Response of cassava to nematicide application and plant-parasitic nematode infection in East Africa, with emphasis on root knot nematodes

Abstract

Cassava (Manihot esculenta Crantz( is attacked by numerous species of plant-parasitic nematodes, among which, Meloidogyne spp. are the most damaging. However, data on the level of damage occurring under field and farm conditions are largely lacking. To examine the impact, upon cassava production, of plant-parasitic nematodes, field experiments were conducted at four sites in East Africa in Uganda (Serere and Sendusu( and Kenya (Mtwapa and Msabaha( during 2002 and 2003. We focused on Meloidogyne spp. (mainly M. incognita and M. javanica(, attacking two popular cultivars in each country. Nematicide (Nemacur® [fenamiphos]( was used against nematodes in control plots. Meloidogyne spp. were the most common nematodes encountered and caused galling damage at harvest at all sites, with Scutellonema spp. and Pratylenchus spp. being the next most commonly observed. The highest root densities of Meloidogyne spp. were recorded at Serere, while Mtwapa had the highest galling damage and intensity. At harvest, nematicide-treated plots had significantly lower densities of the most commonly occurring nematodes and fewer galls on feeder roots (galling intensity(. However, the root galling index score (damage( was not significantly lower compared with untreated plots. The majority of crop growth and yield variables measured at harvest across experiments were unaffected by nematicide application, but fewer storage roots suffered rot. The overall number of variables was minimized using Principal Components Analysis, which identified three major groups. A bivariate plot of the two larger groups across experiments showed that galling intensity, Pratylenchus spp. and Meloidogyne spp. density were negatively correlated with cassava yield (tonnes/ha( and marketable storage root number. Linear regression analysis showed that Meloidogyne spp. density at harvest was negatively associated with yield (tonnes/ha( for cv. Migyera and cv. SS4 in Uganda, but not for cv. Mtwapa1 or cv. Kibendameno (which was positively correlated( at Kenya sites. A strong negative correlation between marketable tuber number and Meloidogyne spp. for cv. Migyera was observed in Uganda, but not for cv. SS4. Meloidogyne spp. density and total nematode species density were each negatively correlated with non-marketable storage root number on both cultivars in Uganda, but not for either cultivar in Kenya.