Tuesday, February 2, 2016

Pecan cultivars and tree size

    In setting out a new pecan cultivar trial, I anxiously wait for the trees to start bearing so I can record nut production data. Nut size, percent kernel, flowering type, maturity date, yield, and disease resistance are all among the key cultivar attributes that can define cultivar success. However, not all cultivar characteristics are revealed within the first decade of nut bearing.
    I was walking through the oldest cultivar trial on the research station when  I noticed obvious cultivar differences in over-all tree size. In the photo above, you can see that Major produced a much larger tree than Dooley. Both cultivars were grafted onto Giles seedling rootstock planted in the field back in 1981. The only difference between these two trees is the scion cultivar. 

    Looking around this 35-year-old trial, I found that trunk diameter was largely a function of cultivar. To create a visual record of cultivar differences in trunk diameter, I photographed the trunks of 6 pecan cultivars placing a one-foot-long ruler across the truck for scale (photo above ). Major and Peruque grew trees so large they seem to dwarf all others in comparision. Colby and Posey grew moderate sized trees while trees grafted to Giles or Dooley seemed small in comparison.
   I would never consider tree size a very important tree characteristic in making cultivar decisions. However, tree growth rate becomes important when planning initial orchard spacing and making subsequent thinning decisions. Large growing trees, like Major or Peruque, should be established at a wider spacing--let say, 40 feet rather than the standard 35 feet. Tree growth rate will also influence tree thinning decisions. The larger a tree grows, the sooner you will need to thin out adjacent trees to make room for future growth.
    It takes at least 30 years of cultivar testing to make accurate tree size determinations. In that time, many cultivars will prove unworthy of future propagation. Of the six cultivars pictured above, I would consider grafting only Major because of its scab resistance and regular nut production. But now, when I graft new Major trees, I'll also be sure to allow more space for these trees to grow.  

Friday, January 29, 2016

Which Kanza trees need thinning this year?

    If you are a long time reader of this blog, you may recall that we have been thinning a block of Kanza trees a little bit each year for the past several years. Today, I took advantage of the warm sunshine to walk through our Kanza block and mark trees for removal this winter.
   In some areas of this 3 acre block of trees the canopies of adjacent trees are beginning to almost touch (photo above, right). In these areas, I flagged some trees for removal.

   But just a couple of tree rows away, trees have not grown as fast and adjacent trees still have plenty of room to grow (photo at left). You might be wondering why, in such a relatively small area (3 acres), are our Kanza trees growing at such different rates. The answers is simple--small changes in soil conditions. All the trees in this block are growing in a soil classified as Osage Silty Clay. However, all soil types contain minor variations that can effect tree growth. The fastest growing trees in this block are growing in areas that are slightly more silty. In contrast, the slowest growing trees are found in soil with more clay.

The initial tree spacing for this orchard was 30 feet by 30 feet. We started thinning this grove in 2012. After thinning, the trees are left at a 42 foot by 42 foot spacing (photo at right). As you can see, the canopies of adjacent trees have ample room intercept sunlight and grow new nut bearing wood.
    After walking the grove today, I marked 9 trees for removal this winter. The blog posts listed below will give you the history of our thinning program in this block of Kanza trees.

2012 Making the decision to thin Trees 
2012 Thinning a Kanza Block
2014 Early spring thinning
2015 Time to think about tree thinning 
2015 Sticking to the plan

Friday, January 22, 2016

Yield data from 2015 cultivar trials

    Harvesting pecans from cultivar trials takes a lot of effort. We harvest dozens of plots separately by cultivar, then work to clean and weigh the nuts from each plot. The results, however, are worth the effort. By the end of the harvest season, we have not only harvested thousands of pounds of pecans but we have also collected the data that allows direct yield comparisons between cultivars. In this post, I present the yields from 3 different cultivar blocks

    Our oldest cultivar trail was established in 1981 by planting one-year-old pecan seedlings. Trees were set on a triangular spacing with 35 feet between trees.These trees were grafted starting in 1984. During the winter  of 2008 (after the 2007 ice storm), we thinned the grove by removing every other row. Today these trees stand at a 35 by 60 foot spacing.
    The 2015 yield data for the nine cultivars in this trial is presented in the table at left. The yield per acre values were calculated from the per tree data based the current tree spacing. By far, scab-resistant Major produced the largest crop among these 9 cultivars. Yields from Colby, Dooley, Giles, Hirschi, and Peruque were terrible. The heavy scab pressure we experience in 2015 negatively impacted nut production from these 5 cultivars.
     In 1983 we transplanted more seedling trees to establish additional cultivar evaluations. These trees were planted on a 30 ft. by 30 ft. spacing then thinned in 2008 to the current spacing of 42 ft. by 42 ft. In 2015, Osage produced the greatest crop among these 9 cultivars (table at right). This was the "on" year for Osage, a severely alternate bearing cultivar. This past summer we thinned the nut crop on our Lakota trees and still produced an excellent crop. Yields from Giles and Chetopa trees were disappointing, producing less than 1/3 the crop as compared to other cultivars in this trial.  

   The table at left presents the yield data from a trial first established in 1991. This planting was established at a 30 ft. by 30 ft. spacing and is scheduled for thinning this winter. This was the first year Oconee produced a big crop for us. Oconee often ripens late in our area and this was the first time we harvested nuts with fully developed kernels. Caddo, Oswego, and Shepherd were other high yielding cultivars this year.
    In reviewing the results of all our cultivar trials make sure to remember that one year's data should not be used to plan a new massive grafting campaign. I've seen some of these cultivars long enough to know to avoid them (ie. alternate bearing Osage and USDA 64-6-182), while others still make me worry about their late ripening dates (Caddo, Oconee).

Saturday, January 16, 2016

Looking inside a swollen lower pecan trunk

I have visited many native pecan groves over the years and have always found at least one tree in each grove that has developed a swollen, disfigured lower-trunk. Trees with this condition are typically weak nut producers and often suffer from significant limb loss. As I work with growers to improve their native groves, I always recommend removing these unhealthy trees.
 The swollen lower trunk condition is most prevalent under two types of growing conditions:  Trees growing under extremely wet soil conditions (photo at right) or pecan trees growing in shallow soils along upland creek bottoms.  In both cases the trees are growing under water stress. I the first case, the tree and its root system is swamped by too much water while the tree growing on the upland site frequently suffers mid-summer drought. Less frequently, trees can develop a swollen lower trunk even when growing in close to ideal soil conditions.

While making plans to thin trees in our native groves this winter, I decided it was time to remove an unhealthy tree with a swollen lower trunk (photo at left).  But I’ve always wondered -- What is going on inside the trunk to make it swell like that? It was time for a closer look.  I started by looking at the bark all around the lower portion of the trunk. The affected tree had several obvious wounds that looked just like old pruning wounds (photo below). However, this tree has not had a limb removed from the lower portion of the trunk for more than 60 years. In that amount of time, an old pruning wound should have completely disappeared. Something else was causing these wounds in the bark. With further inspection of the lower trunk, I also found a nectria canker and a phomopsis gall (photo below).
Wounds visible in the bark of a swollen trunk
Nectria canker on the left and Phomopsis gall on the right
By looking at the bark of the tree, I could tell that several pathogens were infecting the lower portion of this tree’s trunk. However, I really wanted to find out what was going on inside this tree’s trunk to cause the trunk to swell.  I used my chainsaw to cut a section out of the lower trunk area so I could look at patterns of wood growth (photo at right). The piece of wood I cut from the tree extended from just above the point of swelling downwards towards the soil surface. Once I removed the trunk segment from the tree, I took the wood into my shop and sanded the vertical face perfectly smooth to reveal the wood grain (photo below).
Before I show you some of the wood deformities I found inside this tree, I think it is important to show you what normal wood growth looks like when cut longitudinally. Thephoto at right shows a section of pecan wood with a normal wood growth pattern. The annual growth rings, so easily recognized when a pecan tree is cut in cross-section, are a little harder to see this longitudinal slice through the wood. However, you should be able to see the alternating pattern of large-pore, spring wood and finer textured summer wood that provides a visual record of annual cycle of wood growth.

Towards the upper portion of the wood sample, I found what can generically be called a wood burl (photo at left). Growing outwards from a bark inclusion (the black crevasse in the wood) you can see deformed wood growth that almost appears like it is bubbling outwards towards the surface of the outer bark. Lower down on the trunk segment, I found multiple burls both near the surface and deep inside the wood (photo below). In every case, the deformed pattern of wood growth seems to have originated with a wound in the wood.     

After cutting into this tree, it is clear that trunk swelling at the base of pecan trees is due in large part to the development of burl wood. Burls can form in response to either fungal or bacterial infections but how these organisms penetrated the tree trunk is the real key to understanding lower trunk swelling. Each burl was formed in association with an injury in the wood. These injuries were most likely created sometime in the past by the feeding of wood boring insects. Trees that grow under stress (too much or too little water) seem to attract both dogwood borers and flat-headed apple tree borers. The movement of these insects in and out of the tree’s trunk most likely spread the pathogens responsible for burl formation and ultimately leads to swelling of the lower trunk.