Root-lesion,
In recent years, extensive research has been conducted to find replacements for methyl bromide, widely used as a pre-plant soil fumigant before being implicated in the depletion of ozone in the stratosphere (United States Environmental Protection Agency, 1993). A way to reduce the frequency of fumigant use in perennial crops is through prolonging the life of existing orchards. The longer an orchard remains healthy and productive, the less often it will be terminated, fumigated, and replanted. Over time, this reduces the amount of fumigant used. In addition to fumigation, a variety of approaches have been researched for management of plant-parasitic nematodes including development of pre-plant hot water treatments of rootstocks, evaluation of rootstock susceptibility, and evaluation of biological products (Buzo et al., 2009; Giraud et al., 2011; Hasey et al., 2004; Westerdahl and Radewald, 2011).
Only about a dozen nematicidal active ingredients have ever achieved registration in California, and several of the most effective of these lost their registrations owing to groundwater contamination, air pollution, or carcinogenicity (Ferris, 2021). The loss of use of the nematicide dibromochloropropane (DBCP) in 1977, that had been widely used post-plant on bearing fruit and nut tree crops in California, created a tremendous need for replacement products (United States Environmental Protection Agency, 2014).
The goal of this study was to evaluate the potential of DiTera and Nema-Q for post-plant management of root-lesion and ring nematodes in commercial walnut orchards.
Two field trials were conducted to evaluate the potential of two bionematicides for nematode management on walnuts. Because the actual active ingredients in both products tested are uncertain, all rates are expressed in terms of amount of product per ha.
Sutter County Trial: The first trial was conducted in an orchard in Sutter County, CA on Holillipah loamy sand. This orchard was previously used for an own-rooted ‘Chandler’ compared to ‘Chandler’ on ‘Paradox’ rootstock trial (Hasey et al., 2004). In that trial that was planted in 1991, two rootstocks, micropropagated ‘Chandler’ (
Soil samples were taken prior to each treatment date using a 5-cm diameter bucket auger to a depth of 60-cm midway between the dripline of the tree canopy and the tree trunk, in the fall and spring of each year. Nematodes were extracted from a 400-cm3 soil sub-sample with a modified semiautomatic elutriator and sucrose centrifugation technique (Byrd et al., 1976). Extracted nematodes were identified and counted at × 45 magnification under a stereoscopic dissecting microscope (Bausch & Lomb, Bridgewater, NJ). The sampling method used did not yield sufficient roots to permit extraction of nematodes. Yields and trunk circumference were evaluated each year from 2002 (pretreatment baseline) to 2006. Circumference of each tree trunk was measured at 60 cm above the ground and trunk cross-sectional area was calculated from the circumference measurements (Retzlaff et al., 1992). In addition, tree vigor was visually evaluated in 2005 and 2006 using a rating scale developed by the authors: 0=Dead, 1=Very low vigor, dieback, 2=Early yellowing, 3=No new shoot growth, 4=Some new shoot growth, and 5=Most vigorous.
San Joaquin County Trial: The second trial was conducted in San Joaquin County, CA with ‘Chandler’ scion on ‘Paradox’ rootstock. The randomized complete block trial with 6 replicates of 14 treatments evaluated three rates of DT, four rates of NQ and six combinations of DT and NQ for their effectiveness in controlling root-lesion and ring nematodes. Treatments included an UC. Treatments were applied to the soil surface in a 50% banded spray. One day prior to treatment, 1.25 cm of irrigation was applied to the orchard with 30.2 Lph microsprinklers (Rain Bird Micro-Quick Spray Assembly, Rain Bird, Azusa, CA). Each tree was treated individually with 5.7 L of solution from a 7.6-liter watering can (Bloem Classic 2 Gal. Blue Plastic Watering Can, Bloem, Hudsonville, MI). Treatment was immediately followed by an additional 1.25 cm of irrigation. Orchards were irrigated by the grower as needed following CIMIS guidelines (CIMIS station #70) (CIMIS, 2021). Weather data for the trial area is available from CIMIS station #70 (UCIPM, 2021). The orchard was managed by the grower and treated with standard practices with respect to fertilization, insecticides, and fungicides.
Treatment effectiveness was evaluated via soil and root sampling, and trunk circumference measurements. Nematodes were extracted from soil as described for the Sutter County Trial. Nematodes were also extracted from roots that were weighed and placed in an intermittent mist chamber for 72 h (Ayoub, 1977). Treatments were applied in the spring of 2005 following measurement of trunk circumference. Additional nematode sampling followed by repeated treatments were conducted in October 2005, April 2006, and October 2006. Post-treatment trunk circumference measurements were done in October 2005, April 2006, and October 2006.
Data analysis: Both trials were conducted in randomized complete block design. There were five replicates per treatment in the Sutter County Trial and data were analyzed using Repeated Measures Analysis of Variance (ANOVA) (
Sutter County Trial: Pretreatment yield data taken in 2002 indicated that there were no significant differences at the beginning of the Sutter County trial (
Yield data for Sutter County trial.
Yield (kg/tree) | |||||||
---|---|---|---|---|---|---|---|
Year | Treatment | Combined | Paradox | Own-rooted | |||
2002 | UC | 23.4 | 26.3 | 20.5 | |||
DT | 24.0 | 25.2 | 22.8 | ||||
NQ | 28.8 | 32.4 | 25.0 | ||||
2003 | UC | 32.0 | 37.4 | 26.5 | |||
DT | 36.9 | 40.8 | 32.9 | ||||
NQ | 42.1 | 0.0027 a | 47.0 | 0.0338 | 37.1 | 0.0383 | |
2004 | UC | 22.8 | 25.45 | 20.2 | |||
DT | 27.1 | 31.1 | 23.0 | ||||
NQ | 26.5 | 33.1 | 20.0 | ||||
2005 | UC | 33.0 | 33.8 | 32.2 | |||
DT | 39.3 | 43.4 | 0.0325 | 35.3 | |||
NQ | 38.3 | 49.2 | 0.0008 | 27.4 | |||
2006 | UC | 33.0 | 37.7 | 28.3 | |||
DT | 42.3 | 0.0068 | 49.1 | 0.0114 | 35.5 | ||
NQ | 42.6 | 0.0041 | 51.2 | 0.0031 | 33.9 | ||
2007 | UC | 27.0 | 31.2 | 22.8 | |||
DT | 39.4 | 0.0004 | 37.4 | 41.4 | 0.0005 | ||
NQ | 35.8 | 0.0080 | 41.1 | 0.0279 | 30.6 | ||
Yield difference (kg/tree) between UC in 2002, and UC, DT, and NQ in 2007 | |||||||
UC | 3.6 | 4.9 | 2.3 | ||||
DT | 16.0 | 0.0001 | 11.1 | 0.0142 | 20.9 | 0.0001 | |
NQ | 12.5 | 0.0003 | 14.9 | 0.0013 | 10.1 | 0.0494 |
Notes: Data are means of 5 replicates.
Figures indicate the probability of that treatment being different from the UC.
Pretreatment, in 2002, there were no differences in trunk cross sectional area for both rootstocks combined or for own-rooted trees (
Trunk cross-sectional area for Sutter County trial.
Trunk cross-sectional area (cm) | |||||||
---|---|---|---|---|---|---|---|
Year | Treatment | Combined | Paradox | Own-rooted | |||
2002 | UC | 354 | 221 | 488 | |||
DT | 352 | 279 | 425 | ||||
NQ | 386 | 324 | 0.0396 | 448 | |||
2003 | UC | 422 | 292 | 552 | |||
DT | 396 | 363 | 0.0396 | 428 | 0.0003 | ||
NQ | 483 | 0.0064 a | 449 | 0.0001 | 516 | ||
2004 | UC | 448 | 324 | 572 | |||
DT | 468 | 400 | 0.0302 | 535 | |||
NQ | 526 | 0.0009 | 509 | 0.0001 | 542 | ||
2005 | UC | 460 | 354 | 567 | |||
DT | 476 | 425 | 0.0420 | 526 | |||
NQ | 535 | 0.0012 | 542 | 0.0001 | 528 | ||
2006 | UC | 503 | 386 | 620 | |||
DT | 555 | 0.0428 | 487 | 0.0044 | 623 | ||
NQ | 605 | 0.0001 | 613 | 0.0001 | 597 | ||
2007 | UC | 533 | 421 | 647 | |||
DT | 571 | 525 | 0.0037 | 619 | |||
NQ | 648 | 0.0001 | 677 | 0.0001 | 620 |
Notes: Data are means of 5 replicates.
Figures indicate the probability of that treatment being different from the UC.
Visual rating of tree vigor conducted in 2005 indicated ‘Paradox’ trees treated with NQ were more vigorous than UC trees (
Tree rating ranging from 0 (dead) to 5 (most vigorous) for Sutter County trial a .
Year | Treatment | ___Paradox____ | _Ownroot___ | ||
---|---|---|---|---|---|
2005 | UC | 3.6 | 3.0 | ||
DT | 3.8 | 3.6 | |||
NQ | 4.4 | 0.02 b | 3.8 | 0.04 | |
2006 | UC | 3.9 | 3.5 | ||
DT | 4.6 | 0.04 | 4.2 | ||
NQ | 4.6 | 4.1 |
Notes: Data are means of 5 replicates.
Rating Scale: 0=Dead, 1= Very low vigor, dieback, 2=Early yellowing, 3=No new shoot growth, 4=Some new shoot growth, 5=Most vigorous.
Figures indicate the probability of that treatment being different from the UC.
Regression and correlation analysis support the results discussed above. Slopes of the lines are not significantly different, but the y intercepts for yield are significantly different between trees treated with DT and UC trees. Linear regression analysis over time demonstrated positive but not significant slopes for yield (
For trunk cross sectional area (TCSA), linear regression demonstrated positive slopes for all treatments that were significant at
Prior to treatment on the first sampling date, there were no differences in numbers of root-lesion or ring nematode for either rootstock (
Effect of treatments on densities of nematodes per 1,000 cm3 soil in Sutter County Trial.
Sample | ______Paradox_____ | ___________Own-rooted____ | ||||||
---|---|---|---|---|---|---|---|---|
Date | Treatment | Lesion | Ring | Lesion | Ring | |||
October 2003 | UC | 3,030 | 0 | 4,380 | 10 | |||
DT | 3,200 | 0 | 5,740 | 100 | ||||
NQ | 3,820 | 0 | 4,410 | 0 | ||||
April 2004 | UC | 1,440 | 40 | 2,560 | 110 | |||
DT | 1,330 | 540 | 2,020 | 3,270 | 0.004 | |||
NQ | 1,320 | 300 | 1,870 | 0 | ||||
October 2004 | UC | 1,130 | 20 | 1,230 | 450 | |||
DT | 2,250 | 30 | 2,470 | 3,830 | 0.002 | |||
NQ | 1,210 | 420 | 1,580 | 0 | ||||
April 2005 | UC | 870 | 40 | 610 | 300 | |||
DT | 1,410 | 0 | 580 | 2,480 | 0.040 | |||
NQ | 2,290 | 260 | 1,350 | 10 | ||||
October 2005 | UC | 1,470 | 10 | 1,890 | 1,380 | |||
DT | 2,060 | 30 | 2,580 | 2,340 | ||||
NQ | 2,850 | 540 | 4,970 | 0.006 | 30 | |||
April 2006 | UC | 1,380 | 1,610 | 1,640 | 590 | |||
DT | 2,650 | 980 | 1,300 | 2,560 | ||||
NQ | 1,310 | 2,020 | 1,680 | 830 | ||||
October 2006 | UC | 470 | 280 | 740 | 470 | |||
DT | 320 | 1,340 | 0.050 a | 960 | 290 | |||
NQ | 620 | 1,030 | 2,080 | 660 |
Notes: Data are means of 5 replicates.
Figures indicate the probability of that treatment being different from the UC.
San Joaquin County Trial: In the San Joaquin County trial, in October 2005, six months after treatment, numerically, 10 out of 13 treatments showed an increase in trunk circumference ranging from 7.13 to 12.05% compared to a 7.1% increase for the UC trees (Table 5). The largest increases were 12.05% for trees treated with NQ at 56 L/ha (
Circumference of trees in San Joaquin County trial.
Treatment | Initial trunk | Percent increase from initial circumference | |||||
---|---|---|---|---|---|---|---|
(L/ha) | Circumference (cm) | October 2005 | April 2006 | October 2006 | |||
UC | 68.3 | 7.1 | 7.2 | 10.6 | |||
DT 56 | 66.5 | 7.0 | 7.7 | 10.6 | |||
DT 28 | 61.4 | 7.2 | 8.0 | 10.9 | |||
DT 14 | 67.1 | 5.3 | 5.7 | 7.9 | |||
DT 56 + NQ 37.4 | 71.2 | 8.4 | 8.9 | 11.7 | |||
DT 56 + NQ 23.4 | 69.7 | 9.2 | 9.9 | 12.8 | |||
DT 28 + NQ 37.4 | 61.5 | 11.4 | 0.0269 a | 12.9 | 0.0052 | 16.3 | 0.0091 |
DT 28 + NQ 23.4 | 68.2 | 8.5 | 9.5 | 12.8 | |||
DT 14 + NQ 37.4 | 69.0 | 7.3 | 8.1 | 11.1 | |||
DT 14 + NQ 23.4 | 71.7 | 7.1 | 5.9 | 10.0 | |||
NQ 56 | 70.9 | 12.1 | 0.0105 | 12.8 | 0.0062 | 15.8 | 0.0176 |
NQ 37.4 | 64.0 | 7.6 | 8.6 | 11.3 | |||
NQ 23.4 | 72.3 | 6.9 | 7.7 | 10.4 | |||
NQ 12 | 61.6 | 7.2 | 8.1 | 10.8 |
Notes: Data are means of 6 replicates.
Figures indicate the probability of that treatment being different from the UC.
At the time of application in April 2005, although the number of root-lesion nematode in soil was numerically greater than the UC trees for all but one treatment, there were no significant differences between treatments (
Effect of treatments on densities of lesion nematodes per 1,000 cm3 soil in San Joaquin County trial.
Treatment | Date sampled | ||||
---|---|---|---|---|---|
(L/ha) | April 2005 | October 2005 | April 2006 | ||
UC | 2,783 | 4,208 | 2,950 | ||
DT 56 | 6,708 | 4,167 | 2,375 | ||
DT 28 | 4,825 | 8,075 | 6,075 | ||
DT 14 | 4,458 | 4,883 | 3,875 | ||
DT 56 + NQ 37.4 | 4,233 | 7,417 | 1,967 | ||
DT 56 + NQ 23.4 | 3,708 | 6,625 | 2,350 | ||
DT 28 + NQ 37.4 | 8,733 | 4,583 | 1,525 | ||
DT 28 + NQ 23.4 | 3,492 | 5,325 | 1,575 | 0.0500 | |
DT 14 + NQ 37.4 | 5,625 | 8,050 | 2,200 | ||
DT 14 + NQ 23.4 | 8,405 | 2,333 | 0.0005 a | 3,150 | |
NQ 56 | 4,667 | 4,850 | 4,475 | ||
NQ 37.4 | 2,817 | 6,600 | 2,650 | ||
NQ 23.4 | 3,875 | 5,433 | 1,633 | ||
NQ 12 | 2,517 | 3,175 | 1,583 |
Notes: Data are means of 6 replicates.
Figures indicate the probability of that treatment being different from the UC.
At the time of application in April 2005, roots from trees treated with DT at 14 kg/ha (
Effects on lesion nematode per gram of roots in San Joaquin County trial.
Treatment | Date sampled | ||||
---|---|---|---|---|---|
(L/ha) | April 2005 | October 2005 | April 2006 | ||
UC | 8 | 9 | 0 | ||
DT 56 | 142 | 9 | 2 | ||
DT 28 | 121 | 18 | 4 | ||
DT 14 | 203 | 0.02 a | 109 | 4 | |
DT 56 + NQ 37.4 | 11 | 12 | 5 | ||
DT 56 + NQ 23.4 | 18 | 15 | 2 | ||
DT 28 + NQ 37.4 | 8 | 81 | 1 | ||
DT 28 + NQ 23.4 | 45 | 4 | 0 | ||
DT 14 + NQ 37.4 | 36 | 74 | 1 | ||
DT 14 + NQ 23.4 | 1 | 22 | 2 | ||
NQ 56 | 45 | 94 | 13 | ||
NQ 37.4 | 130 | 0.05 | 13 | 16 | |
NQ 23.4 | 165 | 25 | 5 | ||
NQ 12 | 11 | 229 | 0.01 | 1 |
Notes: Data are means of 6 replicates.
Figures indicate the probability of that treatment being different from the UC.
At the time of application in April 2005, there were no differences in number of ring nematode per liter of soil (
Effect of treatments on densities of nematodes per 1,000 cm3 soil in San Joaquin County trial.
Treatment | Ring nematode | ||||
---|---|---|---|---|---|
(L/ha) | April 2005 | October 2005 | April 2006 | ||
UC | 975 | 317 | 1,575 | ||
DT 56 | 1,133 | 7,300 | 2,150 | ||
DT 28 | 1,500 | 950 | 525 | ||
DT 14 | 83 | 1,117 | 900 | ||
DT 56 + NQ 37.4 | 583 | 450 | 2,000 | ||
DT 56 + NQ 23.4 | 3,283 | 2,500 | 350 | 0.03 | |
DT 28 + NQ 37.4 | 83 | 300 | 0 | 0.003 | |
DT 28 + NQ 23.4 | 900 | 1,650 | 1,600 | ||
DT 14 + NQ 37.4 | 575 | 1,183 | 825 | ||
DT 14 + NQ 23.4 | 63 | 0 | 0.01 a | 0 | 0.003 |
NQ 56 | 0 | 0 | 0.01 | 200 | 0.003 |
NQ 37.4 | 150 | 800 | 1,275 | ||
NQ 23.4 | 750 | 4,883 | 475 | 0.03 | |
NQ 12 | 2,900 | 950 | 1,575 |
Notes: Data are means of 6 replicates.
Figures indicate the probability of that treatment being different from the UC.
Bionematicides were evaluated on two walnut rootstocks, own-rooted ‘English’ (‘Chandler’) and ‘Paradox.’ Own-rooted ‘English’ walnut trees can be used in areas where commonly used rootstocks such as ‘Paradox’ (
DT is a killed-microbial product of the fungus
NQ is an extract of
Our trials have demonstrated that bionematicides can improve yield, growth, and vigor in walnut orchards infested with plant-parasitic nematodes. This prolongs the viable life of an orchard and reduces the frequency of pre-plant fumigations. This research contributed to the registration of these organic nematicides (OMRI certified) in California.
As we move away from traditional fumigant and nonfumigant nematicides towards natural products with different modes of action, the most effective application methods, rates, and timing, and interpretation of results become less straight forward. For example, in the trial conducted in San Joaquin County, increasing yields were associated with an increase in populations of nematodes. This could be an indication of the development of a more vigorous root system that is capable of supporting greater populations of nematodes.
For more than 30 years we have searched for a product that would replace DBCP. What we have found after years of believing that “the only good plant-parasitic nematode is a dead nematode” is that products are available that will increase yields in the presence of plant-parasitic nematodes and, may actually permit populations to increase. This opens the door to additional research on how to best utilize the new tools that we have to maximize yields for growers. It also raises questions for additional long-term research on perennial crops. Will yields continue to increase as demonstrated in these trials, will yields stabilize, or will yields crash at some point in the future? Research with bionematicides on annual crops has also shown increases in yield without a reduction in nematode populations (Westerdahl and Radewald, 2011). The current research is also a challenge to others to take another look at data they may have set aside because yields increased, but effects on nematode populations did not match expectations.