Could "Green" Approaches for Scab

Control Improve Profitability?

Reprinted from Southern Tier Produce News; March 2005

David A. Rosenberger, Professor of Plant Pathology, Cornell University

The title for this presentation may have raised expectations that will prove impossible to meet.  What one envisions as a "green" approach to scab control is probably dependent on prior experiences with apple scab, on the company that one keeps, and to some extent, on one's religion.  This presentation will focus on the science of scab control as it relates to measures that can complement or substitute for traditional fungicide programs.  I will not attempt to weigh various options based on their acceptability to groups with widely divergent philosophical perspectives.

As with any business, profitability in apple production requires that income from selling the product must exceed the costs involved in producing, packing, and marketing.  "Green" approaches to scab control that inflate production costs may still be profitable if the "green" crop can be marketed to someone who is willing to pay a premium for food that is produced in a certain way.  Because I cannot assess your markets, I cannot predict which practices will prove profitable for any given farm operation.  The best that I can provide is some estimation of how difficult and expensive it may be to incorporate new practices into existing production systems.

For purposes of this discussion, "green" approaches for scab control are subdivided into four categories.  Those categories are listed below, starting with those I deem least useful and ending with those that have broader applicability:



1) Scab control with new "biorational" fungicides and
nutrient sprays
2) Scab control via scab-resistant cultivars
3) Scab control with copper and sulfur compounds
4) Scab control via inoculum reduction

Scab control with new "biorational" fungicides and nutrient sprays

This approach to scab control requires the least discussion: "biorational," "green" or "soft" fungicides introduced to date are uniformly ineffective for controlling apple scab.  I have personally evaluated Serenade, Oxidate and Messenger and found them less effective than sulfur.  Other scientists have evaluated some of the other oils and natural products with similar results.  These "green" products may be profitable for the manufacturers, but none of those tested to date will improve profitability for apple growers!

Scab control via scab-resistant cultivars

Scab-resistant cultivars provide the ultimate solution for low-cost scab control, but they will prove profitable only if they can be marketed.  Producers of scab-resistant apples will face several significant problems.  First, organic production from the desert areas of Washington State is creating a very low floor for pricing of organic apples.  Producers in non-desert regions will have higher costs for organic production because of greater pressure from diseases and insects.  Second, many scab-resistant apple cultivars lack the taste and quality characteristics that consumers have come to expect in their apples, so finding an acceptable scab-resistant cultivar for your niche-market consumers may

prove difficult.  Finally, scab-resistant cultivars may still require fungicide protection during the summer to prevent sooty blotch, flyspeck and summer fruit rots.  If the objective of growing scab-resistant cultivars is to supply an organic market, then all aspects of pest control must be carefully considered before making a large investment in new cultivars.


Scab control with copper and sulfur compounds.

Methods for scab control with copper and sulfur compounds were perfected more than 50 years ago, and those methods still work very well for anyone willing to expend the effort required.  Copper applied at green tip will provide the same level of scab protection as one would expect from a mancozeb fungicide.  Copper sprays do not have any post-infection or eradicant activity, however.  Copper sprays applied after green tip will often result in severe fruit russetting and/or blackening of fruit lenticels.

The best directions that I have found for controlling apple scab with sulfur compounds was published by Dr. Art Burrell in the 1945 Proceedings of the N.Y. Horticultural Society.  Dr. Burrell suggested that growers should maintain a supply of three different products for scab control.  Wettable sulfur was to be mixed with water and applied as a spray ahead of predicted infection periods.  If the grower had a duster, then a finely ground sulfur was to be applied as a dust when foliage was wet after or between rains.  Dusters could cover an orchard more quickly than a sprayer, and the dust tended to stick to wet trees better than sulfur applied as a spray.  Finally, liquid lime-sulfur was needed to cover trees that could not be protected before the end of a Mill's infection period.

Liquid lime-sulfur provides 60 to 70 hours of post-infection activity, counting from the beginning of a wetting period.  It also acts as an anti-sporulant when applied to trees where primary scab lesions are just beginning to appear.  However, lime-sulfur has a number of undesirable qualities that must be considered.  First, it is caustic and must be handled with caution by applicators.  Second, it can cause severe leaf burn if applied to wet foliage.  Third, even when applied to dry foliage, each application causes a slight reduction in both leaf size and fruit size.  Fourth, application of lime-sulfur any time between bloom and second cover can result in appreciable fruit thinning.  The thinning capabilities of lime-sulfur are not necessarily bad in situations where reduction of crop load is desirable.  However, applications of lime-sulfur might be undesirable if crop load is already light.

Sulfur fungicides can differ significantly in their efficacy.  Among wettable sulfur formulations, the Microthiol Special formulation has proven particularly effective, probably because that formulation includes a bentonite clay carrier that may help to improve resistance to wash-off during rains.

Regardless of the sulfur formulation that is used, sulfur sprays must be renewed frequently during rainy seasons.  Protection provided by sulfur sprays is probably gone after one-half to three-quarters of an inch of rainfall.  Those attempting to use sulfur as their primary scab fungicide should be prepared to re-cover orchards every three to five days between green tip and the third cover spray.



Scab control via inoculum reduction.  This "green" approach offers the greatest potential for improving profitability.  Inoculum reduction is absolutely essential for organic orchards or for  orchards where DMI fungicides (Rubigan, Nova, Procure) are no


longer effective due to fungicide resistance.  Inoculum reduction

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