are important, e.g., our experience with Sovran suggests that the 1.6 oz/100 gallons (dilute basis) rate will be more effective than the 1.0 oz/100 gal rate (this issue was discussed at length in a previous article). Currently, we don't know if it's important for continued suppression to apply a second (back-to-back) application 7-10 days later, but the safe answer is "yes".

One last issue: growers who have been burned by poor fruit scab control with the SI fungicides (without a protectant) are understandably reluctant to apply the strobies without also tank-mixing a protectant. Again, I point to the results from trials that Dave Rosenberger and I have both conducted in multiple blocks over multiple years. Both Sovran and Flint provide excellent control of fruit scab when used alone. If scab is the only issue, I think you'd be better off spending the same money on a full rate of the strobie, rather than a marginal rate plus a half rate of protectant.

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

Powdery mildew is one of the three or four most important diseases of apples in New York State, but the importance of mildew is often overlooked because early season fungicide programs are targeted primarily for apple scab. Considerable attention has been focused on resistance management for scab fungicides, but resistance management for apple mildew may be even more important because we have fewer fungicides that are effective against mildew. The recent registrations of the strobilurin fungicides (Sovran and Flint) provided new tools for resistance management for powdery mildew on apples.

For the past 10 years or more, the only consistently effective mildewcides were sulfur and the SI fungicides (Bayleton, Rubigan, Nova, and Procure). The benzimidazole fungicides (Benlate and Topsin M) were effective in some orchards, but in other orchards they were ineffective because the powdery mildew fungus had developed resistance to the benzimidazoles.

Subjective observations suggest that the SI fungicides have also lost some effectiveness against mildew in some orchards. When it was first introduced, Bayleton controlled mildew very well at rates of 1.5 oz/A. By the mid 1990's, however, consultants and growers noted that Bayleton provided effective mildew control only when used at rates of 3 or 4 oz/A. If apple mildew follows that same pattern that has been observed in grapes, the reduced effectiveness of Bayleton will soon be followed by a similar reduction in effectiveness of other SI fungicides against mildew. In fact, some apple growers reported that Nova did not adequately control mildew in their orchards last summer, although factors other than fungicide resistance may have contributed to some of these failures.

Flint and Sovran have been tested and promoted primarily as scab fungicides, but they are very effective against powdery mildew. In orchards where mildew is a concern, either Flint or Sovran should be rotated with SI fungicides to reduce selection pressure for resistance to both of these fungicide groups. An ideal program would involve applications of Flint or Sovran at tight cluster and pink to reduce sporulation of overwintering mildew. SI fungicides could then be used at petal fall and in cover sprays. In orchards where Nova failed to provide adequate control last year, growers might want to revert to cover sprays of Sovran or Flint after the two SI applications at petal fall and first cover.

In the Hudson Valley and other regions where rust diseases are a concern, the SI fungicides should be used at tight cluster and pink to prevent rust diseases on fruit. Flint or Sovran can then be used at petal fall and first cover. Either rotation (starting with SIs or starting with strobilurins) will provide an effective resistance-management strategy for mildew.

The resistance management strategy can be strengthened by using a third fungicide in cover sprays after first cover. Sulfur at 2-3 lb/A will suppress mildew provided disease pressure is low as would be expected following two strobilurin sprays plus two SI sprays.

In some orchards, Benlate or Topsin M might also provide adequate control of mildew during summer. Benzimidazole-resistant mildew may have been introduced with nursery stock in the early to mid-1980's when nurseries depended on Benlate and Topsin M to control mildew in the nursery row. After the SI fungicides were registered, nurseries shifted away from benzimidazoles. At the same time, growers stopped using benzimidazole for scab control because of resistance problems. As a result, benzimidazole-resistant mildew may be prevalent in orchards that are 15 to 20 years old, whereas these fungicides may still work reasonably well as mildewcides in some of the younger orchards.

There is no easy test for fungicide resistance in apple mildew, so the only way to know if a fungicide is working is through careful field observations. The benzimidazole fungicides are not dependable enough to be recommended for mildew control between tight cluster and first cover. However, they might be worth evaluating as mildewcides after first cover because their failure at that time of year usually will not be disastrous, although it may allow build-up of inoculum for next year.

A primary or over-wintering infection of apple powdery mildew in an apple bud at tight cluster (right) compared to a healthy bud on the left.

The most common mistake that growers make in controlling mildew is omission of prebloom mildewcide sprays. Primary mildew becomes visible on infected flower buds at tight cluster. These primary infections provide inoculum for infecting new leaves, so mildewcide applications should begin at tight cluster or very soon thereafter.

Secondary mildew on an apple leaf after petal fall.

When the benzimidazole and SI fungicides were still at peak effectiveness, many growers found that they could delay the first SI application until pink, bloom, or even petal fall and still achieve reasonable mildew control. Omitting prebloom mildewcide sprays gives the fungus time to establish numerous secondary infections on new foliage. As with apple scab, it is much harder to control a large population than to control a small population. When mildewcide applications are delayed until petal fall, the mildewcides are often being deployed as eradicants against a population that is much larger than it would have been at tight cluster or pink. This not only reduces effectiveness of the program, it also creates much greater selection pressure for fungicide resistance.

The apple industry is fortunate to have the strobilurins registered on apples before the SI fungicides lost all effectiveness against apple mildew. The new fungicides provide an option of rotating mildewcides with different modes of action. If growers fail to rotate mildewcides, apple powdery mildew could conceivably develop resistance to both the SIs and the strobilurin fungicide groups, thereby leaving apple growers with sulfur as their primary mildewcide.

> 4.24 Insects