The photo above is a perfect illustration of the impact fungicide applications have on pecan leaf retention. This morning, I drove into the Pecan Experiment Field and noticed a stark difference between our trees (left of the lane) and our neighbors trees (right of the lane and across the fence). Our trees still had leaves on them.
The difference in leaf retention wasn't due a difference between grafted trees and native trees. Trees on both sides of the lane are grafted. On the left are Posey and Pawnee trees while the neighbors trees are primarily grafted to Mohawk. The difference in fall leaf retention pictured above is entirely due to the application of fungicides made last summer to control scab. Our neighbor did not use any fungicides in 2016.
Even though our primary goal in making fungicide applications was to control nut scab, fungicide sprays have the secondary benefit of keeping leaf diseases under control. It was the cumulative impact of several foliar diseases that caused early leaf drop in our neighbors orchard.
Long before the leaves drop from the trees, foliar diseases reduce the tree's ability to capture the sun's energy and convert that energy into carbohydrates. This usually happens in late summer, exactly at the same time pecan trees need the carbohydrates to fill out nut kernels. The result is an exaggeration of fruiting stress during the current season and a weak return bloom the next.
Over the many years of managing our pecans at the Experiment Field we have found that preserving leaf health with fungicide applications help reduce the intensity of alternate bearing. But remember, all the fungicides applied were aimed at controlling pecan scab. Improving leaf health and reducing alternate bearing is a welcomed bonus.