Etiology, Epidemiology and Management of Early Blight Disease (Alternaria alternata (Fr.) Keissl.) on Tomato (Solanum lycopersicum L.) in Sudan

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Azza Siddig Hussien Abbo, Abbo
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The present research was designed to study the etiology, epidemiology and some management strategies of early blight disease on tomato. Thus, infected leaves of tomato, potato, eggplant and pepper crops, showing symptoms of the early blight disease, were collected from different production areas in Sudan, i.e Wed Medani, Khartoum, Alshehainab, Hajer Alasal, Amrey and Kassala. Seventy isolates of the genus Alternaria were isolated and identified as Alternaria alternata (Fr.) Keissl. The identification was based on cultural, morphological, pathogenicity and genetic characteristics. Spore beak formation is considered as the most important characteristic to identify species. Spore sizes of the 70 Alternaria isolates were within the range of 2.38-13.09_6;m × 12.30-43.63_6;m. Genetic diversity among the Alternaria spp. isolates inciting early blight disease in these crops was determined by using the amplified fragments length polymorphism (AFLP). The genetic analysis provided no evidence of geographical clustering of the isolates. Due to lack of obvious differences among these isolates, representative isolates were sent to the CABI Bioscience, a division of the Commonwealth Agricultural Bureaux CAB International Centre, London U.K. for authentication. A. alternata was identified as the causal agent of early blight disease in these four crops in Sudan. A. alternata was reisolated in pathogenicity test from the inoculated plants, in accordance with Koch's postulates. The cross infectivity of early blight causative agents was studied in a pot experiment of these four solanaceous crops. The causal pathogens were isolated from infected leaves of the four crops. The isolated pathogens were inoculated separately into six weeks old tomato, potato, eggplant and pepper plants. Seven weeks after inoculation all inoculated plants of the four crops were infected at variable incidences. The effects of tomato variety (open-pollinated and hybrid), nitrogen form (urea and ammonium nitrate) and the fungicide Ridomil Gold® MZ 68% on early blight disease on tomato were evaluated. A field trial was conducted for three consecutive growing seasons (2006/07, 2007/08 and 2008/09). In the growing seasons, early blight symptoms showed up naturally 12 weeks after sowing. The application of Ridomil Gold at the onset of the disease significantly (P X04; 0.05) reduced disease incidence and increased tomato yield. The combination of the hybrid variety (Star 9008), use of ammonium nitrate fertilizer as a source of nitrogen and application of Ridomil Gold at the onset of the disease was significantly (P X04; 0.05) superior to all other combinations included in the experiment. The least disease incidences of 44.30%, 78.99% and 74.61% were recorded for this combination in the three growing seasons, respectively. Biocontrol of early blight disease, through the use of natural alternatives to pesticides was studied by using four Bacillus spp. (B. subtilis, B. megaterium, B. pumilus and B. cereus) and the essential oil of tea tree. The antifungal effects of the four Bacillus species were tested against A. alternata in vitro and in vivo. The in vitro growth rates of A. alternata were always lower when it was treated with any of the four Bacillus species in comparison with the untreated control. Statistical analysis revealed that the use of B. subtilis and B. megaterium significantly (PX04; 0.05) suppressed the growth of A. alternata in vitro. In the in vivo pot tests during 2007/08 and 2008/09, tomato plants treated with the four Bacillus species displayed suppressed disease incidence and disease severity. The in vitro inhibition effect of four different concentrations (0.5%, 1%, 2% & 3%) of tea tree oil on the growth rate of A. alternata was evaluated in Potato Dextrose Agar (PDA) medium. The results showed that the antifungal effect of the oil against A. alternata was enhanced significantly (P X04; 0.05) in vitro in comparison with the control. The in vivo pot experiment, which was conducted during 2007/08 and 2008/09, revealed that the use of tea tree oil at 3% concentration 3significantly (P X04; 0.05) reduced early blight disease incidence and severity
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Etiology, Epidemiology and Management of Early Blight Disease (Alternaria alternata (Fr.) Keissl.) on Tomato (Solanum lycopersicum L.) in Sudan