Apr

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1 MISSISSIPPI SOYBEAN PROMOTION BOARD PROJECT NO (YEAR 1) 2014 Annual Report Title: Identification and characterization of mystery disease pathogen that is prevalent in Mississippi soybean fields in 2014 Investigator: Maria Tomaso-Peterson This research addresses the etiology of the mystery disease referred to as taproot decline that is associated with soybean plants throughout MS production fields. The symptoms include foliar chlorosis, interveinal necrosis, stained vascular tissues, and black stroma, which is a matrix of vegetative, mycelial hyphae observed on the stem close to the soil line. The disease is widespread throughout Mississippi and recurs in the same area of a field if soybean is planted in consecutive years. To date, the causal agent has not been determined. A considerable amount of research must continue to successfully determine the etiological agent of taproot decline. Objectives: 1. Identification of the fungal pathogen(s) associated with taproot decline of soybean 2. Assess pathogenicity of fungal pathogen(s) through completion of Koch s Postulates 3. Determine the potential yield loss associated with taproot decline 2014 Progress Soybean plants symptomatic of the taproot decline disease were collected from various MS soybean fields in 2014 just prior to harvest. Three plants from each location were washed with water to remove excess soil and placed in incubation chambers to encourage fungal growth. Sections of infected stem were removed and plated onto water agar to encourage fungal growth. Neighboring Louisiana plant pathologists shared pictures from affected soybean plants which display similar symptoms and fungal signs that we observe in MS. Koch s postulates were initiated 26 March Two inoculum methods were employed infested corn cob grit and infested toothpicks. Soybeans at V2 stage were inoculated separately and placed into humid chambers maintained at 28 o C with a 16 hour day. Soybeans were removed from the humid chamber after 10 days and returned to the greenhouse bench to observe symptoms. The experiment was repeated at V3 to V4 stages. A second inoculation approach included the same inoculum treatments except buried 0.25-in adjacent to the plant stems without wounding. These inoculated soybean plants are maintained on the greenhouse bench to observe symptom development. A disease rating scale was defined to assess symptomology based on a visual 1 to 5 scale, where 1 = healthy plant, 3 = leaf curl, wilt and cholortic leaves, and 5 = dead plant. Soybean inoculated with infested toothpicks began to show symptoms of wilt seven days post-inoculation (Figs. 1 and 2). The undescribed fungus appeared to be inciting leaf curl, wilt and chlorosis 10-days post inoculation with infested toothpicks, and affected soybeans averaged a foliar rating of 3.5 out of 5. Interveinal chlorosis, leaf curl, and wilt are symptoms observed in the field and were also observed on inoculated soybean (Fig. 3). Nineteen days post-inoculation, the root systems were washed to observe below-ground symptoms. Again, similar to symptoms observed in the field, the plants inoculated with the infested toothpick displayed symptoms of taproot decline (Figs. 4 and 5). The final step of Koch s postulates is to re-isolate the pathogen from the infected plant. This is currently taking place in the laboratory. The two remaining experiments will be rated for foliar and root symptoms at their termination. Apr

2 Impacts/Benefits to Mississippi Soybean Producers Results of this research will one day lead to the identity of the cause of the taproot decline malady observed in soybean fields across MS for over a decade. Based on grower communications, this disease is becoming more widespread throughout MS and may be reducing yield. Once a characterization of a causal agent is confirmed, we can then move forward on research into the management of taproot decline. Figure 1. Toothpicks infested with the undescribed pathogen were inserted into the base of the stem causing a wound prior to incubation in the growth chamber. Apr

3 Figure 2. Leaf wilt symptoms were evident seven days post inoculation in toothpick-inoculated soybean (left). Control plants (right) were wounded with sterile toothpicks and did not show adverse effects due to the wounding technique. Apr

4 Figure 3. Interveinal chlorosis and necrosis observed in soybean 19 days post toothpick inoculation. Figure 4. Infection of the taproot and secondary roots of soybean result in root-rot symptoms below the soil line similar to those observed in MS soybean fields. Apr

5 Figure 5. Soybean symptomatic for taproot decline. Apr