The effect of plant extracts as seed treatments to control bacterial leaf spot of tomato in Tanzania
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Bacterial leaf spot (BLS) caused by seed-borne xanthomonads is a serious disease of tomato (Solanum lycopersicum L.), causing significant losses in both yield and quality. To identify more effective control measures, we evaluated crude extracts from 84 plant species in in vitro and in planta assays for antibacterial activity against BLS of tomato. In the in vitro assays, 20.2 % of the tested plant extracts totally inhibited growth of bacteria when seed washings from treated seeds were plated on nutrient agar medium. In the in planta assays, 17.8 % of the tested plant extracts reduced BLS incidence by 100 % in tomato seedlings. The most effective seed treatments were obtained with extracts from Aloe vera, Betula pendula, Coffea arabica, Glycyrrhiza uralensis, Juniperus communis, Ocimum basilicum, Quercus robur, Rheum palmatum, Rosmarinus officinalis, Ruta graveolens, Sinapis alba, Yucca schidigera and Salvia officinalis. Seed treatment of tomato with these extracts completely inhibited Xanthomonas perforans in both in vitro and in planta assays. Extracts from A. vera, C. arabica and Y. schidigera were tested three times using tomato seeds of cultivars Tanya, Cal-J and Moneymaker in Tanzania. Treatment of tomato seeds with these extracts had a positive effect on the number of normal seeds and had no effect on seedling vigor, height and weight. These results indicate that plant extracts from A. vera, C. arabica and Y. schidigera are potential candidates for seed treatment against seed-borne xanthomonads of tomato in Tanzania.