Non-chemical Control of Nematodes in Potatoes

SUMMARY
This project was designed to compare five different green manures crops to see which best control Columbia root knot nematode. At the same time, the green manure crops were compared to determine which control other potato-parasitic nematodes. One quarter of a circle was planted to green manure crops rather than the standard barley rotation. The crops included Sordan 79 sorghum sudangrass, Pacific Gold mustard, Arena oil-seed radish, Arugula and Caliente 119 mustard.

Aside from a small grass infestation, the crops flourished in the field and were thoroughly incorporated in late summer. Columbia root knot nematode (CRKN) numbers decreased only in the radish plots, likely because the grass infestation served as a nematode host. Sordan 79, Pacific Gold and Arena radishes provided excellent CRKN control in other fields.

The project showed that weed control is important in any green manure crop. Arena oil-seed radish provides control not only of CRKN but also of stubby root nematode and lesion nematode.

Because of this study, when another field turned out to have a heavy CRKN infestation in the spring of 2006, Arena was planted instead of applying Telone II. Two-acre grid sampling showed that the radish provided a 99.5% reduction in CRKN and an 87% reduction in lesion nematodes.

OBJECTIVES
The purpose of this project was to compare the effectiveness of five green manure crops and to determine which is most successful at controlling Columbia root knot nematode. The project also served to help establish the required inputs for each green manure as well as determine how well they will grow in the San Luis Valley, and, more specifically, how well they grow in the region around this farm, Three Rivers Ranch in Blanca, CO.

SPECIFIC RESULTS
The five green manure crops were planted June 16, 2005, on about 30 acres. The crops were divided into planter-width (40-foot) strips across the field with three replications of each crop. The rest of the area was planted with radish and mustards. Soil samples were taken on June 17, 2005, and each sample was recorded with a GPS device so that sample sites could be replicated. Soil samples were sent to Oregon State University for nematode testing, composite soil fertility samples were sent to Servi-Tech and samples were also submitted for soil food web testing.

Before planting, the field was fertilized with 100 pounds per acre of nitrogen and received 12 inches of water from irrigation and rain. The field was not treated with an herbicide because none was compatible with all of the crops and small-plot treatment was not cost effective. Weed control had been excellent on the field, but an infestation of love grass (Eragrostis) developed and took over. Harlequin bugs reached heavy levels, so the field was treated and control was effective.

The mustard and radish crops were flail chopped and incorporated with a disk on Aug. 23, 2005, and Sordan 79 was incorporated on Sept. 6. Plots were re-sampled on Oct. 24.

The field was planted with Russet Nugget potato in 2006. The plots were sampled again at planting. Nematode counts were too high to get by without an application of Vydate CLV, so no controls were left in the field. Because of this, no harvest or storage tuber evaluations were done. No nematode damage was noted at harvest. The plots were sampled after harvest, but the results had not been received at the time of this report.

Between June 17, 2005, and Oct. 24, 2005, nematode counts increased in most of the plots and decreased only in the radish plots. CRKN, stubby root nematode and lesion nematode all decreased in the radish plots. The uncontrolled grass may have served as a nematode host. Another issue is that the field was sampled in June when it was planted to match up samples with the individual plots. This is much later than the typical spring sampling time, so there may have been false negative staring values on those plots. Even so, based on experience in other fields, the CRKN levels in October were expected to be near zero in all plots.

BENEFITS OR IMPACTS ON AGRICULTURE
Because of the problems with grassy weeds on this project’s green manure plots, noted at a presentation in January 2006, most local producers who grew green manure crops in 2006 paid strict attention to weed control to avoid the same problem. Other benefits observed from growing green manures in this project included water savings and improved soil tilth.

While the project had hoped to show that planting green manures could help producers avoid chemicals to control nematodes, that results was not achieved. However, other fields were the green manures were grown have shown greatly reduced nematode numbers that did not have to be treated with chemicals. A treatment of Telone II costs more than $26,000 for 120 acres, and a full-season program of Vydate CLV costs about $7,200. Avoiding such treatments not only saves the costs, it can also preserve beneficial organisms in both the soil and the potato canopy.

PRODUCER ADOPTION
Many more local producers adopted a green manure rotation in 2006 as a result of this project. This has results in more samples being taken before and after the green manure crops, which can provide more information on which crops are controlling nematodes. This information will help the farming community by fine-tuning specific crops as a rotation on specific fields.

REACTION FROM PRODUCERS
A number of growers said they were sad that the grassy weeds compromised the project’s results, but they were glad to learn from the mistakes. Most were excited for a non-chemical option for nematode control that also helps save water and improve soil. Many of those growers have included green manures in their rotations.

FUTURE RECOMMENDATIONS OR NEW HYPOTHESES
The project coordinator said he would not consider conducting the small plot research again, but said the information from the trial has been useful in planning future green manure programs. The best information obtained was from the 2-acre grid samples done before and after a green manure crop.

OUTREACH
The project was communicated to the local community with a talk and PowerPoint presentation on Jan. 26, 2006, at the annual Agro Engineering workshop, which is open to the public and occurs near the time of the San Luis Valley Potato and Grain Conference. Another presentation was planned for the Agro Engineering workshop in January 2007, and a handout will be available at the 2007 Rocky Mountain Agricultural Conference.