IPMP3.0, Oregon State University, Copyright 2000





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Pin Nematodes (Paratylenchus spp.) are the smallest plant parasitic nematodes that attack plants. Adult females found in mint average 0.4 mm (0.016 inch). Another pin nematode (Gracilacus spp.) is occasionally found in mint but only differs from Paratylenchus in that it has a much longer stylet. Pin nematodes rarely cause damage to plants unless present in very high numbers. However, mint is one of the best hosts for pin nematodes, and populations frequently exceed 100,000/quart soil. Few species of pin nematodes have been studied in detail on any crop, so the information in this manual is based on information collected from several species and from several crops. Pin nematodes recovered from mint have been labeled as several different species over the years, but current experts on nematode identification have not been able to match the pin nematodes from Oregon to any described species. Until a species name is confirmed, it is only appropriate to label these pin nematodes as Paratylenchus spp. The "spp." indicates that there may be more than one species of unknown identification.

Life Cycle

Pin nematodes are migratory ectoparasites (Fig. 1) which have four juvenile stages. An alternate form of the fourth stage juvenile, often called the "preadult" stage, is a unique resistant or survival stage in which the stylet is reduced and nonfunctional or absent and therefore does not feed (Rhodes and Linford, 1961a). This survival stage allows the population to survive temperature and moisture extremes and long periods without food (Eck, 1970, Rhoades and Linford 1961a). However, the preadults molt rapidly and begin feeding when placed near a plant root, perhaps due to some chemical compound released from the root (Eck, 1970, Rhoades and Linford, 1959). This resistant stage may not form when conditions are ideal for feeding and reproduction (Eck, 1970). Female pin nematodes have been observed to lay an average of 2-3 eggs/day when feeding, but few eggs are laid while females are not feeding (Eck, 1970, Wood, 1973). Eggs hatch in seven-eight days, and egg to egg generation time for P. projectus on ryegrass was 36-38 days at 28-20 C (Wood, 1973).

Feeding Behavior

Once eggs hatch, the juveniles migrate towards the growing root tip as it advances through the soil (Eck, 1970). Pin nematodes easily detect and move towards plant roots, with the majority of inoculated nematodes observed around the root within 30 minutes after introduction (Eck, 1970). Lateral roots appear to be preferred to main roots, perhaps since they are younger and more easily penetrated by this nematode's small stylet. While pin nematodes can be found along any part of the root, they congregate around the early maturation zone, which is the primary feeding area. All feeding stages of pin nematodes feed by inserting the stylet into epidermal cells, often near the base of root hairs (Eck, 1970) and sometimes on the root hairs (Rhoades and Linford, 1959). Often feeding occurs in the depression between two epidermal cells. Feeding stops just before and during molting. Puncturing the epidermal cell walls with the stylet requires 5-20 minutes, consisting of periods of stylet thrusting and periods of rest. From 10-50% of the stylet is inserted into the plant cell. After the stylet enters the cell, there is a period of inactivity for 30-60 minutes before the nematode begins feeding. Changes within the plant cell (accumulation of granular material around stylet tip) begin during this period, presumably due to the release of enzymes into the cell from the nematode. As in most plant-parasitic nematodes, the stylet of a pin nematodes is a hollow tube resembling a hypodermic needle. Feeding is accomplished as rapid (200/minute) pumping by the muscular median bulb withdraws cellular contents through the stylet and esophagus into the intestine. Individuals may feed in the same location for several days without slowing their feeding rate. When the nematode is finished feeding, it removes its stylet and migrates to the new position of the growing root tip to establish a new feeding location. Cells are only fed upon by one nematode at a time, and nematodes do not attack a cell that has been fed upon earlier (Eck, 1970).

Pin nematodes have been observed to feed as endoparasites, entering the exit points of lateral roots (Rhoades and Linford, 1961b), but they do not create their own entrance point as with typical endoparasites. For this reason, pin nematodes may occasionally be reported in mint root samples as well as in soil samples. However, a very small percentage of the total pin nematode population will be recovered from roots, and this generally will not alter a management decision based on results from a soil sample.

Symptoms and Effects on Plant Growth

Pin nematodes may be found in high numbers on many different plants (Eck, 1970) but cause significant damage to only a few crops which are sensitive to feeding by these small parasites. Effects of feeding by pin nematodes on the appearance, structure or function of mint roots has not been studied. In other crop plants, symptoms range from no noticeable pathology at the feeding sites (Rhodes and Linford, 1961b) to shallow localized lesions (Raski and Radewald, 1958). Rate of root growth (root tip elongation) and lateral root development may be reduced or terminated by the prolonged feeding of many individuals on the same root. These changes result in a reduced epidermal surface area, which is important for the absorptive efficiency of the root. This may reduce the nutrients, and thus energy, which can be stored into the rhizome, potentially reducing the health and vigor of future offshoots. This deleterious effect may become progressively evident over several growing seasons (Eck, 1970).

Plant weights of mint may be reduced and flowering delayed by pin nematodes in the Yakima Valley (P. hamatus). Dry weights of potted plants 22 weeks after inoculation with 8,000 pin nematodes/quart soil were reduced 34%, 26% and 20% for peppermint, native and scotch spearmint, respectively. Lesser reductions occurred with lower initial inoculum levels. Infected plants also remain more succulent longer which may make them more susceptible to winter injury (Faulkner, 1964). Thus, pin nematodes may be more damaging to mint in areas with harder winters. In tall fescue, root systems may increase in plants attacked by pin nematodes but even though tillering is increased, top growth is stunted (Coursen and Jenkins, 1958). Tops of celery attacked by pin nematodes can become stunted with extreme chlorosis (yellowing). The root system declines which is followed by a decline in the nematode population (Lowensbery et al 1952). Pin nematodes also cause a decline in carnation (Jenkins and Taylor, 1956), and figs infected by pin nematodes become chlorotic with undersized fruit (Thorne and Allen, 1950). Effects of pin nematodes on other selected crops are reviewed by Merrifield (1990).