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Pathogen Biology

Radopholus similis (Nematoda, Tylenchida, Pratylenchidae) was first observed by Nathan A. Cobb in necrotic banana roots from Fiji in 1891. The species is divided into at least two races, a banana race able to parasitize all hosts except citrus and a citrus race that feeds on citrus and other hosts, including banana. In 1984 the citrus race was proposed as a separate species, R. citrophilus. This was rejected when the ability of the two races to mate and produce offspring was established in 1997 and a molecular analysis of their genomes in 2000 determined they were not distinct species. Three other Radopholus species with a limited distribution and host range are: R. musicola on bananas in Australia, R. citri on citrus in Indonesia, and R. bridgei on turmeric, also in Indonesia.

The burrowing nematode is an obligate parasite and needs a living host to survive, though its various stages can move from root to soil and vice versa. It is classified as a migratory endoparasite, completing its life cycle as it tunnels through the root cortex (Figure 14). Females and juvenile stages are infective but males with their weak stylets do not feed. Radopholus similis usually penetrates roots near the tip, but can invade along the entire length of the root. They move between cells of the root cortex, feeding on them until the cells collapse and form necrotic passages. In most hosts, R. similis does not damage the central cylinder (Figures 3, 5), though the citrus race reportedly feeds on the phloem, girdling and destroying the stele. Migration and egg-laying are stimulated by nutritional factors: females need healthy tissue to feed on, but eggs are laid in root tissue that quickly decomposes. Females usually reproduce sexually, but can also reproduce without males. This phenomenon was once considered parthenogenesis, but recent studies suggest it might be hermaphrodism. Females lay 4 to 5 eggs per day (2 per day in citrus) for several weeks as they move through the root cortex.

Figure 14

Figure 3

Figure 5

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