Walnut quality can be impacted by orchard management practices. Kernel shrivel and changes in pellicle (the covering on the kernel) color can be impacted by both water stress — whether too dry or too wet — and shading. Monetary losses due to these problems can be substantial: We have seen cases where up to 70% of the nuts in an orchard had either shrivel or severely discolored pellicles as a result of severe water stress during late summer. Quality can also be impacted by sunburn, which can also cause kernel darkening and/or various degrees of shrivel, depending on the sunburn severity.
Research conducted in a mature Chandler walnut orchard in San Joaquin County from 2007-2009 showed that photosynthate (mobile carbohydrates) reduction from the leaf was a likely contributing factor to undesirable pellicle color changes. The reduction in photosynthate from the leaf could be caused by leaf loss as a result of either water stress or shading, or by a combination of the two factors.
The most common nut position on the tree to find these quality problems is low in the canopy, near the tree trunk. We also found problems with pellicle color changes related to leaf loss in low canopy positions over the drive row where upper canopy branches had moved downward as the nut weight increased, eventually leading to shading of the lower branches and resultant leaf loss. In 2008, this was the most common position where nuts with yellow pellicles were found.
In 2009, a study was set up in the same San Joaquin County Chandler orchard described above. Spurs in lower canopy shaded positions were artificially defoliated on seven different dates starting in early July.
As expected, defoliation during the time kernel dry weight was increasing resulted in various levels of shrivel. Severe shrivel was worst when defoliation occurred in early July when the kernel was just beginning to fill. Slight shrivel tended to occur with defoliation during late July through August.
It is worth noting that even with total defoliation of the spurs in early July, only 8% of the nuts had severe shrivel, suggesting that the tree was able to move photosynthate to the spur from other positions on the tree. This would agree with our earlier observations that the quality problems are much more severe if a stress event occurs during the July/August period when the tree is bearing a heavy crop as compared to a light crop.
Pellicle Color Changes
Pellicle color changes generally occurred after the hull became black and mushy as a result of lack of photosynthate supply. This condition looks very much like husk fly damage but no husk fly larvae will be found in the husk if the damage is shading/stress related. Yellow pellicles tended to be worst when defoliation occurred in early August but occurred from about mid-July to early-September to some degree.
Black pellicles tended to increase with defoliation from early July with the black pellicles peaking with defoliation in mid-August and the bronze peaking with defoliation in late August. Bronze pellicles tended to increase from the early July to late August defoliation dates.
The condition known as oil-less nuts would be grouped within the bronze pellicle nut category. You can get varying levels of black, yellow, or bronze nuts as a result of defoliation on that date. It is unclear at this point what factors determine which of the pellicle colors occurs. There could possibly be an interaction with the proximity to other, functional leaves that can supply photosynthate to the defoliated spur.
Pee Wee Nuts
In 2011, we investigated the occurrence of pee wee nuts and brown adhering hulls in a mature Howard orchard in Colusa County. We found that pee wee nuts were on spurs that had a low number of much smaller than average leaves. These spurs may open later than the majority of buds on the tree so the small nut size may be associated with the late bud opening — and competition from other nuts that are already much larger — as well as the limited spur leaf area.
Rows that had been mechanically hedged the previous winter tended to have more pee wee nuts, but they occurred in unpruned, unhedged rows as well. The spurs with these characteristics (low number of small leaves) produced pee wee nuts even if the nut position was in a well-exposed location in the sun, suggesting that the problem was related to the condition of the bud as it was formed in the previous season, rather than current year conditions. The bud was likely weak due to having formed in a shaded position the previous year.
The pee wee nuts were generally good quality, light-colored kernels, but just small. Brown adhering hulls were associated with buds that had an intermediate number of leaves — less than buds associated with normal nuts but more than pee wee nuts — and intermediate size of leaves — smaller than buds associated with normal nuts but larger than those associated with pee wee nuts. Nuts with brown adhering hulls were smaller than normal nuts and had variable quality, anywhere from good light color to darkened pellicle.
In general, good irrigation practices will help to minimize quality problems. Avoid starting to irrigate too early in the season. Creating excessively wet conditions in the springtime leads to shallow rooting, poor tree growth, and increased likelihood of water stress-related problems later in the summer. Ideally, you should use a combination of soil- and plant-based measurements in the springtime to determine when to start irrigation. Later in the summer, during the kernel filling period, it is essential to minimize any irrigation related stress, either from too little or too much water.
Although light-related quality problems in the lower canopy can be decreased by selective upper canopy pruning, it is likely that more crop would be lost due to the pruning than would be saved by the improved light in the lower canopy. In general, you should expect quality problems to increase in highly productive orchards (above 3 tons per acre) even with good water management. However, as discussed above, these problems can be minimized with proper orchard tree spacing and irrigation management.