Biology of the pest and damage caused
H. carotae, first described in France in 1955 (Oudinet et al., 1962), is a pest of Daucus carota, certain other Apiaceae related to carrot and three species of Torilis (herbaceous plant of the Apiaceae family). This nematode species requires root exudates for second stage juveniles to hatch from the cysts and eggs to infest roots (Aubert, 1986) and no cultivated Apiaceae other than carrot causes hatching on such a large scale. Since H. carotae is very specific to carrot, the population density in plots can be reduced by a very long crop rotation, which is not the case for other nematodes such as Pratylenchus spp. and Meloidogyne spp.
Eggs usually hatch within 15 days after carrot germination. After hatching, the second stage juveniles (J2) (see 'Life cycle' below), which is the only mobile stage of sedentary female endoparasitic nematodes, penetrate the taproot and secondary roots to develop. After penetrating the tissue, they induce a feeding site, called a syncytium, which consists of a giant cell. The syncytium remains functional throughout the life of the nematode. The resulting injuries and cellular changes, especially functional ones, cause stunted growth, disrupt tuberisation and lead to deformed taproots. The carrot becomes unmarketable (stunted growth, forked carrots, adverse effect on skin smoothness...), with strong development of root hairs around the penetration area, which provide entry points for other soil pests and diseases. These symptoms are all the more severe in sandy, highly draining soils. Moreover, this nematode reduces the plant's absorption of water and nutrients. Symptoms on foliage (lower leaf volume, irregular foliage, yellowing, reddening) can be observed in the case of H. carotae, but without being specific to it (Villeneuve and Leteinturier, 1992a and b). The presence of the parasite in the field is often visible in small, circular patches. In the event of a heavy infestation, the whole plot may be affected.