Some years I get lots of questions on why the pecans are not filled out. Sometimes they are moldy inside, or the kernel is small or wafer-like. There are a few reasons why this could occur. Below are five reasons that are probably most common:
1. Disease. Pecan scab is a fungal disease that infects non-resistant pecan trees. It gets onto the shucks and can take the energy away from the developing kernel leaving it wafery or just plain ugly. Late season pecan scab on the shuck (after the kernel has formed) is usually only cosmetic, although it may inhibit the shuck from opening. The early season pecan scab is the one to prevent.
2. Insects. Pecan weevil and stink bugs are two culprits here. Both penetrate through the shuck and developing shell into the kernel. The weevil will lay an egg, whereas the stink bug feeds. A pecan with a weevil egg (that will develop into a larvae) will be entirely eaten from the inside. A common indicator of pecan weevil is a small, perfectly round exit hole in the shell. At this stage the kernel has been eaten and the larvae has exited the pecan. A stink bug feeds on the kernel as well, often leaving a black spot on the kernel (which tastes really bad). If the damage is done early enough the kernel may not form at all and may mold inside the shell. Timing of control is thus very important stop this type of damage.
3. Water. Too much or too little water can cause poor kernel development. Too much water can cause a rupture that exposes the inner developing kernel to outside elements. When this occurs (we call this water split) the kernel will mold and/or the nut will drop off the tree. Too little water causes the kernel to not fill out entirely within the shell. The kernel may be partially good and partially wafer. Irrigation during dry periods during nut fill is critical in some areas. Too much water (as in from a hurricane, heavy rainfall from a storm, or just too much irrigation) especially after a prolonged dry period can cause water split.
4. Crop Load. Excess crop load can cause problem with kernel development. Too much fruit on a tree will cause the tree to either try to distribute its reserves to all nuts, leaving not enough for any or the tree will drop nuts to try to preserve some nuts. Crop load management is necessary for some varieties in some years. Thinning of the crop at the water stage will help with proper kernel fill. Not all varieties respond to this type of treatment though.
5. Tree Nutrition. Trees that have poor nutrition will tend to produce poor nuts (and a poor quantity of nuts). Periodic soil tests and leaf tests should be done to assess the nutritional requirements of the tree. Pecan trees respond well to annual nitrogen applications, but you will not see an immediate response. It can take a year or two to start seeing the benefits. Other elements such as zinc and potassium are critically important as well.