The influence of cyst nematodes (Heterodera sacchari) and drought on waterrelations and growth of upland rice in Cote d'Ivoire

This study was undertaken to examine the influence of cyst nematodes (Heter odera sacchari) and drought stress, in isolation and combination, on the wa ter status and growth of rice in Cote d'Ivoire. Drought is considered one o f the main yield-limiting factors to upland rice in West Africa, while H. s acchari is emerging as a potentially serious pest. A field study conducted during 1997 at the West Africa Rice Development Association (WARDA) showed that under low water availability in sandy soil, damage to a nematode susce ptible Oryza sativa (cv IDSA6) is significant. Visual drought score and phy siological parameters were significantly affected (P less than or equal to 0.01) in plots with a mean population density of 39.4 H. sacchari eggs ml(- 1) soil at 81 days after sowing, compared to plots with a mean density of 1 .2 eggs ml(-1). Stomatal conductance, leaf water potential and relative wat er content were lower, leaf chlorophyll content was greater, and near total yield losses were observed in higher nematode density plots. In 1-l pots f ollowing 5 days of imposed drought, the H. sacchari-resistant and drought-r esistant O. glaberrima (cv CG14) showed a reduction in stomatal conductance of 73% and in leaf water potential by 0.96 MPa (54%). The effect of H. sac chari (473 eggs l(-)1 soil) inoculated as cysts to soil before sowing was s imilar but less severe. The combination of both stresses had similar but ye t more pronounced effects than the single stresses on stomatal conductance, leaf water potential and leaf dry weight in CG14. IDSA6 responded in a sim ilar manner to CG14, but less acutely to both stresses. Only the combined s tresses resulted in a significant (P less than or equal to 0.05) reduction in osmotic potential and root dry weight compared with the control, for eit her CG14 or IDSA6. It is suggested that H. sacchari increased the effects o f drought and drought-related losses. This may give a false impression of d rought susceptibility in field screening for cultivars, and complicate mode ls which predict crop damage caused by nematodes.