Managing Garlic Bloat Nematode Using Bio-Fumigant Cover Crops
Garlic Bloat Nematode (Ditylenchus dicapsci) was confirmed in sixteen counties throughout New York as well as in Massachusetts and in Vermont in 2010. Garlic Bloat Nematode (GBN) reduces yield and quality of food grade garlic by stunting and yellowing the plants, damaging roots and the basal plate, and increasing secondary infection by fusarium and soft rot pathogens. Damage in 2010 ranged from minor to 80% stand loss. Additionally, infested garlic should not be sold as seed, which is a primary market for many growers.
Garlic is a staple of fresh market growers throughout the northeast, and is a primary crop for growers in each state. Cornell Cooperative Extension, in cooperation with the Garlic Seed Foundation, implemented broad outreach efforts during the 2010 growing season to reduce further spread of GBN to currently uninfested farms, but many farms are already infested with GBN.
In order to assist farms which are infested with GBN we studied the best practices to eliminate the nematode from the soil. Our two objectives were to determine the effectiveness of two bio-fumigant cover crops, Sorghum-Sudangrass and Mustard at eliminating GBN from the soil; and to determine how long GBN survives in the soil when a host plant is not present. These two objectives were accomplished through on-farm trials tailored to our three farmer cooperators’ needs and abilities.
We were surprised to discover that GBN did not overwinter in the soil during the two years included in this study. On all three sites, GBN infested garlic was harvested but extensive spring testing, including sampling directly from the planting rows, yielded no nematodes. Dr. Abawi, Cornell University Nematologist, replicated the field conditions in the lab and yielded similar results.
The lack of soil overwintering nematodes led us to focus our outreach efforts primarily on sourcing clean seed. We believe that this outreach has been effective, based on decreases in the number of positive samples sent to Cornell for diagnosis. The percentage of positive garlic samples submitted for testing has dropped from 30% in 2010 and 2011 to 9.8% in 2012.
Our two objectives were to determine the effectiveness of two biofumigant cover crops, Sorghum-Sudangrass and Mustard at suppressing GBN in the soil; and to determine how long GBN survives in the soil when a host plant is not present. It appeared that GBN was not able to survive in the soil through the winter, based on two years of extensive sampling. Without initial GBN populations, planting cover crops was unnecessary.
Because our objective of using biofumigant cover crops to control GBN in the soil was nullified, we looked for other ways to use biofumigant cover crops to suppress GBN. A secondary use of these cover crops that has been proposed is as a pre-plant treatment for infested garlic seed. However, our grower cooperators struggled to fit these in-season cover crops into their rotations (crops would have been in the ground during the peak of the growing season). One particular barrier was that no diversified grower was willing to plant a brassica cover crop in their fields in the peak of the season due to disease and insect concerns. Additionally, a source of untreated Trudan 8 was impossible to find. Working with the growers, it was determined that focusing on best management practices to grow the garlic paired with replacement of infested seed was the best management practice.
Based on these realities, here are the objectives and performance targets for 2012:
November 2011- January 2012: Compile results from assessments, make recommendations for growers for 2012. Update growers and Extension staff about progress during winter meetings.
We have discussed the results of this trial at the Cornell Ag. Agents Inservice (20 attendees) and at the New England Vegetable and Fruit Conference in New Hampshire (225 attendees). Three garlic schools, held in March, attracted 92 growers from across New York. Management articles, focusing on acquiring clean seed, have been widely disseminated to growers through statewide newsletters and through the Garlic Seed Foundation newsletter (approximate distribution 2700 growers, including both publications).
April 2012: Review updated cover cropping and rotation plants with grower cooperators and order year two seed (actually just retained seed from year 1). Test cooperating farms for nematode, and request they not plant cover crop until results are obtained. All results were negative on all farms.
Revised (April-June 2012): Find growers who intend to re-plant some infested garlic seed, and work to set up cover crop systems with them. Two growers were identified, Peaceful Valley Farms and Quincy Farm. Both received positive results from testing their seed in the spring of 2012, and still intended to plant their saved seed (Quincy) or to use the same source the following year (Peaceful Valley).
May 2012: Collect baseline data on nematode populations and soil health from all sites and plant mustard cover crops. Deliver all year two seed to cooperators. Not completed.
Revised: (May-June): Discuss cropping options with two identified farms. Neither farm was willing to put the brassica cover crop into their fields due to rotation concerns and pest concerns. Quincy farm was interested in trying Trudan 8, but could not use treated seed and no untreated seed was available. Both farms agreed that their best option was to sell of all garlic as food and seek clean seed rather than trying to use biofumigants.
June 2012 : Plant vegetable crops as appropriate and plant sorghum sudangrass covercrop (end of July). Flail mow mustard cover crop and incorporate. After biofumigation period, retest GBN populations in the soil. Not completed.
August10-15: Mow and incorporate sorghum-sudangrass cover crop. Roll soil lightly to improve biofumigation. After two weeks, a winter non-host cover crop may be sown. Not completed.
Late September 2011: Test vegetable and sorghum-sudangrass fields for GBN in the soil. Not completed.
October 2011: Test fall mustard field for GBN in the soil prior to garlic planting. Not completed.
November 2011- January 2012: Compile results from assessments, make recommendations for growers for 2012. Results have been compiled, and recommendations now focus on obtaining clean seed and creating ideal growing conditions for garlic (see attached materials). Crop rotation of 3-4 years out of alliums is still recommended to reduce disease incidence in garlic, but is not the focal point of nematode management.
Finding that GBN has not been overwintering in detectable levels over the past two years was a surprise. We cannot say that there are no conditions under which GBN could overwinter in New York, but it seems clear that when we experience wet starts to the winter as we have the last two years that GBN cannot survive until spring. Dr. Abawi has hypothesized that this is because GBN must enter a specific life phase in order to overwinter, and that dry soils are most conducive to this happening. In wet soils, GBN stays active and susceptible to freezing.
We still recommend that growers test their soil if they will be entering a field which has contained garlic in the last 3-4 years. However, our emphasis has focused on seeking nematode-free seed and providing growing conditions most favorable to garlic.
Our outreach efforts through newsletters, garlic schools (2011), and the Saugerties garlic festival (2011 and 2012) have reached over 2000 growers. Evidence of the effectiveness of the educational efforts can be seen in the samples entering the lab. In 2010, 30 percent of all samples tested were positive for garlic bloat nematode. In 2012, 9.8% of all samples tested positive for garlic bloat nematode.
Impacts and Contributions/Outcomes
Learning that under recent conditions GBN is not overwintering at detectable levels changed the focus of our education and outreach to obtaining clean seed. Outreach to increase grower awareness about the importance of clean seed has been very effective. 419 garlic samples were processed in 2011 and 2012. I have personally taken over 30 calls from growers seeking sources of clean seed, and nearly every grower I have talked to who tested positive for GBN is in the process of eliminating positive seed from their farm by selling it as food and purchasing new seed. The shift out of infested seed is also evident in testing. Only 9.8% of samples tested positive in 2012, a 20% decrease from the 30% positive sample rate seen in 2010.
At Saugerties Garlic Festival, one of the largest festivals in the Northeast, awareness of GBN had increased dramatically. All of the 100+ vendors were aware of GBN from outreach at the 2011 festival, and many had tested their seed. Commercial consumers were also much more aware of the issue, and were asking growers about seed testing prior to making purchases. This shift indicates that growers outside the directly targeted audience are also aware of the problem and that the most important management technique is avoidance.
Growers are focusing more on seed quality than ever before, which is improving the overall quality of garlic. More and more growers are testing, culling poor plants in the field, and grading garlic hard when getting it ready for sale. I inspected approximately 15 acres of garlic this year, and all but one field was exceptionally well maintained.
Additional information regarding impacts will be collected at the garlic schools scheduled for March 2013.
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