One of the most well-known diseases that can harm a pecan tree is a fungus known as 'pecan scab'. It is also the most damaging. The reason for this is because pecan scab can infect the shuck, or pod of the pecan, stems, and leaves. What also makes this disease particularly prevalent is the fact that it can attack anytime during the season and will lay dormant in the leaves during winter waiting to kill your crop.
The cause of pecan scab is water. Too much water will allow the fungus spores to develop and thrive. This can be particularly difficult to control in the southern states where pecan trees are common. Mid to late afternoon rains are typical and those are the worst for spore development because the leaves can stay wet overnight. The longer they're wet, the more likely it is that the spores will develop and turn into tree-killing pecan scab.
The primary indicators that your trees may be infected are splotches that appear on the leaves in various sizes. These lesions are indicative that the pecan scab has killed that part of the leaf and can lead to early drop of the stem. Also, because the splotches are dark brown to black, photosynthesis cannot occur causing the leaf to wither and eventually fall off of the tree.
To prevent pecan scab, there are a variety of fungicides to use to kill the spores. The key is to spray the tree prior to getting it. If you're expecting a heavy rainfall, that's a good time to be preventative since that's when pecan scab tends to develop. However, if you're unwilling to use a fungicide or if it's not economical for your few trees you can still prevent pecan scab by removing dead leaves, pruning infected leaves and clearing the tree and area around the tree after the harvest season.
Pecan scab can be very serious, infecting entire orchards and ruining an entire harvest season. And the fungus is particularly stingy. There are some types of pecan trees that are more resistant than others like the Oconee and Houma varieties. But these may not be the type of tree that will grow in your area so you have to be careful. The truth is that nearly all farmers who grow pecan trees will most likely have to contend with some sort of fungicide and possibly pesticide at some point. It pays to be preventative otherwise fungi and pests have the potential to ruin your entire pecan season.