Summer disease pressures: extended wetting and accumulated wetting hours

In the past two weeks we have had several extended wetting periods with rainfall, and more than 150 hours of wetting from rainfall or dew. Although the rainfall was not excessive, this amount of wetting at relatively warm temperatures does increase summer disease pressure considerably.

These were the extended wetting periods from rain at our AREC:
July 14-15, 19 hr with just 0.01 in. of rain at 73° (very favorable for rots).
July 18-19, 10 hr with 0.09 in. of rain at 68°.
July 19-20, 16 hr with just 0.04 in. of rain at 69°.
July 27, 11 hr with 0.39 in. of rain at 70° (also favorable for rots).
July 27-28, 12 hr with just 0.17 in. of rain at 69°.

We track the number of extending wetting periods above 70° as an indication of potential bitter rot weather. This is not an absolute cut-off temperature but gives a basis for year-to-year comparison of relative bitter rot pressure. Since June 1 this year we have had 13 such wetting periods while last year by this time we had only five.

 As of Monday morning, July 28, at our usual AREC monitoring site at elevation 950 ft, we had accumulated 440 wetting hours since May 18, well beyond the 250-hour threshold for specific treatment against the sooty blotch/flyspeck (SBFS) fungal complex . But at the 910 ft. elevation cumulative wetting hour (CWH) total continues to run about 100 ahead, now standing at 557 CWH. Sooty blotch/flyspeck symptoms (signs of the fungi) are now readily visible. Unprotected fruit that we collected about four week ago have now developed bitter rot as well as SBFS. 

Orchards at lower elevations in Nelson County (Tyro area) have greatly exceeded the 250-wetting hour threshold for presence of the SBFS organisms on unprotected fruit (total 475 CWH at 941 ft elevation). The CWH total at our highest monitoring location (elev. 1465 ft) stood at 272 CWH as of Monday morning, July 28; CWH accumulation at a middle elevation (1165 ft) has consistently lagged behind the other two and is now at 190 CWH. Scout lower areas of your orchards regularly for onset of SBFS appearance and adjust your fungicide program accordingly. 

Below is a picture of bitter rot on a Honeycrisp apple as seen in a commercial orchard in Nelson County June 26. Fruit mummy inoculum, such as that shown with this fruit, and inadequate spray coverage were likely factors that contributed to this problem, but it shows the importance of being vigilant to any and all possible developing problems.

Bitter rot on Honeycrisp apple fruit. Note the presence of small fruit mummies associated with the bitter rot problem.