Northern Rootknot Nematode Woes

The northern rootknot nematode (Meloidogyne hapla) was more prevalent in many vegetable and berry crops grown in Ontario this past season than in previous years. Last year's wet and cool growing season followed by this year's hot summer were ideal for this microscopic worm-like pest to invade and multiply in roots of many host crops. In fact, the optimum soil temperature for reproduction of the northern rootknot nematode is 20-25°C. Unfortunately, these nematodes remain active in soil as long as the temperatures are above 10°C and are not destroyed by our cold Ontario winters.

The northern rootknot nematode has an extensive list of suitable hosts, including strawberries, cane-berries, tomatoes, potatoes, celery, carrots, melons, vinifera grapes, sugar beets, peas and many other legumes, to name a few. Dandelion, thistles and numerous other weeds are also hosts.

In the absence of host roots, the northern rootknot nematode survives as eggs and young nematodes, called juveniles, in soil and leftover infested root debris. When soil conditions become favourable, the eggs hatch and the young female nematodes migrate to the roots of host plants. Young nematodes may travel over a meter in search of host roots, although most live in the top 60 cm of the soil. Once they reach the preferred host roots, the young female nematodes penetrate just behind actively growing root tips using their hypodermic needle-like stylet mouthpart. They then burrow deeper into the root, to the water and nutrient conducting vascular tissue, and develop to the adult stage. Once established at a feeding site in the root, the adult female rootknot nematode remains sedentary for the rest of its life. Males, on the other hand, will move out into and through the soil and along roots searching for mates; males are not known to feed or cause much damage.

Female rootknot nematodes stimulate root cells to enlarge and become galls which act as specialized sinks that supply nutrients to the nematodes as they feed. These enlarged cells may appear as individual 1-mm galls linked like a string of beads or, as more and more nematodes establish feeding sites, they may join together to become one larger root knot, as much as 2.5 cm in some hosts (Figure 1). The size and shape of the knots depends on the host crop and the number of females feeding. Often the roots of some crops, such as strawberries, will produce many small secondary roots around the feeding site giving a "hairy root" appearance (Figure 2). Regardless of the symptom, rootknot nematodes disrupt the transport of water and nutrients from the roots to the shoots resulting in reduced plant growth and yield. Plants that have a slight infestation often do not show any above-ground symptoms. However, severely infected plants can appear stunted, unthrifty and will frequently wilt during the heat of the day. The leaves of heavily-infested tomato plants may appear purplish on the undersides of leaves, resembling symptoms of phosphorus deficiency. Infested carrot roots may be forked near the end of the taproot. In addition to the disorders caused by the root knot nematode alone, several studies have shown this nematode can enhance the severity of diseases caused by other pathogenic organisms or reduce nodulation on legume crops.

Figure 1. Individual knots on strawberry roots caused by the Northern root knot nematode

Figure 2. Root Knot nematode symptoms on strawberry roots appear as small secondary roots produced around the feeding site of female nematodes creating a "hairy root"

So, what can you do if you have a northern rootknot nematode problem? This will depend on the soil type, crop to be grown and soil population levels. The nematode tends to be more prevalent in muck and sandy soils than in clay. The soil population threshold for northern rootknot nematode varies from "zero tolerance" for carrots to 1000 per kg of soil for most other crops including berry crops.

Preventing the introduction of rootknot nematodes into fields by planting healthy transplants is the first step all growers should take. Always inspect transplants for root swellings and tiny knots and never plant seedlings that appear to have symptoms of rootknot damage on the roots. Once this pest becomes established in a field, it becomes more expensive to manage and eliminate completely. Unfortunately, since this nematode has a long list of hosts, crop rotation is often not a good option unless the grower is prepared to rotate to grasses, cereals or corn, which are not hosts. Long periods of fallow have proven to reduce population levels but this is not always an economically viable solution either. Soil fumigation provides rapid and effective control, and certain nematode-suppressing Brassica cover crops have shown promise in reducing populations below threshold levels. In one study, researchers found that planting carrots in the early spring when soil temperatures were around 6 to 8°C resulted in significantly less root knots and increased marketable yields by 20 to 50% compared with planting later when soil temperatures had warmed up to 15°C.

Growers who suspect or have observed northern rootknot nematode damage to their crop should have their soil tested this September or early next spring and take appropriate action, particularly if populations exceed threshold levels. Otherwise, northern rootknot nematodes may become a serious problem next year, particularly if the trend for warm summers continues.

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