IPM News 

L.P.Berkett, IPM Specialist

Disease Management Update

Apple Scab

The unexpected rain and subsequent infection period on May 4 and 5 caught some growers off guard. Hopefully, fungicides were applied within their effective 'kick back'  time and that  the fungicides further protected the orchards from the infection period that occurred over May 8-9.  With the warm weather, degree days were accumulating rapidly and it was estimated that cumulative ascospore spore maturity was between 35% - 50% at the five sites we are monitoring over that infection period.  As I write this newsletter, showers are predicted for May 19-20.  If the showers turn into an infection period, it would again be major one since mature ascospores have been building up in the leaf litter.    The following is the estimated Degree Day accumulation and cumulative ascospore maturity as of 5/16/99:

Please note:  Degree Days and the Ascospore Maturity Tables are posted on our website (http://orchard.uvm.edu/uvmapple/pest/ ) and we are also supporting weather information being sent to Glen Koehler's Orchard Radar (http://pmo.umext.maine.edu/apple/allmodels/Radardirectory.htm ).

We are still in the 'Accelerated Phase' of ascospore maturity (from 300 - 700 DD). This is when ascospore maturity continues rapidly. It is also a period when shoots are extending,  resulting in a significant increase in susceptible leaf tissue.  It is a key time in apple scab management.   From a disease perspective, with any luck, we will continue to have discrete infection periods (not too many !)  which can be effectively managed -- at least we have not had the extended periods of rain that caused so many problems last year.

It is always wise to scout your orchard on a regular basis to determine if your fungicide applications were effective.  The following information is found in Table 4 on page 8 of the 1998-1999 New England Apple Pest Management Guide.  It will give you an indication of when lesions should appear if infection did occur.

Fire Blight

Fire blight usually is not prevalent in Vermont, but it can cause serious damage in some years.  The cooler weather we had during last week lowered the risk of fire blight infection.  Depending on what disease model and weather source are used,  conditions on Saturday, May 8 were the most favorable for infection to occur if blossoms were open at the sites we were monitoring. If infection did occur,  blossom blight symptoms should start to appear when 103 DD (base 55 F) have accumulated from the time of infection.  We will keep track of DD for the five sites and keep you posted when we are nearing the time when symptoms should be visible if indeed infection took place.


Powdery Mildew

'Primary infection sites' for powdery mildew should be visible in your orchard by now.  These are, either fruit or vegetative buds, in which the fungus has overwintered. As the blossom and leaves develop from these buds, the fungus mycelium growths over the surface producing spores that give the leaves or blossoms a white, powdery appearance.  Also, the leaves tend to be slightly deformed.

Source of Picture: K. Yoder.  Index of Fruit Disease Photographs, Biology, and Monitoring Information.  Mid-Atlantic Orchard Monitoring Guide Web Site for Tree Fruit Pathology. http://www.caf.wvu.edu/kearneysville/wvufarm8.html

These spores are the source of secondary infections.  Although weather conditions this spring have been favorable for disease development, we had cold temperatures during the winter that should have killed some of the overwintering inoculum.  However, if you have cultivars that are highly susceptible such as Ginger Gold, Cortland, Gala, Paulared, Idared, Rome Beauty, Granny Smith,  or Jonathan you should be aware of this disease. 

As stated in the March issue of The Vermont Apple Newsletter, 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 1^st 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.  Sulfur is an alternative mildew fungicide but  because of short residual activity,  it has to be applied more often (i.e., every 7 days) for good results under high disease pressure.

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