Morphological and physiological variability of species of Meloidogyne in West Africa and implications of their control

Abstract

The extreme morphological and physiological variability of certain rootknot nematodes (Meloidogyne spp.) and its implication on the development of control methods of these parasites, based upon crop rotations and the use of resistant varieties of otherwise susceptible crops are discussed.In a review of the systematics of the genus Meloidogyne reestablished in 1949 by Chitwood to replace the polyphagous species Heterodera marioni Cornu, 1887, the variability of the so-called perineal pattern of the females, the most important character to distinguish species within the genus, is emphasized.The results of the cytological investigations of Triantaphyllou are briefly discussed. It has been shown that most species studied are characterized by a parthenogenetic mode of reproduction. Two types of non-amphimictic reproduction have been observed: mitotic and meiotic parthenogenesis. Meloidogyne incognita, M. javanica and M. arenaria are characterized by mitotic parthenogenesis.An analysis of the number of publications appearing between 1949 and 1976. dealing with identified species of Meloidogyne has revealed that 93% of the articles concern Chitwood's species of 1949, and that 76% refer to M. incognito, M. javanica and M. arenaria.Faunistic studies in West Africa have shown that the most frequently encountered root-knot nematodes belong to the species M. incognita, M. javanica and M. arenaria. Identification of West African populations are complicated by the occurrence of mixtures of species, the great morphological variability and the existance of populations possessing rather large proportions of individuals exhibiting characters intermediate between species.Certain morphometric characters of perineal patterns (width of vulva and distance between phasmids) were measured in a number of clones of Meloidogyne populations. These observations have demonstrated that although these characters are stable within clones, they cannot distinguish M. javanica from M. incognita.The distance of the excretory pore of females from the anterior end, expressed in stylet lengths might possibly be an aid to distinguish M. incognita from M. arenaria and M. javanica.The author is reluctant to describe populations characterized by unusual perineal patterns as new species and suggests inclusion of host-plant and cytological data in the description of new species.Data presented indicate that length of juveniles could not be used to distinguish among M. incognita, M. javanica and M. arenaria.Observations of juveniles from a population of M. incognita acrita have demonstrated that the inflation of the rectum is not an absolute criterium to distinguish M. incognita and M. acrita. A critical evaluation of the data of TERENTEVA (1967) concerning the height of the lip region of males of M.incognita and M. incognita acrita makes the author reject the statement that this character might be useful to distinguish between these two taxa.The author concludes that up till now, no infallible methods have been found to identify naturally occurring populations of Meloidogyne in West Africa.After the splitting of the polyphagous Heterodera marioni into a number of species of Meloidogyne it became possible to assemble host lists for each species. In principal this information should provide a basis for crop rotation recommendations, intended to reduce root-knot nematodes infestation, provided that the Meloidogyne populations could be identified to the species level. Comparison of the host ranges of M. incognita, M. javanica and M. arenaria emphasized that these species have a great number of host plants in common, many of which are important crops. Only a few species could be used as differential plants to distinguish among these three species.Published data and original results presented show that when many populations of the same species of Meloidogyne are studied, resistance/susceptibility of a given plant species cannot be predicted with confidence. Certain populations are able to parasitize a given plant and others are not.The same phenomenon is observed with resistant varieties of otherwise susceptible crops. When several populations of the same species of Meloidogyne are tested against such a variety, those populations capable of parasitizing the resistant varieties are often called 'B races'. Studies concerning development of 'B races' on resistant tomatoes in Senegal have shown that some populations of Meloidogyne are able to parasitize resistant varieties strongly and immediately. others are able to form 'B races' after a selection has taken place, the majority however, is not capable of parasitizing resistant tomatoes.Indications have been obtained that a gene for gene relation exists between nematode populations and resistant tomato varieties and a code indicating nematode and resistant variety genotype is proposed.It is assumed that most plant species are attacked by Meloidogyne populations; the reaction of the plant attacked towards the specific Meloidogyne population concerned, determines if a successful parasitic relation develops.It is proposed to consider as a group the tropical polyphagous species of Meloidogyne, which are characterized by a certain degree of polyploidy and a mitotic parthenogenetic mode of reproduction. This group should comprise M. incognito, M. javanica, M. arenaria, populations intermediate between these species, possibly undescribed species and certain described species for which information on mode of reproduction and physiological characters (host-range) is lacking.On the basis of an actual field trial in Senegal, it is shown that recommendations for crop sequences or rotations should be based on testing different crops and varieties against naturally occurring populations of root-knot nematodes. Incorrect choice of crops may be made if rotations are based upon Meloidogyne identifications only. It is recommended that different populations in an area be tested on cash, food, and cover crops and resistant varieties.Selection of resistant varieties should be based on reactions to as many different populations of Meloidogyne as possible. Using such a technique 'B races' may be detected, and in addition much material not possessing root-knot resistance in the area where the varieties are developed, may be found to have resistance against other populations elsewhere.In order to obtain the maximum value of non-hosts and resistant varieties, it is recommended to use these plants as a preventive measure rather than as a cure. Therefore, their use should be recommended in slightly infested or Meloidogyne- free soils.The author has observed in fact, that non-hosts planted in heavily infested soil may be badly damaged because of a necrotic reaction to invading juveniles. In extreme cases this may give rise to dead patches in the field and a resultant increase of weed growth, accompanied by an increase of Meloidogyne on susceptible weeds. Thus, usefulness of the non-host can be nullified. Moreover the risk of selecting 'B races' is much higher in heavily infested land than in nonor slightly infested fields.Thus it is recommended to use non-hosts and resistant varieties as preventive treatments within an integrated control including the various chemical and physical treatments available