Soil Biology Battleground! Managing Root Knot Nematode with Anerobic Soil Disinfestation

Funded by the ASCFG Grower Grant Program
Rachel Cross, Spirit of Walloon Market Garden, Boyne City, Michigan

At my flower and vegetable farm in Northern Michigan, we have been noticing an upward trend in root knot nematode (RKN) populations over the last few years, especially in the protected conditions of our hoophouses. This microscopic, tenacious soil-borne parasite infects and malforms roots, resulting in low vigor, reduced yield, and sometimes outright death in susceptible crops. Many cut flowers are prone to RKN damage, including lisianthus, snapdragon, celosia, stock, and sweet pea.

Anaerobic soil disinfestation (ASD) is a novel approach to controlling soil pathogens such RKN and other plant-feeding nematodes, fungi, and bacteria. Dr. Anna Teston and Dr. Sally Miller at Ohio State University (OSU) have researched ASD and developed a method for its use. In this method, a carbon source is added to the prepared, unplanted bed. The soil is brought to saturation, and is then covered with silage tarps to create anaerobic conditions. The microorganisms that proliferate in these conditions create organic compounds that are toxic to soil pathogens like RKN. After 3-5 weeks, and ideally soil temperatures above 85°F, the tarps are removed. Unlike synthetic chemical soil fumigation, this technique is biologically based and suited to the ethos of sustainable flower production. Besides preserving soil health, the process also conserves time; ASD is faster than other RKN reduction strategies such as solarization, a fallow period, or a rotation with a non-host cover crop.

Root knot nematode galls on lisianthus.

The research done at OSU focused on ASD use during the summer months; we planned to utilize ASD during the early season to see if the effect on RKN populations was similar without sacrificing valuable protected space during the main growing season. In correspondence, Dr. Teston noted that RKN was affected in their trials even during cool fall temperatures. We were interested in 3 week, 6 week, and 8 week treatment durations. Two of these were longer than the OSU treatment to account for cooler early season temperatures.

Procedures

We planned to trial the ASD treatment on four beds: HH Bed A for 3 weeks, HH Bed B for 6 weeks, HH Bed C for 8 weeks, and field Bed D for 5 weeks during the summer which was to act as our control compared to the original OSU research. However, we hit a hiccup in our plan when field bed D ended up not containing any root knot nematodes! A good thing for the soil, but not ideal for this experiment. We ended up trialing just the 3 hoophouse beds.

Some materials were slow to arrive, so we ended up starting 3 weeks later than originally planned. On week 13, 2020, beds A, B, and C were sampled for nematodes. These soil samples were sent off to Michigan State’s Plant and Pest Diagnostics lab. As we observed in 2019, many parasitic nematodes were present, at high risk levels (see chart for population counts).

The 3 beds were then prepared with any needed minerals based on our fall soil tests. Based on OSU recommendations, for our carbon source we added wheat bran at the rate of 6 tons per acre and an OMRI-listed molasses product (LCB-F TerraFed) at 24 gallons per acre. For one of our standard 2.5’ x 96’ greenhouse beds, this was an application of 66# wheat bran and 18 oz TerraFed per bed. The wheat bran was easy to source; we found it both at our local feedstore and through our farmer’s co-op. These amendments were worked into the soil, and then the soil was watered with drip irrigation for 5-6 hours, or until we were as close to saturation as we could get on our sandy soils. OSU recommends watering until surface puddling is observed, but this doesn’t really occur with our soil type here. Before tarping, a HOBO Pendant MX Water Temperature Data Logger was buried 1” deep in the middle of each bed to track soil temperature over the time of treatment, and importantly, marked with a flag for easy retrieval!

Bed A was uncovered week 16, bed B week 19, and bed C week 21. At the point of uncovering, soil samples were collected once again and sent to MSU for nematode population analysis.

After uncovering, we intentionally tried to re-inoculate the beds with some beneficial soil biology. The downside to creating anerobic conditions is that you are most definitely killing off more than just pathogenic soil microorganisms; the good guys are going down with them. Before planting, we gave the beds a 5-day recovery period, during which we added compost and made two applications of vermicompost tea and lactic acid bacteria solution

Materials for ASD - molasses, wheat bran, data logger, marker.
Beds B and C after carbon additions and irrigation.
In the process of tarping - next step, bury edges.
Bed B untarped after 6 weeks. Plenty of bran remained, but all was at least partially decomposed. Next step, good microbe additions.

Results

The good: We were very pleased to see huge reductions in RKN numbers immediately after treatment! The longest treatment at 8 weeks had the greatest reduction (at a whopping 95%); this treatment also reached the highest soil temperature at 94.5°F, which may have aided in its efficacy. However, even the shortest treatment at 3 weeks experienced an 88.9% reduction in population. The average soil temperature was actually greater in this treatment than the 6-week treatment, likely due to a cold snap we endured in late April/early May of 2020. Regardless of treatment duration, every treatment was effective, both in terms of population reduction and our own observations of the crops afterwards. The lisianthus crop in bed A, besides not having root knot evidence when pulled in the fall, was also one of our most disease-free plantings ever. Beds B and C were planted to summer tomatoes (which were very vigorous), and then to overwintering flowers (ranunculus, snapdragon, anemone, poppy, larkspur) in October.

The bad: Experiment-wise, this wasn’t the strongest science. No replications, and, in the end, no real control. In hindsight, we should have left a portion of each trial bed untarped and also tested that after the treatment period to account for any seasonal variations in RKN populations. Worst of all, the root knot nematodes came back! Testing in February of 2021 showed increased but still low levels of RKN in beds A and C, but in bed B they were back to nearly 75% of their original pre-treatment problem levels.

Spirit of Walloon ASD Treatment Data

Bed A - lisianthus on the right.
Farmer Jess with an armload of post-ASD lisianthus.
Beds B and C of summer tomatoes before winter flowers; as tomatoes are highly susceptible to RKN damage, it was great to see vigorous plants after treatment!

The Take-away

Anaerobic soil disinfestation wasn’t 100% effective in managing root knot nematode, and it appears that it needs to be a yearly procedure as RKN populations reestablish, but in our trials (of all three durations) it did seem to cause a huge reduction in population levels that lasted at least through the growing season. It was also low cost (50# of bran is less than $10), simple to do, and ecologically sound. We are continuing to utilize ASD at the farm in 2021 (right now, week 11, we are prepping four more hoophouse beds for the process). It’s not a cure-all, but it’s a valuable tool in the sustainable flower farming toolbox.

Acknowledgments

A big thanks to:

  • The folks at the ASCFG for funding grower research.
  • Dr. Anna Teston and Dr. Sally Miller for their original research, and especially Dr. Teston for offering so much invaluable consultation and advice during this project.
  • Dr. Fred Warner, MSU nematologist extraordinaire.
  • Jess and Amanda, super farmers and employees of the decade for putting up with my experiments.
  • Nick and Susan for proofreading and all-around good advice-giving.