Time to set out weevil traps

     Its time to set out pecan weevil traps (photo at right). You can build your own traps by following the instructions given in this previous post.
    With this summer's intense drought pecan weevil emergence will be delayed until your farm gets a good soaking rain. Since the only kind of rain anyone has received this summer has been a pop-up thunder storm, it is very important to have weevil traps set out in all you pecan groves.  It seems like my farm has missed every shower while it has rained just 2 miles away. With this kind of hit or miss rainfall, weevils can emerge in one location while they are still trapped in dry ground at another location.

Pecan tree caught in the act of self-pruning

    During mid-summer, I can usually find a single limb or small branch on a pecan tree whose leaves have turned uniformly yellow (photo at left).  By mid winter, this limb will be completely dead, a victim of the pecan tree's self-pruning habit.
     Pecan is a shade intolerant species. If leaves do not receive enough sunlight to remain photosynthetically active, the tree will shed them. Look at the photo and you'll see that the limb with yellow leaves is completely shaded by the limbs above. After the leaves turn yellow and fall from the tree, all water and nutrient flow through that branch stops and the branch begins to wither and die. Later, the entire limb will be shed from the tree, usually during harvest season with the aid of my trunk shaker.
    A large amount of this natural self-pruning in a pecan orchard can be a indication that the trees are growing too close together, sunlight is limited by excessive shading, and the orchard needs thinning. I usually survey my orchard at this time of year to look for branches that have bright yellow leaves. That way I can make plans to remove trees in areas that would benefit from a tree thinning operation. 

Signs of drought stress on pecan trees

    The entire midwest is suffering from the driest summer since the mid 1950's. Pecan trees are pretty good at handling dry weather conditions but weeks of temperatures in the 100's (F) and very little rainfall have started to take their toll.
    The other day, I drove past some native pecan trees that were growing along a small creek south of Chetopa. Among this small stand of natives, one tree showed the definite signs of drought damage (photo at right). The tree of the left has started to yellow with many leaves turning brown. This tree is shutting down just like it would in the fall in preparation for winter. Going dormant early is the pecan tree's last defense against severe drought.
    Just like every other tree characteristic, reaction to drought varies widely among seedling trees in a native grove. The photo above demonstrates that fact. But soil depth is also a factor. Pecan trees growing along creek banks far removed from major river flood plains will be the first to suffer drought stress. Soils in these locations are relatively shallow (5-7 ft.) compared to the extremely deep soils found in major river bottom flood plains (20-25 ft.). Deep soils have a larger capacity to store water and trees growing in these soils seem to weather drought far better.

    Regardless of soil depth, young trees are not as well equipped as mature trees to weather a drought (see Tree age and drought stress). The photo at left shows a young tree suffering from lack of rainfall.  By just looking at the tree, you can tell it is having a hard time. Leaves are light green with many internal leaves yellow or browning. To protect itself from drought this tree is starting to shed leaves.

    By taking a close look at a single limb on the young tree, you can see see how a tree starts to shut down (photo at right). Trees shed the oldest leaves first (at the base of this year's new growth) while shoot terminals remain green. Looking at a single compound leaf, basal leaflets are shed before the terminal leaflet.
   Note that, in all the photos of this post,  the ground cover is already crispy brown. Like I said before, pecan trees are pretty good at handling drought. Unfortunately, this summer looks to be among the top 5 driest on record and our trees are already suffering for it.

Mid summer nut development

    I collected a few nuts this morning to check on kernel development. I choose three cultivars and noted that all three were about the same size (photo above). In this extremely dry summer, I was surprised to see a few spots of pecan scab on the Maramec nuts. In any case, I needed to slice open each nut to reveal its stage of kernel development. 

    At this point in the season, Osage and Kanza are about 3/4 of the way to full water stage. Maramec is just starting to expand it kernel and is only 1/4 water stage. The most important observation I make when looking at these dissected nuts is that overall nut size is going to be very small this year. Lets just hope we get enough rain to fill out the kernels like we did during last year's drought.

Painting a bark graft

   During the first summer after bark grafting a pecan tree, I try to return to that graft every three weeks or so. In April, I showed you how I make a bark graft. By May, the buds on the scion were bursting with growth. Then in June, I selected the strongest growing bud to become the central leader and later that same month I pruned off stalked buds. Now its mid July and I'm back again (graft in photo at right) for some more  graft maintenance.
    The graft has continued to grow in height but has begun to slow down in response to this summer's dry weather. With a slower rate of growth, the newest part of the shoot is not producing stalked buds. However, I did take the time to add another tie to attach the graft to the 2x2 post.

    In the past, I've seen a new bark graft become the home to a colony of ants (photo at left). Ants like to live in the warm, moist environment found under the aluminum foil and plastic bag that covers the graft union. Ants tunnel into decaying wood found on the upper portion of the stock and weaken the graft union. To combat possible ant problems, I decided to take a bold step and change my normal graft aftercare procedure this summer.

   Lets start by unwrapping the bark graft (photo at right). The first thing I noticed under the wraps was the dark, wet stain located on the top of the stock near the scion. This is bacterial wetwood starting the decay process on the stump of the stock.  Opening up the wound to the air will dry up this wetwood and slow the decay process.
   The second thing I noticed was that callus tissue has formed all the way around the outside ring of the stock, a sign of healthy growth. I also noticed that lenticels on portions of the bark that had been covered by plastic were expanded (lenticels are raised rough patches in the bark that allow stems to "breath").

     Removing the grafting wraps at this time of year would help prevent ant problems but exposing the graft union to full sun in mid-July would cause major sun burn problems. So I decided to apply some sun block in the form of white tree marking paint (photo at left). We have used white latex house paint in the past to protect graft unions but I couldn't resist the convenience of using a spray can with a paint specifically tested for non-toxicity to trees.

    In applying the paint I covered all areas of the graft that had once been covered with aluminum foil and plastic (photo at right).  Now, the ants can't find a cozy place to lay eggs and I have a bright white visual marker to the location of my grafts.
     I have used tree marking paint in the past and found that it fades rather quickly. This is probably not a problem in terms of providing sun protection for the rest of this summer but if you would like to have a more permanent marker of graft location, latex house paint will give you 3-5 years of service.

Summer nut drop

     During mid-summer, there are a few critters out there that can cause pecans to drop. This summer, we set up drop cages to collect dropped nuts so we can monitor the causes for nut loss. Here are a few of the nuts we've found.
    The pecan pictured at right fell from the tree after being attacked by the hickory shuckworm. The bright white spot on the outside of the nut identifies shuckworm as the reason for the drop.  After hickory shuckworm females lay there eggs in the husk of a pecan, they cover the site with some scales rubbed off from their lower abdomen. This results in a raised spot of what looks like white fluff. We see hickory shuckworm damage every July but damage levels are rarely large enough to warrant control measures.

      Stink bugs and leaf-footed bugs can cause mid-summer nut drop (photo at left). The tell-tale signs of activity for these kernel feeding bugs are two fold. On the outside of the shuck, you should see small black feeding scars as well as large areas of damaged husk tissue. Cut open the nut and you will find the internal portions of the nut stained dark brown to black. These insects are primarily a late season pests but this summer's drought has caused an early migration of bugs into pecan groves. Fortunately, we are seeing only low number of these kernel feeding insects at this point. I typically apply an insecticide in early August specifically aimed at controlling stink bugs and leaf-footed bugs.

     We found one surprising nut drop--a pecan half eaten by a squirrel (photo at right). Squirrels usually do not start cutting into pecans until mid-August, when kernels enter the dough stage. However, this extended drought must be putting a lot of pressure on the squirrel's normal food sources. This must also be the reason we have been having great success trapping squirrels over the last month--they are starving.
 

     Nuts that drop from drought typically look dried up (photo at right). The shuck is completely rock hard and dark in color. However, inside you will find that the developing kernel is still white. This is in contrast to the stink bug damaged nut that has a darkened kernel (above). Fortunately, we have not seen a lot of drought induced nut drop so far this year.

Insect Instars

    When talking about insects the term instar often comes up. What is an instar and why does it matter?
    The body of an insect is composed of a hardened outer shell or exoskelton that protects and supports soft vital tissues inside.   
Insects must shed their exoskelton  in order to grow larger and assume a new form. Each time an insect sheds its exoskelton it passes into a new life stage or instar. Differences between instars can often be seen in altered body proportions, colors, patterns, or changes in the number of body segments.
   In the photo above, a colony of second instar walnut caterpillars are feeding on pecan leaves.  The first instar of walnut caterpillars are olive green, while the 2nd, 3rd and 4th instars have a dark red color.  The fifth and final walnut caterpillar instar will be entirely black.
   Knowledge of instars is important important for two reasons. First, insects in the early stages of development (early instars) are much easier to control with insecticides than insects near maturity. Secondly, insects often are most damaging when fully sized during their their final instar.

Vein Spot on Shepherd Pecan

    The Shepherd pecan cultivar has shown promise for growers looking for an early ripening and hardy tree. Besides the regular production of  and good nut quality, Shepherd is noted for its resistance to pecan scab. However, Shepherd seems to be susceptible to another disease--vein spot.
    At right is a photo of the foliage of a Shepherd tree growing at Missouri's Horticulture and Agroforestry Research Center near New Franklin. Vein spot appears on the foliage as elongated black spots on leaf rachii and along the midribs of leaflets. With the dry summer we've had this year, I was surprised to see even this low level of disease on the foliage.
    Vein spot is usually not a problem in pecan orchards that receive scab sprays. Left untreated, a serious infection of vein spot can cause premature defoliation and cause increased alternate bearing. Resistance to pecan scab is a good thing but scab is not the only disease we need to consider when evaluating cultivars.