What causes empty hazelnuts?

A friend of mine has hazelnut bushes. Generally, 1 in 10 or 1 in 20 come up "empty" with no nut or a tiny filament inside the shell. For some reason, nearly 50% are coming up empty from this batch. What are the possible causes for this?

(I have a vested interest in the answer, because they share the harvest with me in good years.)

Thanks!!

This PDF file mentions some reason for undeveloped kernels:

http://www.arehna.di.uoa.gr/Eusipco2005/defevent/papers/cr1151.pdf

Thanks! I have visions of her nephew making a machine to drop hazelnuts, analyze the sound they make, and sort out the empties. LOL

I found this excerpt, "Occasionally, a physiological
disorder such as plant stress from dehydration or lack of
nutrients allow a hazelnuts shell to develop without a
kernel. Also, a physical disorder such as insect damage
can stunt the maturation process and prevent a kernel from
being fully developed at harvest time."

It seems to me I read somewhere that a zinc deficiency can produce empty nuts... or was that for pecans? Maybe zinc is a "thing" with both pecans and hazelnuts?

I don't know about hazelnuts, but pecan can have problems with zinc deficiency. Foliar feeding of zinc is recommended for pecans. Having the appropriate variety for your climate is also important in avoiding undeveloped kernels in pecans. It could be that certain hazelnut varieties are more sensitive to stress or a combination of stresses. The link I sent you was the only one I came across. Probably not using the key words in the correct order.

Our baby pecan trees are still years away from production... but I think we did get two of the "right" varieties for our area, and we'll sure be giving them zinc!

I may recommend that my friends try zinc for their hazelnuts, on the "it can't hurt, might help" assumption.

Thanks, Bettydee!