Fall Walnut Updates
Kari Arnold, UCCE Orchard and Vineyard Systems Advisor, Stanislaus County
Mohamed T. Nouri, UCCE Orchard Systems Advisor, San Joaquin County
Here is a brief overview of important walnut planning topics/tasks. You can contact your local UCCE farm advisor for more locally based information. Your pest control adviser (PCA) will help you make specific pest-control application decisions. More information is available on the UC Integrated Pest Management (http://ipm.ucanr.edu/) and UC Fruit and Nut Research and Information Center (http://fruitsandnuts.ucdavis.edu/).
Special consideration: Prepare and protect trees from future freeze damage
Background
Freeze damage in walnuts is caused by freezing temperatures in the fall. Over the last three years, UCCE Walnut Advisors observed damage in older orchards in Sacramento and northern San Joaquin Valleys where material should be dormant. Often this occurs in young orchards where new growth is still being formed during periods of freezing night temperatures. Freeze damage is brown, necrotic tissue, which can appear to be related to pathogens, but no signs of fungal infection are present. Please see Fig. 1 for photos associated with freeze damage.
Fig. 1. Freeze damage in a 9th leaf Solano orchard. Severity of symptoms is variable across and within orchard blocks (damage beneath the bark appears as brown discoloration). @Figure provided by Mohamed Nouri.
While green tissue is highly susceptible to freeze damage, dormant walnut tissue is believed to withstand temperatures in the low 20s (°F). Yet bud and wood temperature may fall lower than the ambient air temperature, and walnut tissue requires a slow decline of ambient temperature to convert complex carbohydrates to simple sugars during the fall period. In the last two autumn seasons, we experienced rapid declines in temperature, sometimes shifting from 60 °F to 28 °F within 12 to 24 hours. We believe this is why we see the erratic patterns of damage in older orchards as well as the typical damage sometimes seen in young orchards.
Walnut growers are concerned about threats from a possible November-December freeze and increased drought across the state. Such a situation requires growers to be more vigilant in preparing for these unprecedented freeze events. We are hoping for rainfall in early November, which could provide a much needed relief, especially for growers who only use surface water.
What can be done?
Although there is limited field-based research on the topic, institutional knowledge and field observations may be able to help. Suggestions for freeze damage mitigation include the following:
· Promote healthy trees throughout the season but reduce growth in fall. Cutting back on irrigation in September and no longer applying nitrogen after August helps slow growth and may promote the hardening off process needed before a sudden freeze event comes along.
· For young trees, stop irrigating in September to set the terminal bud (Fig. 2) and harden off the trees, later resume irrigation if needed to avoid tree stress.
· If there is no rain after harvest, apply regular irrigation before a freeze event, so the soil is moist in November. This should keep the orchard slightly warmer and store heat during warm, sunny days. Hydrated trees are expected to be less susceptible to freeze damage.
Fig. 2. Example of a set terminal bud. Photo provided by Janine Hasey.
Managing Walnut Mold
Something has been plaguing walnut orchards for many years without a known cause. Often referred to as Brown Apical Necrosis, or BAN (Fig. 3), growers and PCAs have scratched their heads for years when walnut grades come back dinged due to moldy, off-color nuts. Dr. Themis Michailides, a UC Davis Plant Pathologist, recently decided to take a stab at this issue and can now offer a solution. Here is what he and his lab have found. After collecting samples and isolating various types of fungi from nuts, hulls, and BAN tissues, the Michailides lab at the Kearney Agricultural Research and Extension Center in Parlier, CA consistently found Alternaria, Fusarium, Aspergillus niger, Botryosphaeria, and Phomopsis present (Fig. 4.). Botryosphaeria and Phomopsis, we know from previous work, can be managed by pruning dead/ diseased wood/spurs, pruning after harvest in the fall, reducing sprinkler angles to avoid wetting tree limbs, and depending on severity, fungicide sprays applied in mid-May, mid-June, and mid-July (further product information can be found at http://ipm.ucanr.edu/). Yet the other three fungi, Alternaria, Fusarium, and Aspergillus niger were not considered pathogens on walnut. By performing a variety of tests both in the lab and in the field, the Michailides lab discovered these fungi are responsible for walnut mold. Additionally, walnut blight, caused by Xanthomonas arboricola pv. juglandis may exacerbate the problem, leading to larger lesions on the hull and the potential for greater damage to the hull and nut.
Fig. 3. Brown Apical Necrosis is shown on the left, not to be confused with Walnut Blight, shown on the right, and caused by the bacterial pathogen, Xan- thomonas arboricola pv. juglandis. Internal tissues in nuts with BAN at this stage do not show any decay and/or black discoloration as do nuts with walnut blight. Figure provided by Themis Michailides.
Fig. 4. Moldy, off-color nuts which lead to economic loss due to down- grading. Figure provided by Themis Michailides.
What can be done?
Applying Merivon at three weeks prior to hull split reduces mold related to Botryosphaeria, Phomopsis, and Alternaria. Adding Tebuconizole to the tank mix will increase efficacy against Phomopsis. To further increase efficacy, apply Rhyme at 20-30% hull split. If this high level of control is not needed, only apply Rhyme at 20-30% hull split.
*Note: Please refer to current label recommendations and restrictions when applying pesticides.
Updates on Botryosphaeria-Phomopsis Diseases of Walnut in San Joaquin and Stanislaus Counties
For the past several years, Botryosphaeria and Phomopsis canker and blight diseases have been increasingly observed in walnut orchards in almost all walnut-growing regions in California. Main symptoms include cankers in branches and dieback of spurs resulting from infections moving from affected fruits (fruit blight) via the peduncle or shoots through leaf and peduncle scars. The infected branch turns black, cankers enlarge, and the pith of the branch is black or dark brown (Fig. 5). Growth in the pith, at least for Botryosphaeriaceae spp, moves beyond the killed woody tissues (external margin of canker) for 1 to 2 inches, which is obvious when one splits a shoot along the long axis.
Fig. 5. Symptoms in walnut trees associated with Botryosphaeria and Phomopsis fungi; (D, shows the growth of the fungi within the pith beyond the margin of the canker -dead tissues).
Serial inoculation experiments indicated that pruning wounds are susceptible for at least four months. The wounds of 3- to 4-year-old shoots are more likely to develop larger cankers than those of 1- to 2-year-old shoots. This long-lasting susceptibility may be due to the hollow pith inside the walnut branches, which can provide a favorable condition for the fungal spores to germinate and continue to cause infection. As the infection or the pith cankers on spurs or branches continue growing during fall, the dead part of the branch may become covered with a dense layer of pycnidia (Fig. 5D).
You may find dead branches in the lower canopy of orchards, which may be caused by abiotic problems that may include shade/low sunlight or freeze. In the latter case, dead branches will not show any vascular discoloration (Fig. 6). However, the surface of these branches eventually will be covered with scale, Botryosphaeriaceae, and Diaporthaceae fungi.
Fig. 6. Symptoms in walnut trees associated with abiotic problems that may include shade/ low sunlight or freeze
New findings in 2020:
Despite several management practices implemented to prevent major yield and economic losses caused by Bot/ Phomopsis diseases, recent field survey results showed Diaporthaceae fungi to be the most prevalent fungal pathogen isolated from diseased walnut samples in the San Joaquin and Stanislaus Counties. Botryosphaeriaceae fungi were found occasionally in these orchards. Although growers are making several fungicide applications per season to control these diseases with emphasis in controlling the Botryosphaeriaceae, the persistence of Diaporthaceae spp. in walnut orchards has raised the question of whether the Phomopsis fungi have emerged as the main blight/ canker/dieback disease of walnut in San Joaquin County. A spore-trapping study was undertaken to determine when and under what environmental conditions spores of these fungi are released. Based on colony counts, the population of Botryosphaeriaceae fungi was significantly lower than that of the Diaporthaceae fungi – which corresponds to the results of the field surveys. Molecular work revealed the occurrence of three new species of Diaporthe recovered from both spore trapping and diseased tissues. The occurrence of these new species in walnut orchards represents new reports in California. In this spore-trapping study, we analyzed the correlation between precipitation events, irrigation, and grinding of infected branches between tree rows (following maintenance/cleaning pruning) and Bot/Phomopsis spore release. Among these variables, we found a strong correlation between spore release and precipitation: as precipitation increased, spore release also increased, and spores were mainly captured from March to May, a period that coincided with late-season rainfalls.
We also detected high aerial dissemination of Phomopsis spores after grinding of the prunings (among which were also infected branches), which were placed between tree rows in a mature orchard with a high incidence of Phomopsis disease (Fig. 7). This information is of great importance as it helps to identify production practices responsible for the spread of these fungal pathogens within walnut orchards.
Fig. 7. High aerial dissemination of Phomopsis spores when grinding of infected branches that are placed between tree rows in a mature walnut orchard.
In orchards with sprinkler irrigation systems, low number of spores were captured during and following the first irrigation of the season. In addition, our results showed no correlation between further in-season irrigation events and the release of fungal spores of Diaporthaceae and Botryosphaeriaceae fungi. However, the wetness/ humidity in the orchard resulting from the first irrigation may cause spores to ooze and be released from pycnidia in diseased tissues within the orchard.
Ongoing research:
Based on the spore-trapping study, a new fungicide program was initiated this year to investigate whether an early spray timing would be effective to reduce the disease incidence. Pruning wound protection trials were also initiated this year to evaluate the efficacy of some old and new chemical and biological compounds to protect pruning wounds from infections by canker pathogens.
Disease management practices:
Cultural control:
· When pruning dead branches, pruning cuts should be made into healthy gree
n wood during the summer or immediately following harvest allowing enough time before rains occur and spread inoculum to susceptible fresh cuts.
· For young orchards not infected with Bot/Phomopsis pathogens, after pruning (pruning for training), you can shred prunings and leave wood chips in the orchard. No sprays are needed.
· For heavily infected orchards, it is advisable to remove infected prunings from the orchard and shred or burn them if permitted.
For orchards/trees affected by the November 2020 freeze damage, remove dead limbs and prunings from the orchard as they may eventually become infected with Bot/Phomopsis pathogens.
Chemical control:
· Timely application of effective fungicides adjusted for weather and Bot/Phomopsis inoculum level in the orchard.
· In orchards with a high incidence of Phomopsis, emphasis should include a triazole fungicide in the spray program.
· It may be good to consider applying a Bot/Phomopsis spray before the first irrigation of the season. Irrigation may create a microclimate that encourages potential infection and sporulation of these fungal pathogens.