First Report of Bipolaris oryzae Causing Leaf Spot of Switchgrass in Mississippi.

Abstract

'Alamo' switchgrass (Panicum virgatum L.) seedlings growing in a soilless mix exhibiting dark brown, irregular-shaped foliar lesions with black borders were submitted to the Mississippi State University Plant Disease Diagnostic Laboratory in the summer of 2009 from a local forest products company. Symptomatic tissues were plated onto water agar (WA) and incubated for 21 days on a laboratory bench top with a 12-h photoperiod at 22°C. An asexual, sporulating, dematiaceous hyphomycete identified as Bipolaris oryzae (Breda de Haan) Shoemaker was observed. Conidiophores were single, mostly straight, multiseptate, brown, and ranging from 138 to 306 × 7.7 to 15.3 μm and averaged 221.6 × 10.7 μm. Conidia were golden brown, multiseptate, ranging from 3 to 10 septa, straight to slightly curved to fusoid, wider midway, and tapering toward the terminal cells. Conidia measured 40.8 to 109.7 × 10.2 to 20.4 μm and averaged 75.8 × 13.8 μm. Morphological characteristics of B. oryzae were similar to those described by Drechsler (1) and Sivanesan (3). The internal transcribed spacer (ITS) region of ribosomal DNA from four pure culture colonies derived from single conidia was amplified by PCR using ITS1 and ITS4 primers. The resultant 572 bp was sequenced for isolates 86 through 89 (GenBank Accession Nos. GU222690-93). The sequences were 99% similar to the sequence of B. oryzae strain ATCC-MYA 3330 (GenBank No. FJ746665) isolated from P. virgatum. Pathogenicity of isolates 86 and 88 was confirmed by inoculating sterile potting mix with a fungal slurry. Sterile Alamo switchgrass seeds were sown into the infested soil in Magenta boxes and incubated for 6 weeks in a growth chamber with a 14-h photoperiod at 30°C. Leaf lesions and leaf blight were observed in seedling stands growing in B. oryzae-infested soil. Lesions were excised and plated onto WA. Sporulation of B. oryzae was observed on symptomatic tissue. In the interim, 300 nonsterilized Alamo switchgrass seeds of the same seed lot as the original symptomatic seedlings and originating from Oklahoma were plated onto WA (10 seed per plate). The seeds were incubated on the bench top as previously described. The experiment was repeated and B. oryzae colonized 1.4% of the total switchgrass seeds evaluated, indicating seed transmission and subsequent seedling infection as previously observed in the original seedlings. Leaf spot, caused by B. oryzae, was first reported as a new disease of switchgrass in North Dakota (2). In the summer of 2009, the authors observed leaf spot in four cultivars of switchgrass, including Alamo, growing in research plots in Webster County, MS. Twenty-two isolates of B. oryzae were recovered from diseased leaves of these switchgrass cultivars. To our knowledge, this is the first report of B. oryzae causing leaf spot of switchgrass in Mississippi, which broadens the natural distribution of this disease.