The call of the week has been concern over leaf drop, not exclusively, but primarily on ‘Desirable’. This often occurs after we go through a hot, dry period followed by rain or an increase in irrigation. Desirable is sensitive ot just about anything that can affect a pecan tree and they respond to heat/drought (yes, even when irrigated) with some leaf drop. Usually the drop doesn’t occur until you get rain because water is required to break down that abscission layer at the point of attachment to the tree. We often see this occur in June/July but it can happen at any time and late in the season like this it is often heavier because those dry, hot conditions late in the season accelerate the process of leaf senescence. Usually lower leaves in the shade will be the first to come off because, again, these are often the most stressed leaves and are generally the firsst to senesce. This can be observed in trees where those internal, lower leaves are yellowing already.
I have seen a lot of yellowing of leaves over the last month in orchards throughout the state. The rain in July likely contributed to some N leaching at that time and then the heat/drought later in the month and first part of August has all combined to contribute to a lot of early leaf senescence. When this occurs other issues like Magnesium deficiency and anthracnose begin to appear as well. The leaves are simply worn out. It is not super critical to address either the Mg deficiency or the anthracnose this late in the season. Mg is adequate or high in most of our orchards from liming with dolomitic limestone ebut it too can leach out with heavy rains. Deficiency symptoms may result when heavy K applications are made or as Ca levels in the soil get too high or pH gets too low. Thus, the problem is most likely related to some sort of imbalance in the soil. A soil test should reveal the most likely culprit. Mg deficiencies appear as yellowing along the leaf edges and between the leaf veins creating a Christmas tree pattern on the leaf. Such deficiencies are often temporary and tend to correct themselves in the course of normal management (liming when needed, adjusting K, etc.). If soil tests look good, you can make foliar Mg sprays to address this issue but this late in the season, with leaves hardened off, it is likely a waste and should be addressed the following spring.
Any anthracnose showing up now likely infected the foliage weeks ago and fungicide sprays for it now will do no good. Bear in mind that thought we would like to keep all the leaves on the tree we can until frost, a low percentage of leaf loss (10-15%) at this point is not the kiss of death. Foliage naturally begins to senesce more rapidly as we get into October. If trees got too dry at some point, got stressed from crop load, had too much disease, etc. this will accelerate the senescence and loss of those leaves. If you still have 85-90% of the leaves on your first shake you are fine.
