Occurrence and molecular characterization of root-knot nematodes on tunnel-grown cucumber (Cucumis sativus) in Balochistan province of Pakistan

Abstract

Cucumber (Cucumis sativus) is mostly used as a fresh vegetable around the globe. Root-knot disease caused by Meloidogyne spp. is a serious menace in cucumber production worldwide. In Balochistan province of Pakistan, cucumber is mostly grown under plastic tunnels, and the pilot survey indicated extensive symptoms of root-knot disease on the roots of tunnel-grown cucumber. However, there is no information that which root-knot nematode species are associated with tunnel grown cucumber in Balochistan. Therefore, the present survey was conducted to document the prevalence, severity, and incidence of root-knot nematodes (RKNs) in cucumber grown under plastic tunnels in the districts of Pishin and Quetta. The study showed differences in prevalence, severity, and incidence in different districts. The average prevalence was recorded at 52.05 % in both districts, with the highest occurrence in the Pishin district (57.5 %). The disease severity was measured in terms of the galling index, the average disease severity observed in Pishin district was 5.71, while in Quetta, the average disease severity was 4.73. Similarly, the disease incidence in Pishin and Quetta districts was 66 % and 59 %, respectively. Morphological and molecular identification confirmed that the cucumber associated nematodes belong to Meloidogyne incognita and M. javanica species, while morphometric characterization showed limited resolution for species differentiation. The DNA was isolated from the female nematodes and subjected to polymerase chain reaction (PCR) through molecular markers COX1 and 18S rRNA. The PCR products for 9 isolates were sequenced. Based on morphometric data, perineal patterns and molecular identification, out of 9 isolates, 5 were characterized as M. javanica, and 4 were characterized as M. incognita. However, maximum likelihood based phylogenetic trees from both COX1 and 18S rDNA genes could not separate the M. javanica and M. incognita isolates into different clades and showed lower specific discrimination. This is the first report of the association of these root-knot nematode species from the tunnel grown cucumber in Balochistan.