Plant Nematology Problems in Tropical Africa

COMMONWEALTH INSTITUTE OF HELMINTHOLOGY HELM INTHOLOGICAL ABSTRACTS Series B, Plant Nematology Vol. 45 December, 1976 No. 4 PIANT NEMATOLOGY PROBLEMS IN TROPICAL AFRICA by Donald P. TAYLOR Laboratoire de Nématologie O.R.S.T.O.M. B.P 1386 Dakar, Senegal Author’s Note. The intention of this review is to present the most significant information relating to plant nematology problems in tropical Africa. It was never intended to be a review of all the literature concerning plant-parasitic nematodes of this region. In general, taxonomic and faunistic studies have not been included because they seldom apply to the topic under consideration. Many subjective decisions were made as to whether or not to include a particular paper. I have attempted to include those which best applied to the subject. I apologize in advance for errors of omission. I also wish to express my appreciation to Dr. Michel Luc for reviewing this article and for his many constructive suggestions. For the sake of simplicity, tropical Africa will be considered as that portion of the continent south of the Sahara - admitting that certain areas, primarily because of the influence of altitude, do not fit the precise definition of tropical. With the great variation in total rainfall, rainfall patterns, temperature, soils and altitude, virtually all cultivated crops are produced in this vast region. Thus, the scope of this paper includes a wide array of food crops important as locally consumed dietary staples, export fruits and vegetables, and oil, fibre and other industrial crops. In recent years greater attention has been given to nematode problems in tropical Africa, but as was stated at a symposium on nematodes of tropical crops, knowledge of this subject lags behind that available in temperate climates because “nematologists and appropriate research laboratories are insufficient or non-existent in the humid tropics where plant nematodes and their damage are greatest” (Chiarappa, 1969). It is the purpose of this article to summarize briefly the present state of knowledge of economically important nematode problems in tropical Africa. Undoubtedly many additional problems will come to light as nematology research intensifies in the future; however, the examples and discussion presented below are sufficient to indicate that nematodes are frequently a limiting factor in crop production in many parts of tropical Africa. Other reviews on nematode problems in the tropics have approached the subject on a crop by crop basis (Smart & Perry, 1968; Peachey, 1969) but because many of the most important pathogenic nematodes in tropical Africa have extensive host ranges, I prefer to present the material according to nematode genera. 269 270 Helminthological Abstracts - Series B 1976 Vol. 45 No. 4 Melo idogy ne The Meloidogyne, or root-knot nematode, problem is beyond doubt THE most important nematode problem in tropical Africa. Although many other pathogenic or suspected pathogenic genera occur within the region, -none can approach the economic importance of species of Meloidogyize when the entire range of crops affected is considered. Luc (1968) stated that in the former French territories in tropical Africa, “the most widely distributed, and the most damaging nematodes are the different Meloidogyne”. A similar statement was made by Addoh (1971) concerning Meloidogyne in Ghana. Species of Meloidogyne occur in every country in tropical Africa; a genera- lization based upon the fact that root-knot nematodes have been found in all African countries in which they have been sought. M. incognita and M. javanica are the forms most frequently identified (Luc, 1968; Whitehead 1969a). An interesting aspect of the Meloidogyize problem in tropical Africa is the rapidity with which the problem becomes severe. From Kenya, Jackson (1962) has reported root-knot nematode damage to vegetables in the first crop planted on land not previously cultivated. Edwards (1953) reported complete failure of tomato in Ghana in the first season on freshly cleared land due to Meloi- dogyne. He traced the source of infestation to indigenous weeds that served as reservoirs of the nematodes. Whitehead (1969a) reported the occurrence of Meloidogyne in 37 of 51 samples from indigenous forests iq East Africa. Leiper (1939) had previously pointed out that in Africa, traditional farming methods had not seriously increased soil-borne parasite problems, but that the introduc- tion of the European system of intensive cropping led to an increase in plant parasite populations and to crop failures. The taxonomy of this genus is extremely diificult. Whitehead (1969a), for example, has recorded 9 species from tropical Africa, primarily East Africa: M. javanica, M. incognita, M. lzapla, M. decalineuta, M. africana, M. megadora, M. kikuyensis, M. ethiopica, and M. arenaria. In addition, M. acronea has been recorded in South Africa and Malawi (Coetzee, 1956; Bridge, et al., 19769. Notwithstanding arguments concerning the validity of some or these species, the point remains that the genus Meloidogyne, as represented in tropical Africa, consists of several morphologically distinct groups, that can be identified with soine certainty. More importantly, each taxon has a distinct host range which may or may not overlap those of other taxa. Thus, it is not at all unusual for a single plant species to be attacked by several species ofMeloidog.yne. In attempt- ing to reduce losses through cultural means, it is imperative to determine the host range of the population under consideration, even or especially if it consists of a mixture of species, which is frequently the case. Even if a monospecific population of MeloidogyEe can be identified, considerable physiological intra- specific variation has been reported in tropical Africa so that one cannot depend with confidence upon the results of published host range data. To illustrate this point I cite the occurrence of biotypes of Meloidogyne capable of breaking resistance in tomato (Netscher, 1970; Taylor, 1975) and the occurrence of biotypes of M. javanica capable of attacking strawberry, a “non-host’’ of this species (Martin, 1962a; Taylor & Netscher, 1975). Thus, prior to implementing a crop rotation to reduce root-knot nematode populations, it is always prudent to test the reactions of the proposed rotational crops to the local Meloidogyne population. From the practical point of view, one must ask whether or not we are much further ahead today in understanding the Meloidogyne problem than when this group of pathogens was referred to as “Heterodera marioni” with several recognized physiological races. Plant Nematology problem in tropical A frica 271 As is true for most nematodes, accurate crop loss figures attributable to Meloidogyne are difficult to obtain. On the basis of soil fumigation experi- ments in Kenya, Ngundo and Taylor (1974) concluded that Meloidogyne could cause up to a 60% yield reduction in green beans. There are also well known cases in which species of Meloidogyne have caused total economic loss, i.e. unmarketable root crops or crops so badly damaged that harvesting was un- economical. Luc (1968) cited an example from Seqegal where it was iE‘IpDssible to produce tobacco in two consecutive years because of the presence of Meloi- dogyiie. Following are short summaries defining the extent of the Meloidogyne problem on several important crops of tropical Africa. Tobacco. Luc (1968) has recorded “heavy” attacks of Meloidogyize on tobacco in Madagascar, Senegal and Ivory Coast. The most complete data on this crop comes from Rhodesia where Martin (1960) estimated that annual losses of tobacco due to Meloidogyne were El,OOO,OOO. On the basis of a survey in 1961-62, Daulton (1962) stated that Meloidogyize was responsible for the loss of 18-25 fnillion pounds (8-1 i ,000 tonnes) of cured tobacco leaf per year. In addition he reported that 84% of tobacco seedbeds were fumigated annually, primarily for root-knot control, and that 46.8% of the total 224,000-acre tobacco crop was fumigated annually. In the 1963-64 season, 56.4 % of the tobacco acreage was fumigated (Daulton, 1964). In Tanzania, tobacco losses were estimated to be 30 % or more due to the attacks of M. javaizica and M. incognita (Whitehead & Ledger, 1961). The complex Meloidogyne problem on tobacco in Madagascar has recently been reviewed (de Guiran, 1970). Vegetable Crops. Considered as a group, vegetables are among those plants most damaged by Meloidogyne in tropical Africa (Luc, 1968). Netscher (1970) maintained that Meloidogyne constituted the “main problem” for vegetable growers in Senegal and that members of this genus were commonly associated with tomato, eggplant, potato, cabbage, cauliflower, celery, red beet, leek, pepper, lettuce, parsley, carrot, okra, onion, cucumber, peas and beans. In South Africa, van der Linde (1942) considered that root-knot nematodes were the most serious pests of potato and other vegetables. At the eastern extreme of the region, Meloidogyne has been reported to cause severe damage to tomato, eggplant, carrot and red beet in the Seychelles (Mathias, 1971); and damage to beans in Kenya has already been mentioned (Ngundo & Taylor, 1974). From Ghana, Addoh (1968) reported yield reductions in tomato, potato, okra, yam and other vegetables when attacked by Meloidogyne. Many other examples could be cited to establish the point that a very serious Meloidogyne problem exists in vegetable production throughout tropical Africa, whether in established production fields or in fields recently cleared and put into production . Vegetables differ in their susceptibility to Meloidogyne. Some crops, such as onion, are usually only slightly susceptible; others, like cabbage and cauliflower, are considered moderately susceptible; whereas, such crops as tomato, eggplant and lettuce are highly susceptible (Netscher & Luc, 1974). In addition, Meloidogyne resistance has been bred into certain tomato lines that are adapted to conditions in tropical Africa. However, as previously indicated, resistance- breaking biotypes are known to occur in the region. Nevertheless, in Senegal a resistant tomato variety, Rossol, yielded as well in non-treated M.

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