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© 2011 Plant Management Network.
Accepted for publication 4 March 2011. Published 1 June 2011.


Evaluation of Phosphonate Treatments for Control of Phytophthora Crown Rot of Walnut


G. T. Browne, USDA-ARS Crops Pathology and Genetics Research Unit, Department of Plant Pathology, T. L. Prichard, Department of Land, Air and Water Resources, L. S. Schmidt, USDA-ARS Crops Pathology and Genetics Research Unit, Department of Plant Pathology, University of California, Davis, CA 95616; and W. H. Krueger, University of California Cooperative Extension, Glenn County, Orland, CA 95963


Corresponding author: G. T. Browne. gtbrowne@ucdavis.edu


Browne, G. T., Prichard, T. L., Schmidt, L. S., and Krueger, W. H. 2011. Evaluation of phosphonate treatments for control of Phytophthora crown rot of walnut. Online. Plant Health Progress doi:10.1094/PHP-2011-0601-01-RS.


Abstract

Preventive foliar and soil (chemigation) applications of phosphonate were evaluated in a factorial manner for control of cankers caused by Phytophthora citricola in a Persian walnut orchard, cultivar Chandler. In each of two experiments, a foliar treatment was applied once in the second week of September, and a soil chemigation treatment was applied three times at weekly intervals from the last week of August to the second week of September; the treatments were applied separately and in combination to the walnut trees. At 1 and at 7 months after completion of the treatments, the trees were wound inoculated with P. citricola. Treatment efficacy was assessed by measuring the area of resulting trunk cankers. Trees that had received a foliar spray with phosphonate consistently developed less necrotic bark area than trees that had not received the treatment. This effect was evident following the inoculations made 1 and 7 months after phosphonate treatments. Chemigation with phosphonate contributed to canker suppression only following inoculations made 1 month after treatment in experiment 1; the suppression was not evident in experiment 2. The results suggest that a late-summer foliar spray treatment with phosphonate, but not necessarily short-term chemigation treatments, may help to reduce losses caused by P. citricola in Persian walnut.


Introduction

Phytophthora crown and root rot is among the most serious diseases of Persian walnut (Juglans regia) worldwide. In California, more than 10 species of Phytophthora have been implicated in the disease, but Phytophthora cinnamomi and P. citricola are especially aggressive and difficult to manage (9,11,12). Incidence and severity of Phytophthora crown and root rot generally can be minimized by careful soil water management and use of Paradox hybrid rootstock (typically Juglans hindsii × J. regia) (11). Compared to the rootstocks Northern California black walnut (J. hindsii) and Persian walnut (also known as English walnut) , the hybrid rootstock is more resistant to several species of Phytophthora (7,10). In practice, however, the resistance of Paradox hybrid has been insufficient to prevent losses caused by P. cinnamomi or P. citricola (11). Improved management strategies are needed for these two pathogens on walnut.

Phosphonates (i.e., fungicides that include phosphonic acid or phosphonate as the active ingredient) (14) and mefenoxam (i.e., a phenylamide fungicide) have been used for systemic control of many oomycete pathogens, including Phytophthora species on walnut (5). Under greenhouse conditions, the phosphonate Aliette (Bayer Crop Science, Research Triangle Park, NC) and the phenylamide fungicide metalaxyl (Syngenta Crop Protection, Greensboro, NC; metalaxyl contains mefenoxam as well as its inactive isomer) were found to suppress disease caused by P. citricola and P. cinnamomi on walnut (8). However, to our knowledge, replicated field evaluations of these materials for control of Phytophthora diseases of walnut have not been reported. Mefenoxam treatments are relatively expensive and for this reason have not been used widely for treatment of mature walnut trees. Due to their affordability, phosphonate treatments have been of widespread interest to walnut growers, but data on efficacy of the treatments under walnut orchard conditions are needed.

Phosphonates have excellent systemic activity in plants due to their mobility in both xylem and phloem (3). Effective application methods for phosphonates in trees have included foliar spraying, soil drenching or chemigation, and trunk injection (4,5). The relative efficacy of these treatment methods has varied among host and pathogen combinations. The mode of action of phosphonate is incompletely understood, but evidence suggests that it disrupts growth of Phytophthora and intensifies host defenses (6).

Distinctions between phosphonates, which have broad activity against oomycetes, and phosphoric acid and its salts, which provide P for plants, were reviewed recently (15). Reduced sensitivity to phosphonates has been reported in a few populations of oomycetes (1,13), but, in part due to the apparent complex mode of action of phosphonates, it is difficult to assess implications of the reduced sensitivity for disease control.


Application of Phosphonate Treatments

A walnut orchard planted at near Davis, CA, in February 2000 was used to evaluate efficacy of foliar spray and chemigation treatments with phosphonate. The orchard area encompassed two soil series, a Yolo silt loam and a Brentwood silty clay loam. The orchard was planted for experimental purposes but generally was managed as a commercial orchard. The trees were spaced at 11.3 m apart between rows and 7.1 m apart within rows. The orchard was divided into two halves, one being used for experiment 1 in 2005-2006 and the other being used for experiment 2 in 2006-2007 (a repeat of the first experiment). In each trial, chemigation and foliar spraying with phosphonate were tested in a factorial manner using a nested experimental design as described below.

In each experiment, the phosphonate chemigation treatment and a water-control irrigation treatment each were applied to 16-tree-long mainplots arranged in six randomized complete blocks. Each mainplot was served by a dedicated irrigation line with one microsprinkler per tree (Bowsmith Inc., Exeter, CA; full circle, 3.0-m-diameter pattern, 21.5 liters/h, head placed 0.9 m from the tree trunk). Within each mainplot, there were four groups of four trees; two groups of trees consisted of Persian walnut cultivar Chandler grafted on Northern California black walnut rootstock, while the other two groups were of ‘Chandler’ grafted on Paradox hybrid rootstock. In experiment 1 the phosphonate formulation Fosphite (J. H. Biotech, Ventura, CA), which contained 53% mono- and di-potassium salts of phosphorous acid (equivalent to 0.47 kg of phosphonic acid per liter), was applied at a rate of 7 liters/ha to each chemigation plot on 29 August, 6 September, and 12 September 2005; in experiment 2, the applications were made 28 August, 5 September, and 13 September 2006. The Fosphite formulation was diluted with water (1:4) before injection with a Chem-Tech Pulsafeeder 250 (Punta Gorda, FL). Each application injected Fosphite into the irrigation system during the first 45 min of a 24-h irrigation using the resident microsprinkler system. The total volume of water applied per tree in the 24-h period was approximately 520 liters. The water control plots received the same schedule and amounts of water through their microsprinklers as the chemigated plots. Between chemigation treatments, all trees were irrigated with their resident microsprinklers at weekly intervals during the growing season to fully meet evapotranspiration needs.

The foliar spray treatment and a non-treated control each were applied to two-tree plots randomized within the four-tree rootstock subplots described above. In experiment 1, the foliar spray treatment consisted of one application of Fosphite at 7 liters/ha in 935 liters of water per hectare on 12 September 2005. In experiment 2 the same foliar treatment was applied 13 September 2006. The foliar treatment was applied with a backpack sprayer (Mistblower SR420-Z, Stihl Inc., Virginia Beach, VA) to wet all aboveground parts of the trees, and care was used to avoid spray drift to adjacent control trees.


Inoculations

One month after completion of the phosphonate treatments (7 October 2005 in experiment 1, 3 October 2006 in experiment 2), eight of the 16 trees in each phosphonate chemigation and water-control mainplot were wound inoculated on one side of the trunk with a 1-cm × 1-cm V8 juice agar [200 ml V8 juice (Campbell Soup Co., Camden, NJ), 2 g CaCO3, 16.9 g agar, 800 ml distilled water] patch covered with mycelium of P. citricola (this was the beginning of assessment period 1). The isolate of P. citricola, GB1281, was from a Persian walnut tree near Stockton, CA. The other side of the tree trunks was inoculated with a sterile square of V8 juice agar (the control). Four of the inoculated trees had received the foliar phosphonate treatment while the other four had not. Also, the different walnut rootstocks were equally represented among the inoculated trees. The inoculations occurred about 30 cm above the soil surface and 15 cm above the graft union. A 1-cm-wide chisel was used to remove a 1-cm × 1-cm square of bark (the wound), and an agar patch was placed in each wound. The sides of the tree trunks were assigned randomly to the inoculants (i.e., P. citricola and the sterile control). The inoculated wounds were covered with the previously removed patch of bark and wrapped with silver duct tape (no. 3939, 3M, St. Paul, MN) to prevent drying of the wound. Two months after inoculation (13 December 2005 in experiment 1 and 12 December 2006 in experiment 2), the resulting canker areas were measured (this was the end of assessment period 1). After the surface bark was shaved off with a hatchet to reveal the entire margin of each canker, sheets of transparency plastic (no. 3R2780, Xerox, Norwalk, CT) were used to trace each canker’s margin. The area of each canker was determined by digitally scanning its trace and applying APS Assess software (St. Paul, MN).


 

Fig. 1. Canker incited by Phytophthora citricola in Persian walnut. The trees were inoculated at either 1 or 7 months after application of preventative phosphonate treatments, and efficacy of the treatments was assessed according to the area of cankers 2 to 3 months after inoculation.

 

Nearly 7 months after the completion of the phosphonate treatments (28 April 2006 in experiment 1, 30 April 2007 in experiment 2) the eight remaining non-inoculated trees in each phosphonate chemigation and water control mainplot (i.e., those not used as described above for assessment period 1) were inoculated with P. citricola and the agar control as described above (this was the beginning assessment period 2). A little over 3 months after the second inoculation (8 August 2006 in experiment 1 and 15 August 2007 in experiment 2) the resulting canker areas were measured as described for the first inoculations above (this was the end of assessment period 2).

At the times of canker measurement, the mean tree trunk circumference in experiment 1 at inoculation points was 511 cm (range 355-670 cm), whereas that in experiment 2 was 555 cm (range 225-780 cm). On multiple occasions at the time of canker measurement, samples of bark were collected from wounds inoculated with P. citricola and the agar control in order to confirm the association of the pathogen with the disease. The pathogen was identified according to its morphology and published descriptions (5).


Analysis of Treatment Responses

Analysis of variance was conducted on the canker area data using the MIXED procedure of SAS (SAS Institute Inc., Cary, NC). Canker necrosis area was affected by interactions of experiment × foliar spray treatment × assessment period (P = 0.02). Therefore, the final analyses of variance were conducted separately by assessment period within experiment. For the latter analyses, the modeled fixed effects included chemigation treatment, foliar spray treatment, rootstock and the interactions among these variables. The random statement included block, and the two-way interactions of block with chemigation, foliar spray, and rootstock. Separate analyses were conducted for the data from inoculations with P. citricola and the data from control inoculations.


Efficacy of Phosphonate Treatments

In the first assessment period of experiment 1 (began 1 month after phosphonate treatments) , there was an interaction of foliar spray and chemigation treatments (P = 0.0009, Table 1). The foliar spray and chemigation treatments reduced canker area by 59 and 41% respectively, compared to the control, whereas the combination of the two treatments reduced canker area by 70% (Table 1). In the second assessment period (began 7 months after phosphonate treatments), there was no significant effect of chemigation (P = 0.11 to 0.95 for main and interactive effects), but the foliar treatment reduced canker area by 53% compared to the control (P = 0.04). Rootstock had no significant main or interactive effect in either assessment period of experiment 1 (P = 0.10 to 0.98).

In experiment 2, chemigation had no significant main or interactive effect in either assessment period (P = 0.11 to 0.95), but foliar spraying reduced canker area in both the first and second assessment periods (P < 0.0001 and 0.006, respectively, Table 1). In assessment periods 1 and 2, the foliar treatment reduced canker area by 31 and 34%, respectively, compared to the control. Overall, canker areas in the second assessment period were much greater than those in the first period. Rootstock had no significant main or interactive effect in either assessment period of experiment 2 (P = 0.09 to 0.92).

Our isolations from cankers and the wounded control inoculation points confirmed the association of canker development with infection by P. citricola and the absence of the pathogen in tissues around the control wounds.


Summary

The results of this study indicate that phosphonate foliar spray treatments may contribute to integrated management of P. citricola on walnut. In our trials, a single preventative foliar spray with phosphonate in September measurably suppressed canker development in the walnut scions over a 10-month period. These findings, obtained under field conditions representative of commercial walnut production in California, confirm previous results obtained with phosphonate under greenhouse conditions (8).

In contrast to the single foliar spray, the triple series treatment of phosphonate chemigation did not consistently suppress canker development. Inconsistency of phosphonate chemigation effects was reported previously for almond (2). In the almond study, chemigation was more effective during a period of peak evapotranspiration in July than in the fall, when transpiration was less, perhaps reflecting the amounts of phosphonate taken up in the transpiration stream during the respective periods. In the study reported here, the fact that chemigation improved canker suppression in the first assessment interval of experiment 1 suggested that further work could lead to optimization of chemigation treatment approaches and more consistent efficacy for some periods of application.

We chose September for the foliar spray thinking that at that time the tree shoots would compete less with the roots as a sink for phosphonate, but effects of different application times were not evaluated. It seems likely that successive foliar sprays made according to product labels during the summer and fall would be more effective than the single foliar spray we tested. Due to the persistence of phosphonates in hosts for many months, additional foliar treatments would likely increase tissue concentrations of phosphonic acid, which have been correlated with improved disease control (4).

Because it would have been very difficult conduct the inoculations and canker measurements on the belowground portion (i.e., the rootstock) of the trees used in this study, we conducted these procedures aboveground in the scion. This is potentially important, because although infections by P. citricola often extend into Persian walnut scions, the infections typically originate in the rootstock (11). The downward mobility of phosphonate would be expected to facilitate its movement into the rootstock (14), but using our methods, it was not possible to directly assay effects of the treatments on development of disease in the rootstocks.

It is unknown why canker size in the second assessment period of experiment 2 two was so much greater than that in the other experiment and assessment periods. Since the trees were a year older in experiment 2 than in experiment 1 they were, on average, larger and perhaps carried a greater nut load, but it is unknown whether tree size or crop load affected canker development. Air temperature data for Davis, CA, suggested that experiment 1 had greater exposure to high temperatures suppressive or lethal to canker development caused by P. citricola (i.e., greater than 100°F), but this was not explored thoroughly. Nevertheless, despite the large differences in canker size across experiments and assessment periods, the effect of foliar phosphonate treatment was relatively stable.

Further work to optimize and validate foliar and chemigation phosphonate applications for commercial walnut production would be desirable. For example, it would be useful to examine whether multiple foliar sprays and chemigation treatments timed during periods of peak evapotranspiration would improve disease control compared to that observed in this report. Also, it needs to be determined whether efficacy of the foliar treatment against P. citricola, a pathogen that primarily invades the root crown and trunk of walnut trees, is effective for P. cinnamomi, also an aggressive pathogen of walnut but one that that primarily invades walnut roots.


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