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Disease Management --
Apple Scab -- Reducing Overwintering Inoculum
The wet weather made it a very challenging growing season for scab management. One way to reduce the amount of overwintering inoculum is to implement a sanitation program. For example, flail mowing or applying 5% urea to fallen leaves in the autumn can decrease the overwintering inoculum by 50-75%, which means that for any one infection period next year, there could be approximately 50-75% fewer scab lesions developing. As outlined in the 2000-2001 New England Apple Pest Management Guide (page 13), sanitation options include:
Flail-mow after leaf fall or in spring. If leaf-fall is very late, then flail-mowing should be delayed until early spring but as soon as possible after the snow cover is gone.
If flail-mowing is not possible, a 5% urea solution (42 lbs. urea/100 gals. water) should be applied to fallen leaves at 100 gals. per acre so that the leaf litter is thoroughly wet.
Another option is to flail-mow and then apply urea to the "in-row" area that could not be reached by the flail mower .
Are any of your Orchard Blocks a Candidate for the "Delayed First-Spray" Strategy?
There are a number of pages in the 2000-2001 New England Apple Pest Management Guide dedicated to describing how to predict an orchard's "scab-risk" level for next spring through a new sequential sampling procedure conducted after harvest but before leaf-fall. A description of how to assess for scab including how many trees and shoots you need to look at initially is given on page 13. Once you sum up the number of leaves that had scab on 10 trees (10 shoots per tree) or 20 small trees (5 shoots per tree), the charts on page 14 or 15 will tell you: (1) if the level of scab is too high and thus, you should follow a protective strategy for the next growing season; (2) the level of scab is low enough that you can delay your first scab spray to after the third infection period (but before the fourth) or until Pink, whichever comes first; or (3) whether you need to sample another 10 trees to determine (1) or (2). Note that Chart 2 should be used if no sanitation practices will be conducted; Chart 3 should by used if you will be using sanitation practices to lower the level of overwintering inoculum. To help you determine how to distinguish scab lesions from the many spots that can be on leaves at this time of year, there are a series of colored pictures on pages 57-59.
Jessica Reardon, a graduate student on the UVM Apple Team, is conducting research to field verify this new sequential sampling procedure. Last autumn, she used the technique to identify three "low scab-risk" orchard blocks and then set up an experiment in each block to see if delaying the first scab fungicide to Pink would result in an increase in scab on the foliage and fruit. She is just now finishing up data collection on the fruit and we hope to report on the results this winter after we fully analyze them but, a preliminary look at the data is encouraging, even in a year that was very challenging for scab management. She will repeat the experiment again next year to further evaluate the technique.
If you do want to start to use this new sequential sampling procedure this autumn, please heed the warning that appears in the New England Pest Management Guide:
It is recommended that the first time you try this strategy you should try it in only one block, preferably a block of small to moderately-tall trees on semi-dwarfing rootstock . Old blocks of standard trees are not recommended for this strategy. It is also recommended that you do not consider this strategy if a sterol-inhibiting (SI) fungicide program was used during the previous growing season. SI fungicides inhibit rather than kill the pathogen, so the fungus may be active in the leaf even though visible scab lesions do not develop. Thus, significant ascosporic inoculum may develop in leaves that would be assessed as "scab-free" in the autumn, perhaps placing the orchard at "high-risk" although the assessment indicated the orchard was "low-risk."
Fire Blight Disaster in Southwest Michigan ...
What can happen when highly susceptible cultivars are planted on highly susceptible rootstocks in an area where the environment is favorable for infection.
This is the second time in ten years that Southwest Michigan has been hit by a fire blight disaster (1991 was the last severe fire blight epidemic). Mark Longstroth, Extension District Horticulture and Marketing Agent for Southwest Michigan, has pulled together some staggering figures on the cost and impact of the fire blight epidemic that occurred in Southwest Michigan this spring. The following information is from his web page ( http://www.msue.msu.edu/vanburen/fb2000.htm) which describes in detail the devastation:
"The fireblight epidemic in Southwestern Michigan is as severe as anyone can remember. Many acres of high-density apple orchards have been destroyed with the death of almost all the orchard trees. From 350,000 to 450,000 apple trees will be killed and 1,550 to 2,300 acres of apple orchards will be lost. The development cost of these orchards was over $ 9 million. Apple yields will be reduced by 35% over the region. Some growers will suffer 100% losses in some plantings. The Southwest region produces an average of 4.5 to 7 million bushels and the expected crop loss is 2.7 million bushels about $ 10 million. It will take at least 5 years for yields to recover with a cumulative loss of yield of nearly $ 36 million. The region's total economic loss is almost $ 42 million. "
If you have been fortunate and have not seen the damage fire blight can cause, I would highly recommend that you visit Mark Longstroth's web page listed above and his web page (http://www.msue.msu.edu/vanburen/fb2000p.htm) that documents through pictures the devastation that fire blight has caused.
In many respects, it was a disaster waiting to happen. Over the last ten years, growers, to remain competitive, were planting high density orchards with Gala, Fuji, Braeburn and other highly susceptible cultivars on M9 and M26, which are rootstocks considered highly susceptible to fire blight. Warm growing conditions in the spring make fire blight a perennial disease problem in the region; Southwest Michigan growers typically apply 2-3 antibiotic sprays during bloom. These orchards were at a high risk of disaster from fire blight the moment they were planted.
What is the lesson here for us in Vermont ? Be aware of the susceptibility of the newer cultivars to disease problems. Fire blight can have a severe impact in Vermont but, fortunately, severe problems have not been that frequent to date with our current cultivar/rootstock combinations. However, planting cultivars highly susceptible to fire blight on highly susceptible rootstocks will increase disease risk. In terms of having conditions favorable for fire blight infection, we are fortunate in Vermont in that we have cooler springs, but we still can have conditions favorable for blossom infection. If we experience a climactic change wherein our springs become warmer -- watch out. In other words, the potential for a fire blight disaster in Vermont rests partly in your hands, by what new cultivars/rootstocks you plant and how you manage them, and partly in the hands of Mother Nature. (At least our cooler weather is good for something !!)
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