First Report of Target Spot of Tobacco Caused by Rhizoctonia solani AG-2.1

Abstract

In August 2010, lesions similar to those reported for target spot were observed on Nicotiana tabacum L. plants produced in float systems in Cerrillos, Salta, Argentina. Tobacco leaves with characteristic lesions were collected from different locations in Cerrillos, Salta. Symptoms ranged from small (2 to 3 mm), water-soaked spots to larger (2 to 3 cm), necrotic lesions that had a pattern of concentric rings, tears in the centers, and margins that often resulted in a shot-hole appearance. Isolation of the causal agent was made on potato dextrose agar (PDA) acidified to pH 5 with 10% lactic acid and incubated at 25 ± 2°C in darkness for 2 to 3 days. Hyphal tips were transferred to a new medium and the cultures were examined for morphological characters microscopically (3). Eight isolates were obtained. The rapid nuclear-staining procedure using acridine orange (3) was used to determine the number of nuclei in hyphal cells. Multinucleate hyphae were observed, with 4 to 9 nuclei per cell. Molecular characterization was conducted by examining the internal transcribed spacer (ITS) region from all of the isolates of the pathogen identified as Rhizoctonia solani based on morphological characteristics (1). Fragments amplified using primers ITS1 (5′TCCGTAGGTGAACCTGCGG3′) and ITS4 (5′TCCTCCGCTTATTGATATGC3′) (4) were sequenced and compared with R. solani anastomosis group (AG) sequences available in the NCBI GenBank database. Sequence comparison identified this new isolate as R. solani anastomosis group AG 2-1. Previous isolates of target spot were identified as AG 3 (2). The isolates that were studied were deposited in the “Laboratorio de Sanidad Vegetal” INTA-EEA-Salta Microbial Collection as Rs59c, Rs59b, Rs59, Rs66, Rs67, Rs68, Rs69, and Rs70. The ITS nucleotide sequence of isolate Rs59 has been assigned the GenBank Accession No. JF792354. Pathogenicity tests for each isolate were performed using tobacco plants grown for 8 weeks at 25 ± 2°C with a 12-h photoperiod. Ten plants were inoculated by depositing PDA plugs (0.2 cm) colonized with R. solani onto leaves; plants inoculated with the pure PDA plug without pathogen served as controls. The plants were placed in a 25 ± 2°C growth chamber and misted and covered with polyethylene bags that were removed after 2 days when plants were moved to a glasshouse. After 48 h, symptoms began as small (1 to 2 mm), circular, water-soaked spots, lesions enlarged rapidly, and often developed a pattern of concentric rings of 1 to 2 cm. After 8 days, all inoculated plants showed typical disease symptoms. Morphological characteristics of the pathogen reisolated from symptomatic plants were consistent with R. solani. Control plants remained healthy. These results correspond to the first reports of the disease in the country. Compared to other areas in the world, target spot symptoms were only observed in tobacco plants produced in float systems and were not observed in the field. The prevalence of the disease in Salta, Argentina was 7%. To our knowledge, this is the first report of R. solani AG2.1 causing target spot of tobacco. References: (1) M. Sharon et al. Mycoscience 49:93, 2008. (2) H. Shew and T. Melton. Plant Dis. 79:6, 1995. (3) B. Sneh et al. Identification of Rhizoctonia species. The American Phytopathological Society, St. Paul, MN, 1991. (4) T. J. White et al. Page 282 in: PCR Protocols: A Guide to Methods and Applications. Academic Press, San Diego, 1990.

Molecular identification and pathogenicity of Rhizoctonia spp. recovered from seed and soil samples of the main bean growing area of Argentina

“…One Rhizoctonia solani isolate from soil and one from bean seeds identified as AG 2-1 and AG 7 caused root rot and web blight symptoms in our inoculation trials. There are reports of AG 2-1 and AG 7 isolates causing disease in tobacco and cotton (Bacharis et al, 2010;Mercado-Cárdenas et al, 2012), but not on bean plants, as we observed under controlled environment conditions. Moreover, AG 2-1 isolates were found to be associated with damping-off, target spot, and sore shin in tobacco in the same area (Mercado-Cárdenas et al, 2015).…”

Molecular identification and pathogenicity of Rhizoctonia spp. from tobacco growing areas in northwestern Argentina

“…We have recently reported that R. solani AG 2-1 is associated with target spot on tobacco seedlings produced under float systems, but made no field observations (Mercado Cárdenas et al 2012).…”

“…Multiple AGs of R. solani are pathogenic on tobacco (Shew and Melton 1995;Nicoletti and Lahoz 1995;Gutierrez et al 1997;Nicoletti et al 1999;Zhao et al 2013). AG 1, AG 2-1, AG 2-2 and AG 4 cause root rot, stem rot or sore shin, and damping-off, while AG 2-1 and AG 3 cause target spot on leaves of tobacco (Shew and Melton 1995;Gutierrez et al 1997;Nicoletti et al 1999;Mercado Cárdenas et al 2012;Zhao et al 2013). In our study the R. solani isolates identified as AG 2-1 were obtained from plants with damping-off, target spot and sore shin symptoms.…”

Soil properties related to suppression of Rhizoctonia solani on tobacco fields from northwest Argentina

“…As expected, foliar symptoms of target spot were not observed on tobacco fields because these symptoms mainly affect young plants before transplantation. Previously, R. solani AG2‐1 was only found to be associated with foliar symptoms in younger plants grown in float systems (Mercado Cárdenas et al , ); in addition, R. solani AG4 was shown to cause damping off in younger plants and root rot in older plants (Mercado Cárdenas et al , ).…”