Assessment of the identity, distribution and control options for seed- and leaf-gall nematodes in grass in South Africa

Abstract

The presence of seed- and leaf-gall nematodes (SLGN; Subanguina spp.) in seed of Eragrostis spp. grass is currently a matter of great economic concern to the local grass seed industry. The adverse effect of these parasites on export as well as local markets since 2001 necessitated i) a survey in the Eragrostis spp. production areas of South Africa and ii) an investigation of control strategies for this parasite. Eragrostis spp. seed samples were obtained from 13 localities and soaked in tap water for 48 hours at 25° C for nematode extraction. The extent of SLGN infestation was quantified by calculating population density, frequency of occurrence (%) and prominence values (PV). For identification purposes, DNA fragments from these parasites were sequenced using the forward primer TW81 and reverse primer AB28. Separation of galled seeds from uninfected seeds was done in two separate trials by subjecting SLGN-infected seed to i) sieving using 250-um-, 500-um-and 600-um- aperture mesh sieves and ii) flotation and sedimentation using eight NaCI concentrations, viz. 2.5%, 5%, 7.5%, 10%, 12.5%, 15%, 17.5% and 20%. In addition the efficacy of various nematicides was evaluated under semi-controlled micro plot conditions against SLGN-infected E. curvula (Ermelo variety) seed. SLGN from 12 of the 13 localities were identified as S. wevelli, with population densities and PV both ranging from 0.1 to 205 in 1g seed samples. Phylogenetic tree data indicated that these 12 SLGN populations had bootstrap values of less than 90% and did not differ significantly from each other nor from the S. wevelli reference population (USA). Low similarity values obtained between the ITS regions of these populations and the reference population substantiated their identification further. Subanguina wevelli populations from all the localities are, however, reared in vivo on E. curvula (Ermelo variety) in order to obtain mature specimens for confirmation of molecular identification. In terms of sieving, the number of S. wevelli contained in seed did not differ significantly for the three sieves. All three sieves did, however, contain significantly fewer S. wevelli than the unsieved control treatment. The seed on the 250-um-mesh sieve had the cleanest seed (13% infested), followed by the 500-um-mesh sieve (28% infested) and the 600-um-mesh sieve (31% infested). Flotation- separation, using a NaCI technique resulted in treatments not differing significantly from the untreated control (tap water only) in terms of S. wevelli numbers. Subanguina wevelli numbers in floating seed ranged from 702 to 1 952 for the 7.5% and 20% NaCI treatments, respectively, and from 84 to 392 for the 12.5% and 5% treatments in sunken seed, respectively. Combination of sieving and a 15% NaCI solution still resulted in high numbers of S. wevelli in seed batches. Planting of such seed will therefore still lead to S. wevelli infection in Eragrostis grass. Treatment of E. curvula seed with 0.25mg avermectin per seed, carbofuran (1,5g/m) and terbufos (0.66g/m) resulted in the greatest reduction of S. wevelli numbers per 2g seed and these differed significantly from the untreated control. Aldicarb, cadusafos, ethylene dibromide, ethoprophos and oxamyl treatments still had relatively high numbers of S. wevelli and did not differ significantly from the untreated control. Results from this study indicate that a single control strategy is not adequate to ensure that grass-seed consignments are free of SLGN. More than one control strategy, included as components of an integrated pest management system has to be applied to address this nematode problem.