Luke Milliron, UCCE Orchards Advisor Butte, Glenn, and Tehama Counties
Katherine Jarvis-Shean, Orchards Advisor Yolo, Solano, and Sacramento Counties

When prices are lean walnut farming is tough business. Farm advisors are not economists or financial advisors. However, farm advisors can be useful in providing best practices for economically sustainable production. Two key economic decisions growers face are when to replant individual trees and when to remove and replant entire orchards.

Learn from Dying Trees to Improve Management of the Whole Orchard:
Former Advisor Carolyn DeBuse, and Emeritus Advisor Bill Krueger wrote about the need to pause before replanting, by asking why the tree died in the first place. Saturated soils and soil borne pathogens are common culprits for tree loss. It is important to carefully evaluate recently declining trees, particularly evaluating their root systems for a possible cause. Your pest control advisor or local farm advisor can help with this diagnosis. Can changes be made to reduce the chance of additional trees declining and increase the chance of success for replanted trees? For Phytophthora and nematodes, these changes are often genetic (i.e. replant rootstock selection) but may also include a chemical approach. However, the needed action is just as often cultural, whether it is adjusting irrigation set time to prevent ponding or installing stream-splitters to keep water off trunks.

Water logging is a common cause of walnut tree decline. The Howard variety appears particularly sensitive to saturated soils. In addition to tracking saturated conditions with soil moisture monitoring, a key practice to not over-watering is irrigating only when demanded by the trees, as indicated through regular pressure chamber use.

Tree Replanting Decisions:
When a tree dies and is removed, there is always a question of whether to replant in that spot. The decision of whether to replant an individual tree is a gamble that the replant will become established and produce enough to offset its associated costs by the end of the orchard’s life. A decision that is an automatic “yes” early in the orchard’s life becomes more complicated as the orchard ages.

The shading from surrounding trees, and the ensuing low probability of successful vigorous growth weighs against the desire to replant and establish lost production. DeBuse and Krueger noted that if the orchard floor has over 75% shade at midday, the chance of a successful replant is slim. However, UC Davis Walnut Specialist Bruce Lampinen believes that a successful replant is dubious in orchards with over 60% shading. If the decision to replant has been automatic for you, pause to consider the cost of an unsuccessful replant and first evaluate how much light is available to the new tree.

After weighing the success of the replant and evaluating why the original tree died, if you do replant, follow the best practices to help nurture this investment. Root removal, possible spot fumigation, nursery product selection, and correct planting are the key steps. Of these, nursery product selection is a particularly critical step, if possible, tailor the choice of rootstock to the main source of tree loss (see rootstock table in this newsletter). As part of nursery selection, choosing a bareroot over a potted tree is typically ideal because you are starting with a tree that is larger and easier to manage. Once the replant is in place, the tree stands little success without paying extra attention to the need to modify irrigation and fertilization. You can learn more in the excellent guide by DeBuse and Krueger.

Orchard Replanting Decisions:
In addition to a lack of available light, general orchard decline is a second major reason to rethink replanting individual trees. General orchard decline in old age, frequently the result of weak trees facing an onslaught of multiple pests and diseases (figure 1), means replants make little sense because of the short window remaining in the orchard’s life. Instead, the orchard should continue to be farmed without replants until it is no longer economically feasible to do so.

Figure 1. An orchard facing general decline, with a high rate of tree loss. Extensive crown gall, trunk cankers, and high nematode counts all present in this orchard.

Amidst lean prices, if this is the year that you slate an orchard for removal, there are several steps you should consider before removal. A critical initial step, just as with replanting individual missing trees, is to ask why the orchard is in decline, and if those problems may follow you into the next planting. In addition to assessing the risk of the replant problem, and the persistence of crown gall, a nematode sample taken while the current trees are still in the ground is a great start. Learn more about nematode sampling and result interpretation and the next steps for replanting amidst high lesion nematode counts.

Building upon this initial diagnosis step, consider the following steps:

1. Assess potential carry-over problems before harvest.
2. Kill the roots of the old orchard. The established best practice for this is cutting down trees above ground during October and within 5 minutes painting the stump with Garlon3A, or a mixture of 1:3 mixture of Garlon3A and MorAct.
3. Wait a full growing season before replanting walnuts. This is the step where there are a range of management choices, such as the decision to fumigate, the use of a spring-summer crop to dry down deep soil moisture, or even exploration of a new technique like anaerobic soil disinfestation.
   This is also the step, where you could consider whole orchard recycling (WOR), whereby the previous trees are chipped and incorporated back into the soil. In almonds, this has been successful both for the performance of the subsequent planting and for soil health. Problems and success with WOR in walnut have yet to be demonstrated, with only a single pilot demonstration of young second generation WOR trees to-date. Until we have data on the persistence of crown gall and lesion nematode in roots of a recycled orchard, this practice is not advised for orchards with heavy cases of these afflictions. You can learn more about WOR at orchardrecycling.ucdavis.edu. WOR is an expensive practice, however the practice is now recognized for potential financial support through the CDFA’s Healthy Soils Program.
4. Fumigate, if necessary. The choice of whether to fumigate is centered around the concerns of root lesion nematodes, the replant problem, and to a lesser extent crown gall when replanting walnut orchards. Specifically, if lesion nematode is present in the old orchard, nematicide (e.g. Telone) fumigation, in addition to all the other steps may be necessary to have a successful replant to walnut. Fumigation is expensive, however the carryover problems from the previous orchard can prove far more costly. Cost savings with fumigation can be achieved by opting for strip fumigation specifically mapped for the tree rows, instead of broadcast fumigation. Adequate fumigant distribution can be problematic especially on clay-type soils. To ensure a successful fumigation, it is critical to dry down the soil to 12% moisture for optimal fumigant efficacy.
5. Replant on an appropriate rootstock. Consider the potential benefits of clonal Paradox rootstocks, particularly against nematodes (VX211) and Phytophthora (RX1). See detailed rootstock traits in the table in this newsletter.

Find more about orchard removal and replant steps. Finally, if you are removing and planting a new walnut orchard, a key document to have on hand when financially planning is the Sacramento Valley UC Walnut Cost Study.