Nut development: 26 August 2013

   This summer, I've been following kernel development of three pecan cultivars; Osage, Kanza and Maramec (photo above). I choose these three cultivars because they represent early, mid season, and late ripening pecan cultivars. Previously, I had cut open nuts longitudinally to reveal the progress of kernel expansion from from the small heart stage to full water stage. However, with the kernel development process starting to switch over from the expansion phase to the kernel filling phase, its time to look at nut development from a new angle. At this stage, a cross sectional cut through the nut gives a clearer picture of the kernel filling process. Lets take a closer look at nuts from each of these cultivars.

    Osage is the earliest ripening of the three cultivars and at this point in the 2013 growing season, kernel deposition has begun. In the photo at right, you can see that a translucent band of kernel tissue has formed just under the seed coat. As time progresses, this kernel tissue will continue to thicken growing inward and eventually filling the entire kernel cavity.
    After comparing the photo of Osage to the photos of Kanza and Maramec (photos below) I discovered something new about kernel expansion I hadn't seen before. Once the kernel becomes fully extended in length, the kernel continues to expand but in an outward direction. This outward expansion compacts internal packing material up against the inside of the shell. By comparing the close-up photos of Osage, Kanza, and Maramec you can easily see this process in action.

    Taking a closer look at the cross-section of a Kanza nut reveals that kernel deposition has not yet started. And since the packing material surrounding the kernel was only partially compressed, this Kanza kernel was still expanding radially.

     Late-ripening, Maramec had no kernel deposition and no compaction of the packing material. I'm now wondering if Maramec will even ripen for first fall freeze.
   Hopefully the return of hot summer temperatures this week will help stimulate pecan crop development. Its nearly September and most cultivars are still weeks behind in terms of the normal kernel filling period. Lets hope for a long warm Fall.

The many faces of pecan scab

Scab lesions on Giles pecan shucks

   Most growers recognize the black scruffy spots on pecan shucks of as being caused by the pecan scab fungus, Cladosporium caryigenum. This year we are seeing plenty of scab develop of susceptible pecan cultivars (photo at right). 
    However, scab lesions can develop on several plant parts other than on pecan shucks. Depending on the location of the infection, scab lesions take on several different looks. While photographing scab infections on Giles nuts, I stopped to record the appearance of scab on other parts of the tree.

Scab lesions on Giles leaf

    





     First, I looked at the leaves. The scab fungus had created lesions on the midrib on the underside of a leaflet (red arrows at left). These lesions were small, black, and limited to growing on just the midrib. Scab also infected the leaf rachis (inside red circles at left). These infection sites were not as intensely black as the midrib lesions and contained brown scruffy areas. Lesions on the rachis were much larger in size than the lesions found along the leaflet midrib.

Scab lesions on Giles stem

    








    On current season's wood, I found very-small, black depressions in the bark (red arrows, at right). Because of their small size, stem lesions are easily overlooked but these infection sites are extremely important for the spread of scab the following spring. Leaves and shucks fall to the ground every winter but stems and stem scab lesions remain in the tree's canopy ready to help spread scab to next year's nut crop.

Collecting pecan leaf samples for nutrient analysis

   Each summer we collect leaf samples to determine the nutritional status of our pecan trees. Only by analyzing leaves, can we discover how well our trees are mining each of the essential elements needed for plant growth from the soil. We can also determine if additional nutrients in the form of soil applied fertilizers need to be added to correct nutrient shortages.
   Collecting a leaf sample is simply an exercise in choosing leaf parts that can represent the "average" nutritional status of the tree. In sampling pecan, locate the middle leaf along this year's shoot growth (circled in red, at right). Then locate the middle pair of leaflets on that particular leaf (yellow arrow point to leaflet pair) and remove that pair of leaflets from the leaf.

    In the photo at left, the yellow arrow points to the location where I pulled off the leaflet pair. That pair of leaflets is shown within the red circle.
   When sampling a pecan grove a single leaf sample should contain at least 50 pairs of leaflets. Collect leaflets from a couple of locations on a single tree but then move on to other trees in the grove. Remember your goal to to collect an "average" sample for the entire sampling area.
   If a certain portion of your pecan grove seems to be struggling, collect a separate sample from the trees in that particular area.  As soils vary across the field, tree performance can vary. Leaf analysis is the best tool to identify possible nutritional problems at the root of poor tree performance.

Interpreting pecan leaf analysis results

    Annual nut production is only possible if pecan trees have a sufficient supply of soil nutrients available for uptake by tree roots. Mid-summer leaf analysis is the best way to assess the nutritional status of pecan trees and to determine the fer­tilizer needs. Table 1 provides the major and minor nutrient sufficiency ranges for pecan trees grown for nut production. Tables 2-4 provide fertilizer recommen­dations for the major nutrients based on leaf test results. At the minimum, pecan trees require annual applications of nitrogen fertilizers twice a year—in early March and again in early October.  If potassium and phosphorus fertilizers are needed a single application can be made in the Spring (March).
  
Table 1. Leaf Elemental Concentration Sufficiency Ranges for Pecan Orchards

N%               2.4-3.0

P%               >0.12

K%               0.75-1.00

Ca%             >0.7

Mg%            0.3-0.6

Zn ppm         >60

Fe ppm         >50

Mn ppm        >100

B ppm          20-45

Cu ppm        >7

Table 2. Nitrogen fertilizer recommendations based on leaf test results. Nitro­gen fertilizers should be applied both Fall and Spring every year.

	Pounds of actual N to broadcast per acre
% Nitrogen in leaf samples	October application	March application
below 2.0	60	90
2.0 to 2.3	50	90
2.4 to 2.5	40	80
Above 2.5	40	60

Table 3. Phosphorus fertilizer recom­mendations based on leaf test results. Phosphorus fertilizers should be applied in the Spring.

% Leaf P	Recommended P2O5 lbs./Ac
< 0.12	60
0.12 and above	none

Table 4. Potassium fertilizer recommen­dations based on leaf test results. Potassium fertilizers should be applied in the Spring.

 

% Leaf K	Recommended K2O lbs./Ac
< 0.75	60
0.75 and above	None

 

Correcting Other Common Nutrient Deficiencies

Magnesium.  When leaf levels drop below 0.30%, add a magnesium con­taining fertilizer to the soil at the rate of 20 lbs Mg/ Acre. Magnesium containing fertilizers include magnesium sulfate, magnesium oxide, K-Mag, Sul-Po-Mag and kieserite. Dolomitic lime can be used in acid soils needing a pH adjust­ment. High N and K fertilization rates can lead to Mg deficiency.

Zinc. Zinc deficiency problems  (leaf Zn < 60ppm) most commonly encountered on sandy, soils low in organic matter or soils with a pH above 7.2. Increasing the organic matter in soils and adding Zinc sulfate to the soil at the rate of 5-10 lbs. ZnSO₄ / Ac. is recommended for acid infertile soils. In high pH soils, zinc must be applied annually as a foliar spray. Growers should mix 2lbs of ZnSO₄ per 100 gal of water (or use commercially prepared liquid Zinc prod­ucts) and spray their trees starting at leaf burst. Three to five sprays are needed and should be made at 2 week intervals.

 

Pecan scab infection decreases nut size

Giles nut clusters with and without fungicide sprays

    It's been several years since we've had the weather conditions needed to promote the spread of pecan scab. However, the four weeks of rainy weather we experienced this year during late July and early August was prefect for promoting scab infections. Scab susceptible cultivars that were not protected with applications of fungicides are now covered with scab lesions.

    Two clusters of Giles pecans are pictured above. The cluster on the left received two fungicide applications this summer. The nuts on the right were not sprayed with a fungicide at any time this year. Besides covering the shuck with black lesions, scab infection during the nut sizing period has had  dramatic impact on nut size. The scab covered nuts are nearly one half the size of the healthy nuts.

    This fall, the yield losses due to scab infection on untreated Giles trees will be immense. Scab will increase the number of harvested sticktights, shrink average nut size and reduce percent kernel. Yes, fungicides are expensive but growers that made well timed applications will be rewarded with more pounds of high quality nuts this Fall.  

        

Pecan weevil nut drop

   We discovered this cluster of Posey nuts while collecting leaf samples today (photo at right). The nut cluster had 5 nuts with three of the nuts streaked with brown necrotic areas. This didn't look good.
  

    I pulled off one of the damaged nuts for a closer look (photo at left). On one side of the nut I found an obvious puncture wound in the shuck (at end of red arrow). I could tell this puncture extended all the way into the kernel because of the shiny black stain on the outside of the nut husk. The kernel was punctured allowing liquid endosperm to leak out of the nut, onto the shuck and turning black as it oxidized.  In three days we would be finding dry, blackened pecans on the ground.

      I cut open one of the damaged nuts to inspect the kernel. The interior of the nut had lost its normal, light-green coloration. The entire contents of the nut had turned brown. Between the size of the puncture wound and the brown discoloration of the kernel, I can state with confidence that this nut was a victim of  pecan weevil feeding. Not only that, it was probably just one adult weevil damaging all three nuts in the cluster.
   Because nut development is so far behind normal, pecan weevil feeding damage will be an all too common problem this year. This is exactly the kind of feeding damage we were trying to prevent when we sprayed last week. Looks like we missed one.

Nut Development: 19 August 2013

    After a week of below-average temperatures, kernel development has proceeded at a painfully slow pace (photo at right). Osage kernels are a little fuller than last week but are still in the water stage. Kanza has achieved full size and and is also in the water stage. Late ripening Maramec has grown in size but the kernel is still only half way expanded.
    The weatherman is predicting seasonally warm temperatures this week with plenty of sunshine. It will be interesting to see if plentiful heat and light get get pecan development moving. The first fall freeze will be here before we know it.