Management of Ditylenchus africanus (Nematoda : Tylenchidae) on groundnut (Arachis hypogaea)

Abstract

MANAGEMENT OF DITYLENCHUS AFRICANUS (NEMATODA: TYLENCHIDAE) ON GROUNDNUT (ARACHIS HYPOGAEA) Groundnut production has decreased in South Africa over the past seven years in spite of an increase in grain yield potential of the crop. This decrease in production, associated with a decrease in area planted to the crop, is ascribed to poor kernel quality that renders production of groundnut uneconomical. Ditylenchus africanus has a major effect of groundnut downgrading and is suspected to be the primary cause of the decline in national groundnut production. The objective of the study was to find an economically feasible management strategy for the nematode on groundnut. In order to evaluate and develop control strategies for D. africanus, relationships between nematode population densities and damage symptoms with groundnut downgrading disorders and calculation of income, as stipulated by the local agricultural products standards act, was established. This relationship was used throughout the study to determine the economical feasibility of control strategies in field or near field conditions, viz. microplots. Nematicides applied at prescribed rates on groundnut gave inconsistent results in terms of D. africanus population reduction and yield improvement. Nematicide efficacy was higher under irrigation than rain-fed conditions. Inconsistent efficacy of systemic nematicides under dry land production conditions lead to the conclusion that groundnut production under marginal conditions is uneconomical, irrespective of nematode control. Production of the crop under marginal conditions should be discouraged because of the great economic risk. Spread of D. africanus may be enhanced as a result of abstinence from additional nematode control inputs under such conditions. Neither reduced nor increased dosage rates of systemic nematicides justified concomitant reductions or increases in cost of application. Integration of different planting dates of groundnut and systemic nematicide application is not a feasible management option for D. africanus. Nematicide efficacy and variation in environmental conditions are too unpredictable to serve as a basis for planning nematode control measures and optimum groundnut yield. Tolerance levels of the cultivar Kwarts are not adequate to be included in a management strategy for D. africanus populations.

Integration of chemical control with differential nematode susceptibility of groundnut cultivars that are currently available is also not economically feasible. Application of ethylene dibromide at planting at reduced rates of 10 to 15 //ha proved to be effective in D. africanus management and groundnut yield improvement. The reduced costs of these application rates and the increase in income obtained by these applications provide an economically feasible alternative management strategy to groundnut producers. Adoption of a strategy such as this may serve to increase national groundnut production and restore areas lost to groundnut production over the past few years.