Pathogenicity of yam parasitic nemtodes in Uganda

Yam (Dioscorea spp.) is a native tuber crop in Uganda that is considered a good candidate for overcoming food deficiencies in the famine stricken country. Despite its glory, nematodes have been a major hindrance to increased and sustainable production of yams in Uganda. This study was therefore conducted to assess the pathogenicity of major parasitic nematodes to yams in Uganda. The specific objectives of the study were to: i) identify yam germplasm resistant/tolerant to the major yam nematodes in Uganda. ii) Establish the influence of Pratylenchus sudanensis and Meloidogyne spp. On yam production; iii) examine the nature of association between Pratylenchus sudanensis and Meloidogyne javanica and: iv) establish the variations in reproduction and host range of three different P. sudanensis isolates in yams in Uganda. Generally, the introduced yam lines were more damaged and infested (P≤ 0.05) by both Pratylenchus sudanensis and Meloidogyne spp. than the landraces. Kaama, a landrace, was highly infested with both nematode species but had minimal damage on the roots and tubers. A subsequent pot study confirmed that this yam species is tolerant to Pratylenchus sudanensis and Meloidogynes spp. attack. To study the influence of nematodes on yam production, several trials were conducted under pot and filed conditions. The effect of Meloidogyne spp. on yams was examined on three cultivars i.e., Dioscorea rotundata, Dioscorea cayenensis and Dioscorea alata. In both the pot and field trials, tuber weights od D.rotundata were significantly reduced at a minimum initial Meloidogyne spp.population (Pi) of 1,000 nematodes/plants. As part of this study also, D.rotundata tubers with different damage incidences (i.e., clean, moderate and severe galling) were stored for four consecutive months. On a monthly basis, tuber weights and densities of the different developmental stages (i.e., eggs, juveniles and females) of Meloidogyne spp. were assessed. For the first three months of storage, the eggs were the most abundant followed by juveniles while the females were the least isolated. Additionally, no females and juveniles were obtained from the tubers 3 months after storage. Further, the severity galled tubers lost more (P≤ 0.05) weight than the moderately galled tubers. Similarly, the moderately galled tubers lost significantly more weight than the clean tubers. Pot and field trials also revealed that the damage by P.sudanensis depended on the initial nematode population densities used but a threshold of 1,000 was required to crack the tubers in the field. The pathogenicity of P.sudanensis isolates collected form Rakai, Masaka and Jinja were tested on weeds commonly found growing in yam fields. The nematodes infested all the weed species except those of Androgogon gayanus, Bidens pilosa, Crassocephalum crepidiopes and Cyperus rotundus. However, there were no differences (P≤ 0.05) in reproduction rates of the 3 isolates on the different weed species tested. However, when the trial was repeated on carrot discs, the Jinja isolate exhibited a lower reproduction than the Rakai and Masaka isolates. It is therefore suspected that the Jinja isolate is a different biotype from the other two isolates. The study also established that P.sudanensis exacebates the reproduction and damage by Meloidogyne javanica on yams.