The evaluation of plant extracts, biocontrol agents and hot water as seed treatments to control black rot of rape in South Africa

Black rot disease, which is caused by the pathogen Xanthomonas campestris pv. campestris (Xcc), is a major challenge to brassica vegetable production by smallholder farmers. The pathogen is seed-borne making it difficult to control the disease. In this study various plant extracts, commercial biocontrol agents (BCAs) and hot water treatments were evaluated for their antibacterial activity, and as seed treatments of rape (Brassica napus L.) against Xcc in vitro and under greenhouse conditions. The microtitre double-dilution assay showed that acetone extracts of Cymbopogon citratus had strong antimicrobial activity with the lowest minimum inhibitory concentration (MIC) of 0.19 mg/ml, which was comparable to the antibiotic neomycin (0.2 mg/ml). Using the agar well diffusion method the BCA Paenibacillus sp. (3 × 109 cfu/ml) recorded the highest antibacterial activity with a maximum zone of inhibition of 17 mm. Seed treatment with hot water at 50 °C for 30 min reduced the bacterial population to 3.1 cfu/ml compared to the untreated inoculated control (6.0 cfu/ml). Significantly higher germination percentage (84%) was recorded after seed treatments with acetone extracts of Agapanthus caulescens (15 mg/ml) and hot water at 50 °C for 30 min. In the greenhouse trials, acetone extracts of A. caulescens (15 mg/ml), Paenibacillus sp., and hot water at 50 °C for 30 min significantly increased seedling emergence and reduced black rot incidence and severity on rape leaves. The present study showed that plant extracts, commercial BCAs and hot water have potential as seed treatments for the control of Xcc and black rot disease.