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Disease Cycle and Epidemiology

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Epidemiology

Like all nematodes, lesion nematodes have six life stages -- egg, four juvenile stages, and the adult stage (Figure 11). The duration of the lesion nematode life cycle runs from 4-8 weeks, but this may be influenced by environmental conditions such asadequate temperature and moisture. After embryonic development within the egg to the first-stage juvenile (J1), the nematode molts to the second-stage juvenile (J2) and hatches from the egg. All juvenile and adult life stages of lesion nematodes are worm-shaped and motile, and all life stages (except the egg and J1) can infect plants. Adult males are numerous in some species and rare in others, and it is believed that lesion nematodes usually reproduce sexually (amphimixis) but can reproduce asexually (parthenogenesis). Although lesion nematodes can invade plant tubers, rhizomes, pods, and infrequently some aboveground plant structures, they are parasites of roots of all plant species attacked.

Figure 11

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Lesion nematodes penetrate plant roots completely and migrate throughout the root tissue, mainly the cortex, as they feed. They can penetrate anywhere along the roots, but they show some preference for the region near the start of the root hair zone (Figure 12). They penetrate the root epidermis either intra- or intercellularly, but once inside, they migrate intracellularly (Figure 13). The nematodes feed on cells within the root, usually until the cells lyse and cavities are formed, and then the nematodes move forward within the root to feed on healthy plant cells (Figure 14). There are no elaborate plant cell modifications induced by lesion nematodes for feeding as there are with many sedentary plant-parasitic nematodes. The migration of the nematode within the root is usually ahead of the developing zone of necrosis that culminates in a visible lesion. Lesion nematodes at any life stage (except the egg and J1) can move in and out of the root into soil, and the entire life cycle (egg to egg) can also occur within a root (Figure 15). The nematodes can multiply within roots to populations up to 1,000-3,000 nematodes/gram of root (Figure 13)!! They can overwinter in infested plant parts or in soil at any life stage, although fourth-stage juveniles seem to be the optimal survival stage.

Figure 14

Figure 15

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The wounds inflicted on plant roots and other belowground plant parts by lesion nematodes (Figures 3, 4, and 7) can serve as infection courts for pathogenic soil microbes, primarily fungi. This appears to be particularly true in disease complexes that involve lesion nematodes and wilt fungi such as Fusarium and >Verticillium. Complexes with Fusarium in which lesion nematodes are the initial invaders result in high populations of lesion nematodes, whereas the opposite timing seems to inhibit the accumulation of large populations of lesion nematodes (likely because they feed on living cells - i.e. they are "biotrophs"). The combination of P. penetrans and V. dahliae in potato induces a synergistic interaction that results in a disease syndrome termed "potato early dying". Neither pathogen induces severe damage at low populations, and reduction of nematode populations with nematicides has been reported to reduce the severity of the disease.

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Epidemiology
As with most plant-parasitic nematodes in soil, lesion nematodes do not usually migrate more than 1-2 meters from the root zone that they infect. In plantings where root grafts may occur (i.e. fruit trees), the nematodes may travel from plant to plant through roots. In most cases, the movement of lesion nematodes is defined as "contagious" -- small foci of infested areas gradually enlarge to encompass significant areas of disease (Figure 17). Areas of disease become more pronounced in adverse environmental conditions such as water and nutrient stress, or if secondary pathogens simultaneously infect the roots. The spread of these nematodes within fields is usually accelerated by the cultural practices of the grower, such as soil cultivation. Lesion nematodes can be introduced to noninfested sites by poorly sanitized farm equipment and contaminated planting stock (i.e. tubers, seedlings). Since many species of Pratylenchus are endemic to native vegetation in many locations, new planting sites may already be infested with the nematode prior to cultivation.

Figure 17

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