Powdery Mildew -

During 1998, some orchards saw more powdery mildew than in previous years primarily because the previous winter and last spring were favorable for disease development (i.e., relatively warm temperatures).  The fungus overwinters as mycelium predominantly in the apical buds of vegetative shoots. Winter temperatures of below -11F negatively impact overwintering inoculum by killing the bud in which the fungus overwinters and thus the mycelium dies.  The colder temperatures we have experienced this winter can be expected to kill some of the overwintering inoculum but, if you had an increase in the disease last year and have highly susceptible cultivars such as Ginger Gold, Cortland, Gala, Paulared, Idared, Rome Beauty, Granny Smith,  or Jonathan, you may have to target at least two sprays at managing this disease, particularly if we have another warm spring.   [It is interesting to note that Ginger Gold is to powdery mildew, as Jersey Mac is to scab - very susceptible.]

The most effective fungicides for powdery mildew are the SI fungicides (Bayleton, Nova, Procure and Rubigan).  They will provide good control when applied from bloom through the time when the apical bud sets on vegetative terminals (about 24 days past Petal Fall). During this period, the Petal Fall and 1st Cover sprays are particularly important in mildew management because they coincide with the period of active shoot growth.   Note that all of the SIs listed except Bayleton are effective against apple scab,  so, if you are applying SIs for scab, you will be getting the added benefit of mildew management. However, as indicated under the apple scab section of this newsletter,  SIs should be used judiciously to curb the development of scab resistance. Please note that sulfur is an alternative mildew fungicide.  It also is effective against powdery mildew but applications have to begin earlier (i.e., Tight Cluster) and, because of short residual activity, reapplied every 7 days for good results under high disease pressure.  Benlate and Topsin-M have been used against powdery mildew but it has been reported in New York that they have provided poor control in orchards in recent years.   
Please note that mildew will only infect young, immature tissue.  Once the leaves harden off, the threat of infection is gone.  If you are not sure what the symptoms of the disease looks like and have access to the Internet, the Vermont Apple IPM Focus web page (http://orchard.uvm.edu/uvmapple/pest/) has a link under "Apple Diseases" to West Virginia University's Index of Apple Disease Photographs and Biologies. There are a number of good pictures at that site that illustrate powdery mildew symptoms.

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