Control of Alternaria leaf spot of coriander in organic farming

Increased consumer awareness on the means of production of food in recent decades has intensified pressure for growth of organic farming particularly for its benefit on the environment due to minimal use of synthetic chemicals. In this regard, non-chemical seed treatments, viz. biocontrol agents (Trichoderma and Bacillus), hot water treatments and plant extracts, were studied as alternatives to synthetic chemicals for the management of Alternaria alternata (Fr.) Keissl. causing Alternaria leaf spot affecting organically produced coriander (Coriandrum sativum L.). Antifungal activities of acetone, ethyl acetate and water extracts of Allium sativum, Carica papaya, Datura stramonium, Lantana camara, Tagetes minuta and Zingiber officinale were evaluated using the disc diffusion assay. Discs impregnated with acetone extracts of Allium, Datura and Zingiber at a concentration of 15mg/mL completely inhibited growth of A. alternata, whereas discs impregnated with Tagetes recorded the lowest antifungal activity. Ethyl acetate extracts of all plants except Carica and Tagetes at 15mg/mL showed antifungal activity which was comparable to Celest (R) XL, a synthetic fungicide. A comparison of water extracts showed that discs impregnated with Lantana extract at 15mg/mL had the highest zones of inhibition (16.5mm); however, discs impregnated with Tagetes at a concentration of 5mg/mL yielded the lowest antifungal activity against A. alternata (0.3mm). The greenhouse trial showed that all non-chemical seed treatments significantly improved percentage seedling emergence, except for seeds treated with Lantana extracts and hot water at 48 degrees C for 60min, when compared to untreated controls. The study showed that seeds treated with Trichoderma sp. yielded seedlings with the longest shoots, which were significantly higher than seedlings grown from seeds treated with Celest (R) XL. There was no incidence of Alternaria leaf spot disease on seedlings from seeds treated with Bacillus sp. and an extract of Allium, which compared well with seeds treated with Celest (R) XL. Since there are limited chemicals registered for management of diseases affecting herb production, the results of this study have shown that soaking coriander seeds in a hot water bath set at 54 degrees C for 15 mins, and biocontrol agents (Trichoderma and Bacillus) and extracts of Allium and Zingiber are potential replacements of synthetic fungicides in controlling Alternaria leaf spot disease on coriander produced under organic farming.