Response of yam landraces and hybrid lines to nematode attack in Uganda

Yam (Dioscorea spp.) is a major food crop in Africa and in Uganda, it is considered a famine-researve food crop. However, the crop faces a complex of constraints, especially susceptibility to nematodes and limited research attention. In an attempt to increase the productivity of this crop in Uganda, the National Agricultural Research Organisation (NARO) in collaboration with the International Institute of Tropical Agriculture (IITA), East and Southern Africa Research Centre (ESARC) based at Namuloge have launched a breeding program at Sendusu farm, Namulonge. As part of the collaborative study, this study was initiated with the following objectives: a) to establish the nematode species profile in yams in Uganda; b) to develop an early screening technique for nematode resistance evaluation and c) to screen yam lines and cultivators for resistance to the major yam parasitic nematodes in Uganda. The first part of the study involved a survey of the major yam growing regions of the country to establish the nematode species profile. During the survey, the common yam cultivators in east and central Uganda were sampled as plant roots, tiubers and soil surrounding the selected tubers. Nematode indentification and counting was done under a compound microscope and the data for all the trials were analysised using the Statistical Analysis Systems (SAS). The common nematode species observed in the survey included Pratylenchus sudanensis, Meloidogyne javanica and Helicotylenchus dihystera with P.sudanensis being the most dominant nematode species. P.sudanensis isa new record of a nematode species attacking yams in Uganda (possibly in East Africa). Te survey also showed inter specific competition between P.sudanensis and M.javanica. Root knot galling on tubers was mainly attributed to M. javanica whereas cracks were highly correlated with P. sudanensis. Infestation during field screening under natural nematode infestation revealed M.javanica as the dominant nematode species followed by a mixture of Pratylenchus pratanensis and Pratylenchus coffeae. Also Rotylenchulus spp. And Scutellonema brachyurum were encountered but in very low numbers. The different lines responded differently to nematode attack suggesting that cultivators with sources of nematode resistance could be present. Seven hybrid lines and 5 local cultivators were subsequently artificially inoculated for screening. These plantlets were raised form tissue culture. Minisetts weighing between 60-70g were pre-sprouted and then planted in perforated 8 litre plastic pots filled with sterilized soil. The pots were arranged in a complete randomized design with 2 replications with each replicatencomprising of the control, and P.sudanensis and M.javanica inoculated plants. The plants were inoculated one month after planting. The inoculate were extracted form tubers collected during the survey. Since M.javanica density was low, it was first multiplied on 2 months old tomato plant roots. P.sudanensis was abundant and was applied without multiplication. Two months after inoculation, the plants were harvested and plant growth parameters, i.e., root length, root weight, tuber weight and dry shoot weight were recorded. Also, nematode damage indices of galls and cracks; and nematode counts were taken. Findings revealed no significant difference (P<0.05) in plant growth parameters. All the lines were susceptible to both nematode species but P. sudanansis exhibited higher densities and damage levels compared to M.javanica both the roots and tubers were found infested. This implies that when handling a large germplasm in screening for resistance to nematodes, only the roots may be taken since nematode extraction form tubers is tideous and takes long time.