My Garden Disaster
Root-knot Nematode
In April 2021, I planted my spring garden in Oracle, AZ. I have had a vegetable garden in the same location for 14 years. I have eight beds: Six are 4’x8’, raised about 4”; two are 3’x6’, raised about 3’. Last winter, I grew a cover crop of peas and oats. I planted my garden about April 15, and about a month before, I amended the soil of all the beds with my home-made compost, composted horse manure and worm castings.
All began well.
Although I believe I did everything right, I – somehow, some way – introduced root-knot nematode into my garden. I don’t think I’ll ever know exactly how, but it was devastating to find. RKN is a microscopic roundworm that forms galls (knots) on the roots of plants infested by it (see picture below). The galls are filled with the roundworms, which pierce the roots of some plants, especially summer crops like tomato, pepper, cucumber, squash, eggplant, and okra, and basically sucks the life out of them.
Okra plant infected by root-knot nematodes.
An important note about nematodes: Not all are harmful. There are two types: Beneficial (predatory) or parasitic. Beneficial nematodes are good to have in our soil because they attack an assortment of garden pests like cutworms or squash vine borers; the parasitic nematodes are the ones that are damaging.
I follow the rules of crop rotation, which means not planting a vegetable in the same plant family in the same location year after year. Crop rotation is one of the oldest and most economical methods of controlling nematodes. I have grown okra each year but never in the same bed. I germinated the seeds from last year’s plants. I planted the okra in one of the low beds. In late July, I noticed the lower stems/leaves on most of my eight okra plants were turning brown and falling off. The plants continued to produce fruit so, at first, I thought it might be all the extra water from our abundant monsoon. Within a week, most of the stems and leaves would just come off by a gentle touch. I did a little research and thought they may need some extra fertilizer, as they were continuing to produce, even though the plant looked chlorotic (yellowing of the plant’s green tissue because of nutrient deficiency). It didn’t occur to me that RKN could be the culprit because I’d never had that problem. At the same time, I noticed that the Mexican sunflower bush, planted in the same bed, looked a little peaked. The carrots in that bed looked healthy, but when I pulled them up, some were fine, others forked, and others had extremely bumpy skin.
At this point, I started to suspect RKN. I was loathe to do it (because I really didn’t want to know), but I finally I pulled up one of the okra plants, and there it was: Root-knot nematode. I started looking at plants in other beds that were failing to thrive, despite other vegetables in the same bed flourishing (I’ve heard that RKNs are lazy and may just stay in one corner of a vegetable bed). The cucumber was infested, two tomato plants, and one of the squash plants, all in different beds. I wondered how, after 14 years, my garden could be so contaminated. How had I introduced this ruinous pest? RKN is soil-borne, so there were a few ways it could have happened. Here are some thoughts I had:
The two three-foot raised beds in my garden were new. I filled the bottom 18” of the beds with dirt from my property. The top 18” of soil was from a landscaping company in Tucson (composted soil). I only used this soil in the two beds, but one of the six squash plants in one of the beds had RKN. All the plants in the other 3’ raised bed have flourished with no visual issues, and it was the exact same soil. Because only one of the 3’ raised beds was infected, I’m not convinced the new soil is the cause.
It's possible that I, unknowingly, spread the RKN from one bed to another, either by walking on one bed to another or by using infected tools.
The worst bed was the one with the okra, Mexican sunflower, and carrots. All came from seed I germinated, so the contamination was not from a source outside of my garden. But, there was a sorrel plant in that bed, which had been planted in the prior fall. That plant came from a very reputable source so, again, I’m not convinced that was the origin.
I immediately started researching, and found these recommendations to manage RKN:
Plant mustard and French marigolds in the infected area. Let them grow until ready to flower, then cut to the ground and till everything in. There are substances in their roots that are toxic to some nematodes. Most of the effect, though, is not from being a natural nematicide, but rather that it acts as a trap crop, i.e., if the nematode enters the roots of the marigold or mustard, it will be killed. If it enters a nearby plant that is susceptible, it will survive. Planting marigolds and mustard as a cover crop may only be effective for one season; they may need to planted year after year.
Let the infected beds lie fallow for one year. Till the soil a few times to expose the beasts to cold temperatures.
There are many vegetable varieties that are resistant to root-knot nematodes and will produce a good crop even in the presence of nematodes. Effectiveness of this method is increased when combined with crop rotation. By alternating root-knot resistant and susceptible vegetables within a given portion of the garden from one year to the next, the overall nematode problem can be reduced by preventing a build-up of high populations.
Plant winter vegetables now, and be happy with a winter garden. RKN is inactive in low-temperature soil (60
Keep garden tools clean so as not to unwittingly spread insect eggs, fungi, weed seeds or other garden problems to another area. I always have a spray bottle of disinfectant (two parts alcohol and 1 part water) in my tool kit, and I’m careful about cleaning my pruners and lopers after use; I wasn’t as careful with my spade and shovel or walking from bed to bed.
I haven’t decided what to do, yet, but I may use a different remedy in each bed.
Cindy Bieger, Oracle Master Gardener
