Bitter rot and summer disease pressures

This week we have had several reports of bitter rot showing up in Frederick County. For several weeks we have also seen it in our research plots at the AREC. Some may be associated with fire blight strikes, but more frequently they seem to be related to the presence of small fruit or flower mummies that served as the likely inoculum source.

Bitter rot on an Idared fruit with a small mummy, a probable inoculum source

Although it was quite dry throughout most of August, the bitter rot infection relates to warm wetting conditions as early as mid-May. From May 16 to Aug 11 we had 21 extended wetting periods at temperatures in the 70s and higher 60s, favorable to bitter rot activity. At our AREC we had only 0.14 in.of rain from Aug 11 to Sept 2, but some county locations have recorded considerably more rainfall than we have had at the AREC in the past month, so that adds to the potential problems. Although these conditions could also favor Glomerella leaf spot development, so far we have not had reports of this from Frederick County.

While we lacked wetting from rainfall, we continue to add accumulated wetting hours from dew. As of the Monday morning, Aug 31, total accumulated wetting hours (ACW) at the AREC were: at 909 ft elevation, 975 hr; at 952 ft elevation, 673 hr; and at the 983 ft elevation, 576 hr ACW. Since accumulation of wetting hours began May 14, this represents an increase of 5.5 wetting hours per foot of elevation drop from 983 to 909 ft. Continue to scout for sooty blotch, flyspeck, and fruit rots, especially at lower elevations in an orchard.

At Tyro, in Nelson County, total ACW as of Monday morning, Aug 31, were: at 941 ft elevation, 792 ACW; at 1165 ft, 415 ACW; and at 1465 ft, 473 ACW. An extended wetting event with substantial rainfall was recorded at Tyro Aug 18-19, 25 hr wet at 74-69° at the highest elevation.