Nematode survey in Busia and Bungoma districts and the response of three tobacco varieties to infection by meloidogyne spp. on clay and sandy soils under field and pot plant studies
Ogallo, Leopold Juma
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Plant-parasitic nematodes in the tobacco-growing areas of Busia and Bungoma districts were identified, and the response of three commercial tobacco varieties to increasing initial population densities of rootknot nematodes, Meloidogyne spp. on clay and sandy loam soils were investigated during 1983. Nematode survey was conducted over an area of approximately 4,000 km2, during which some 238 pre-plant soil samples were collected on smallholders and the BAT (K) Company experimental tobacco plots. Nematodes were extracted from the soil samples by the Baermann’s pan and filter method and then identified with the aid of various manuals. Rootknot nematodes, Meloidogyne spp., were found to be widely distributed in the area, whereby the frequency of detection on the 238 pre-plant soil samples was 61%. However, on some 60 tobacco plots which were sampled further at midseason and postharvest for both soil and root samples, the Meloidogyne spp. did not occur in all, including 10 plots in which they were not detected at preplant soil sampling. Identification of Myloidogyne species was by the perineal pattern technique, for which some 100 mature females which were extracted from galled tobacco roots collected randonmly over the area of study, were examined. Meloidogyne javanica (Treub). Chitwood was predominant, with an occurrence frequqncy of 85%. The other 15% of the specimens wee M. incognita (Kofoid and White). The composition ratio was hence 6:1 for M. javanica and M. incognita, respectively. About 6 genera of the non-galling plant-parastic nematodes were commonly found in soil around tobacco roots. The nematodes were assumed to be variably parasitic on tobacco roots. They were, in descending order of abundance; root-lesion nematodes, Pratylenchus sp., stubbey-root nematodes, Tylenchorhynchus sp., spiral nematodes, Scutellonema sp., Helicotylenchus sp., and Rotylenchus sp., and sheath nematodes. Hemicycliophora sp. The population densities of either individual or combined genera of the non-galling plant-parasitic nematodes at the pre-plant, mid-season and post-harvest soil and tobacco root samplings were always too low to be of economic importance. At post-harvest sampling, they constituted only 15% of nematodes. Meloidogyne spp. alone made up the remainng 85%. Three commercial tobacco varieties; air-cured “Burley 181”, fire-cured “Heavy Western” and flud-cured “Speight G28”, were tested against Meloidogyne javanica and incognita concomitantly (6:1), at increasing initial densities on clay and sandy loam soils on field and pot plants. There were 60 field plots, which had natural nematode infestations with initial inoculum densities (Pi) ranging from 5.9 to 10.2 g of soil. The soils of pot plants were artificially infested at five levels of Pi = 0, 5, 10, 20 and 40 nematodes/100 g of soil. Each inoculum level for every tobacco variety was replicated four times. Tobacco damage by the rootknot nematodes were determined by the galling of roots, stunting and leaf yield reductions. The rootknot nematodes caused significant (P = 0.05) damage on tobacco both in the field and pot plants. Greatest damage was among plants grown on sadny or gravel soils where stunting and leaf yield reductions were 50% or more. Plants in clay loam soils tolerated nematode infections considerably at the Pi’s tested, as they rarely showed significant (P = 0.05) stunting inspite the fact that the roots were moderately galled at post-harvest.