Studies on the interaction between root-knot nematodes and the bacterial wilt pathogen in potato production systems in Kenya

Abstract

A survey of bacterial wilt and root knot nematodes associated with potato was carried out in three potato producing districts of Kenya namely Nyeri, Nyandarua, and Meru. Sampling was done in 30 randomly selected. farms in each district. The incidence of bacterial wilt was highest in Nyeri district (18.8%), intermediate in Meru (167%) and lowest in Nyandarua (10.4%). The disease was observed at altitudes between 1825 and 2695 M. Isolates obtained from infected potato plants were found to be predominantly Ralstonia solanacearum race 3 biovar II. Root-knot nematodes population tMeloidogyne spp.) was highest (45.2) in 200cm3 soil samples collected from Nyeri and Nyandarua and lowest in Meru (30.7). Correlations between bacterial wilt incidence and the population density of Meloidogyne spp. were significant (r = 0.26, P = 0.05) and positive indicating that the two organisms interact. Greenhouse tests for reaction of potato cultivars to Meloidogyne spp. revealed that none of the cultivars screened was immune. Significant (P = 0.05) variations, however, existed among the cultivars both in root galling and nematode reproduction. The highest nematode populations were recorded on cultivars KP93739.26, B53, Desiree CIP- 800048, KP92733.26 and Kerrs Pink. Plant performance depended on inoculum density. Galling and egg mass indices also increased with increase in inoculum density. Differences in shoot weight between nematode-treated and the nematode-free plants were insignificant (P = 0.05). Reduction in shoot weight ranged between 2.5 - 25.2 %. Root weight and final population of Meloidogyne spp. increased with inoculum density. Field and greenhouse tests revealed that none of the fifteen cultivars was resistant to R. solanacearum. Cu1tivars Kenya Dharnana, Mauritius clone (89016) and Cruza CIP- 720118 were rated as tolerant, while the other cultivars were considered moderately susceptible to susceptible. The relationship between infection by root-knot nematodes and severity of bacterial wilt of potato cultivars Asante CIP 3813 81.20 (susceptible), B53 (moderately resistant), and Kenya Dhamana (resistant), was investigated in a greenhouse experiment. Plants grown in soil infested with both pathogens had more wilt than those grown in soil infested only with R. solanacearum.