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Saving Young Pistachios From Dieback

Saving Young Pistachios From Dieback

As both the citrus and pistachio farm advisor in Kern County, CA, who would have thought that over the past two years I would be spending more time investigating what appears to be frost damage to pistachio than frost damage to citrus?

While frost has not been scientifically proven to be the cause of this pistachio winter tree dieback, the evidence certainly points in this direction and corrective actions are being taken assuming frost is the culprit. To date, heavy metal toxicity and disease have been investigated, only to be ruled out as causes of this dieback. Affected trees have been juvenile and have ranged in age from those planted the spring before the damage occurred to seven-year-old trees.

Symptoms of frost damage to pistachio trees are varied and depend, apparently, upon factors such as the elevation and surrounding topography of the orchard, the age of the tree, the degree of vigor or dormancy of the tree when the freeze event occurs, how low temperatures fall and the duration of the freeze event and, probably, a multiplicity of other factors.

Trees in affected orchards appear healthy and vigorous as winter approaches. The first unusual symptoms reported in an affected orchard usually occur about 10 to 14 days after the first freeze event in the fall. In the orchard, leaves dry and fall as the trees begin to go into dormancy but, on affected trees, remain stuck to the branches. The prolonged attachment of leaves suggests this damage occurs early, before natural leaf fall, probably with the first sub-freezing temperatures in late October or early November.

Many Symptoms

Temperature data suggests that with vigorously growing trees, temperatures do not have to drop much below freezing for freeze symptoms to appear. Associated symptoms include a water-soaked appearance of the bark, the growth of blackish mold on the affected branches and the trunk, and small ribbons or pustules of white gum extruding shallowly from the bark. Trees exhibiting these symptoms usually leaf out and flower, if old enough, earlier in the spring than neighboring unaffected trees. Although alarming, trees exhibiting these symptoms usually fully recover, with perhaps some minor branch dieback.

The most heavily damaged trees are not readily apparent until spring leaf-out, usually in early May. There are no post-freeze symptoms such as dead leaves stuck to branches, bark gumming, or blackening. Beginning in May, after unaffected trees have begun pushing normal, new growth, the most severely damaged trees produce only rootstock suckers or no growth at all. In some three and four-year-old orchards, up to 20% of the trees have been damaged in this way.

Trees that are one year old or younger usually have less of a problem with frost damage. However, when affected, very young trees can be killed outright by frost because they do not appear to have sufficient carbohydrate reserves to push new growth from the crown or roots.

In this more severe form of winter dieback, it is not clear when severe freezing occurs, although the extent of the dieback suggests that cold temperatures are required for an extended duration. Over the past few years, temperatures below 24ºF, with durations below freezing for eight hours or more occurring in late November through mid-December, have been sufficient for severe tree dieback to be present.

New Planting Locations

Isolated instances of what was thought to be frost damage have been described in the past. The reason we are seeing so much more of it during the past few years is probably attributable to several factors. Growers have become more confident in the salinity tolerance of pistachios and are planting them in saltier ground.

In the pistachio growing areas of California, areas with high salinity are also often at low elevation. Salt, like cold air, tends to move and collect downhill. In addition, acreages of young trees in what appear to be the susceptible age range have increased greatly, and, again, much of this new acreage has been planted in low-elevation areas on the west side of the San Joaquin Valley.

The drought in the southern San Joaquin Valley has compounded the problem in that reduced rainfall means low dew points and less fog. In the absence of high humidity, night temperatures can drop rapidly in the evening, producing lower low temperatures and longer durations of subfreezing temperatures.

Observational evidence suggests that rootstocks with some Pistacia atlantica heritage are more frost-tolerant than those without. However, for the southern San Joaquin Valley, P. integerrima heritage is a necessary component of a successful rootstock to provide resistance to Verticillium wilt, a fungal disease of pistachio. Examples of available rootstocks with both sources of germplasm include UCB-1 seedlings, the UCB-1 Duarte clone, and Platinum clone. Whatever rootstock is chosen, additional efforts will have to be made to reduce juvenile tree vigor as fall approaches.

Reduce Vigor

Initial observations suggest that reducing the vigor of juvenile pistachio trees as fall approaches, with the goal of encouraging earlier dormancy, may prevent or greatly reduce frost damage. The current most effective method of reducing tree vigor is to cut off irrigation in mid to late August on most soils. On our current rootstocks, fully irrigated pistachio trees do not appear to approach full dormancy until late December or early January.

Current observations suggest that to reduce the chance of frost damage, juvenile pistachio trees, especially those in heavy soils with high water-holding capacities, poor drainage due to high salinity, or other factors such as perched water tables or hard pans, should not be irrigated again until January or later. Some growers defoliate juvenile trees with foliar applications of zinc sulfate with the objective of encouraging earlier dormancy. While there is little scientific evidence to support the practice, defoliating the trees in a timely fashion — not too early so as to encourage a flurry of new leaf production and not too late to have an effect — should reduce tree vigor going into winter.

Frost protection may be another option. Trunk wraps are widely used in citrus production, and, depending upon the materials used, can provide up to 8ºF of protection, for the portion of the tree covered. Trunk wrapping materials vary greatly in durability and flexibility, and, to my knowledge, are not currently being used to reduce frost damage commercially in pistachios.

Older, producing pistachio trees have demonstrated much better tolerance of cold temperatures than juvenile trees. Why this is so is not clear. Production on large crops of nuts may reduce the vigor of older trees or perhaps, nut production itself, through hormonal activity or carbohydrate partitioning and relocation within the tree, encourages earlier dormancy.

For trees that are three or more years old demonstrating severe dieback symptoms, it may be beneficial to white-wash the south and west sides of tree trunks to prevent sunburn in late spring and summer. Bark that was previously shaded by leaf canopy can burn rapidly. Always wait in the spring to observe the extent of frost-related dieback before taking corrective action involving pruning or tree replacement. As temperatures increase, trees can continue to die back as a result of previous damage to the vascular tissue. For tree trunks that have been severely frozen and exhibit extensive bark damage, rebuilding the tree by grafting onto a sucker growing from the ground may be superior to grafting onto a sucker emerging from damaged wood.

One Tough Nut To Study

Science, properly conducted, seeks to isolate variables that appear to be important to the question at hand, with the objective of discovering and quantifying those that clearly influence cause and effect. Unfortunately, with respect to the problem of pistachio juvenile winter tree dieback, isolating variables in the field is difficult.

A major problem is the nature of freeze events. Freeze events vary in calendar timing, on how far temperatures fall and their duration, and how quickly the temperature falls before the event or rises afterward. Obviously, scheduling freeze events in the field is not possible.

The state of the tree is also important. General tree health, the degree of hydration, tree age, tree vigor, or state of dormancy when the freeze event occurs, will all affect how the tree responds. The location of the tree can be important with respect to existing soil conditions and surrounding topography. Is there cold air drainage or is the tree located in a basin where cold air collects? Salinity has been implicated in pistachio freeze damage, but low elevation areas also tend to be those that are saline, again making the isolation of variables difficult.

The scientific literature is replete with attempts to mimic the real world in growth chambers and even these efforts are overwhelmed by the complexity of the natural interplay between the tree and its response to environmental cues as it moves into dormancy. Particularly difficult to overcome in the growth chamber are differences in temperature between the surrounding atmosphere and roots insulated by field soil as opposed to a chamber pot, variations in sunlight and temperature with time during the day and season versus heating and bulb light on a timer, and other unavoidable differences between outside and inside.

For these reasons, readers of this article should be cautioned that the information presented is largely observational, and does not meet the standard of being scientifically proven. The causes and solutions to the winter dieback of juvenile pistachio trees will surely be refined in the future, and the observations made in this article should be thought of as a framework upon which to build future findings.

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