Disease Management Update

Apple Scab -

Protective Schedule to Prevent Infection

Given last year's scab situation, the focus in terms of disease management in 1999 will be on apple scab.   Throughout New England and the Northeast, 1998 was considered one of the worst years for this disease.  This means that there is potentially higher than 'normal' overwintering inoculum in the orchard for the 1999 growing season.

Similar to last year, we have had a relatively warm winter where either snow cover or melting snow/rain have kept the leaves on the orchard floor insulated or wet - these are conditions that are favorable for overwintering inoculum. 

This situation points to an aggressive, protective strategy this year during the primary scab period using materials such as captan, mancozeb (Dithane, Manzate, Penncozeb), and/or metiram (Polyram) and making sure the first fungicide is applied before the first infection period and continuing tight protection until the ascospores are released.   Note that if you choose to use the "Extended" program for the EBDC fungicides (i.e., mancozebs, metiram) and use them as a 'stand alone' fungicide, your dilute gallonage/A based on your tree row volume calculation should not be over 250 gallons nor should your trees have dense canopies.  In these situations, you risk not having enough material on the trees to be effective and should include another material, such as a half rate of captan with the EBDC.  This combination has worked well in the Hudson Valley according to Dr. Dave Rosenberger.   Another option with the EBDCs on larger trees would be to follow the 'Prebloom/Bloom Only' program which allows a higher rate.  Whatever rate of EBDCs you may choose, they will have less of an impact on the predacious mite, T.pyri if you do not apply them past bloom (see below under European Red Mite section of this newsletter for more information on T.pyri in Vermont.) More information on these programs can be found on page 59 of the 1998-1999 New England Apple Pest Management Guide

Dodine (Syllit) would also be an option around your oil spray if resistance has not developed in your orchard.  Unfortunately, there is no practical laboratory test to determine level of resistance in an orchard.  Drs. Koller and Wilcox state in a recent article that appeared in New York Fruit Quarterly (vol.6, no. 4, 1999) that the level of resistance is related to orchard history (i.e., the total number of dodine treatments an orchard has received).  From their research, they have estimated that the total number of dodine treatments that would drive an orchard "over the edge" appears to be around 60 sprays.  It does not matter whether these were sprayed over a 10 or 30 year period.   In their studies, they found that the level of resistance in an orchard declined over time but using dodine only once would drive the orchard back to "resistant" very quickly.  The bottom line is that if you do not know the history of dodine use in your orchard, any use of dodine is risky.

And that leads into the topic ….

Judicious Use of SI Fungicides --- Concerns of Resistance

The following table illustrates the percentage of strains of the scab fungus resistant to the SIs (i.e., sterol-inhibiting fungicides such as Nova, Rubigan, Procure) and dodine in New York orchards based on research conducted by Drs. Koller and Wilcox:
   

All of the orchards had comparable histories of SI use over seven to eight years.   Orchards A-E were considered fully sensitive to SIs, Orchards F-I had considerably higher percentages of SI resistant strains than baseline but much lower than what would qualify them as 'resistant', and the last three orchards (J-L) had reached a status that the researchers considered as being close to SI-resistant.  The researchers speculated that allowing  high levels of scab to come through and using low application rates of SIs either intentionally or by having overly long spray intervals or due to incomplete spray coverage are driving the more rapid development of SI resistance in some orchards.

It was noted that the first commercial orchard that was SI-resistant was discovered in Michigan in 1995, after SIs had been used for nine years which translated into a total of approximately 50 applications.

That number - 50 applications - should be kept in mind and underscores the fact that SIs should be used judiciously since resistance development is a reality.

The researchers proposed the following "rules" to help the SIs remain effective:

Bottom line --- given the potential for a high level of overwintering inoculum this year and concerns regarding resistance,  following a tight, protective schedule with mancozeb, metiram and/or captan appears the best option scab disease management.

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