Brassica Green Manure Systems for Weed, Nematode, and Disease Control in Potatoes
1. Determine the efficacy of Brassica green manure systems for disease, nematode, and weed control in potatoes
2. Determine the nitrogen contributions from Brassica green manures
3. Conduct an economic analysis of Brassica green manure systems compared to costs of current pest and nitrogen management practices
4. Demonstrate the Brassica green manure system to potato growers, fieldmen, and extension educators through use of on-farm trials, seminars, workshops, extension publications, and a video.
Brassica green manures have the potential to provide biological control of several common potato pests, including soil borne diseases, nematodes, and weeds. Therefore, a three-year, multidisciplinary study to evaluate pest control in a Brassica green manure-potato system was conducted at three locations, Aberdeen, Idaho, a short season, high elevation, irrigated site; Prosser, Washington, a long season, low elevation, irrigated site; and Mt. Vernon, Washington, a long season, low elevation, rainfed site. At each location, three pest management systems were compared in a split plot design with four replications. Main plots were low, medium, or high pest control input levels, and subplots were green manure treatments: no green manure, winter rape (Brassica napus cv Humus), and white mustard (Sinapis alba cv Martigena). Low input main plots were not treated with weed or disease control chemicals; medium input plots were treated with a low-rate herbicide application; and high input plots were treated with a standard herbicide mixture and at Aberdeen also were fumigated with metam sodium. Winter rape or white mustard green manures were planted mid-August to early September and incorporated the following spring one to three weeks before planting potatoes. The Mt. Vernon site was flooded during the winter of 1995-1996, so data are reported for the full-term studies at Aberdeen and Prosser only.
Winter rape produced more biomass than white mustard at both Prosser and Aberdeen because white mustard was winterkilled at both locations. However, both green manure systems provided good to excellent soil cover (live plants or residue) from about three weeks after planting the green manure until green manure incorporation the following spring. Weeds were the main pest present at Prosser, while weeds, Rhizoctonia, Verticillium wilt, and root lesion nematodes infested the experimental areas at Aberdeen. Incorporating winter rape reduced common lambsquarters growth 78 percent compared to the low input, no green manure control at Prosser, but provided little or no season-long weed suppression at Aberdeen. White mustard provided little season-long weed suppression at either location. However, weed control with a combination of an incorporated green manure plus a low rate, post-emergence application of rimsulfuron + metribuzin (medium input treatment) was excellent (99%) and equal to the high input, standard practice herbicide treatment. The winter rape system had a higher percent of Rhizoctonia-free tubers (64%) than the white mustard (27%) and no green manure (28%) treatments in the low input (unfumigated) plots. Visual evaluation of Verticillium wilt showed less wilt incidence with winter rape incorporation (7%) than with white mustard (21%) or no green manure incorporation (22%) in unfumigated plots. In 1997, potato stems from the winter rape treatment were less colonized by V. dahliae than stems from the white mustard or no green manure treatments. However, V. dahliae populations in the soil were not different among treatments, suggesting that winter rape incorporation may not have a direct toxic effect on V. dahliae but may reduce potato root infection by the disease. Winter rape is a host for root lesion nematode, and root lesion nematode populations were 3.8 times higher in the winter rape treatment than in the white mustard and no green manure treatments after green manure incorporation in unfumigated plots. However, populations in the unfumigated winter rape treatment were below the economic threshold both years of the study.
U.S. No. 1 tuber yields were ten to 45 percent lower in the low input management system than in the medium or high input system at Prosser (P=0.07) and Aberdeen (P£0.01) due primarily to poor weed control. Green manure treatments did not affect U.S. No. 1 yields at Prosser, but yields were reduced 43 percent, averaged over management levels, in the winter rape treatment compared to the no green manure treatment one year out of two at Aberdeen. Reduced yields in the winter rape treatment at Aberdeen likely were the result of nitrogen mineralization, which elevated soil nitrate levels in the top 30 cm of soil during potato tuber initiation. Elevated nitrate levels delayed potato tuber initiation at both Prosser and Aberdeen, but at Aberdeen in 1996, the initiation delay resulted in reduced yield. The longer growing season at Prosser may allow time for full recovery from nitrogen-delayed tuber initiation while there may or may not be time for recovery under the shorter season conditions at Aberdeen.
Data from the Aberdeen experimental site were used to modify a commercial farm-based enterprise budget. The 1998 Southeastern Idaho Non-Storage Russet Burbank Costs and Returns Estimate (enterprise budget) from the University of Idaho was modified to reflect the changes in management practices and inputs used in the treatments. Revenue was calculated using an incentive-adjusted price and a paid yield. The low input system with or without a green manure had the lowest net return (highest negative return). Both the high input system and the medium input system provided substantially higher net returns than the low input system. The medium input system with the white mustard or no green manure treatment produced higher net returns than the high input system with these green manure treatments. The negative impact on net returns was greatest for the winter rape system because tuber yields were substantially reduced by this treatment one out of two years at Aberdeen.
Our studies suggest that for potato production areas with longer growing seasons and low root lesion nematode populations, a combination of a winter rape green manure plus a low-rate, post-emergence herbicide application could provide excellent soil erosion control over winter, excellent weed control, and Rhizoctonia and Verticillium wilt suppression. However, under shorter growing season conditions, additional nitrogen management studies are needed before a winter rape green manure system can be recommended for potatoes. For shorter growing season conditions, the combination of a white mustard green manure plus a low-rate, post-emergence herbicide application could provide good soil erosion control over winter and excellent weed control.
Farmer Adoption and Direct Impact
Winter rape green manures were not tested on-farm by either Idaho or Washington growers. In Idaho, studies conducted at the Aberdeen Research and Extension Center showed that nitrogen mineralization by the incorporated winter rape green manure treatment might delay tuber initiation long enough to result in reduced tuber yields. Therefore, additional nitrogen management studies are needed before a spring-incorporated winter rape green manure system can be tested on-farm because of the risk of serious yield loss. In 1997, three growers in the Columbia Basin of Washington used a white mustard green manure system. Good ground cover was noted by all three producers, and one also reported weed suppression and lower nematode populations early in the season. He noted two potential drawbacks to the white mustard system, increased wireworm populations and overwintering of green peach aphids if white mustard does not winter-kill.
This summary was prepared by the project coordinator for the 2000 reporting cycle.