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As I See it- Powdery Mildew, Apple Scab, Fireblight and Thinning

Win Cowgill - County Agricultural Agent

This seasons strange weather has made disease control and apple chemical thinning difficult at best. We are seeing an extensive amount of apple powdery mildew in north Jersey orchards.
The dry weather in March and early April was the cause. Hudson Valley NY orchards are poised to have a lot of powdery mildew infections as well, based on observations by Dr. Rosenberger, Cornell. Dr. Dan Cooley UMASS, reports that powdery mildew is prevalent in Massachusetts orchards as well.

Powdery Mildew and Dry Weather 
We may be seeing more PM due to the dry weather this year but also due to the shift in apple scab fungicides use of new scab materials to manage resistance. We were very dry until March 30-April 1 and then 3 more weeks until April 22. As a result some growers do not have as good a control of apple powdery mildew as we would like.

There are a few options, the SI fungicides like Rally and Procure. These are also good choices on cedar apple rust and will provide 72-96 hours of back action against apple scab. If you have already used 2-3 applications of an SI this spring, consider the use of Sulfur if you are having a powdery mildew application. Sulfur is an excellent mildewcide. Caution, do NOT use if the temperatures are forecast to be 80F or higher like they Microthiol is one brand and is OMRI approved for organic growers at 5lbs/Acre. On pumpkins sulfur is one of my preferred choices of materials for PM control.

Apple Scab
We have been using the NEWA apple scab forecaster this season to look at the maturity of the scab ascospores. It appears that the two dry periods above affected the NEWA model for predicting scab. Dr. Dave Rosenberger, Cornell picked this up last week and alerted us. Monday, 5/21, I had Rebecca Magron, RCE of Hunterdon County, run our spore tower again and calculate the number of spores released. 120-to 140 indicates that we can still have Scab Infection, we found over 200 mature scab ascospores. This means the NJ growers should stay covered up for apple scab. 
Dr. Rosenberger suggests the following, 
“A cover spray the middle of this week (5/21-26) should carry orchards through at least two inches of rain and/or for 14 days, and that should be enough time for more spores to mature and for more leave litter to decay."
"Furthermore, as we approach the end of May, there is more likelihood that both scab and mildew activity will be at least partially arrested by days when high temperatures exceed 85 degrees.” 
We have had 5 days of wetting with showers forecast for the next two days in Hunterdon County, NJ.

Maintain fungicide coverage
Captan and Mancozeb can be sprayed in light rain of if the weather breaks. Consider adding a systemic fungicide for back action if you were not adequately covered up before this last wetting event.
See Table 6.1.3 Characteristics of apple scab fungicides, 2012 Cornell Pest Management Guidelines for Commercial Tree Fruit Production.
(Left click to view, Right click to download)

Apple Orchards with no crop - in Northwest Jersey there are some orchards with no crop - This advice from Dr. Rosenberger and Mike Fargione, Cornell 
“In orchards that were clean last fall and have no crop, it might work out to walk away from the more scab-resistant cultivars (e.g., Empire, Red Delicious, Golden Delicious, Honeycrisp) without a crop at this point, but scab-susceptible cultivars should still be protected against both scab and mildew through the Memorial Day weekend.”
The NEWA fireblight forecaster is calling for high or extreme infection periods all this week if you have any open blooms. Be sure to check newly planted trees for flowers as well! Strep is only good 24 hours before and after an infection.

Streptomycin should not be used on the shoot blight phase of fireblight as it is infective and will hasten resistance buildup. The exception is if we have a hail event this week, then strep must be applied within 24 hours. Growers should be keeping an eye open for fire blight strikes and remove them ASAP if they appear. Several south Jersey orchards are reporting infections appearing.

Shoot Blight Phase of Fireblight
The only chemical control of the shoot blight phase is a combination of copper plus mancozeb. This can help to dry up infections but note, it will russet the fruit!

Apple Thinning 
Fuji fruit at Rutgers Snyder Farm is 22-25 MM in size- above 16 mm the only PGR that will have some effect from 16-25MM is ethephon. See last weeks Plant and Pest. Fruit above 25MM in size will need to be hand thinned as PGR's will be very ineffective.