Tuesday, October 21, 2014

Up in the pecan tree canopy

    While I'm hand harvesting nut samples using a hydraulic lift, I often spot interesting and unusual things up in the tree's canopy that I wouldn't notice with my feet firmly on the ground.

    We have all noticed nut shells on the ground a harvest time that have been chewed by mice. You can see the gnaw marks on the shell and a hole chewed large enough to allow easy access to the kernel inside. But I never realized that mice would actually climb up the tree and start feeding on nuts long before they drop to the ground. The photo above shows two pecans neatly cleaned out by a hungry mouse. Since this was the only nut cluster I found like this, its likely that red-tailed hawks find tree-climbing mice an easy catch.

    In the canopies of unsprayed trees, I find lots of nuts infested with hickory shuckworm larvae. The small white caterpillar with a red head tunnels through the shuck of the pecan and can cause the shuck to stick to the shell (photo at left). 
   
      Hickory shuckworm larvae eventually pupate inside one of their tunnels before turning into an adult moths. But have you ever though about how the moth gets out of the shuck. The photo at right shows a shuckworm pupal case partially shoved out a hole created at the end of a tunnel. Still trapped in its pupal case, this shuckworm moth managed to wiggle out of the shuck until it could break free outside. I've seen several of these pupal cases and every time the case emerges from the shuck tail-end first. I guess makes sense. The moth could use the force of spreading wings to crack open the case then simply back out.   


    In one tree, I found nuts with unusually shaped shucks (photo above). The nuts appeared flattened with a distinctive crease down the middle. Although these nuts had not yet split shuck, the shuck had separated from the shell and I could easily peel out the nut. What I found inside was a small nut with a kernel that had grown too big for its shell. 
    This condition occurs when nut size and final shell dimensions are determined during a dry period in late July. A lack of soil moisture during the nut sizing period causes the creation of smaller-than-normal nuts.  Later, ample rain fell during the kernel filling process in late August which, in turn, promoted rapid kernel expansion to such an extent it popped open the shell.  What is interesting to see is how the shuck reacted to the over-expanded kernel. The crease in the shuck is located directly above the crack in the shell. Fortunately, not all the nuts on this tree appeared like this. Most nuts had normal shucks and already shuck split.

    Every fall, I enjoy taking photos of ripe pecans on the verge of falling out of their shucks. This year, I found a nut that was not only ready to fall but one that tickled my fancy (photo at right). Almost like a Rorschach diagram this pecan reminded me of something totally unrelated to pecans. Can you see the helmet, eye slits, and open mouth with pecan falling out?  Or maybe I have an overactive imagination!

Friday, October 17, 2014

Shuck split acoss the midwest

   Last week, I visited 6 pecan orchards across 3 states to check on how pecan shuck split varies with location. Below is a map of the orchards I visited all in the space of 3 days.


      By looking at the map you can see that I traveled both east and north from my home base in Chetopa, KS. What the map doesn't show you is how the seasonal climate differs between these pecan orchards. In the Table below I have listed the annual average cooling degree days for each location as well the the average number of frost free days (above 32 F), and the average number of freeze free days above 28 degrees F.
    Cooling degree days are calculated by the National Weather Service (NWS) as a measure of summer heat used to estimate the energy demand needed for refrigeration and cooling. But cooling degree days also can be used to estimate the amount of heat a pecan tree receives during the summer. Pecan is a heat loving crop and even the earliest ripening pecan cultivars require significant summer heat to fill out kernel and ripen (see pecans and climate).
    The freeze free days above 32 degrees is the most common measure of length of growing season and a number most gardeners are familiar with. But, unlike common annual garden plants, the green tissues on a pecan tree do not freeze at 32 degrees. Pecan leaves and shucks freeze at about 26 degrees which makes the freeze free days above 28 degrees (data reported by the NWS) a closer approximation of average length of the pecan growing season.        


    As you might expect, locations farther north had a shorter growing season than more southerly locations. The two most southerly locations (Chetopa and New Madrid) have the most summer heat. However, there seems to be east-west heat gradient among the 4 other locations that all lie approximately on the same northern latitude. Paola, KS is the hottest during the summer while Carlyle, IL  is the coolest.
    At each location, I checked on the development of two of our most popular pecan cultivars -- Pawnee and Kanza. Lets look at the photos, starting with Kanza.

     On Average, Kanza ripens a few days after Pawnee so I was little surprised to find shuck-split Kanza nuts all across the Midwest.  As you look at the photos you'll note that Kanza nuts look pretty much the same anywhere they are grown. The one exception was at New Madrid where the nuts were covered with black sooty mold that developed following a huge aphid outbreak.
    The one orchard I did not find Kanza fully shuck-split was located in a bottom-land orchard near Paola, KS. From the table above, you can see that Paola has the shortest growing season of all the sites I visited but this location gets plenty of summer heat. What's interesting is that just a few miles away and on top of a limestone ridge, Kanza nuts were fully split open.
    Now here's a valuable lesson. Macro-climatic conditions (as measured by the NWS) may be useful for predicting cultivar adaptation on a general level but micro-environmental conditions will determine the exact timing of pecan bud-break and nut shuck-split at any specific location.
    In the Spring and Fall, cold air flows like water down hill and accumulates in the valleys and low areas. This natural downhill movement of cold air is the reason we have areas in the landscape called "frost pockets".
    The cumulative impact of cold air movement into the bottom-land site near Paola means that things warm up later in the spring and they cool off sooner in the Fall than the nearby upland site. As a consequence, a location where cold air settles has a slightly shorter the length of growing season and a reduced number of cooling degree days. This movement of cold air also explains why the upland Kanza nuts were ripe last week while the bottomland Kanza nut were still a few days away from splitting shuck.

     Now let's look at the Pawnee photos (at left and below). Unlike Kanza, I found a wide range of ripening stages among Pawnee nuts across the Midwest.  At the warmest and most southerly locations, Pawnee was split wide open and had been split for quite some time. When I drove northwards into Illinois I found Pawnee was just starting to split shuck. This was exactly the kind of observation I was expecting to make on the trip.
   Locations with a shorter and cooler climate should ripen pecans later in the year.


     But then I moved into central Missouri and onto eastern Kansas. At these locations, I found that micro-environmental conditions impacted Pawnee nut ripening. The Columbia orchard is in an upland position while the New Franklin site is located in the Missouri river flood plain. The Pawnee nuts I saw in Columbia were similar in ripening stage to the Pawnee I saw in Illinois. But just down the road from Columbia at the Horticulture and Agroforestry  Research Center (HARC) near New Franklin, Pawnee was still tight in the shuck.
    In this case, Pawnee nut development at New Franklin was delayed in comparison to Columbia by two factors--position in the landscape and soil type. The HARC orchard lays at the foot of the river hills in the Missouri river flood plain--a perfect spot for cold air to settle. Pawnee trees at this location are also growing in a heavy clay soil. This type of "gumbo" soil warms so slowly in the Spring that bud-break can be delayed pushing back the entire nut development process.  
    When I arrived back in Kansas, I found that stage of Pawnee ripening mimicked the observations I made for Kanza at the same two orchards. Pawnee growing at the upland site in Kansas was split much like Columbia, MO and Carlyle, IL. At the bottom-land site in Paola the shucks were still tight just like the nuts I saw at HARC.
    In comparing Pawnee and Kanza across all locations this year, the ripening dates for these two cultivars were nearly similar at the four most northerly locations. In Chetopa and New Madrid, Pawnee ripened significantly earlier than Kanza. But every season is a little bit different. If I were to repeat this trip next Fall, I would probably find slightly different patterns of pecan ripening.


Tuesday, October 14, 2014

Pecan Cultivars that ripened by mid-October

Chetopa, 10 Oct. 2014
Dooley, 10 Oct. 2014
Giles, 10 Oct. 2014
Greenriver, 14 Oct 2014
Lakota, 14 Oct. 2014
Oswego, 14 Oct. 2014
    The rain started late last week and by Monday evening nearly 9 inches (228 mm) fell in total. Between the rain and the dark skies, I was unable to photographic several cultivars that split shuck by October 10. However, the sun popped out today and I was able to record shuck-split photos for the 3 cultivars that ripened last Friday and three more cultivars that ripened over the wet and deary weekend.
    Last week, I checked on shuck split and found that Chetopa, Dooley and Giles had ripened (photos at right and below).  The photos reveal that all three of these cultivars are susceptible to pecan scab but the infections did not prevent normal shuck split.
   This week, Greenriver, Lakota, and Oswego opened their shucks (photos below). Note how nice and clean the shucks of these three scab-resistant cultivars appear in the photos.
   When I was looking for a cluster of Oswego nuts to photograph, I came across a cluster that contained a double nut (photo at bottom of post). Twin nuts originate as two fused pistillate flowers that share a common shuck wall but have separate stigmas and ovaries. At harvest, the twin nuts fall as a pair fused only at the very base of the shell. They often break apart during mechanical harvest.  

Friday, October 10, 2014

Air movement and scab infection

    This past week I've been visiting pecan orchards in Kansas, Missouri, and Illinois. During this trip I was able to see how air movement through a pecan orchard can impact pecan scab. The photo at right shows heavily infected Colby nuts produced by a young orchard in southern Illinois. The orchard had been planted next to a red cedar hedge row that served to prevent the wind from moving through the trees and drying off leaf moisture deposited on the tree by summer rains or heavy morning dews. Extended periods of leaf wetness helped to promote the growth and spread of the pecan scab fungus to the point that every nut was covered by the disease.
   Just 1/2 mile away from the scab infested Colby trees, I was able to photograph Colby nuts with just a minimal amount of scab lesions (photo at left). This tree was growing out in the open, adjacent to a row-crop field and widely spaced from other pecan trees. Scab was kept in check on this Colby tree, not by fungicide sprays, but by great air movement. With ample sun and wind to dry plant surfaces, the scab fungus did not have the right environmental conditions to infect and spread on this open grown Colby tree. 
   

    Down in the Bootheel of Missouri I found that air movement within a single tree's canopy can also impact scab infection. The photo above shows two clusters of Faith pecans; a scab infested cluster at the top and a relatively clean nut cluster at the bottom.  Note my hand in the photo. I was holding the scab infested cluster next to the clean cluster so you can see how scab infection inhibits nut size.
    The scab infested cluster was originally located on a lower, interior limb. The clean cluster was out in full sunlight and fully exposed to drying winds.
    This Faith tree is fairly young and was weighted down by a heavy crop of nuts. Limbs on the tree hung low which prevented good air movement under the tree and trapped moisture on lower interior branches. This trapping of moisture provided optimal conditions for disease spread leading to a localized scab outbreak on interior limbs. As this tree grows larger and lower limbs are removed, air movement through and under the canopy will improve decreasing the conditions for this type of scab problem.