We labeled the bed where we found nematodes, to ensure good sanitation practices.
Growing in a hoophouse has all the benefits of a warmer climate and sometimes the disadvantages, such as pests and diseases not seen in outdoor crops. For us in central Virginia, winter-hardiness Zone 7, that now includes root-knot nematodes (RKN). Nematodes are tiny soil-borne worms that have a wide host range and are hard to control. They move only 3'–4' (1–1.2 m) per year on their own, but people move them on shoes, tools, etc. It’s been hard to find an organic approach to management of this pest. I’m offering what I’ve learned so far, so other growers have a starting point.
Tilling every 10 days (bare fallow) is one option, but we didn’t want "dead" soil. RKN are active at soil temperatures between 50°F–95°F (10°C–35°C), but reproduce slowly if at all below 64°F (18°C); their optimum temperature is about 90°F (32°C). At the lower end of their active temperature scale, RKN can complete a life cycle in 50 days. At 85°F–90°F (29°C–32°C) that time can be reduced to only 20 days.
In February 2011 we found peanut root-knot nematodes (Meloidogyne arenaria) in our hoophouse, in young spinach, which we were digging up to transplant outdoors. Some of the roots were misshapen with lumps in them. We sent a sample of plants with roots and soil to our plant diagnostic clinic at Virginia Tech and they confirmed our fears.
We formulated a two-year plan foregoing food crops for two years, using nematode-suppressive cover crops and summer solarization. Thinking we’d then be done with nematodes, we grew lettuce there the next winter. We hoped the solarization would have killed off the sclerotinia that has plagued our winter leaf lettuce. That worked well!
But meanwhile we found nematodes in the other half of the same bed, in a crop of green bush beans. (It was easy to distinguish the lumpy nematode-containing roots from the nodules of nitrogen-fixing bacteria, which are spherical, external to the roots and attached to them.) We started the same two-year plan for the new area. For those two areas, our plan felt manageable and cautious: we could afford to lose a half-bed of food crops for two years, in the cause of eliminating this pest.
Our Initial Two-Year Zap IPM Approach
1. Prevent increase by removing all weeds, often (they are common nematode hosts).
2. Promptly remove crops, including roots, as soon as harvest is over, destroying infected plants (we laid ours out in the driveway to bake in the sun).
3. Prevent spread to other areas — wash tools after use, being careful not to move soil to other areas.
4. Suppress nematodes by adding compost at every crop transition to build the levels of organic matter and micro-organisms such as Rhizobacter and mycorrhizae, which can induce systemic resistance to nematodes.
5. Grow a series of nematode-suppressing cover crops. In fall sow wheat. In spring cut it down every time it reaches 12" (30 cm), to keep it manageable. In April spread compost, cut down and dig in the wheat, sow sesame (particularly good with peanut RKN) and interplant OP Lemon Drop French marigolds at 7" (18 cm) spacing. In mid-June spread more compost, dig in the cover crops, water and solarize the soil from June to September. In September remove the plastic, add compost and sow wheat again.
6. For year three, grow non-susceptible crops, or nematode-resistant varieties of susceptible crops, and monitor — watching for plants that wilt during the day and recover overnight, watching for general decline, doing root checks, looking for lumps.
Other Nematode-Suppressing Cover Crops
We might have included Pacific Gold mustard (B. juncea), if we’d found it in time. Don’t confuse this with Ida Gold Mustard, which kills weeds, and is susceptible to nematodes. Other cover crops that suppress nematodes include some other OP French marigold varieties (but avoid Tangerine Gem or hybrid marigolds); Iron and Clay cowpeas; chrysanthemum; white lupins; black-eyed Susan; gaillardia (blanket flower, Indian blanket); oats; sesame/millet mix. We decided against sorghum-sudangrass (too big), winter rye (harder than wheat to incorporate by hand), bahiagrass, Bermuda grass (both invasive), castor bean and Crotolaria (sunn hemp) (both poisonous, although newer varieties of Crotolaria have lower toxin levels), partridge pea, California poppy (both require at least one full year of growth) and some obscure vetches that weren’t available locally.
Brassicas for Disease and Pest Management
All brassicas contain compounds with wide-ranging biotoxic activity against insect pests, bacteria, fungi, nematodes and weeds. Because the active chemicals are released only when the plant cell walls are ruptured, brassica cover crops are usually tilled in or plowed under to maximize their natural fumigant potential. The level of biofumigants varies, depending on the species, variety, planting date, growth stage when terminated, climate and weather. This is still a relatively new area of research, with growers having widely different results.
This “mustard-gas effect” is caused by glucosinolate compounds in brassicas. Breakdown products of glucosinolates, such as isothiocyanates and nitriles, suppress nematodes by interfering with their reproductive cycle. Oilseed radish as a plowed-in cover crop can reduce stubby-root nematode (Trichodorus) and root-lesion nematode (Pratylenchus). Root exudates from incorporated cover crop radishes stimulate hatching of sugar beet cyst nematode eggs, but the larvae that emerge are unable to develop into reproductive females, so the population is reduced. Radish does not always completely control the nematodes, but it can be part of a longer-term strategy to eliminate them. See SARE Growing Cover Crops Profitably and ATTRA Nematodes: Alternative Controls.
Avoiding Susceptible Crops in Warm Soil
We considered using the infested bed for susceptible crops during winter, waiting until soil temperatures dropped below 64°F (18°C), when it is too cold for nematodes to reproduce. I found that as of October 6, our soil was still too warm. It would certainly mean no early greens. We decided against this, as it implied a late start to harvesting, and perhaps an early finish too. Later-planted crops such as radishes and lettuce can be grown through most of their cycle without suffering nematode infection, finishing before spring when the nematodes become active, but we want greens for early winter too. Many spring crops can be planted before soil temperatures warm to 50°F (10°C) and severe damage can be avoided. In spring we'd pull up the susceptible crops when soil temperature reached 64°F (18°C) again.
Planning for Nematode Management
In August 2014, when we pulled our early tomatoes in the bed next to the one we were treating, we found nematodes on the roots of four out of 44 plants. It’s possible they had made their own way there from the next-door bed, or that we accidentally transferred them with tools. Having two beds out of six not producing food was too much of a hit. We decided we needed to learn to live with a certain level of nematodes and only take an area out of production if we got a level of nematodes above the Action Threshold, where we were obviously losing yield. Also, we’d use soil amendments to make conditions suitable for nematode antagonists, added before planting crops, so materials would be in place at root level.
Our New "Two Years Nematode-Resistant, One Year Susceptible" Plan
We planned a system of two years of resistant food crops only (particularly in warm weather), followed by one year of somewhat susceptible crops. We made up one beds-worth of the most resistant crops in our plan. This includes Russian kales (B. napus), mizuna (B. rapa var. japonica) and frilly mustards (B. juncea), Yukina Savoy (classification is now B. rapa, not B. juncea as we originally believed) and radishes. We kept this set of crops as a future nematode-resistant winter combination. In the spring, we grew West Indian gherkins (resistant), followed by Mississippi Silver or Carolina Crowder cowpeas (resistant).
We added Monterey Nematode Control once the soil was warm enough. Biocontrols need to be in place in the soil as it reaches a warm enough temperature for the RKN to be active. Essential oils from wormwood (Sweet Annie), caraway, fennel, applemint, spearmint and oregano are said to be helpful but I have no proof. Jerry Ross of Maui, Hawaii, reported that molasses seems to drive down populations of RKN in the soil. He found that applying some dried molasses (used as a horse supplement) to the soil seemed to really help.
After two years of resistant crops, we did a year of nematode-resistant varieties of susceptible crops, or something from the “Somewhat susceptible” list in the Susceptibility Chart below. The crops at the top of their columns are more susceptible than those below them. The chart can be read as a continuous list, working from top left to bottom right.
My new book The Year-Round Hoophouse also contains charts of RKN-resistant varieties of various vegetable crops.
Pam Dawling works in the vegetable gardens at Twin Oaks Community in central Virginia. She often presents workshops at MEN Fairs, as well as sustainable agriculture conferences. Pam also writes for Growing for Market and other magazines. Her book, Sustainable Market Farming: Intensive Vegetable Production on a Few Acres, is available at www.sustainablemarketfarming.com, Pam's second book The Year-Round Hoophouse is now also available. Her blog is on her website and also on facebook.com/SustainableMarketFarming.
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