Follow-up on Scab Development

Rains today (March 10th) pose no threat for apple scab in the Hudson Valley because we still do not have any green tissue on apples buds.  According to one degree-day model*, McIntosh can be expected to reach the green tip bud stage after degree day accumulations starting from January 1 reach 97 to 147 degree days, base 43 (using the Baskerville-Emmons degree-day calculation model). I used the degree day accumulations shown on the NEWA website for the Hudson Valley Lab this morning and then used a spread sheet to add degree day accumulations expected for the next 10 days based on the Accuweather forecast.  Those calculations suggest that we should have green-tip sometime between Saturday, March 12th, and Thursday, March 17th.

I checked another batch of overwintering leaves from another abandoned orchard for spore discharge in the shooting tower on Tuesday and found no spores discharged from that sample. However, there’s a hitch that negates the value of that observation. Although the leaves were from an abandoned orchard just north of the lab, last summer had such light scab pressure that those leaves apparently had very little scab on them at the end of the season. As a result, I was unable to find any scab pseudothecia in these leaves when scanning them under the dissecting scope. Now I don’t claim to be an expert at spotting scab pseudothecia because throughout my career that job was handled by my very capable technician, Mr. Frederick Meyer. However, I do think that even with my limited skill-set, I would have been able to find at least one pseudothecium in those leaves if any had been present, considering that I spent an hour searching over the surfaces of nearly 30 leaves. The bottom line is that I have not duplicated my finding of mature spores as indicated in my post on March 7th, but I still believe that my observation of mature spores on March 7th is adequate to serve as a biofix for RIMpro.

Rains are predicted for early next week, March 14-15, and according to RIMpro as I have run it using virtual data from the MeteoBlue weather service, that wetting period could trigger a light scab infection if trees actually do reach green tip on Saturday. However, any such infection period (if the rain develops and trees have green tissue by then) would only be a concern in orchards with a lot of overwintering scab. Given our dry year last year, I suspect that most orchards have a very low inoculum load, and my observations of leaves from abandoned orchards suggest that the influx of spores from abandoned trees will also be low this year in the Hudson Valley. Furthermore, I suspect that green tip in most blocks in the Hudson Valley will be delayed until the middle of next week, thereby negating any scab risk from potential rains on Monday-Tuesday of next week.

The bottom line: It appears that apple growers can enjoy one more week-end before we need to worry about apple scab.

*The degree-day model referenced above is from the last page of Scaffolds Fruit Journal (13 April 2015) vol. 22, issue 3, where Art Agnello has posted the green tip date observed at Geneva over the past 26 years calculated as noted above and expressed as a range that is derived from the mean +/- one standard deviation.  Thus, as of 2015 the  range for green tip was 97-145 base 43F (i.e., the mean value is 121, with a STD DEV of 24).

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