Cycles et développement des Meloidogyne
Authors: Ritter M. 
Published: April 2008

Sommaire

Le cycle des Meloidogyne est bien connu depuis longtemps, il ne diffère de celui des Heterodera que par le nombre habituellement beaucoup plus élevé de générations annuelles et une polyphagie qui permet à beaucoup d'espèces de se multiplier sur des plantes très diverses. L'apparition des cellules géantes nourricières suscitée par la présence des larves dans le système radiculaire s'accompagne généralement de déformations et de galles plus ou moins caractéristiques selon l'espèce de Meloidogyne et le végétal en cause. Les générations se succèdent partiellement dans ces galles.

Le taux de multiplication varie aussi selon les espèces et selon l'hôte; la durée du cycle dépend surtout de la température. En fait, chacune des séquences qui le compose (éclosion, mobilité, invasion et croissance des larves) possède un optimum thermique propre qui peut être très different chez une même espèce. Chez de nombreux Meloidogyne le sex ratio est influencé par l'hôte et les conditions du milieu. Le déterminisme du sexe apparaît donc généralement épigamique et souvent les males ne jouent aucun rôle dans la reproduction qui peut rester parthénogénétique en dépit de leur nombre. Malgré l'abondance de la littérature mondiale consacrée aux Meloidogyne, on ne possède que des informations très fragmentaires sur les particularités de leur développement et de leur cycle.

Resume

Life‐cycles and Development of Meloidogyne Species

The developmental stages of Meloidogyne, already well described many years ago, are very similar to those of Heterodera, although the number of generations per year is often far higher and the host‐range wider. Nevertheless, slight differences in the description of specimens, supposed to belong to one species, indicated the desirability of seeking more precise morphological characteristics in order to revise the taxonomy of the genus.

The larvae, which are already in the second stage when hatching, develop within the galls or outside, where they fix themselves rapidly in rootlets or older roots, and sometimes even in the root neck or stalk, of the host. Their présence provokes the formation of giant cells and galls. During the fourth, and last, moult the males become filiform, but the females are pear‐shaped. M. incognita (Kofoid et White) Chitwood and M. javanica (Treub) Chitwood were found to always reproduce parthenogenetically, even when males are numerous; other species, such as M. naasi Franklin, are amphimictic or, as in M. hapla Chitwood, have facultative parthenogenesis. The laying of up to 2882 eggs has been recorded, but the average number is between 300 and 500, depending on the species and the host‐plant.

The host‐plant, apart from being or not being susceptible, can influence the speed of development and fecundity, while temperature can act on both the host and the nematode, such as in certain tomatoes which loose their resistance above 36°C.

As far as Meloidogyne itself is concerned temperature has a major influence on all stages. Intermittent cold and hot spells favour egg‐hatching in M. naasi, but inhibit it in M. hapla. Second stage larvae die when ice forms within the plants, but they are rather tolerant of external frost; in this respect, M. incognita is more susceptible than M. hapla which shows a 16% survival after an exposure to — 15°C for 10 minutes.

Many data have been published about the thermic influence on the duration of the endoparasitic phase, from penetration into the host to the appearance of the mature female, but the figures given are not consistent, and need further consideration. Recent work showed that while the development of M. naasi at 26°C took 39 to 51 days it was as short as 21 days in M. graminis (Sledge et Golden) Whitehead at 27°C.

Under unfavourable conditions, L2 larvae which would normally have developed into females can develop into males. Temperature, level of inoculum and manuring of the host‐plant are apt to change the sex ratio, such as in M. graminis, where at 16 and 21°C all larvae develop into females, but at 27°C, 4% males appear and 80% at 32°C. Since males do not nourish themselves and do not provoke gall‐formation, changes in the sex ratio can evidently determine the pathogenecity of populations, but the reasons behind these changes, as well as the role of the males, still have to be clarified.

Detailed studies on the biology of each species are needed, but it should be remembered that they often occur together and that some might escape attention when outnumbered by those whose ecological requirements are better met. Thus, M. hapla, which had never been recorded on the Côte d'Azur in France, was frequently observed when tomatoes resistant to the other species were used.

In order to arrive at comparable results, techniques for breeding on seedlings under aseptic conditions must be developed, so that the optimal conditions for each species and each population can be determined. Moreover, close contacts between research people must ensure that they handle completely identical material.