Pecan cultivars: Checking on scab infections

    I  spent some time collecting nut samples from 24 different pecan cultivars to check on pecan scab infection levels. This season we applied 3 fungicide sprays to our pecan orchard. So when you look at the photos keep that in mind. Cultivars with zero scab lesions are either scab resistant or our scab control efforts were able to keep the disease in check. In many cases, the shucks of some pecan cultivar show signs of scab infection but the level of infection is not severe enough to impact nut size, kernel fill, or shuck opening. Peruque is an example of a cultivar that was not protected by our scab spray program and scab has engulfed the entire nut (photo above). Colby is scab susceptible but our scab control program limited the disease to a few small of scab lesions. Norton and Osage are scab resistant--the black spots on these shucks are the result of limb rub.
 

    Major is a scab resistant cultivar (photo above). Kanza, Lakota, and Hark all have Major as one of their parents and are also scab free. Even though these four cultivars are scab resistant that does not mean that they are immune to all pecan diseases. The fungicides we applied to our grove this year have kept the shucks of Major and her daughters from contracting pecan anthracnose.

     The photo above show four early-ripening northern pecan cultivars. Warren 346 and Lucas were completely clean showing zero scab lesions. In contrast, Mullahy and Goosepond had scab lesions despite our spray program.

    Oswego is a seedling of Greenriver and both cultivars are scab resistant (Photo above). Posey and Surecrop are only slightly susceptible to scab and it appears that our spray program has prevented all disease on the shucks.

  
    Giles is a cultivar selected from a native grove just 2 miles from our research station. Giles shows scab susceptibility despite our efforts to control the disease (photo above).  Jayhawk and SWB617 have Giles parentage but Jayhawk is scab resistant. The scab resistance of Jayhawk must come from its unknown parent.  Chetopa is another local cultivar, originating as a native tree at the Pecan Experiment Field. Chetopa is not scab resistant but three fungicide applications have kept this cultivar clean.

   
   The final photo of cultivars show that Mandan, Pawnee, and Faith are all susceptible to pecan scab. Mohawk was free of the disease.  
   With the exception of Peruque, we have been able to control scab on susceptible cultivars to a point where we will see no economic losses. This year we invested about $60 per acre in disease controlling fungicide sprays. Judging from the crop I see on the trees, we should average over 1200 lbs. of pecans per acre. That more that enough nuts to pay for our spray costs.

Pecan cultivars: Checking on kernel development

    I collected nuts from 24 pecan cultivars and cut open nuts to check on kernel development. The four cultivars shown above are some of our early ripening pecans. All four have developed kernel tissue but they still have a ways to go until the nut becomes fully packed with kernel. The Peruque nut was heavily infected by pecan scab which translated into a smaller nut and poor kernel filling.

     The four cultivars pictured above represent some of our earliest ripening cultivars. Warren 346 and Lucas look pretty well packed with kernel but judging from the width of the inner-wall partition these two cultivars are still developing more kernel. Mullahy and Goosepond are only slightly behind Warren 346 and Lucas in terms of kernel fill.

   
    The four cultivars in the photo above illustrates the wide differences I observed in kernel development during late August. Pawnee and Faith have laid down a thin layer of kernel with Pawnee being further advanced than Faith. The Mandan nut has a developed a thin layer of translucent kernel (under-developed cells) while Mohawk is still in the water stage.

    These next four cultivars are all in the early stages of kernel deposition (photo above). Posey and Surecrop are ahead of Greenriver and Oswego. It is interesting to note that both Posey and Surecrop have prominent wings on the sutures of the shuck. Surecrop originated as a seedling in Carlinville, IL. Could it be that Surecrop is a seedling of Posey. Large suture wings are fairly rare among pecan cultivars.

 
    Although SWB617 is a Giles seedling, this cultivar is far ahead of its parent in terms of kernel development (photo above). On the other hand, Jayhawk, another Giles seedling, lags behind its parent. Chetopa is in the earliest stages of kernel development.

 
    This last group of four cultivars include Major and three of her progeny; Kanza, Lakota, and Hark (photo above).  Kernel development of Major and Kanza is roughly the same, while Hark appears slightly advanced. Lakota was still in the water stage and showed no signs of kernel development at this point in the growing season.

Pecan kernels filling - pests on the prowl

    The other day I cut open pecans from a few cultivars of pecans and discovered just what I expected. My pecans had started the kernel filling process (photo above, right). At this point during the growing season, the seed inside the shell has formed a thin layer of kernel tissue just under the seed coat. The amount of kernel deposition by late August is both weather and cultivar dependent. Earlier ripening cultivars will fill their kernels sooner that later ripening cultivars.

     As soon as pecans start to fill their kernels, two important pecan pests attack: Pecan weevils and squirrels. Since we've experience above normal rainfall for the month of August pecan weevils have been emerging continuously since late July (photo at left). However, female weevils will not lay their eggs inside the pecan until kernels start to fill. So with the kernel filling process underway, we applied our second weevil spray to our trees this week (August 23 & 24) to control both pecan weevil and stinkbugs.  For growers with high weevil populations be prepared to make a third weevil spray in early September.
    

    The kernel filling process also marks the time when squirrels start cutting open pecans in search of a meager meal of partially formed nut meat. This kind of squirrrel damage largely goes unnoticed by growers but we have begun to collect lots of nut fragments in drop cages set under native pecan trees (photo at right). Although, we might never claim complete victory over squirrels, we've been trapping squirrels in coni-bear traps since early summer to help reduce the resident squirrel population.    

Same grafting technique, different results

    The other day, I was unwrapping some bark grafts when I noticed big differences in graft union appearance. The photo above shows two Kanza grafts made the same day on similar-sized seedling rootstocks. I used the exact same bark grafting technique on both trees. The graft on the left seems to be growing from just the outside of the stock tree while the graft on the right has grown callus over the top of the stock tree's stump. The tops of both grafts are growing vigorously and have made good diameter growth. Why such a noticeable difference in graft union appearance?
    It all comes down to cambial contact. As you might remember, the cambium is the layer of cells found between the bark and the wood. During the spring flush of new growth, cambium cells grow and divide to create a new layer of wood and a new layer of bark. This annual flush of cell division is how trees grow in diameter.  When grafting trees, cambial tissues of the scion and the stock are placed so close together that the cells grow together and form callus tissue. Once callus is formed the trees starts to lay down co-joined layers of new wood and bark to form the graft union.
    The tree on the left made good cambial contact under the bark of the stock tree but callus did not form along the upper edge of the stock tree. This usually happens when an arched scion pulls away from the upper edge of the stock during the grafting process. When the scion lays tightly against the stock all the way to the top of the stock, callus forms along the upper edge of the stock and the graft union heals over rapidly. Just look at the tree on the right. The cut surface of the stock is almost completely covered in just one growing season.
    Although I would prefer every tree to look like the graft pictured on the right, I can live with a less that perfect graft like the tree on the left. As long as I'm getting good top growth, a poorly formed graft union can be managed by pruning dead tissue off the stock and the tying the scion to a strong stake.

Marking successful grafts

     By the first part of August, scions I grafted last spring using a bark graft have grown 5 feet in height (photo at right). Over the summer months I've pruned the scion down to one shoot, trimmed off trunk sprouts, and removed grafting tape to prevent girdling. Now its time to remove all graft wraps and paint the graft union to make field identification of successful grafts easy.

   At this point in the growing season, my bark grafts are still covered with aluminum foil and a plastic bag. To remove these wraps I take my knife and make a vertical cut through plastic, foil, and grafting tape. After making the cut, I simply peel off all the graft wraps together at one time.

   Once uncovered, you can see how moist the tree is under the wraps (photo at right). By mid August, the graft union is fully formed and is beginning to slow its growth rate.  The reason I like to unwrap the grafts at this time is because ants and dogwood borers like to make a home in the moist environment under the wraps. Since both insects can damage tree cambial growth, exposing the graft union to the air helps prevent this kind of insect damage.

   Unfortunately unwrapping a graft union in mid summer can cause sun burn to recently exposed bark and cambial tissues. I use latex house paint to prevent sun scald and to mark successful grafts (photo at left).

    One advantage of painting the graft union is that I can now easily see which trees have been successfully grafted from a distance (photo at right). I also use different color paints to identify the cultivar grafted. On my farm, Kanza trees are white, Lakota yellow, Hark green, and USDA 61-1-X is red. Once painted, I don't need to worry about missing ID tags or trying to draw an accurate map of an orchard created from a field of randomly spaced volunteer pecan seedlings.

    We are rapidly approaching the season of the year when buck deer begin rubbing young trees. So after marking my new graft with paint, I replace the deer cage over the tree to protect this fine young Kanza graft (photo at left).

Crop load management

   This year, several of our pecan cultivars have set an over-abundance of nuts. So many nuts in fact that the tree couldn't possibly fill all those kernels and nut quality would suffer. So, I've been cutting nuts to determine when pecans enter the water stage (photo above) and when it's the right time to shake trees to reduce the crop load.  Today, with the nuts at the right stage and the sun shinning we used our tree shaker to remove a portion of our crop.

Lakota before nut thinning

   The cultivars in our orchard that required nut thinning this year were Pawnee, Gardner, Faith, Lakota, and Osage. Even some of the trees in our Kanza block needed crop load reduction. We used a Savage PTO shaker equipped with doughnut pads to give each tree a light shake. When the nuts are at full water stage, it only takes a short burst of vibration to rain down green nuts.  

Lakota after nut thinning

    Summer shaking is not an exact science but once you become accustomed to the practice you get a feel for the technique. During shaking you can actually see heavily laden limbs spring back upwards as the weight of nuts is reduced. Look closely at the before and after photos of shaking a Lakota tree, you can see the limbs have moved upwards.

   The one thing you can't do is look at the ground. Seeing hundreds of green nuts on the ground can make you feel like you've just thrown away a good portion of your crop (photo at left). But just remember, the remaining nuts on the tree can now fully pack kernel inside the shell resulting in a yield equal to or greater than if the tree was never shook. In addition, better quality nuts will command a better price and the tree will return with a good crop next year.
    One advantage to shaking for crop load regulation is that you can always skip over trees that are not overloaded. This year we shook about 80% of our Pawnee trees and 15% of our Kanza trees.

   The greatest danger it using a trunk shaker in mid summer is the risk of bark damage (photo at right). Clamp on the shaker improperly or not tight enough and you can tear off bark. This causes lasting tree damage that is hard to reverse. My advice is to not get in a hurry, and clamp on and off the tree with great care.

Pecan leafminers

   Have you every noticed a brown spot on the upper side of a pecan leaflet (photo at right)?  What looks like a foliar disease is actually the home of a small caterpillar. The upper-surface blotch leafminer (Cameraria caryaefoliella) feeds on leaf cells just under the epidermis of the leaf and creates an irregular shaped  brown blotch. Look carefully at the blotch and you'll see that under the papery thin epidermis you'll find a pile of black frass left by the caterpillar.

     I carefully pealed back the upper layer of the blotch to see if I could locate the caterpillar underneath (photo at left). I found the worm and moved him out to the leaf surface. The larva was black in color and very sluggish. The color and behavior of this caterpillar indicated that this poor fellow has been parasitized by a miniature wasp. That's one leafminer that won't be metamorphosing into a adult moth.
  

   Less common in my orchard is the pecan serpentine leafminer, Stigmella junglandofoliella (photo at right). This leafminer also feeds on leaf tissue just under the epidermis of pecan leaves but creates a meandering tunnel. As the larvae grows in size, the tunnel becomes wider making it look like a small snake is sitting in the surface of the leaf. Eventually the larvae in the tunnel will pupate and a small moth will emerge for the widest portion of the tunnel.

    Leaf miners are typically a non-economic pest. I commonly find them on young trees that receive little or no insecticide treatments. Once trees start bearing nuts and are regularly treated for major nut feeding pests, leaf miners largely disappear from the orchard.

Kernel development: 8 Aug 17

   This time of year I like to monitor pecan kernel development pretty closely for two reasons. The first is to determine when nuts reach the full water stage which is the optimum time for shaking trees to reduce an excessive nut crop. Then when kernels transition into the dough stage, I'll know when pecan weevils will begin laying eggs inside the nut.

   Today, I cut open the nuts of several well known pecan cultivars and a few new and emerging cultivars (photos at right and below). Let see how their kernels are developing.
   The first set of cultivars I placed in a photo together are the progeny of Major.
Kanza, Lakota, and Hark all dripped out liquid endosperm when cut open but the kernels inside have not yet reach the full water stage.

    In the second photo, I placed three early ripening cultivars to see if one of these pecans had reached the full water stage. Osage and Goosepond are close but only Mullahy has reached what I call the water stage. Notice how the kernel halves of Mullahy have grown almost all the way to the base of the shell.
  

     The next photo shows a collection of USDA cultivars. I've arranged the nuts in this photo by ripening date--Pawnee being the earliest to ripen and Mohawk being the latest. I was surprised to see that Pawnee was still only three quarters of the way to full water stage. Kanza and Lakota were both ahead of Pawnee this year. I guess that why I still need to check kernel development every year.

   The final photo show a couple of old time cultivars (Giles and Posey) versus two recently discovered cultivars (SWB617 and Waccamaw). SWB617 is a seedling of Giles but it ripens way before it's parent. As you can see SWB617 has nearly achieved full water stage while Giles is still at one half water. Waccamaw is a large nut similar to Posey and their stage of kernel development is close to the same. I'll be interested to see what Waccamaw does in the future.

Spraying for weevil, stinkbug, and scab

    We had a good rain shower over the weekend which came at a perfect time to promote nut growth and kernel fill. But 1.5 inches rainfall also provided plenty of moisture to softened the soil and allow pecan weevil to start emerging. So today we fired up the sprayer (photo at right).
   Today's pesticide application was aimed at controlling three key pests; Pecan weevil, stinkbugs, and pecan scab.  We generally spray the grove in early August for stinkbug control but the wet weather dictated that we also control weevil and scab. We used Warrior 2 for insect control and Quilt Xcel for disease control.   
    Applying a fungicide this late in the season has proven beneficial during wet summers to control not only pecan scab but also pecan anthracnose and downy spot. Judging from the forecast we are in for a cool wet August.

Pecan development and summer shaking

    Many orchards in northern pecan states are blessed with excellent pecan crops this year. Some cultivars have even set excessive crop loads increasing the likelihood of poor kernel quality this fall and  increased risk of cold injury this winter. Using a trunk shaker to remove a portion of the crop in mid summer is the best way to avoid kernel quality issues this year and increase return bloom next year.  However, for summer trunk shaking to be effective growers must time their nut thinning operations by carefully monitoring nut development.
    Trunk shaking to thin the crop works best when pecans have reached full size and the kernel is in the water stage. This means that growers must monitor their crop to check on kernel development. Yesterday, I collected nuts from four pecan cultivars (photo above). Let's see what inside.

    To check kernel development, hold the nut so you can see the attachment scar on the bottom of the nut (photo at left). Note that the attachment scar is oval in shape. The long axis of the oval was aligned with the stem of the pecan shoot.

   To reveal both sides of the kernel and the progress of kernel development, I cut the nut in half perpendicular to the long axil of the oval attachment scar (photo at left). If you find the mid point perfectly, your knife will split the shell on the suture. 

      The nuts I cut open yesterday, revealed that kernels have not yet reached full water stage (photo at left). Currently, Pawnee is at 1/2 water stage and Giles 1/4 water stage.  Nut development can move quickly at this time of year so be prepared to monitor nuts about every 5 days or so. From past experience, we have thinned Pawnee about a week before Kanza and about 10 days before Giles and Lakota.