Spiral Nematodes

Nematodes can be a threat to turfgrass, and have been found in golf courses, parks, and lawns in the San Francisco Bay Area. Spiral nematodes (Helicotylenchus spp.) are one of many nematode species that are found in the soil. Spiral nematodes have a wide host range, and do not cause excessive damage, but they can reduce the vigor of plants in a monoculture host like turf grass. Mature spiral nematodes are up to 1.2 mm long, and are so named because they curl up into a spiral when dead or relaxed. They are ectoparasites, meaning they feed outside the cell, although some may act as partial endoparasites—inducing growth of a food cell in which they bury their head to feed for long periods of time, before moving on to create a new food cell. These food cells do not grow like a gall, but the cell composition does change.

Spiral nematodes are all females, reproduce without mating, and lay individual eggs in the soil (unlike the root rot nematode that lays an egg mass). The first molt occurs within the egg (J1). If moisture is present in the soil when the egg hatches (J2), the nematode is motile and works its way to a plant root where it can feed. The nematode undergoes three more molts (J3 and J4), before becoming a mature adult. Populations are highest in late summer and early fall and lowest in spring when cold temperatures inhibit reproduction.

Spiral nematode turf damage is associated with bluegrass and Bermuda grass. Above ground symptoms include stunting, reduced plant vigor, yellowing or brown patches in the turf, and areas that succumb to weeds. There is typically a “hot spot” or dead zone, which is heavily infested. These symptoms can have other causes, so soil analysis is required to diagnose the problem: http://youtu.be/taW-VM-4IRw

For management tips, see the Integrated Pest Management(IPM) site. Prevention is the best approach, but strategies like crop rotation are difficult for turf. Chemical treatments exist, but some of them cause damage to turf, or must be repeated, since they also kill beneficial organisms that keep other diseases at bay. See UC IPM Pest Management Guidelines: Turfgrass (page 72) for additional information.

Root Rot Nematodes

Root-rot nematodes are from the genus Meloidogyne, and thrive in hot climates with short winters. They are not host-specific, and infect the roots of a wide host range, including flowers and vegetable crops (tomato, green pepper, corn, grains, and so forth). They are sedentary endoparasites—living and feeding from within their hosts.

Females lay their eggs in a gelatinous mass on a root gall surface; ensuring a protective, moist environment in which the eggs develop. The first molt occurs within the egg (J1 stage). After the eggs hatch, the second-stage juveniles (J2) are motile. They leave the egg; penetrate the root with their stylets, just behind the root cap; and secrete a liquid that stimulates growth hormones in the cells, causing them to divide and swell. This forms root galls that become permanent feeding sites. After molting through two more juvenile stages, (J3 and J4), reproductively mature females lay a new batch of eggs. This lifecycle occurs over the course of several weeks, so, in the right environmental conditions, plants can become infected very quickly. If the galls become separated from the roots, the eggs can exist in the soil for years, until the right conditions and a suitable host are available.

Galls on carrot from root-rot nematode. Photo by Claudia Nischwitz.
Copyright, ASPnet.

Above ground, plants exhibit symptoms that are similar to any disease that harms the root system, including stunted growth, wilting, and yellowed leaves. Reduced crop yield also results. To help with diagnosis, it is necessary to have the soil analyzed and to check the roots for galls, if at all possible.

For management tips, see the Integrated Pest Management (IPM) site. Since the eggs can remain in the soil for years, management is difficult. The solution is prevention – using resistant plant varieties; rotating crops so the nematodes cannot continue their lifecycles from season to season; leaving soil fallow periodically; amending the soil to keep it healthy; planting when nematodes are less active; and keeping infected plants or material from contaminating uninfected areas. Soil solarization, the process of heating the top 8 - 12 inches of the soil, can temporarily reduce the nematode population. Currently no chemical or soil fumigants are available to the home gardener.

Plant Diseases - Nematodes

This time I’m researching nematodes, which are non-segmented round worms found in soil, water, and extreme environments such as deserts and ice fields. A few can be seen with the naked eye, but most are tiny and require magnification. For resources I’m using Essential Plant Pathology, information from my plant diseases course at Merritt College in Fall 2012, and Five Kingdoms. Keep in mind that most nematodes are beneficial – they aerate the soil, consume detritus, feed on bacteria and other small life forms, and circulate organic and mineral soil components. Others cause considerable damage to crops, turf, and plants in the landscape. Nematodes can be found in the San Francisco Bay Area.

Nematodes are categorized as Eukarya in the Animalia kingdom. About 10% of all nematodes are plant parasites, and require a living host (obligate parasites). They are aquatic and typically live, feed, and move in the soil. They use a specialized feeding structure—called a stylet—to penetrate plant tissue, probe, secrete proteins and polysaccharides, and ingest cellular contents. Most nematodes are migratory and move from cell to cell to feed (ectoparasites). Others are sedentary and remain embedded in a plant throughout their lifecycles (endoparasites); some endoparasites induce formation of specialized feeding sites, such as galls, where they live and feed. A few do not fit either of these categories.

The female nematode lays hundreds of hardy eggs (some species do not require male fertilization of the eggs). The eggs can exist in soil for years or decades, until the right environmental conditions exist for them to infect a plant. Their life cycle includes four juvenile stages. Nematodes molt, shedding the outside cuticle and part of the lining of the stylet, from stage to stage as they grow. Juveniles look like miniature adults through all stages (they do not undergo a metamorphosis from stage to stage).

Take a look at several nematodes; of special interest (1) views of the stylet at 57, 107, and 145 seconds, (2) a root rot nematode at 159 seconds, and (3) a needle nematode at 202 seconds: http://youtu.be/Qf8oGjqyXFc