Disease Management Update

Apple Scab   - -  Lesions appearing on youngest terminal leaves

In my visits to orchards so far,  it has not been difficult to find foliar scab -- unfortunately, it has been very easy in some orchards.   Lesions have been predominantly on mid-cluster leaves and on about the  3rd -5th leaf from the base of terminals.  When scab was present on the earliest cluster or terminal leaves -- 'sheet' scab followed.  The good news (relatively) is that most scab was seen on the foliage and not on the fruit.  In one orchard we visited the foliage and fruit looked beautiful  -- no scab -- and not one fungicide was applied this year.  The secret to this grower's success is that the whole orchard is planted to scab-resistant cultivars !!

Please Note:   Secondary scab lesions are just beginning to appear on the youngest leaves at the apical end of vegetative terminals.    They are very difficult to see but appear as slightly brown, velvety areas on the upper surface of the leaf.    It is important to check these leaves in your orchard to make sure that your program to stop secondary spread has been successful.

Right now we are in the midst of  scab assessment in our research trial that was designed to test the sequential sampling method for determining the  "scab risk" of an orchard.  Last autumn,  the level of scab was assessed in three orchards.   Based on the autumn  assessment,  the orchards were deemed at "low risk"  for scab this spring and thus fungicides were delayed until Pink. It has turned out that this year will be a good  test year because of the wet weather.   We will keep you posted on the results !!!


Fire Blight  - -  Good News

I am happy to report that we have not seen any blossom blight  in  orchard visited to date nor has any been reported to us from Vermont growers !! 



Lest we forget what Powdery Mildew looks like . . .

The following picture shows a Cortland tree severely infected with powdery mildew.   You can see how the disease affects the canopy of the tree.   In this particular block,   no  SI or Stroby fungicides had been used during the growing season.   It is a reminder that even in Vermont where cold winter temperatures can reduce overwintering inoculum, powdery mildew can build up and be severe on susceptible cultivars.  Note that the affect on total leaf area in the canopy -- it is reduced. 

Attention turns towards Sooty Blotch and Flyspeck

These two fungal diseases blemish the fruit.    If we continue to have a wet, humid growing season and if orchards continue to have very high grass,  we can expect to see sooty blotch and flyspeck on fruit if they are not protected.  Cultural practices such as summer pruning and mowing can significantly reduce disease incidence by allowing better air movement in the trees.  However, in a wet year,  fungicide protection becomes more important.   Table 15 (page 87) of the 2000-2001 New England Apple Pest  Guide  outlines fungicide options and timing of sprays.   Note that sooty blotch is more easily managed with fungicides than is flyspeck. 

Based on the assumption that most of the inoculum for flyspeck infections comes from conidia produced on wild hosts surrounding the orchard,  summer fungicides for controlling flyspeck are not needed  until 270 hours of wetting have accumulated from 10 days after petal fall.  At that point, presumably, flyspeck conidia will become available in the orchard perimeter and will begin blowing into the orchard.   [This assumes that  infection by flyspeck ascospores within the orchard was prevented by fungicides applied for apple scab through peak flyspeck ascospore release, which occurs approximately 10 days past petal fall.]

Note that captan and ziram do not have any eradicant activity against flyspeck and therefore must be applied before the conidial infections, i.e., before 270 hours of wetting have accumulated. The benzimidazole fungicides (Benlate and Topsin M) provide about 100 wetting hours of eradicant activity against flyspeck.  As a result, development of flyspeck on fruit can be arrested if Benlate or Topsin M is applied sometime between 270 and 370 hours of accumulated wetting counting from 10 days after petal fall.

As a measure for comparison, as of June 19th, 184 hours of wetting had occurred at the UVM Hort.Res.Center in South Burlington.