Heterodera sacchari: field population dynamics and damage to upland rice in Cote d'Ivoire

In field studies in Cote d'Ivoire, West Africa, Heterodera sacchari severely reduced upland rice (Oryza sativa cv. IDSA6) grain yield. In 1995, pre-sowing treatments with carbofuran did not affect crop height, number of tillers at 84 days after sowing (DAS) nor grain yield of O. sativa cv. IDSA6 or O. glaberrima cv. CG14. The protection of eggs within cysts and a delayed hatching pattern are thought to have contributed to this. In 1996 and 1997, cultural methods were used to modify initial population densities and grain yield was correlated negatively with mid-season (56 and 84 DAS) H. sacchari population densities. The relationship between yield (Y) and population densities at 56 DAS was described by the equation Y = Ymean.Ymin + Ymean(1-Ymin).z((P-T)). The crop tolerance limit at 56 DAS was calculated as 1.67 and 2.39 eggs/cm(3) soil, and z as 0.15 and 0.65 in 1996 and 1997, respectively. Population densities of H. sacchari at sowing, 28 DAS, 42 DAS and harvest were not correlated with grain yield in regression analysis of individual plots, but grain yield was significantly greater in groups of plots which had lowest initial egg density. Initial egg population densities were correlated negatively with leaf area index at 84 and 96 DAS in 1996 and 1997, respectively, with implications for growth of and competition from weeds. In 1997, mean initial egg density was 63% more and mean yield 57% less than in 1996. Drought in 1997 exacerbated yield losses. In 1996 and 1997, crop height, number of tillers and relative leaf chlorophyll content were significantly less in plots with higher mean egg density at sowing.