Tuesday, June 26, 2012

Summer training a bark graft

    A bark graft placed on a large tree can grow very fast. Today, I returned to the tree I had  topworked this past April for some additional directive pruning. Just 4 weeks ago, I pruned this graft to a single central leader and now the new leader has grown to nearly 5 feet tall (photo at right).
    The graft has also grown in girth. The photo at left gives you some idea of how much this graft has grown. The original scionwood I placed for this graft was about 3/8 of an inch in diameter. Now that scion is almost 1 inch in diameter and still growing.
    Note the size of the central leader compared to the size of the lateral branch I pruned off the last time I visited this tree. Four weeks ago all the shoots growing from the scionwood stick were roughly the same diameter. Now, with all the tree's energy focused on just one shoot, my central leader is becoming well defined.
   To maintain the growth of a strong central leader, I spent some time to remove stalked buds and practice the two foot rule for pecan tree training.  Looking over the central leader of this new graft, I noticed that stalked buds became more prevalent and further extended as I looked up the stem (photos above). Left alone, these stalked buds would have grown into branches, giving the tree a overly-heavy, bushy top that is more prone to wind breakage.

    Caught early in their development, stalked buds are easy to remove. I just break them off with my fingers (photo at right). I took me just a few seconds to clear the entire central leader of stalked buds. Those few seconds were a great investment, saving me from having to spending hours trying to prune out the weakly attached lateral branches with bark inclusions that would have developed from the stalked buds.
     The final step in today's tree training operation was to place an additional tie to attach the central leader near the top the 2 x 2 inch post (photo at left). I may have needed a step ladder but I sure don't want to loose such a strongly growing graft to the winds of a summer thunderstorm     

Saturday, June 23, 2012

Pecan vs. Hickory: Nut Development

     Earlier this week, I posted a photo that illustrated how differences in nut development between pecan cultivars can be seen early in the growing season. Today, I drove by our Wilson hican tree (a pecan/hickory hybrid) and was surprised by the size of the nuts hanging from the branches. Just for fun, I decided to compare nuts from our hican, a shagbark hickory and a Mohawk pecan (photo above). It is obvious, by looking at nut size, that the hican and hickory are way ahead of the pecan in terms of nut development.

   My next step was to cut open the nuts an look at kernel development. The kernels of the hican and hickory are about 1/2 way expanded, while the Mohawk pecan hasn't even started kernel expansion yet (photo above). The hickories will have fully developed kernels by late July while Mohawk pecan won't start to fill until the 3rd week of August.  Why the big difference between species? Some folks have theorized that pecan weevil may have had something to do with it.
     If you have ever collected hickory nuts in the wild you will find that most nuts have been attacked by pecan weevil. In fact, the life cycle of pecan weevil seems to be more in tune with hickory nut development than with pecans. Pecan weevils start emerging from the soil in late July. At this time, hickory nuts are filled with kernel and provide a great spot for female weevils to lay eggs. But in a pecan grove, early emerging weevils often need to wait around until pecans fill their kernels in mid-august to finally start laying eggs. Since female weevils don't like to wait, they will migrate to trees that fill their kernels first. This leads to weevils populations to become clustered around early ripening pecan trees.  
    Pecan weevil has placed evolutionary pressure on pecan as a species to delay fill kernel later in the season. This happens because the nuts of early kernel filling species (the hickories) or cultivars are often completely destroyed by weevil larvae making it difficult for these trees to produce viable seed to create a new generation of trees. In contrast, late filling pecans avoid weevil attack and are able to spread thousands of viable seed into the environment.
    Filling kernel late in the growing season creates another set of problems for pecan trees that is not associated with pecan weevil. Pushed too late into the fall, late kernel filling cultivars often have trouble creating a full, plump kernels and nuts can fail to split their shucks before first fall freeze. From my experience, the best compromise between late kernel fill for weevil avoidance and nut development early enough to avoid frost damage are pecan cultivars that fill their kernel by mid-August and shuck split one week before the average date of first fall freeze.

Friday, June 22, 2012

Fall webworm hatching

   Last week we found an egg mass of the fall webworm. This week we found hatched out larvae feeding comfortably in their loosely spun web (photo at right). Just like newly hatched walnut caterpillars, first instar fall webworms do not have large enough mouth parts to bite through an entire pecan leaf. Instead these young larvae just scrape off the outer layer of cells on both sides of the leaf. In the photo, the leaflets nearest to the cluster of caterpillars has just been scraped clean of its outer layer. As these leaflets dry, they will turn brown just like the other leaflets under the web.
   We are not finding a large number of first summer generation webworm colonies at this time. However, since it looks like we are facing a walnut caterpillar outbreak this summer, insecticides applied to control walnut caterpillar will also control fall webworm.

Reaction wood

     Ever look at a big, old, native pecan tree and wonder how the branches can hold up all the weight of leaves and nuts. Trees respond to the force of gravity by producing what has been termed, "reaction wood". Reaction wood is basically the production of wider growth rings in response to the pull of gravity.
   The photo above is a cross section of a lateral branch cut from a large native pecan tree. When this branch was young, growth rings around the pith are fairly uniform in thickness. But as the branch grew larger (and heavier), the growth rings along the bottom of the branch became much wider than the same rings at the top of the branch. This is especially true for the last three years of growth.
   Reaction wood is also the reason crooked graft unions, so common in young trees, seem to grow into nice straight trunks with age. The tree simply grows thicker growth rings is areas under gravitational stress, eventually growing into a well balanced trunk.

Thursday, June 21, 2012

Walnut caterpillar hatching

    Just like fall webworm, walnut caterpillars have arrived right on schedule--late June. Today, we found a couple of newly hatched walnut caterpillar colonies (photo at right). Newly hatched larvae are quite small and olive green in color.
    I looked around the foliage near the colony of feeding caterpillars and found the egg mass from which they hatched (photo at left). Looking closely at the egg mass, I learned two things. First, the egg mass is much larger than normal. And second, not all the eggs hatched indicating possible parasitition by the wasp, Trichogramma minutum. The laying of large egg masses is often a precursor for an outbreak of walnut caterpillars. The fact that less than 10% of the eggs were parasitized means that wasp parasites were not numerous enough to keep walnut caterpillar in check.  Everything I saw today, points to a walnut caterpillar outbreak, at least in and around the Pecan Field.

     To scout for newly hatched walnut caterpillar colonies look for brown, skeletonized leaves in the canopy of your pecan trees (note brown leaf in center of photo at right). The mouth parts of newly hatched larvae are so small they can not eat the entire leaf. Instead, the young insects scrape both upper and lower surfaces of the leaf, turning the leaf brown. The photo below shows a close up of the brown, skeletonized leaves and a feeding colony of first instar walnut caterpillars.


Wednesday, June 20, 2012

Early maturing cultivars start kernel expansion quickly

    During the growing season, I like to cut open pecans to check on their development. This morning I was out in a pecan cultivar block cutting nuts and noticed that the earliest ripening cultivars had already pulled ahead of later ripening cultivars in terms of kernel development. Below is a photo of two nuts I cut this morning.

    At this time of year, pecan kernels appear as a small, water-filled sacks imbedded inside the nut. Like the photo in my previous post, the kernel starts out in the shape of a tear drop. However this week, the kernels have grown larger and started to change shape. From the tear drop shape, the kernel grows into a more blocky shape like the kernel of the Oswego nut pictured above. With continued kernel development, the water-filled sack takes on a heart shape as two kernel halves start to develop (Osage nut above).
     This fall, Osage will ripens 3 weeks before Oswego. But even today,  you can see an obvious difference in nut development phenology between these two pecan cultivars. Osage has already advanced further than Oswego in terms of kernel expansion. All pecan cultivars seem to become fertilized at about the same time (see fertilized vs. pollenized post), but the early ripening cultivars seem to develop far more rapidly than cultivars that end up maturing later in the fall.

Saturday, June 16, 2012

Pecans start to exlarge

    After looking at nut clusters for signs of pecan nut casebearer larvae for the last 3 weeks , I've seen a sudden increase in nut size just this past week (photo at left). Our pecans have how entered the period of rapid nut enlargement. It is during this stage of nut development that pecan nuts become most susceptible to pecan scab.
    Nuts are expanding in response to signals set out by healthy, developing embryos. If you carefully slice open a nut at this time of year, you'll see a tear-drop shaped structure imbedded inside (photo above). This is the earliest stage of kernel development. At the apex of the small tear-drop structure is the embryo. The round, water-filled sack below the embryo is the developing nut kernel.    

Friday, June 15, 2012

Fall webworm right on schedule

     This year, pecan trees broke bud nearly 3 weeks early. First-summer-generation pecan nut casebearer came as least 2 weeks earlier than normal. So what should we expect when it comes to summer-foliage feeding caterpillars? The answer came today when I found my first fall webworm egg mass (photo at above).
    Mid-June is the normal time period for the very first webworm egg masses to be spotted. It seems fall webworm moths didn't get the memo that bud break was early in 2012.
    Fall webworm egg masses are easy to identify by the white "fluff" scattered over the top of the egg cluster. This "fluff" is actually moth scales that the female scrapes off her body in an effort to disguise the eggs to avoid attacks by egg parasites.

    By early July you should see webworm colonies in their dirty white webs (photo at left). Fall webworm has two generations per year in our area. A second generation appears in late August.
    Depending on the number of colonies we see per acre, we can control this pest with a wide range of insecticides. Currently, the action threshold for fall webworm is 5 colonies per acre. Make sure to scout your orchard carefully to spot the colonies when they are still small (the size of your fist). Young larvae are much easier to control than larvae nearing the end of their life cycle.

Thursday, June 14, 2012

Removing stalked buds

    We have been summer pruning the young trees in our orchard this week and find ourselves removing lots of stalked buds.  Both primary buds (upper bud at each node) in the photo at left are elongated and held on a short stalk. Allowing these stalked buds to grow into branches will create a tree with narrow, weak crotch angles (read this previous post for a detailed explanation of the problems stalked bud can create).
   At this time of year stalked buds seem to break quickly after they form and start to form new branches. Below are before and after photos demonstrating the removal of stalked buds. On the left side you can see stalked buds that have grown into branches at every leaf node. The stalked buds I removed are marked with the letter "R". There is one stalked bud I decided to save (marked with a "S") because this bud was located at the very top of the tree and represents the growing point for my central leader. After pruning I have preserved my central leader and prevented the tree from developing a leaderless, bush top (right side of photo).

Wednesday, June 13, 2012

Insect assassin: The wheel bug

    Not all the insects you can find in pecan tree canopies are harmful to your crop. Here's an example of a large insect predator that can be found searching pecan foliage for other insects to eat (photo at right). This large, grey bug features a pronounced "cogged wheel" protruding from its back and strikingly red antennae. Arilus cristatus is the largest member of the assassin bug family (Reduviidae) growing nearly 1.5 inches long.
    The wheel bug feeds on caterpillars, sawfly larvae, aphids, or any insect not fast enough to get out of the way. This insect has a long, red, tube-like mouth part that is normally held under the head.  When a wheel bug attacks another insect, it holds back its head then throws out its tube-like mouth parts like a javelin into the body of its prey. The wheel bug immediately injects a neurotoxin into the body of its victim, paralyzing it. Once the prey is immobilized, the wheel bug secretes its digestive fluids into the insect's body. After the insect becomes pre-digested, the wheel bug sucks all the fluids out through its tube like mouth parts.
    The wheel bug is one of our major predators of walnut caterpillar and fall webworm. But be careful around this beneficial insect. If you even get bitten by a wheel bug, the neurotoxin secreted in this insect's bite will leave a painful welt on you that can will last for days.

Tuesday, June 12, 2012

Target canker starts with pruning wounds

   It looks almost like a platter attached to the trunk of a tree; but this is target canker caused by the fungus, Nectria galligena (photo at left).  The fungus doesn't attack healthy trees but requires a pruning wound or broken limb to enter the tree and attack the callus tissue that grows to seal over a wound.
   We usually don't make massive pruning cuts on pecan trees like the one in the photo, but following the 2007 ice storm, we removed limbs torn off by the weight of the ice. This wound became infected by the Nectria fungus. Each year, the tree forms new callus in an effort to seal over the wound but every year the slow growing fungus kills the callus. The result are concentric rings of dead callus tissue.
    Here's an example of a canker that demonstrates why they call this disease "target canker" (photo at right). This canker must have started on a small broken limb and the battle between tree and fungus continued over 15 years. In this case, the wound will never heal over correctly. The "target" will continue to grow in diameter right along with the tree.
    A single target canker on a tree seems to have little effect on nut production. However, trees with multiple cankers become obviously weakened and should be thinned from the grove.
    Normally a tree wound will become sealed over by a single large roll of callus tissue. Here's a photo (at left) of a tree wound that is healing properly. It will take just a few more years and this wound will be completely covered over.

Monday, June 11, 2012

Pecan Nut Casebearer: June 11 report

    Looking for pecan nut casebearer damage this year has proved only one thing--we have a great nut set and very few casebearer larvae (photo at right). The final tally for this year's casebearer population at the Pecan Experiment Field was 3% damaged nut clusters. The population of casebearer was so low it didn't even provide a good level of nut thinning.
    With a heavy nut set this year, we are in desperate need for some decent rainfall. Dry weather in June and July will make for small nut size. Continued drought into August and September means poor nut fill. Lets hope we don't repeat last years dry conditions.

Wednesday, June 6, 2012

Training a new bark graft

    Back in April, I used a bark graft to top work this tree (photo at right). By the first part of May, the scion had broken bud and began rapid growth.  Now in June, the scion's new growth is nearly 3 feet in length and its time to start the tree training process.
     I began working on this graft by removing all suckers sprouting from the trunk below the graft. I next removed the bird perch and cut off the grafting tape that sealed the plastic bag around the scion (photo at below). With the extraordinary growth rate this graft has already demonstrated, it is extremely important to make sure the scion doesn't become girdled by green tape.
   You can see that I have 4 strong shoots growing from the scion. As usual, the bud closest to the graft union received the biggest push from the stock. In this case, both primary and secondary buds at the base of the scion produced new shoots (lower left side of the scion) diluting to energy that could have been directed to a single bud. Regardless, my job is to pick the strongest shoot to become my new central leader and remove all others. 
    The uppermost bud had produced the largest shoot on the scion and is my choice to save for the new central leader. I removed the lower 3 shoots and pruned off the small stub at the top of the scion (photo at right). 
   Pruning to a central leader at this point will force the scion to grow even more rapidly. To prevent the wind from breaking out the new growth, I attached a eight foot long piece of 1 x 2 inch board to the side of the tree with duct tape (photo at left).  I then used one inch wide flagging tape (white) to tie the new shoot to the board. As the shoot continues to grow upwards, I'll tie the shoot in several places further up on the board.
    Before leaving this tree, I decided to prune a portion of the original tree back.  I concentrated on removing limbs and shoots that were pointing upwards and competing with the scion for sunlight. I removed one entire branch all the way back to the trunk, the pruned off several upward growing side shoots. You can see in the photo at right that the scion is now receiving full sunlight. 
    All summer long I will be using directive pruning techniques to ensure the formation of a strong trunk. By the end of the growing season, I fully expect this new graft to grow at least 7 feet in height and 1.5 inches in diameter. But, it will only grow that large in I keep the graft tied to the stake and competing limbs pruned off.

Tuesday, June 5, 2012

Intercropping with soybeans

     Last week we received some much needed rain. So, this week we had enough moisture to plant soybeans in our double pecan row/intercrop demonstration plots. The pecan trees in these plots are now 10 years old and nearing the point where they are getting large enough to warrant mechanical harvest. Depending on tree growth this summer, this may be the last year we crop this field. If this turns out to be the last crop of soybeans, we will establish a cool season grass in the tilled areas and switch over to hay production for 3 to 5 years before completely ending all intercropping practices. By year 16, our entire focus will be on pecan production with the trees producing more income per acre than any field crop that can be grown in SE Kansas. 

Monday, June 4, 2012

Pecan Nut Casebearer: June 4th Update

     We have been scouting for pecan nut casebearer damage for a couple of weeks now and have found that the damage level has stayed at 2% for the past week. When the damage level stays low for more that a week, it usually means that total cluster damage for the entire casebearer season will not exceed 6%. Looks like our decision not to spray for casebearer control this year will save us some hard earned cash.
     I'm still waiting to make the first fungicide application for disease control. We've been on the dry side, with long periods of low humidity between infrequent rainfall. We'll continue to watch the weather and apply a fungicide when the weather turns wetter (we need the rain!).