La culture du Gombo (Abelmoschus spp.), legume-fruit tropical (avec reference speciale a la Cote d'Ivoire)

The data for this study have been gathered in Ivory Coast during the period 1977-1980.The first purpose of the study was a description of the traditional okra cultivation and, in particular, the evaluation of the local planting material in order to determine its potential for crop improvement.The second purpose of the programme was a study of the agronomic limitations in the case of intensive cultivation and in what ways such problems might be solved.The inventories of vegetables offered for sale in 18 important regional markets (cf. chapter 2) proved that okra is one of the most important fruit vegetables in Ivory Coast. The young fruits are sold either fresh or dried. Especially in the western part of the country okra is also an important leaf vegetable.The indigenous vegetables are traditionally grown in the fields of foodcrops. From the inventory of a large number of these fields in different parts of the country, it appeared that vegetable growing is particularly important in yam and groundnut fields. Yam and groundnut are usually grown at the beginning and at the end of the rotation respectively. In whatever region or type of field, okra is one of the most common plants in this mixed cropping.The popularity of this vegetable is even more clearly shown in the bibliographical data on vegetable consumption in Ivory Coast.Contrary to the ample bibliographical information on okra cultivation in the subtropics, much less is published on its cultivation in the tropics, especially in the (sub)equatorial zones. General recommendations for intensive cultivation in Ivory Coast have been embodied by SODEFEL ("Société de Développement de la Production des Fruits et Légumes").In our experimental programme, we have been able to study in detail the local planting material (chapter 4), the dry matter production and its distribution (chapter 5), ecological influences (chapter 6), the mineral nutrition (chapter 7), cultural practices (chapter 8), pests and diseases (chapter 9) and breeding and selection (chapter 10).Based on a number of 314 local okra lines collected by the author, our experiments show that the local planting material in Ivory Coast is made up by two taxa which appear to be very distinct as to their morphological, cytogenetical and agronomic characteristics.The morphology of the taxon, called "SOUDANAIS", corresponds with existing botanical descriptions of Abelmoschus esculentus (L.) Moench. Its chromosome number (2n = 130-144) lies well between the extremes reported for this species.The taxon, called "GUINEEN", has morphological characteristics intermediate between Abelmoschus esculentus (L.) Moench and A. Manihot (L.) Medikus. Its chromosome number (2n = 184-200), the highest observed in the genus, supports the hypothesis that it might be an amphidiploid of these two species.The crossing behaviour of the "GUINEEN" type in crosses with the supposed parental species resembles the behaviour of the artificial amphidiploid of Abelmoschus e s culentus and A. Manihot , realized in Japan.Interspecific hybridization between the "SOUDANAIS", type and the "GUINEEN" type succeeds well in both directions. The hybrids are vigorous, but have a very reduced fertility. However, we obtained considerable amounts of seed on these hybrids under conditions of open pollination, probably due to backcrossing (pollination by insects with the fertile pollen of the parental species).In the traditional vegetable growing of the forest zone, the "ordinary" okra "SOUDANAIS", type) has been almost completely superseded by the "GUINEEN", type.The growth analysis (cf. chapter 5) indicated that the first month can be decisive for the success of the crop. The relative growth rate (RGR) in general decreases with the age of the plants and retardation in initial growth may hardly be caught up. The fragility of the young plantlets makes them very sensitive for modifications in cultural practices.The high yield of young fruits and seed (second experiment) of the "GUINEEN", line as compared to the three cultivars/lines of the "SOUDANAIS", type, points to the capacity of the first-mentioned type to maintain longer a considerable leaf area during a longer period. The long duration of the generative period of the "GUINEEN"-type may be very interesting for home gardens where the objective is precisely a production spread over a long period.No significant differences in dry matter production have been observed between crops grown for the young fruits and those grown for seed, although dry matter production tends to be higher in the vegetable crop than in the seed crop in cultivars with a long generative period ("GUINEEN" type).In the crop grown for seed a greater proportion of the total dry matter is channeled to the generative plant parts. From this it can be deduced that, for use as a vegetable, the young fruits should be harvested as late as possible, thus as late as permitted by the quality requirements of the product.The differences between the cultivars in the ratio vegetative parts/generative parts were mainly due to the differences in the duration of the vegetative and the generative period. Early flowering and late senescence are important selection objectives.The distribution in roots, leaves and stems of the dry matter present in the vegetative plant parts, was independent of the cultivar and of the purpose of the crop (young fruits or seed). Differences between cultivars were observed rather within the main organs of the plant, for instance within the leaves between blade and petiole, and within the ripe capsules between seed and fruitwall.From the study of ecological influences (cf. chapter 6), it appeared that in rain- conditioned cultivation the most favourable period for sowing of the "SOUDANAIS" type lies directly at the beginning of both the long and the short rainy season (march and august). The less favourable results gained from other sowing dates mainly seem to be due to a less favourable rainfall distribution (excess or shortage of water) and to a higher incidence of pests and diseases. The second rainy season, however, is more capricious than the first one, and in some years, sowing in august risks suffering from insufficient rainfall.The number of the leaf-axil bearing the first flower-bud is independent of the growth rate of the plant, but positively correlated with the duration of the photoperiod and/or with the temperature. In South Ivory Coast, the reaction on the photoperiod finds expression in a quantitative way, as none of the "SOUDANAIS" lines of our collection appeared to have a critical photoperiod shorter than 12½ hours.On an average, the "GUINEEN", type has shorter critical photoperiods than the "SOUDANAIS", type and sowing at the beginning of the long rainy season in South Ivory Coast, may result in vegetative periods of up to 8-9 months. Such a long cropping period explains why its geographical distribution is limited to the most humid parts of west Africa.Three greenhouse experiments on the photoperiodical reaction were executed in Wageningen.It appeared that flower initiation in the three subtropical cultivars 'Clemson Spineless', 'Pusa Sawani' and 'Long Green' is little influenced by photoperiods ranging from 10 to 18 hours, but these cultivars show a quantitative reaction for anthesis. The delay of the flowering date under long photoperiods is caused by flower-bud abortion and slower growth of the persistent buds.However, 'Perkins Long Pod' Ivoirien, the cultivar recommended in Ivory Coast, has a critical photoperiod between 13½ and 14 hours for flower initiation and between 13 hours and 13 hours 15 minutes for anthesis and fruit setting. Below the critical photoperiods this cultivar reacts quantitatively. Already at photoperiods longer than 12 to 12½ hours, both anthesis and fruit setting become negatively affected in a serious way. Although this cultivar does rather well in the South, it seems risky to recommend it for the entire territory of Ivory Coast.In accordance with its flowering behaviour under natural conditions, a critical photoperiod between 12 hours and 12 hours 40 minutes was observed in the greenhouse for Adiopodoumé la, a line of the "GUINEEN", type.A shade experiment showed that a reduction of the radiation with 50% during the first three weeks of the cropping period reduces yield. 25% shade during the rest of the cropping period also tends to reduce yield, but this could not be shown statistically.The uptake of the macro-elements N, P, K, Ca and Mg (cf. chapter 7) was investigated by means of chemical analysis of plants from the second experiment of the growth analysis. The evaluation of the uptake by two cultivars grown for the young fruits as well as for the seed, did not reveal any selective absorption of any element as a consequence of either the type of cultivar or the crop-purpose. The differences in the uptake could be explained by the differences in total dry matter production.The mineral content of the total cumulative dry matter is high in young plants, decreases with age, but gets more or less stabilized from the age of 11 weeks. In the present study, the total amount of dry matter produced during the entire cropping period contained 1.9% nitrogen, 0.2% phosphorus, 1.2% potassium, 1.6% calcium and 0.7% magnesium on average.The course of the mineral requirements indicates that a basal dressing before sowing of those elements that are leached easily such as nitrogen and potassium, is little effective, because the uptake of elements is negligible during the first month of the cropping period. Even after the first month the duration of the cropping period justifies fractionated fertilizer applications which, however, can be restricted to the vegetative period in the case of cultivars with a relatively short generative period "SOUDANAIS" type).In the case of cultivars with a long generative period ("GUINEEN", type), grown for seed, the decline of dry matter production as soon as the fruits start to mature, thus about one month after anthesis, suggests that a final fertilizer application at the beginning of the generative period should equally be sufficient. Several applications after anthesis seem to be justified in the case that the need for mineral elements continues to be considerable during the generative period, as was observed for the "GUINEEN", line, Djiroutou l, grown for the young fruits. This line produced 7.4 t/ha of dry matter, containing 133 kg nitrogen, 14 kg phosphorus, 80 kg potassium, 114 kg calcium and 49 kg magnesium. More than half of these quantities were taken up during the generative period.Compared to the rest of the plant, the useful product, viz. the young edible fruits or the seeds, is rich in nitrogen, phosphorus and potassium and poor in calcium and magnesium. Export of nitrogen, phosphorus and potassium can easily reach 50% of the total needs of the crop.In South Ivory Coast, soil temperatures are often supra-optimal, and they have, directly or indirectly, a negative effect on germination and initial growth.The experiments (cf. chapter 8) showed the beneficial effect on initial growth of cultural practices which bring down the soil temperature: mulching, watering before the hottest part of the day, sowing on that side of the ridges which is least exposed to direct radiation of the sunlight.Okra is known for its capricious germination and emergence. The best attention during initial growth can be realized on the nursery bed on account of the small acreage under care. Sowing on a nursery bed followed by transplantation, however, is hardly practised, due to the fact that okra-plantlets older than two weeks almost do not tolerate transplanting without a ball of soil. This has been clearly demonstrated in a transplanting experiment.According to references in literature, optimal plant density for the modern commercial cultivars is about 150 000 plants/ha. It is generally assumed that the optimal density for the late and robust local cultivars is much lower.In an experiment with densities from 20 000 to 80 000 plants/ha and ratios of the interrow and in the row spacings varying from 1 : 1 to 6 : 1, the best spacing as to yield and easiness of harvesting appeared to be 100 x 17 cm (60 000 plants/ ha) for the cultivar 'Perkins Long Pod' Ivoirien, recommended in Ivory Coast.The most important fungal diseases observed in our experiments in South Ivory Coast (cf. chapter 9) were damping-off ( Macrophomina phaseoli (Maubl.) Ashby), vascular wilt ( Fusarium oxysporum Schlecht) and Cercospora-blight ( Cercosporaabelmoschi Ell . & Everh.). Oidiumabelmoschi Thüm. is important especially in the northern half of the country.Insect damage was mainly due to the cricket Brachytrupes membranaceus (Drury), to flea beetles of the genus Podagrica , to the bollworms Earias biplaga Wlk. and Pectinophora gossypiella (Saund.) and to the beetle Anomaladenuda Arrow.Okra Mosaic Virus (OMV), transmitted by Podagrica , is widespread but the damage is much less important than the damage due to Leaf Curl, transmitted by Bemisia tabaci (Genn.).Nematodes of the genus Meloidogyne constitute a major problem in okra cultivation. The fact that this pest did not appear in our experimental fields might be due to the predominance of Panicum maximum Jacq., an inferior hostplant, during the fallow periods between successive okra crops.The routine phytosanitary treatments applied in the experiments (quintozene against damping-off; maneb + carbatene against leaf fungi; HCH against field- and mole-crickets; carbaryl + parathion against other insects) generally allowed good yields in the experiments.From a varietal screening it appeared that the "GUINEEN" type of okra is much more tolerant to pests and diseases than the "SOUDANAIS" type which explains why in the traditional cultivation (without phytosanitary measures) in the forest zone, the former is preferred as planting material.Selection work (cf. chapter 10), especially in the United States and in India within the species Abelmoschus e sculentus, has produced very attractive cultivars such as 'Clemson Spineless' and 'Pusa Sawani' which are well suited to intensive cultivation. The use of these cultivars in Ivory Coast, however, should be discouraged as long as no intensive phytosanitary control can be excercised, because they are very susceptible to local pests and diseases.Our preliminary observations on the resistance or tolerance potential of the local planting material of Abelmoschus e sculentus ("SOUDANAIS" type) were disappointing.On the other hand, the tolerance of the "GUINEEN", type of okra to pests and diseases is impressive. This species is, as a consequence of its adaptation to extensive cultivation, in general later flowering, more vigorously developed and with a longer cropping period than the "SOUDANAIS", type, but it presents sufficient variation to prompt to selection directed towards desirable characteristics for intensive cultivation.After the first stage of straightforward selection of attractive lines present in the local planting material, such as we have been starting, a further breeding programme might comprise ultimately line selection in hybrid populations, with a view to combine the desirable characteristics of a 'Clemson Spineless' with the tolerance to pests and diseases of the local planting material.