Hunting Down the Cause of Black Pecan Kernels

This month’s column deals with the issue of black pecan kernels, which I have seen throughout my entire career. This problem is not the same as stink bug damage which is localized black spots on the kernel. This black kernel issue as shown in the pictures can have different degrees or levels of blackening. Some of the kernels have a fungal growth but this is thought to be a secondary issue. Some kernels are shiny black, and for other kernels, only the skin or integument is black while the meat is still white.

Unfortunately, I still do not have or know of a definitive answer on what causes this problem. In some years for growers that I visit, it has been a minor issue while in other years somewhat serious.

Early in my career, I thought this situation might be associated with stink bugs where a feeding puncture allowed access for pathogens; however, I don’t think this is an answer. Years ago, I took some black kernels to the Texas Plant Disease Diagnostic lab and was told (at least for samples submitted) that it was not a disease problem. Another idea for the cause is that the problem was from pecans laying on a wet orchard floor, but I don’t think this is the entire cause either for I have found pecans with black kernels still in the tree.

Also, I remember Benton Storey, Ph.D., relating an incident where there was an ammonia leak in a pecan storage facility which caused pecan kernels to turn black. This has made me question whether or not ammonia could be an issue. There are lots of ideas and thoughts on this problem but no real answer.

Since the pecan kernels are fully formed, affected pecans cannot be separated by weight which presents a problem for inshell sales for the retail market and to wholesale buyers.

Because this issue has been brought up again this year and there has been some discussion among pecan pathologists, horticulturists, and entomologists, I want to keep the ball rolling on this discussion and so I’m looking for some feedback. If you would like to provide some input or thoughts on this, please send me an email at w-ree@tamu.edu. Information that will be helpful includes:

1. Location (state) of the orchard. I have not heard of this being a problem in New Mexico so maybe not an issue in arid regions
2. Do you feel this is an issue for your operation and if so, do you have an estimate of percent loss.
3. Do you see the problem on certain varieties?
4. Do you make late season fertilizer applications?
5. Any information on fungicide applications
6. Information on post-harvest handling of the crop.
7. Any other comments.

Hopefully, more input and ideas on this issue can guide any future research or investigations into more specifics. Thank you in advance for your input.