NEMATOLOGICAL ENVIRONMENT AND SELECTION PRESSURE ON SIX SASRI RESEARCH FARMS
Current estimates indicate that the loss in yield of sugarcane from root-feeding nematodes amounts to approximately R250 million per year. Tolerant varieties provide an inexpensive and long-lasting option to reduce crop loss from nematodes. Currently, genotypes are selected on the SASRI research farms without direct attention being given to their tolerance or susceptibility to nematodes. Not much is known about the nematode communities that exist on the research farms and whether these nematode communities exert sufficient selection pressure on the genotypes in the selection programme. The objectives of this work were (i) to characterise the plant breeding selection sites according to their nematode communities, and (ii) to measure the effect that nematodes have on the yield of varieties at plant breeding selection sites and thus identify the selection pressure for nematode tolerance. To achieve this, the 2nd stage single lines at six research farms were surveyed for nematode populations. Results showed that Empangeni, Gingindlovu and Kearsney had similar nematode profiles, whereas other research farms exhibited differences in nematode communities. For example, Bruyns Hill and Glenside had higher numbers of Helicotylenchus, whereas Pongola had higher numbers of Hemicycliophora and Rotylenchulus. The common variety at all sites, NCo376, exhibited a similar nematode community to other sugarcane clones at a particular site, but a substantially different community between sites. Yield data from single line trials showed that nematode communities influenced selection at four of the six sites. Two variety x nematicide trials were planted to assess the loss due to nematodes. At Empangeni, the yield responses (which reflect crop loss) in the plant crop ranged from 10 to 27%. At Gingindlovu, the responses in the plant crop ranged from 44 to 52%.
Les nématodes parasites des racines associés Au riz en Basse-Casamance (Sénégal) et en Gambie
Au cours d’une enquete SUT les Nematodes radicicoles parasites du riz au Senegal et en Gambie, 82 echantillons ont ete etudies, dont 6’7 en Basse-Casamance (Senegal) et 15 en Gambie. Les trois types de rizieres existant en Basse-Casamance ont presente entre eux des differences sensibles tant dans les peuplements que dans les populations de Nematodes parasites du riz. Les rizieres de mangrove ainsi que certaines rizieres moyennes en sont pratiquement exemptes, probablement a cause des phenomenes de sulfatoreduction dont elles sont le siege. Les rizieres moyennes hebergent un petit nombre d’especes a la fois frequentes et abondantes alors qu’elles sont sensiblement plus nombreuses dans les rizieres hautes. En Basse-Casamance, les especes frequentes et abondantes sont : Tylenchorhynchus mashoodi, Hirschmanniella spinicaudata, Criconemoides palustris, Xiphinema bergeri, Heterodera spp. Pratylenchus SP., Paratylenchus sp. et Trichodorus spp. alors qu’en Gambie une seule repond a ces criteres : Hirschmanniella oryzae.
A Perception of Tolerance
In this speculative essay, tolerance to nematodes is perceived as a defensive process that has evolved by selection pressure from abiotic factors and not from nematodes themselves. It is suggested that nematodes do not cause peculiarly nematodal stresses in plants hence the plant only responds to the consequences of their disruptive activities. While most native plants are probably tolerant and susceptible to the nematodes with which they are associated, many agricultural plants have lost this capacity during selection for yield and quality. It is further suggested that plants that are tolerant to particular stresses caused by abiotic factors may also be tolerant to nematodes which cause the same stresses. In natural ecosystems it is possible that tolerance has enabled a lower level of resistance to be evolved hence in breeding plants for improved yields maximising tolerance and minimising resistance may be a useful strategy. Tolerance is perceived as a function of the physiology of the whole plant in which there are many contributing factors.