During about the third week in June, young Howard walnut trees experienced nut drop of perhaps as many as 200-300 nuts per tree. These nuts were full size with the shell beginning to harden. Cutting open an immediately dropped nut, you could see darkening of the inside watery kernel material, and termination of shell development that appears to have been triggered about 10 days before the drop was observed. This nut drop, wide spread with the Howard variety, was similarly noted in 2003 and 2011. Several possible causes are explored in this article.
Lack of Pollination? From studies that have been done on pollination, we know that un-pollinated walnut flowers fall off about 4 or 5 weeks after bloom. We do not believe that pollination is likely to be a limiting factor in walnuts, especially under most typical situations where there are many walnuts in the area and lots of pollen in the air. Further, these dropped nuts were much larger than nutlets aborted due to a lack of pollination or fertilization. There also appeared to be as many dropped nuts from trees near pollenizers as there were from trees farther away. Pollination is not a likely explanation.
Internal walnut blight? There was some speculation that this nut drop was “internal walnut blight”. Although walnut blight does progress into the nut it’s doubtful this was the case because there were generally no external walnut blight symptoms. Affected orchards were mostly young with little opportunity to have established walnut blight bacterial populations. Older Howard orchards, where blight bacterial populations could be well established generally had little nut drop. Samples sent to the UC plant pathology lab from this recent drop and previous Howard drop events with similar symptoms were unable to detect any evidence of walnut blight bacteria and this was a dry year as well. So, internal walnut blight does not appear to be the problem.
Wet soil? In both 2003 and 2011, spring conditions were wet, and it has been suggested that the wet soils resulted in stress due to saturation. Pressure bomb readings in some orchards with significant drop in 2013 indicated the trees were too wet, possibly due to over irrigation. This may have contributed by aggravating the problem in some cases, but not all soils were saturated yet apparently the nut drop was widespread over several counties. Howard nut drop was also observed on sandy soils that weren’t retaining water. If saturated soils alone were at fault it would seem other cultivars could be affected in the wettest most saturated areas and this doesn’t seem to be the case.
Abrupt temperature change? Only Howard was affected with nut drop in 2003 and 2011 when sudden high temperatures occurred at about the same time of year. A sudden increase in temperature at a critical stage in nut development may have killed the developing kernel resulting in the nut drop. This year, the daily high temperatures from bloom through May were generally in the mid-70s to a high of about 90°F. These temperatures were followed with temperatures in the high 90s on June 1st and 3rd, mid-80s on June 4th – 6th, and a sudden rise of temperature on June 7th – 8th to 104°F accompanied by dry north winds with humidity dropping from around 70% to 44%. These temperature and humidity fluctuations can also trigger mesophyll collapse sometimes observed in leaves.
Combination of factors? It’s possible that a sudden increase in temperature coming where soils are saturated might have contributed. Perhaps Howard is more sensitive to any stress – too much or too little water, sudden temperature extremes, heavy crops, or soil type may be playing a part as well. Environmental stress is the suggested cause and Howard may just be more sensitive to stress than other cultivars. Unfortunately, these are our best guesses as there is no way to do an experiment that will clarify the situation.
We’ve observed this problem before under similar environmental conditions…so what caused it? The short answer is that no one knows for sure. It may be nothing more than “June drop” on heavily set young Howard trees; perhaps triggered by the sudden rise in temperature and drop in humidity. Perhaps this explains it; even after the drop, the Howard crop still looks pretty good in most mature orchards.