Penn State announces scab period through March 17

Thanks to the early spring weather last week, some degree of green tip is occurring throughout Pennsylvania and Maryland. Last week’s scab spore test was not a fluke and we recovered mature spores from our scabby overwintered leaves from the 2015 season yesterday. No—I am not ready for this yet. However, many trees are not at green tip, so sanity can prevail somewhat.

Please don’t be fooled by the cooler weather and think apple scab infection is not an issue. If the temperature is averaging 45 degrees, 18 hours of leaf wetness is needed. Since it has been persistently wet since March 13, it hasn’t taken long for leaf wetness hours to add up. At that temperature and wetness hours, symptoms will show up in about 17 days. We do have a little bit of a silver lining: since last year was a relatively light scab year, we are coming into this season with less available scab spores ready for action for 2016.

Warmer temperatures and rain are forecasted for the next two days, which will lessen the number of leaf wetness hours needed for scab spores to cause infection. Green tip is a vulnerable time for scab and it is important to be vigilant, especially in blocks with known history. Once scab gets established, it is very challenging to get ahead of the disease.

For folks that have green tip peeking through:

Your best bet right now would be to use mancozeb at 6 lb/A. Copper use at green tip could run the risk of phytotoxicity.

If you are not at green tip:

Another copper application (2 lb metallic copper/A) would be a good idea since any copper applied last week most likely has washed off with the hard rain the last couple of days.

– Kari Peter, Penn State University