Integrated Soil and Crop Management for Organic Potato Production
The yield and quality of a potato crop are the result of complex interactions amongst crop nutrition, cultural practices, and pest damage. In this project we are developing a participatory process to share knowledge, experience, and farmer innovation; to illuminate new strategies for farmer-identified problems in whole farm systems; and to enhance their adoption and adaptation. This project aims to enhance the ability of farmers to: 1) identify and manage pest and nutrient problems, 2) keep on-going records and enterprise budgets, 3) synthesize “systems” information, and 4) plan and evaluate alternative management strategies, both individually and in collaboration with other farmers.
1. Pilot a participatory approach to learning and adaptation of novel farming systems strategies.
2. Evaluate the effects of soil management on tuber insect pests and diseases, weeds, nitrogen availability, and profitability.
3. Extend project findings to a larger audience of farmers.
Objective 1. Pilot a participatory approach to learning and adaptation of novel farming systems approaches.
Before the first meeting, growers were sent a potato production survey in which they described all aspects of their production system from seed sourcing to market. All group participants received all surveys before the meeting. During the first and second meetings in December 2005, farmers and the project team collaboratively 1) identified and prioritized the issues reducing potato production profitability, 2) identified and discussed any known solutions, 3) generated hypotheses to be tested during the first growing season in on-farm trials, and 4) identified who would participate in on-farm trials. Staff met monthly from December through August to plan, trouble-shoot, and discuss project activities, and bi-weekly from August through December to interpret data and plan the December 2006 meeting. Before each December meeting, draft agendas were drafted by staff for grower review; the agenda for the December meeting was finalized after incorporating grower input. On-farm reports (whole group and farm-specific) were sent to each participant before the meeting. At the meeting, staff presented research reports with considerable interaction from farmers and staff. One farmer presented his potato enterprise budget, and the group discussed the production surveys in the context of the enterprise budgets. In the final morning session, farmers and staff prioritized research issues for summer 2007 and evaluated the year’s work and the meeting process. Staff are now drafting research and extension activities to address those priorities with farmer input when needed; these proposals will be presented to the group at the second winter meeting in February 2007. The last winter meeting will be held in December 2007, when second year and final project results will be presented and discussed. All participants are surveyed before each December meeting to track changes in knowledge and attitudes.
Objective 2. Evaluate the effects of soil management on insect dynamics, tuber disease, and nitrogen availability.
A. Nitrogen management
On-farm data on nitrogen and yield were collected in 2006, including: pre-plant soil testing, laboratory soil incubations, potato petiole nutrient analysis, potato nitrogen uptake from on-farm fertilized and unfertilized plots, end-of-season 3-ft soil nitrate sampling, irrigation water nitrate, and fresh tuber yield.
Pre-plant soil testing
Eleven farms had complete soil nutrient testing prior to the growing season. Results and interpretations were distributed to the farms. None of the results were unusual for organic farms with compost-based nutrient management.
Laboratory soil incubations
Soil was collected for laboratory incubations in spring prior to planting and again in the summer. Approximately 500 g soil on a dry weight basis was added to Ziploc bags and then incubated at 22 C for 9 weeks. Soil was subsampled in three-week intervals and extracted with 2M KCl and analyzed for nitrate nitrogen. The results suggest that the mineralization rates of organic farms in our study groups are high, with an average of 0.8 mg nitrate-N/ kg soil/ day at 22 C from the spring sample.
Potato Petiole Values
Potato petioles were collected at most farms during tuber growth. Our 2006 petiole nitrate-N values were below published recommended ranges for Russet Burbank potatoes grown in the Columbia Basin.
Crop Nitrogen Uptake
Crop nitrogen uptake (vines + tubers) was measured weekly during tuber growth to determine typical N uptake rates using normal grower fertilization practices. During mid-season (40 to 100 days after planting), crop N uptake averaged 2 lb N per acre per day. Six farms also participated in an unfertilized N uptake study, in which a field plot was marked off before planting and no compost or other fertilizers were added. Crop N uptake in the absence of current season fertilization ranged from 100 to 200 lb N per acre.
At harvest, soil samples down to 3 feet in 1-ft increments were collected from most of the farms. Most fields had low to medium levels of nitrate-N at harvest, suggesting that overall N supply was not excessive.
Tuber Yields at our farms ranged from 15 to 25 tons/acre for fresh tuber yield, based on 36-inch spacing between hills.
Summary of N management findings
In this season, the project quantified plant uptake, yield, nitrogen mineralization, petiole nitrate values, and soil nitrate testing at different times during the growing season. This information was new to both farmers and researchers. The largest source of nitrogen for the potato crop appears to come from in-season mineralization of N from decaying soil organic matter.
B. Tuber insect pests
After a number of discussions with the participant growers, regional potato experts, and some preliminary sampling on project farms, it was determined that the most likely insect pest groups were flea beetles and wireworms (click beetles as adults). Five farms were sampled for insect pest incidence and damage.
The three flea beetle species in the Willamette Valley that are capable of inflicting economic damage on tubers are the tuber flea beetle (Epitrix tuberis), the tobacco flea beetle (E. hirtipennis), and the western potato flea beetle (E. subcritina). The first two of these species were found in the potato fields on all five of the sampled farms, and the third species was present in very low numbers at a few of the farms. Tuber flea beetle numbers were about twice as high as those of tobacco flea beetles.
Flea beetle populations increased at the edges of the potato fields more rapidly than at the inner areas of the fields. This indicates that flea beetles initially came from overwintering sites outside of the potato fields, and gradually spread into the inside of the fields from the edges. The ‘edge’ area of the field was defined as potato plants within 5 meters of the outside edge of the field.
The extent of tuber damage in each of the five fields relative to flea beetle populations and the timing of their arrival was also assessed. Three of the fields showed the expected higher or lower amount of damage with higher or lower flea beetle populations respectively, but two of the fields showed the opposite trend, with high populations yet comparatively little tuber damage. There are many possible management and/or biological factors in these fields that could cause the large flea beetle populations to not reach their typical damage potential, and this also shows that the beetle population levels are not always reliable predictors of damage in a field.
Since tubers are more susceptible to damage from increasing numbers of flea beetle larvae as the season progresses, the time of arrival of the beetle adults to a field in the spring could also be an indicator of the extent of damage in a given field. The timing of arrival of flea beetle adults also turned out to not be a reliable predictor of tuber damage, but the sampling at the beginning of spring may not have been intensive enough to catch the first individuals. The plan for year 2 is to increase the intensity of flea beetle sampling in the early spring to confirm the actual timing of arrival, as well as to provide biofix information for predictive tuber and tobacco flea beetle degree day phenology models that are being developed.
Sampling and diagnostic methods for flea beetles were tested and developed, and these have been summarized in an identification and monitoring worksheet for use in the field. Sweep netting of potato plants was more efficient than simple visual observation for assessing beetle numbers in the crop field. Yellow sticky traps yielded some information on flea beetle numbers, but they were not as efficient as sweep netting, and are probably more useful for assessing the timing arrival of the first flea beetles rather than population levels. Since flea beetle populations were shown to increase at field edges first, monitoring efficiency can be increased by focusing on these parts of the field. A flea beetle damage rating system was also developed to quantify damage for the consistent diagnosis of flea beetle damage.
Since wireworms have a 4- to 5-year life cycle, fields that were planted to potatoes in the 2005 season were also sampled in addition to the 2006 potato fields at each of the 5 farms. Adult beetles of the wireworms were sampled with pitfall traps, white sticky traps, and pheromone ground traps for two invasive species from Europe (Agriotes lineatus and A. obscurus), which have a reputation for causing more consistent damage to tubers and other crops than the other local wireworm species. Wireworm larvae were sampled with underground bait traps of germinating grain.
Several different species of wireworms were obtained at the 5 farms, but efforts for species-level identification and confirmation focused on the two invasive species due to their economic importance in British Columbia and Washington State over the past several decades. In 2005 the known distribution of these two species in Oregon consisted of only a few nurseries and ports near the Columbia River, as reported in a survey by the Oregon Department of Agriculture (ODA). One of the project farms in this same area near the Columbia River had A. lineatus adults in the pheromone traps. Another farm about 15 miles south of this area also had A. lineatus in the pheromone traps, and this was a new county record of this invasive species that was reported to the ODA.
Although species-level identities of the other wireworm species were not confirmed in most cases, the extent of overall wireworm damage relative to overall wireworm and adult numbers in a given field was recorded. As for the flea beetles, numbers of wireworms were not always associated with the extent of wireworm damage in a given field. Wireworm damage in general was not as prevalent as flea beetle damage among project participants.
Pitfall traps and white sticky traps trapped very few wireworm adults, but the pheromone traps and larval bait traps were useful for wireworm monitoring and should be continued in project fields where wireworms are a concern. As was done for the flea beetles, a wireworm damage rating system was developed for consistent diagnoses, and discussions with the growers provided them with information about how to tell that damage apart from the damage of other insect and other tuber skin problems.
The flea beetle management strategies the farmers are most interested in investigating in the 2007 season are entomopathogenic nematodes, covering the base of the potato plants with soil or a mulch to prevent oviposition, and early planting and harvest to get the tubers out before they accumulate significant damage. We will also try to examine the overwintering sites on each farm as sources, and combine this information with the information produced from the other experiments into a set of IPM recommendations that will be presented in discussions and summarized in either handbook or information sheet form.
Since the wireworms are less of a priority for most of the farms, we do not have any experiments planned to generate information about specific management tactics. We will, however, continue to monitor the farms to track the potential spread of the two invasive species. We will also look into monitoring source areas of wireworms on the farms and track the extent of their damage in the potato fields. This information will also result in IPM recommendations that will be discussed and summarized in the same way as those for the flea beetles.
C. Late blight (LB) management
All farm fields were scouted for LB and any other diseases. No farms in southern and eastern OR, one farm in the Willamette Valley, and both farms in NW WA experienced LB epidemics in 2006; the differences in farm incidence are related to differences in temperature and rainfall patterns in these distinct regions – LB is more prevalent in cooler, moister environments. In addition, NW WA is a major potato growing region with high inoculum potential. Farms in LB-conducive regions use cultural methods such as wide row spacing, row orientation, irrigation management, and destruction of inoculum sources (e.g. cull piles) to manage LB. Only one farmer collaborator has ever applied copper fungicides. Disease incidence in the one farm with LB in the Willamette Valley may have been due to row-covering of an early potato planting, a narrow in-row spacing, late afternoon irrigations, and late spring rainfall. One of the WA farms intends to investigate the use of copper fungicides for LB control in 2007.
A European Union project (BlightMOP) investigating organic LB management strategies reported that the most promising strategy is to identify LB resistant germplasm. Ospud identified 3 commercially available LB resistant cultivars: Island Sunshine (IS), Jacqueline Lee (JL), and Defender. These were grown in non-replicated trials on 5 cooperating farms in 2006 as well as in inoculated replicated trials at the OSU research farm. Defender was highly resistant to foliar LB, and IS and JL were somewhat to moderately resistant in on-farm and research station trials in 2006. In general, the farmers who trialed the three cultivars considered them to be viable alternatives to the russet and yellow-fleshed cultivars they were currently producing and 7 farmers intend to evaluate them in on-farm trials in 2007. The OSU potato program continues to evaluate specialty potato germplasm (commercially available and research clones) for LB resistance; Ospud will continue to partner with them to identify LB resistant clones with organic market potential.
Objective 3. Extend project findings to a larger audience of vegetable farmers. The project intends to extend experiences and findings to a larger audience of diversified vegetable farmers through:
1) The project website, www.ospud.org , which describes current and upcoming project activities.
2) Outreach workshops
A. Workshop One: A potato evaluation and tasting was held at one of the Ospud farms in collaboration with the OSU potato breeding program. Approximately 50 organic potato farmers, retailers, chefs, processors, and researchers drafted organic potato germplasm selection criteria and evaluated approximately 25 potato cones for appearance, flavor, and texture (steamed, chipped, and fried). See www.ospud.org for summary and data.
B. At least one outreach workshop will be held in winter of year two, during which project farmers will present results and experiences.
3) Extension publications:
Three extension publications will be generated in year two. Possible topics include: A) nitrogen management in diversified organic potato production systems, B) LB resistant potato germplasm, C) recordkeeping for diversified vegetable production system management.
Impacts and Contributions/Outcomes
This project in its first year has:
1) developed and evaluated a participatory process for this group of farmers and researchers – a process that will continue to evolve over the next 11 months;
2) identified at least 2 commercially available potato cultivars with LB resistance and good market quality for organic fresh market production;
3) measured the balance between N supplied by different sources (preplant nitrate-N, fertilization, mineralization of soil organic matter, and irrigation water) and crop N uptake. During mid-season (40 to 100 days after planting), crop N uptake averaged 2 lb N per acre per day. At 100 days after planting, unfertilized crop N uptake averaged 150 lb N per acre. Potato yields of 20+ ton/acre were produced at farms with relatively low levels of soil nitrate–N throughout the year. Petiole nitrate-N associated with typical yields was much lower than suggested by PNW Extension publications. These findings are being incorporated into a worksheet to assist growers in managing the supply of nitrogen for their potatoes;
4) identified tuber flea beetle (TFB) as the most important insect pest in western OR and WA; wireworm is a significant problem only on fields with a rotational history including long-term sod. TFB is an intractable pest, requiring further research on cultural, biological, chemical, and integrated control methods.
All of these findings in the long term will contribute to improving the economic and environmental sustainability of organic potato/diversified vegetable production in this region.
47th Ave. Farm
Oregon State University
Corvallis , OR 97330
Fry Family Farm
Days Creek, OR
Postdoctoral research associate
Oregon State University
Corvallis, OR 97331
Sauvie Island Organics
Oregon State University
Dept. of Crop and Soil Science
Corvallis, OR 97331
Mt. Vernon, WA
Blue Fox Farm
Potato specialist, emeritus
Oregon State University
Dept. of Crop and Soil Science
Corvallis , OR 97331