“Off-type” Golden Hills Trees Found in Some Orchards. A Bud Sport is Suspected
Craig Kallsen, Pomology Advisor for Kern County
For a number of years, growers have been bringing to my attention what appears to be trees that originated from a “sport mutation” in their orchards. Buds may mutate spontaneously and some of the most valuable varieties of our cultivated tree species have originated in this way. For example, navel orange originated from a bud sport. For the purposes of this article, I am assuming that what appears to be off-type trees seen in some orchards are, indeed, the result of propagation of a bud sport. However, proving that these trees are bud sports is not easy to do and is outside my area of expertise. Please consider this article a “heads up” for those collecting budwood for future orchards.
Unfortunately, this off-type tree present in ‘Golden Hills’ orchards is unsuitable for horticultural use. This sport, based on current observations, appears to produce small yields of very small nuts that will likely pass through the 30/64 inch screen when harvest quality is evaluated at the processing plant. Many of these small nuts are blank, as well. The picture below demonstrates the difference in nut size seen in an orchard on June 22, 2022, between the unknown sport (on the left) and a “normal” tree (on the right).Nuts should be full size at the end of June and shell hardening is occurring at this time.
Usually, in the orchards I have visited, four or five of these sport trees are found lined up in a single row, as if they came from the same bud stick. Sometimes there is a “normal-looking” male tree separating the sport trees in the row. The observation that the male tree is unaffected, suggests that the affected female sport trees on each side came from the same bud stick. There may be a number of locations within an orchard where the sport trees are found, all, again, lined up in an interval along a single tree-row. More recently, longer rows of these trees have been observed, suggesting that the budder had multiple sticks of affected budwood.
Differences in canopy growth are noted in these trees. The age of the trees appears to effect symptoms. Juvenile trees demonstrate long shoot growth with ragged, misshapen leaves and leaflets as in the two photos below.
Also, this kind of growth is present in the upper canopy shoots of trees just coming into bearing (beginning in fifth leaf). In addition, in trees just coming into bearing, lower areas of the canopy on older shoots may demonstrate “bunchy” growth as shown in the shoot on the left in the photo below.
In general, identifying this specific sport in an orchard of trees is not easy before they come into bearing. Poor yields of small nuts are the major identifying characteristics.
The similarities in growth between affected and non-affected non-bearing trees is demonstrated in the photo below. The affected trees are in the row on the right (note painted rings on the trunk) and “normal” trees in the row on the left.
In an attempt to define where the sport originated, sample leaves of the affected trees were sent to Dr. Dangl, at the UC Davis Foundation of Plant Services - Foundation Plant Services (ucdavis.edu). Dr. Dangl has a protocol for genetically identifying and separating the major pistachio varieties and rootstocks. However, this protocol will not determine if a tree is a sport or not. His results showed that the affected trees were ‘Golden Hills’’. If we assume then, that these trees are sports, then the logical conclusion is that these trees are a sport of Golden Hills’, which explains how something like this ended up in a pistachio orchard in the first place.
Pistachio trees take a long time from planting to when they begin to produce nuts. Understandably, growers are hesitant to remove a tree and replace it without sound evidence. Minor difference in canopy growth are not usually enough to warrant tree removal. For this reason, affected trees may remain in an orchard for a long time. Typically, in the pistachio industry, budwood for new orchards comes from existing commercial orchards, and usually from non-bearing third or fourth-leaf trees. If the trees shown in the above picture were used as sources of budwood for other orchards, it is not difficult to imagine how this sport was and could be spread far and wide, with the severity of occurrence among orchards and within a given future orchard, increasing with time. It is not possible that the sports in this orchard mutated in this orchard. The original mutation, undoubtedly, occurred many years ago in an orchard far away and has since traveled through budding. Fortunately, pistachio does not appear to mutate as commonly as navel orange but even a single sport can be a problem, especially considering the number of pistachio trees that have been propagated in the past decade. The existence of even the possibility of bud mutations should emphasize the importance of care in budwood selection for future orchards.