Disease Update: April 10, 2013

[Future Harvest - CASA]

Disease Update: April 10, 2013
Dr. Kari Peter, Penn State Fruit Research and Extension Center

http://extension.psu.edu/plants/tree-fruit

Posted: April 10, 2013

Green tip is here and the next ten days are forecasted to be ideal
conditions for apple scab infection. Growers are encouraged to apply
protectant fungicides during this critical period.

For the last several days, I’ve had the song “People Get Ready”
playing over and over in my head…
Once again, another year where the warm temperatures appeared
overnight before I had a chance to put my winter coat back in the
closet! It’s official: We’ve entered the season where apple scab
ascospores are ready to do damage. How do we know this? In the
1940s, W.D. Mills published the “Mills Tables,” which uses wetness and
temperature to predict primary apple scab infection, as well as the
time for secondary infections to develop. As a result, The Apple Scab
Model was born. What makes ascospores happy? The optimum (and
minimum) conditions for spores to germinate and infect are six to nine
hours of leaf wetness at an average temperature of 61-65ºF. Although
infection decreases at or above 80ºF, very long wetting periods during
this temperature can encourage infection. For those interested in
determining infection periods, monitor the duration of the wetting
period by noting when the wetting period starts and when the leaves
dry, and add the maximum and minimum temperatures during the wetting
period and divide by 2. For reference, The Mills Table can be found
here under the apple scab “control” section.
Gala is at green tip at Penn State’s Fruit Research and Extension
Center. In addition, we are starting to see ascospore release from
overwintered scab infected leaves. Spore maturity is low; however,
the warmer temperatures will help speed up the spore maturation
process, which will lead to high numbers of ascopores released. Based
on the SkyBit Ag E-Weather IPM Apple Disease Report at FREC, the next
ten days are forecasted to be prime conditions for apple scab
infection. Although the Apple Scab Model is used to predict
infections, weather and tree growth need to be monitored closely at a
very local level within one’s own individual orchard. If conditions
are seen similar to those at FREC, growers are encouraged to apply a
protectant fungicide (i.e., copper, mancozeb, Polyram, captan).
Copper at green tip will also help with fire blight control in
orchards where fire blight has been problematic.