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.
Garlic Bloat Nematode (Ditylenchus dicapsci), or Stem and Bulb Nematode, was confirmed in sixteen (16) counties throughout New York as well as in Massachusetts and in Vermont in 2010. This damaging nematode was first detected in New York in early 1930’s, when it caused significant damage to the onion industry until the 1960’s when direct seeding of onion became the prevailing production practice. In this infestation nematodes are being spread through garlic seed, which is all propagated asexually through division. All new seed from infested plants is also infested. We suspect that nematode has been spread throughout the Northeast and Canada through movement of seed at garlic festivals and through direct farm sales.
Garlic is a staple of fresh market growers throughout the northeast, and is a primary crop for growers in each state. 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. Additionally, infested garlic should not be sold as seed, which is a primary market for many growers. This year in New York hundreds of growers were unable to sell their garlic as seed at festivals because of the fear of spreading Garlic Bloat Nematode (GBN).
Cornell University Cooperative Extension, in cooperation with the Garlic Seed Foundation, has been working since 2010 to prevent introduction of GBN onto additional farms. However, farms that already have GBN infestations needed to know the best management practices to again become nematode free.
Prior to this project there were no specific and demonstrated recommendations for the Northeast for crop rotations and cover crop selection to eliminate GBN under local production conditions. Limited work on the management of this nematode has been done in Ontario, Canada, which also had a recent outbreak and damage by this nematode. However, conclusive recommendations about rotation lengths and specific crop rotations did not exist for our growers.
In order to create research-based best practices for farmers throughout the northeast to eliminate GBN from their infested soils, we worked with farmers who currently had infested soils to achieve two specific objectives. The first objective, determining a timetable for nematode persistence in the soil, actually negated the need for the second objective of testing the effectiveness of two different cover crops with biofumigant activity at eliminating GBN. The focus of our outreach accordingly moved from using cover crops to eliminate GBN in the soil to sourcing clean seed while still maintaining a 3-4 year rotation out of alliums as a general best practice.
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.
April 2011: Establish tentative two year cover cropping and rotation plans with grower cooperators and order year one seed. This was accomplished according to plan.
May 2011: Collect baseline data on nematode populations and soil health from all sites and plant mustard cover crops. Deliver all year one seed to cooperators. The mustard cover crops were planted and seed was delivered, all prior to receiving results from nematode testing. Sorghum was not planted, because the results indicating that GBN was not present had been received by this point.
June -July 2011: Plant vegetable crops as appropriate and plant sorghum sudangrass cover crop (end of July). Flail mow mustard cover crop and incorporate. After biofumigation period, retest GBN populations in the soil. This step was not completed because there were no GBN present in the soil. Mustards were incorporated normally.
September 10-15: 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. This step was not completed because there were no GBN present in the soil.
Late August 2011: Test vegetable and sorghum-sudangrass fields for GBN in the soil. This step was not completed because there were no GBN present in the soil.
October 2011: Test fall mustard field for GBN in the soil prior to garlic planting. This step was not completed because there were no GBN present in the soil.
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.
Details of cover crop block layout and sampling procedure: Cover crop blocks were set up differently at the different farms, depending on how the planting was set up (see attached diagram). On the farms where cover crops were going to be incorporated with a walk-behind rototiller the treatments were set up in a randomized complete block design, since the tiller could move from block to block easily, allowing for different blocks to be incorporated at different times. The farm which incorporated materials with a tractor had larger, homogenized blocks to accommodate the need to incorporate all of the mustard separately from all the sorghum/sudan grass. Plots were established in early spring, at the same time as GBN sampling took place. The earliness accommodated the need to plant mustard.
Year one: All three grower cooperators had garlic and soil samples tested in the fall. All samples tested positive for GBN. In the spring, each farm was sampled in April by taking 10 samples per block in large fields and 6 samples per block in small fields. Sampling was completed using a soil probe and sampling to a depth of 5 inches. Samples were aggregated in plastic bags. Each treatment was sampled separately to establish unique baselines. When the first set of samples all came back negative, each farm was sampled a second time in March to ensure that the samples had not been damaged in handling. Samples were increased to 15 per large block and 10 per small block. Care was taken to keep soil cool, and samples were overnighted to the Geneva NY lab for testing. Again, all samples came back negative.
Year two: Due to the extra sampling in the spring of year one, we decided to skip fall sampling in year two and only do spring sampling. In the spring, blocks were again set up in the previous year’s garlic ground and sampling was conducted in the same fashion as during year one. We asked growers not to plant mustard until the results were available, since they might not need to plant mustard at all. Again, all samples tested negative for GBN. Plots were removed and grower cooperators were able to use the fields normally.
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.
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.
Education & Outreach Activities and Participation Summary
Best management practices have been developed for Garlic Bloat Nematode. The most recent publication by our state nematologist reflects the lack of concrete research results from this study, but does not abandon the idea that biofumigant cover crops would be beneficial if the nematode ever is found to persist. The research did not disprove the efficacy of biofumigant cover crops, so this approach makes sense.
GBN has been discussed at meetings in New York, Vermont, Pennsylvania, Maine, and New Hampshire. We have received calls from across the country regarding the issue, and have been working to steer growers towards testing seed as a first and best practice.
Additional presentations and publications are above.
: If we use the testing rates as a benchmark for the industry in New York, and assume that growers lost 15% of their crop on average if infested with GBN, (Actual number is difficult to quantify, even based on survey numbers, because growers also lost garlic to various diseases and struggle to separate out the cause of loss. 15% represents an average loss from the fields that extension personnel have examined, which ranged from no loss to 80% loss) we could say that 30% of growers representing a 26.5 million dollar industry lost 15% of their crop in 2010. The dollar value assigned to that loss would have been $1,192,500. In 2012, only 9.8% of growers tested positive. Applied to the industry-wide $26.5 million sales, 9.8% of growers would have lost 15% of their crop, for a total industry loss of $389,550. This represents an industry-wide change of $803,000. The impact across the northeast would be even greater, though New York represents a majority of the industry.
The primary change made by farmers has been the adoption of seed testing as the best way to avoid being contaminated with garlic bloat nematode. The impacts of the testing program were detailed during the impacts section. Growers still consider biofumigant cover crops to be a useful part of the rotation, but are not relying on them to eliminate GBN from soils.
Areas needing additional study
The use of other cover crops in the garlic rotation to suppress weeds and supply nitrogen