Evaluation of fungicide seed treatments to control seedling diseases of cowpea

Abstract

Cowpea is an important food crop and is increasingly being cultivated by small-scale farmers in South Africa. Cowpea is susceptible to a wide range of seedborne diseases, which causes damage to the crop at all stages. Seedling diseases caused by pathogens like Rhizoctonia solani (Kuhn), Pythium ultimum (Trow) and Fusarium solani (Mart) App. and Wol attack cowpea, and result in low yields especially in rural areas where little or no control measures are taken against these pathogens. Different concentrations of Celest® XL [fludioxonil (25gai/ L) and mefenoxam (10gai/L)] were evaluated against these pathogens and their effect on germination. Thiram (500gai/L DS) was used as a standard fungicide. In the in vitro assay, media was amended with Celest® XL at 1x (0.06ml), 1.25x (0.75ml) and 2x (0.12ml) the recommended rate. Growth diameter was measured on day 3, 6, and 9. All treatments significantly inhibited mycelial growth of P. ultimum, F. solani and R. solani when compared to the control. A germination test was performed according to the rules of the International Seed Testing Association (ISTA) (2005). Celest® XL improved cowpea germination, and increased shoot and root length. Disease incidence was significantly lowered by all the treatments when compared to the control. In greenhouse trials, seedling trays were filled with pasteurised growing medium and assigned randomly with four replications per treatment. Each replication consisted of 25 plants. The growing media was artificially inoculated with each of the three pathogens by placing two mycelial plugs in each cell of the seedling trays. Fungicides were applied as a seed slurry treatment at a concentration of 0.6g/500g thiram and 1x (100ml/100kg seed), 1.25x (125ml/100kg seed) and 2x the recommended rate (200ml/100kg seed) Celest® XL and mixed for 5min. The control was treated with water using the same procedure. It was found that all the treatments significantly inhibited mycelial growth of all three pathogens in the in vitro test. Germination performance was enhanced by treating the cowpea seeds with thiram and Celest® XL. The results also showed that percentage emergence was increased by all treatments when compared to the control. All the treatments, significantly reduced disease incidence on cowpea seedlings in the greenhouse. All the treatments when compared to the inoculated control, significantly increased plant height and dry shoot mass. This study provides sufficient data to warrant further testing of the treatments under field conditions. The residual effects of Celest® XL and thiram also needs to be determined before the fungicides can be registered as seed treatments of cowpea in South Africa.