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.

Dogwood borers on young pecan trees

   Last weekend I was unwrapping bark grafts and I discovered a graft union covered by  a pile of nasty insect frass (photo at right). Looking closely at the grains of insect excrement, I found they were held together by fine silken threads. I pulled out my pocket knife to scrape off  all the debris when I discovered a cream-colored larvae with a red head.

    The larvae was a dogwood borer (Synanthedon scitula). This larvae definitely didn't like being out in the sunshine and immediately attempted to burrow down under the remaining frass piles (photo at left). Cleaning off all the frass, squashing the larva, and painting the graft union probably saved this graft union from being girdled by the dogwood borer.

    Once I found a dogwood borer in one of my grafts, I started to look at other trees for sign of borer activity. Dogwood borers feed on the inner bark and cambium of numerous hardwood trees and can become a serious pest in young pecan orchards. Adults are clear-winged moths that lay eggs on the bark of susceptible tree species. After hatching, young larvae search for a tree wound, branch attachment crevice, or graft union to enter the tree and star feeding on nutrient rich inner bark. The red arrow in the photo at right points to a column of reddish-brown frass pushed out of a dogwood borer's feeding gallery deep inside a branch attachment crevice. The frass pile usually takes on a tubular shape and is held together by fine, silken strands.

    I also found evidence of dogwood borer infestation on the trunks of young trees. the yellow arrows in the photo at left point to tree wounds created by dogwood borer activity. Piles of reddish-brown frass have pushed out of active borer sites while old borer wounds seem to ooze tree sap (dark black staining on bark).
   Dogwood borers seem to more of a problem in young, non-bearing orchards. Problems with this insect seem to disappear when trees begin nut production and a regular insecticide program is adopted to control nut feeding pests. Insecticides aimed at pecan nut casebearer and pecan weevil will serve to control both dogwood borer adults and emerging larvae.
   Because I found extensive dogwood borer damage all across my young orchard, I will be applying a trunk spray of chlorpyrifos (Lorsban) to knock back the existing borer population. However, if I had been paying closer attention and recognized the borer threat earlier, an early May trunk spray would have been most effective.

Flat headed apple tree borers make a mess.

   I was mowing the other day when I spotted what looked like a long steak of mold growing on the bark of one of my young pecan trees (photo at right). Oh no! Is this something new I need to worry about? Am I going to lose this beautiful, well-shaped tree? I finished mowing, put up the tractor, then came back for a closer look.

   Upon closer inspection, I found several trees with prominent stains on the bark,  usually centered around the upper side of a branch connection (photo at left). The stain appeared water soaked (dark brown) on the outer margins and fuzzy light-grey near the center. It definitely looked like I had an actively growing fungus on the surface of the bark.

   It all became clear when I looked at the other side of the tree and found a neat pile of insect frass in the crotch of the tree (see red arrow, photo at right). Now I know whats going on here. I don't have a strange new disease on my trees I simply have some actively working flat-headed apple tree borers.
   These common wood boring insects are often found in young pecan trees, especially in orchards that have yet to receive regular applications of insecticides aimed at controlling nut feeding pests. However, the staining on the outside of the bark is not normally associated with apple tree borers.
   The stains are a result excessive sap flow that occurred in response to the heavy rains we experienced from July 21 to August 12 (> 15 inches). Sap flowed into the wood borer's tunnel then spilled out onto bark surrounding the branch crotch. Since tree sap contains several forms of carbohydrates, the sap-soaked bark combined damp weather conditions provided an excellent environment for the growth of the common grey mold fungus.
    Since mid-August, the weather has turned off hot and dry. No new sap is flowing from wood borer tunnels and the grey mold fungus has stopped growing. However, the stains have remained and will remain until winter rains scrub the bark clean.
   One final note. Apple tree borers do only slight damage to the tree and rarely require chemical control.

The anatomy of natural limb pruning

   I was walking through the grove checking for scab lesions when I came across a butchered off limb on one of our Giles trees (photo at right). This now-dead limb was the product of the pruning job we were forced to perform in response to massive limb breakage caused by an ice storm in Dec. 2007.  You can see that the limb tried to resprout a few new shoots, but the entire branch stub soon became shaded and died from lack of sunshine.
    While I walked back to the barn to grab a chainsaw, I though-- "why don't I take a look inside this limb to see how pecan trees naturally shed large limbs broken by storms".

    First, lets take a closer look at this branch stub at the point it connects with the trunk (photo at left).  At a point around 2.5 inches out from the trunk, I spotted a line in the bark marked by a slight change in bark color and texture. The yellow arrows point to the top and bottom of this line which actually marks the boundary between living and dead bark tissue.

   Using my chain saw, I pruned off the limb just outside the branch collar revealing nice healthy wood tissue inside (photo at right). Now that this limb has been pruned off, the tree can start callusing over the wound.

    I picked up the pruned-off branch stub to take a closer look. Except for a small portion of the branch that had been closest to the trunk, most of the branch stub was in various stages of wood decay. A nice, white shelf fungus was growing out from cracks in the bark (photo above). The wood was also riddled with insect holes that seemed to be serving as a home for a colony of ants. With all these agents of wood decay and destruction so close to the main trunk, how does the tree prevent additional wood damage? 

   To find out,  I used the chainsaw to dice open the branch stub so I could look at the wood inside. The photo at right shows about 8 inches of the original 30 inch branch stub. However, this is where the action is--where live wood tissue meets decaying wood. The white wood on the left is living tissue that was removed from the tree when I made the pruning cut (described above).
    I mentioned earlier that I could see a difference in bark color and texture between areas of living and dead tissues. Well, that line also shows up in the cross section of the bark. The orange arrow at the top of the photo points to a dark black line in the bark that marks the boundary between living and dead tissues.  And remember those pretty white fungi?  Fungal growth inside the wood leaves a series of fine black lines in unique patterns that wood-workers would recognize as a condition known as "spalting" (see yellow arrow). All the holes in the wood are the result of insect activity; wood borers at first, then a colony of ants. 
    However, the most important thing to see in the wood of this mostly rotten branch stub is the very prominent boundary layer (blue arrow) between healthy white wood and decaying branch. By filling all the pores in the wood with water repelling organic compounds, the tree prevents wood rotting organisms from advancing into the trunk. This plugging of the pores has stained the wood a chestnut brown in a layer about 1/4 inch wide. The outside limit of the boundary zone is marked by a fine black line indicating the limit of fungal activity.  
    If I had let nature take its course, the rotten portion of the branch stub would eventually fall off the tree (years from now). Only then, could the tree start to callus over the wound. However, with proper pruning the callusing process can begin immediately. 

Hickory shoot curculio

    Have you seen flagging leaves and shoot dieback on this year's new pecan shoots (photo at right). If you have, you are most likely seeing an attack of the hickory shoot curculio.
     After resting dormant all winter in ground cover thatch, adult curculios (Conotrachelus aratus) become active in the spring as pecan shoot growth commences. When the first leaf on new pecan shoots is about one half expanded, female curculios start searching of locations to lay eggs.

     Hickory shoot curculio females use their mouth parts to carve a moon shaped depression into the base of a leaf petiole (photo at left). In that depression she will lay an egg. The larvae that emerges from the egg will first tunnel into the leaf petiole causing the leaf to wither and die. It will continue to feed by tunneling into the center of the new shoot making its way towards the terminal, often killing the entire shoot.

     If you see signs of hickory shoot curculio activity, cut open an infested shoot and you should find a white, legless larvae with a red head (photo at right).
    Once the larvae hatches and begins to burrow into pecan shoot tissue, it is impossible to control this pest with an insecticide. In other words, once you see the damage it too late to take action this year. If your pecan grove has a history of curculio attack plan on controlling adults early next spring.

     You will probably never see an adult hickory shoot curculio in the field (photo at left) but watch the development of springtime pecan shoot growth to time an insecticide treatment. Spray when the first leaf on the new growth starts to unfurls all its leaflets.
     Hickory shoot curculio is one of those sporadic pecan pests that seem to appear out of nowhere and then a couple of years later disappear altogether. I've only seen a few curculio infestations that warrant chemical control. It seems that naturally occurring biological control agents usually keep this pest in check.  

Flatheaded appletree borer

 

 

   The flatheaded appletree borer is a common pest of young pecan orchards. You rarely see the insect, but during early spring, you can easily spot the results of its activity in the crotch of a branch. In the photo at right, you can see a pile bark shavings created by the adult phase of the borer, a  buprestid beetle called  Chrysobothris femorate.  The adults chew their way out of the tree to escape from tunnels created by the insect's larval stage during the previous growing season.

   Even old exit holes created by adult flatheaded appletree borers are easily spotted near branch attachments. In the photo at left, note the oval shaped exit hole in the side of the tree at the point of a branch attachment. Once the adults emerge, they mate and female beetles begin laying eggs about a week later. Females beetles can lay as many as 100 eggs and search the tree's bark for deep crevices that offer some protection for the eggs. This female behavior is the reason larvae tend to develop at branch attachment points especially branches that have developed a bark inclusion.
    Eggs hatch in 15-20 days and the resulting larvae start to create tunnels under the bark feeding on the inner bark, cambium, and new wood. Healthy pecan trees fight flatheaded appletrees borers by producing excessive sap flow that can prove lethal to developing larvae. Trees weakened by stress often suffer the greatest damage from these borers, sometimes resulting in limb loss.

    The larvae are legless and can grow to 1.25 inches long. Several body segments just behind the insect's head are enlarged and flattened (photo at right). These segments are actually part of the insect's thorax, but appear to be a large flat head, thus the common name. Once larvae become fully developed they burrow deep into the wood of the tree to form a pupal chamber, where the insect over winters. Larvae pupate in the early spring and the adult beetles chew their way out as described above. 
    Although we find flatheaded appletree borers in most young pecan orchards,  the damage caused by this insect is usually not serious enough to warrant a control program. The best defense against borers is the keep your trees healthy with adequate nutrients, water, and weed control.