In order to understand the difference between root nodules, that are caused by nitrogen fixing bacteria and root-knot disease, caused by nematodes, one must first have a clear understanding of what each is.
Root Nodules Caused by Nitrogen Fixing Bacteria
Nitrogen-fixing bacteria are able to transform atmospheric nitrogen (a form the plants cannot absorb) into fixed nitrogen (a form plants can absorb). There are 2 main kinds of nitrogen fixing bacteria 1. nonsymbiotic and 2. symbiotic. I am going to focus here exclusively on symbiotic. According to Oxford dictionary symbiotic involves the “interaction between two different organisms living in close physical association”. In this case the interaction is between specific soil bacteria, (namely Rhizobium, Frankia and Azospirillum species) and specific host plant roots. Rhizobium bacteria are predominantly associated with members of the pea family (Fabaceae), with one exception being Parasponia of Cannabaceae family. The Fabaceae family includes nearly 20,000 species of trees, shrubs, vines, and herbs such as beans, peas, soybean, clover, alfalfa, indigo, Kentucky coffee tree, golden chain tree, pagoda tree, locust, redbud, smoke tree, lupin, peanuts, mimosa and many more. Frankia alni bacteria are associated with actinorhizal plants such beech, ash, walnut, birch, roses, pumpkin, squash, watermelon, cucumbers and many more. Certain Azospirillum species are associated with cereal grasses (Britannica, T. Editors of Encyclopaedia (2020))
I am going to focus on Rhizobium bacteria for it is the most common and it has been studied and researched the most, but the concepts here can be applied to Frankia alni bacteria and also to certain Azospirillum species. Rhizobium bacteria normally live in the soil and are quite capable of living independent of plants, and the plants that are capable of associating with them are quite capable of living without the Rhizobia, as long as there is enough nitrogen in the soil to meet the plant’s needs. The interaction between Rhizobium bacteria and compatible plants (like legumes) comes about when there is insufficient nitrogen in the soil. When this occurs, the plants release flavonoids to attract the Rhizobia and in response the Rhizobia release nodulation factor. Nodulation factors are signaling molecules that cause the plant to modify it’s root hairs to allow the Rhizobia to form an infection thread and enter the plants roots. Once inside the root cells they begin to divide rapidly, forming a nodule. Within these nodules the Rhizobia are able to sequester atmospheric nitrogen and convert it into ammonia (the form plants can take up). In exchange the plant provides the bacteria with sugar. In this symbiotic relationship both plant and bacteria benefit, however maintaining nodules is resource intensive for the plant and as such it controls how many nodules form.
Root Nodules Appearance
Root nodules take on many shapes and sizes depending on bacteria and plant species. Nodule size can vary from 1-10mm. Healthy nodules, that are actively fixing nitrogen are pink on the inside. They turn green when they are no longer fixing nitrogen and may be discarded by the plant. When nodules appear white or grey on the inside it means they have not yet started fixing nitrogen. The nodules appear on the root hairs or on the side of the root and are easily detached. Plants that have these nodules typically appear healthy even robust.
Root-Knot Nematode Disease
By contrast, root-knot nematode disease is caused by microscopic worm like, parasitic nematodes of the Meloidogyne species. These nematodes live in the soil of warmer climates or ones with short winters. These nematodes burrow into the roots of some 2,0000 species of susceptible plants to feed on the plant’s sugars, causing abnormal swellings on the roots. The appearance of the galls varies according to species but “generally, fast growing annuals will have a large flesh gall and woody perennials, small hard galls” (Texas A&M AgriLife Extension, (n.d.)). These swellings, unlike the nodules discussed above, occupy the entire circumference of the root rather than on the side of the root. The individual galls can also get larger measuring from 3mm-50mm. They can also fuse together causing large sections of the root to be swollen. Other root symptoms may include a darkening of the roots, rotting, stunting and lack fine feeder roots or a proliferation of lateral roots on root vegetables (“hairy roots”). Above ground symptoms may include chlorotic or pale green leaves, stunting, wilting (especially during the heat of the day), reduced yield, reduced resistance to disease and nutrient deficiencies. Root-knot nematodes can even attack legumes and interfere with root nodulation and nitrogen fixation.
Some susceptible plants include carrots, peas, beans, bell peppers, tomatoes, grape vines, roses, morning glory, zinnia, petunia, hollyhock, shasta daisy and many more. To view a more extensive list visit Rating of Ornamental Plants to Root Knot Nematodes by Texas A&M AgriLife Extension Service.
Life Cycle of Meloidogyne Root Knot Nematode
Root-knot nematodes do not become active in the soil until the soil temperature reaches 15 C. Eggs that have over wintered in the soil hatch and second stage larvae penetrate the root tip area (this step take about 5days). The nematode then releases an enzyme that causes the root cells to enlarge in that area. The female nematodes remain in this same location feeding and molting for about 20 days at which time they begin laying eggs.
Root nodules caused by nitrogen fixing bacteria differ from the root knots (galls) formed by root-knot nematodes in the following ways:
- Root nodules are caused by nitrogen fixing bacteria namely Rhizobium, Frankia and Azospirillum species. Whereas root-knots are caused from microscopic worm like nematodes namely Meloidogyne species.
- Root nodules and the nitrogen fixing bacteria benefit the plant by fixing nitrogen. Root-knot nematodes do not benefit their host, they are a parasite and can weaken, even kill a plant.
- Host plants with a healthy symbiotic relationship with root node forming bacteria typically appear healthy and robust. Plants whose roots have been infected by root-knot nematodes appear distressed, stunted and chlorotic.
- Root nodes are typically smaller and appear on the root hairs or on the side of the root and are easily detached. Root-knots occupy the entire circumference of the root and can fuse together causing large sections of the root to be swollen.
- The roots of plants with root nodes are healthy and typically white in colour. The roots of plants infected with root-knots appear darken, rotting, stunted and may lack fine feeder roots or have a proliferation of lateral roots such as on root vegetables (“hairy roots”).
- Root nodes are pink in the center or may be green. Root-knot galls are brownish.
- The plant roots with root nodes may be turned back into the soil in the fall to nourish future plantings. The roots of root-knot must be removed and destroyed.
- You can continue to plant susceptible plants in soil containing nitrogen fixing bacteria. You cannot plant susceptible plants in soil containing root-knot nematodes.
Photo Credits: all photos by the author.
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