First report of leaf blight of Brachiaria brizantha in Brazil caused by Bipolaris cynodontis

D.M. Macedo and R.W. Barreto^*

Clinica de Doenças de Plantas, Departamento de Fitopatologia; Universidade Federal de Viçosa (UFV); 36571-000, Viçosa, Minas Gerais

^*rbarreto@ufv.br

Accepted for publication 18/12/06

Brachiaria brizantha cv. Marandu (bread grass, bread signalgrass; local names: braquiarão, capim marandú) is one of the most widely utilized pasture grasses in Brazil.  In July 2005, specimens of this grass showing severe foliage blight symptoms were brought to the Plant Clinic for identification.  Necrosis started at the leaf apex and extended towards the base resulting in complete blight of the leaf.

Figure 1: Leaf blight symptoms on Brachiaria brizantha cv. Marandu.

A Helminthosporium-like fungus was directly associated with and isolated from necrotic tissues.  Specimens from host and cultures were mounted with lactophenol and examined under a light microscope.  All showed typical features of a Bipolaris sp. with conidiophores single or in small groups, cylindrical, 70-234 x 4-8 µm, dark brown, 6–11 septate; conidia single, fusiform, straight or slightly curved, 32–106 x 11–19 µm (most 75 x 11 µm), 4–11 distoseptate (most 5–7), chestnut brown, hilum thickened, darkened, truncate; germination bipolar or through basal cell, germ tube oriented along the main conidia axis.  Based on these characteristics, in particular the conidial length, the fungus was identified as Bipolaris cynodontis.  A sample was deposited in the local herbarium (VIC 29370).

Figure 2: Bipolaris cynodontis - germinated and non-germinated conidia (Bar =12.5 µm).	Figure 3: Bipolaris cynodontis – conidiophores bearing geniculate condiogenous cells with attached conidia (note ongoing conidiogenesis) (Bar =12.5 µm).

The fungus was grown on vegetable broth-agar, where it sporulated abundantly.  Thirty-day-old B. brizantha plants were brush-inoculated with a conidial suspension (10⁶ conidia per ml with 0.05% Tween 20).  Plants treated with tap water served as control.  The plants were left in a dew chamber for 48 hours and then transferred to a greenhouse.  Descending leaf blight symptoms similar to the original field symptoms appeared after seven days, only on inoculated plants.  Bipolaris cynodontis was the only fungus reisolated from the diseased tissues.

Several species of Bipolaris have been recorded worldwide on other species of Brachiaria: Bipolaris panici-miliacei, B. bicolor, B. zeicola (Sivanesan, 1987), B. setariae, B. zeae (Lenné, 1990) and B. cynodontis (Pratt, 2006).  So far, B. cynodontis has only been recorded as a pathogen of one Brachiaria species, namely Brachiaria platyphylla (broadleaf signal grass) in the USA (Pratt, 2006).  In Brazil, Bipolaris cynodontis has previously been found only on Cynodon dactylon (Bermuda grass) (Mendes et al., 1998).  This is the first record of this fungus attacking B. brizantha in Brazil and worldwide.

References

Lenné JM, 1990. A World list of fungal disease of tropical pasture species. Phytopathological Papers 32, 1-161.

Mendes MAS, Silva VL, Dianese JC, Ferreira MASV, Santos CEN, Neto EG, Urben AF, Castro C, 1998. Fungos em plantas do Brasil. Brasília, Brazil: Embrapa-SPI/Embrapa-Cenargem.

Pratt RG, 2006. Johnsongrass, yellow foxtail, and broadleaf signalgrass as new hosts for six species of Bipolaris, Curvularia, and Exserohilum pathogenic to bermudagrass. Plant Disease 90, 528.

Sivanesan A, 1987. Graminicolous species of Bipolaris, Curvularia, Drechslera, Exserohilum and their teleomorphs. Mycological Papers 158, 1-261.

The British Society for Plant Pathology