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.
