Plant-parasitic nematodes and yellowing of leaves associated with black pepper plants in Vietnam

Thirty-five nematode species belonging to 19 genera were extracted from 432 soil and root samples collected in 19 districts in six provinces in the North Central Coast, Central Highlands and Phu Quoc Island. Based on our study, the predominant nematode species associated with black pepper plants in Vietnam is Meloidogyne incognita: almost every soil and root sample examined during our study was infected with this root-knot nematode species. No difference in frequency of occurrence of M. incognita among the three agro-ecological regions surveyed was observed. However, the root population density of M. incognita in black pepper plants was on average about five times higher in the North Central Coast and Central Highlands than in Phu Quoc Island. Large differences in root population density of M. incognita on black pepper plants were observed in different districts within the same agro-ecological region. Black pepper roots infected with M. incognita showed the same type of galls as described in the literature. The percentage of root galling averaged about 40% in all the three agro-ecological regions. The percentage of black pepper plants with yellow leaves was on average about 20–25% in the North Central Coast and Central Highlands, which was somewhat higher compared to Phu Quoc Island (16.9%). Large differences in percentage of black pepper plants with yellow leaves were observed in different districts within the same agro-ecological region. The same type of yellow leaves as described in the literature for black pepper plants infected with Meloidogyne species was observed in our study. In general, a positive relationship between the soil and root population densities of M. incognita on black pepper plants and percentage of plants with yellow leaves was observed. However, in one district (Quang Tri) a relatively low nematode population density was associated with a high percentage of plants with yellow leaves. Five nematode species were recorded for the first time on black pepper plants in Vietnam. Our study reconfirms previous reports that Radopholus similis does not occur in Vietnam.