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.

Extended wetting for fruit rots July 8-9; accumulated wetting hours.

July 8-9 we had another extended wetting period: 15 hr with 0.24 in. of rain at 72°. This wetting period was favorable for rots.

As of Thursday morning, July 10, we had accumulated 293 wetting hours since May 18, well beyond the 250-hour threshold for specific treatment against the sooty blotch/flyspeck (SBFS) fungal complex at our usual AREC monitoring site at elevation 950 ft. But at the 910 ft. elevation cumulative wetting hour (CWH) total continues to run about 100 ahead, now standing at 391 CWH. Early sooty blotch symptoms (signs of the fungi) are now visible.  

Orchards at lower elevations in Nelson County (Tyro area) have also exceeded the 250-wetting hour threshold for presence of the SBFS organisms on unprotected fruit (total 345 CWH at 941 ft elevation). CWH total at the higher elevation stood at 200 CWH as of Thursday morning, July 10. Scout lower areas of your orchards regularly for onset of SBFS appearance and adjust your fungicide program accordingly.

Sooty blotch symptoms; extended wetting for fruit rots July 3-4.

Last week July 3-4 we had another extended wetting period: 9 hr with 0.6 in. of rain with a mean 71°. This wetting period was favorable for rots.

As of Monday morning, July 7, we had accumulated 270 wetting hours since May 18, well beyond the 250-hour threshold for specific treatment against the sooty blotch/flyspeck (SBFS) fungal complex at our usual AREC monitoring site at elevation 950 ft. But at the 910 ft. elevation we have recorded 372 CWH and early sooty blotch symptoms (signs of the fungi) are now visible.  

Orchards at lower elevations in Nelson County (Tyro area) have also exceeded the 250-wetting hour threshold for presence of the SBFS organisms on unprotected fruit. Scout your lower orchard areas regularly for onset of SBFS appearance and adjust your fungicide program accordingly.

Sooty blotch and flyspeck on unprotected fruit; rot infection period

Last night we had an extended wetting period following afternoon rains: 16 hr with 0.22 in. of rain at 75°. This wetting period was favorable for rots.

As of this morning, July 3, we had accumulated 258 wetting hours since May 18, surpassing the 250-hour threshold for specific treatment against the sooty blotch/flyspeck (SBFS) fungal complex at our usual AREC monitoring site at elevation 950 ft. This is the elevation from which we have reported cumulative wetting hours since 1994, and this is the latest we have reached that threshold since 1999. But we have recorded 352 CWH at a lower AREC site (elevation 910 ft).  

The wetting hour threshold indicates the time at which we would expect the SBFS organisms to be present, but not yet visible on unprotected fruit but we expect symptoms in the more advanced area shortly. Remember to scout your lower orchard areas regularly for onset of SBFS appearance and adjust your fungicide program accordingly.

Accumulated wetting hours for sooty blotch and flyspeck; mildew remains active; fire blight and rot inoculum levels.

 Each year we record accumulated wetting hours (CWH) from rainfall or dew, starting ten days after petal fall for purposes of predicting development of the sooty blotch and flyspeck (SBFS) fungal complex,. This past week wetting at our AREC occurred almost entirely as dew, with only traces of precipitation which dried quickly. 

As of yesterday morning, June 30, we had accumulated 228 wetting hours since May 18, approaching the 250-hour threshold for specific treatment against the SBFS fungal complex at our usual AREC monitoring site at elevation 950 ft. But at a lower AREC elevation (910 ft), we have greatly surpassed this threshold with 334 CWH.  Again this illustrates the importance of elevation for wetting hour accumulation. Remember to scout your lower orchard areas regularly for onset of SBFS appearance and adjust your fungicide program accordingly.

With the continued dry weather this past week, powdery mildew remains active where shoots are continuing to grow. Since mildew spores were first observed Apr 9, we have had 49 dry weather “mildew infection days”.

This past week in visits to Frederick County and central Virginia orchards, we saw more fire blight than we would like to see. This seemed to occur as a result of marginal protection at late bloom. For the most part, it does not appear to be spreading actively any more, but the presence of dead wood in the trees raises the concern of increased inoculum levels for the rot fungi, especially bitter rot, as we approach harvest. Bitter rot was already evident in one Nelson county orchard and small, over wintering fruit mummies seemed to the likely inoculum source in that situation.