Bioactivity of Allium sativum Extracts Against the Plant Pathogens Alternaria solani and Sclerotium rolfsii

Allium sativum (garlic) is considered as a vegetable and as an herbal crop throughout the world, including South Africa. Yield and quality can be improved through nitrogen (N) and sulphur (S) nutrition. The objective of the study was to determine the influence of ammonium sulphate and calcium nitrate fertilization on the bioactivity of A. sativum plants against Alternaria solani and Sclerotium rolfsii. The experimental layout was a randomized complete block design (RCBD) with four replications. A. sativum plants were treated with ammonium sulphate or calcium nitrate fertilizers applied as topdressing to give a total of 0, 50, 100, 150 and 200 kg/ha, divided into three applications at three week intervals. Crude extracts were prepared separately from the leaves and bulbs of A. sativum. The results obtained indicated that leaf extracts of the plant which were treated with calcium nitrate fertilizer demonstrated low bioactivity when compared to plants that were treated with ammonium sulphate. A. sativum bulb extracts were found to have very low bioactivity at 54 days after planting (DAP) and high at 175 DAP, however leaf extract bioactivity increased from young (54 DAP) to full vegetative maturity of the shoots (82 to 112 DAP) and declined with maturity of the bulb (140 to 175 DAP), regardless of N source supplied to the plants. Calcium nitrate failed to improve the medical properties of A. sativum while ammonium sulphate enhanced the bioactivity.