The other day down at the nursery, around July 21st, 2010 yellowing leaves on two Summercrisp pear trees were observed. Within three days, the leaves on one of the trees were completely brown but still attached and by this time, the leaves of a third tree were yellowing.
What is going on with these trees?
To give a little background, these pear trees were purchased from Bailey Nurseries, are Summercrisp scion on a semi-dwarf rootstock (Old Home S333) and were planted bare-root in the spring of 2008. However, it wasn't until 2010 that foliar problems were first observed. One thing that had happened in 2010 that didn't happen in 2008 or 2009 was a swing of temperatures down below freezing while the leaves of the trees were expanding and an incredibly wet early summer through mid-summer with intermittent extremes of heat. With these climatic extremes could any of them be causing problems later in the growing season of 2010? What other observations have been made?
Other observations that have been made include the scratch test on branches and stem. When done on the lower branches, the scratch test reveals a green cambium. Well, if green it must be alive so what's the problem?! Well, the scratch test done around the circumference of the stem shows something different- a brown cambium. Brown cambiums = dead as door nails.
Now not all areas of the cambium on the stem are brown, only a specific region is showing a brown cambium but it is also this area that's awfully precarious for other reasons. If you look at the adjoining image, you see a compressed area of the stem flanked by swollen areas above and below the compression. It's obvious- this compressed area of the stem is dead thereby restricting the movement of water from the roots to the leaves and photosynthates from the leaves to the roots. But how or why is this compression happening...and on only a few of the trees?
At this moment, the answer is unclear. With the help of Dimitre Mollov from UMN Plant Disease Clinic, Chad Giblin, Dave Hanson and Gary Johnson a few potential problems have been given as options. First off, for critter damage all of these trees were protected by white stem guards during the winter months, which happened to be left on into May. Were these guards on so long that the moisture buildup was too intense under the guards to damage the cambium? The cold snap the Twin Cities experienced in April was after early growth was seen on the pears. Active growth was occurring on the trees so were out of winter dormancy, did this cold snap kill the cambium of the stem in areas already weakened by possibly the stem guard?
But another thought has been raised that looks back at 2009 as the time problems arose on the trees. The noticeable swelling of the stems points to more than just a few months of growth in 2010- all this growth would possibly have to include the growing season of 2009. Could possibly a pathogen (canker?) of some sort girdle the stem in 2009 and the tree, with enough stored energy, leaf out this year and then fail in 2010?
It has been observed by two great pathologists, Dimitre Mollov and Dr. Blanchette that this cambial death looks mostly like winter damage caused by the extreme lows. It doesn't completely make sense why these particular trees were affected in this particular area of the stem, but of anything, it's the best observations I have. It would make more sense if Bailey's had used a non-cold tolerant interstem in their grafting but that doesn't look to be the case.
Lesson learned? Maybe even U of MN releases aren't 100% cold hardy?
Photos: Top- Rebecca Koetter, Middle- Dave Hanson, Bottom- Dave Hanson