If your plants look stunted and its leaves exhibit chlorosis and lack vigour, yet you’ve done everything right and you can’t figure out why, it’s time to check the roots of your plants. If the roots are knobbly and have galls, you have a case of root knot nematodes in your soil. I have experienced this on a few occasions, fortunately in my planters and not in-ground. Read on to find out how to control root knot nematodes organically.
What are they? These plant parasites are microscopic roundworms that damage plant roots and feed on its vascular system. This causes growths to form, affecting the plant’s capacity to absorb water and nutrients.
Shrubs, trees and herbaceous plants are susceptible to root knot nematodes. These include begonia, azalea, hibiscus, gloxinia, hydrangea, impatiens, cyclamen, coleus, some cacti, rose, and edible plants like tomato, capsicum, chilli, eggplant, rosella, okra, cucumber, pumpkin, melons, Malabar spinach, passionfruit, banana, pineapple, sweet potato among others.
What are your natural options? Here’s how to get rid of root knot nematodes organically, you can consider using a mix of methods to improve efficacy rates.
African marigolds (Tagetes erecta) are popular garden flowers known for their bright colors and ability to repel certain harmful nematodes. However not all nematodes are deterred by marigolds. Some may actually thrive on the plants, forming galls causing wilting, stunting growth, and reducing flowering. As a gardener, it’s important to know how to identify and control damaging nematodes on marigolds. With a few preventative and control measures, you can have a vibrant, nematode-free flower bed.
What are Nematodes?
Nematodes are microscopic roundworms that live in soil and plant tissues. While some are beneficial, others parasitize plants. The most common plant-parasitic nematodes that affect marigolds are root-knot nematodes like Meloidogyne incognita. These burrow into roots, causing knotty galls that impair nutrient and water uptake. Other nematodes like Pratylenchus and Rotylenchulus species can also be problematic.
Nematode feeding damages the vascular tissue, providing entry points for secondary pathogens like fungi and bacteria Above ground, symptoms include wilting, stunting, yellowing, poor flowering, and dieback. Severe infestations can kill plants Careful inspection of roots reveals small galls or lesions on otherwise healthy-looking plants.
Prevent Nematodes in Your Garden
The old adage “an ounce of prevention is worth a pound of cure” certainly applies to nematode control. Here are some tips to keep your soil and marigolds nematode-free:
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Test your soil to determine if and which nematodes are present before planting. Send samples to your local cooperative extension office.
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Remove weeds regularly as they can harbor nematodes.
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Sterilize soil and tools with heat or solarization before planting.
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Add organic matter like compost which promotes beneficial microbes that antagonize nematodes.
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Plant nematode-resistant marigold varieties like ‘Crackerjack’, ‘Toreador’ and ‘Polynema’.
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Rotate marigold planting locations and avoid replanting in nematode-infested soil.
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Plant nematode-suppressing cover crops like marigolds, then till them into the soil before planting your garden.
Control Options for Existing Nematode Problems
If preventative measures have failed and your marigolds show nematode damage, take action quickly to save your plants and avoid spreading the infestation. Here are some effective control options:
Organic Treatments
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Apply nematode-destroying beneficial nematodes like Steinernema species.
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Water in applications of neem oil which reduces nematode motility.
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Mix diatomaceous earth into the top 6 inches of soil to mechanically damage nematodes.
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Solarize soil under clear plastic in summer to heat treat nematodes.
Chemical Treatments
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Apply targeted nematicides like fluopyram or abamectin.
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Fumigate soil with metam sodium prior to planting. Note fumigants require special PPE and training to apply.
Cultural Practices
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Rogue heavily infested plants and destroy rather than composting.
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Amend soil with composts containing black cumin, marigold, chrysanthemum or alfalfa.
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Allow soil to dry out between waterings to stress nematodes.
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Apply salt fertilizers like calcium nitrate to create a temporarily hostile environment.
Prevent Reinfestation
Getting rid of nematodes doesn’t ensure they won’t return. Use these tips to keep populations down after treatment:
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Rotate between marigold varieties, as nematodes can adapt to resistances.
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Leave vacant beds fallow for a season to interrupt nematode life cycles.
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Maintain proper plant spacing and prune to increase air circulation and dry out soil.
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Clean tools and equipment between use in different beds to avoid spreading nematodes.
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Monitor regularly for symptoms and send new soil tests annually.
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Time plantings so susceptible plants don’t follow others infected with the same nematodes.
The Bottom Line
Catching and treating nematode issues early is key to saving marigolds and stopping the spread. Combine preventative cultural practices with monitoring and targeted control measures for the most effective management. While eradication is difficult, following integrated pest management practices can keep nematodes from getting out of hand. Paying attention to proper site preparation, plant selection, and hygiene will go a long way in promoting vigorous, nematode-free marigolds.
Add compost and organic matter
Good quality aerobic compost contains plenty of microbes. These include larger, predatory micro-organisms, including beneficial nematodes that can ingest root knot nematodes. By applying it directly as a top dressing or some prefer, compost tea, these microbes are introduced into the soil. Adding organic matter such as manure and leaves also encourages fungi, bacteria and naturally occurring good nematodes to inhabit soil.
In Singapore, you can get good quality compost from Soil Social, it is more pricey than other composts on the market, however it is aerobic and contains a high amount of beneficial microbes. I tried it and have seen improvements in my plants as soon as within a month. My stunted okra grew more lush and began fruiting more regularly.
You can also consider trying out this soil drench recommended by Jerry Coleby-Williams of Gardening Australia.
This method makes it very inhospitable for root knot nematodes, however it kills many organisms in the process, including pathogens, nematodes, weeds, and seeds. Soil solarisation involves laying clear (more effective) or black plastic sheets over wet soil for 4-6 weeks, therefore trapping heat and raising soil temperature. Nematodes are mobile, so this method will reduce numbers but it may not fully eradicate the population.
Grow biofumigant crops for root knot nematode control (Photo by
Brassicas such as Brassica rapa (field mustard) and Brassica juncea (mustard greens) have bio-fumigation properties and are effective at root knot nematode control when grown as a cover crop, however there are some other plants, like Tagetes patula (French marigolds), Tagetes erecta (Mexican Marigold) and oats, which are highly capable too.
How it works is it releases compounds to suppress pests and pathogens in the soil. It is effective when in its early flowering stages, plants are chopped or pulped and incorporated into the soil and watered. The soil needs to remain moist for a few weeks, and isocyanate gases are released by the plants as it breaks down.
Not all plants are susceptible to root knot nematodes. Through rotating the types of plants grown over two years or more, specifically incorporating plants that are not host plants, you can starve these nematodes. Root knot nematode resistant plants include corn, millet, grain sorghum, asparagus, and chrysanthemum and zinnia.
Using Marigolds to combat root knot nematodes
FAQ
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Chemical name
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Trade name
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Formulation
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Thionazin
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Nemafos
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Granular or emulsifiable liquid
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Ethoprophos
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Mocap
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Granular or emulsifiable liquid
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Fenamiphos
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Nemacur
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Granular or emulsifiable liquid
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Fensulfothion
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Dasanit
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Granular
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