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Disease Cycle and Epidemiology

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The life cycle of R. similis in banana roots, from egg to egg, occurs in 20 to 25 days at 24 to 32°C (75 to 90°F). Eggs are reported to hatch in 8 to 10 days with juvenile stages completed between 10 and 13 days. As with all nematode species, there are four juvenile stages: the first stage (J1) develops within the egg, then molts and emerges as a J2 nematode. In citrus, the life cycle is 18 to 20 days with egg hatch reported between 2 and 3 days.

Since the beginning of the 1800s, the wide distribution of R. similis in tropical and subtropical regions has followed the transfer of infected plant material between countries. Banana is vegetatively propagated and infected suckers and rhizomes continue to be the main method of nematode distribution within and between fields. When roots rot, nematodes either move out into the soil and actively invade nearby healthy roots, or continue feeding into the rhizome (banana), corm (taro), or lower stem (anthurium) of their hosts. Though populations in the soil are usually low, burrowing nematodes can be passively transported in soil water or infested soil clinging to roots, tools, machinery, footwear, et cetera.

Radopholus similis is short-lived in the soil, six months or less. As with all biotrophic organisms, it depends on living host plants for survival (Figure 15). These plants include the current crop, volunteers and weeds. In India, for example, nematode populations in black pepper plantations are sustained by the practice of interplanting with alternative hosts, such as coconut, banana, turmeric and ginger. In the South Pacific, taro, banana and coconut are host to R. similis (Figure 16).

Figure 15

Figure 16

Root rot initiated by burrowing nematode is often increased by the invasion of other soilborne microorganisms. For instance, the fungi Cylindrocarpon musae and Rhizoctonia can increase damage to banana roots infected by R. similis. It is also known to interact with weakly pathogenic isolates of another fungus, Fusarium solani, increasing root rot and death of black pepper seedlings.

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