Phoma sorghina, a new pathogen associated with Phaeosphaeria leaf spot on maize in Brazil

Phaeosphaeria leaf spot (PLS) is a major foliar maize disease in Brazil, with reports of up to 60% yield reduction (Dourado & Fancelli, 2000). The cause has been attributed to Phaeosphaeria maydis (Fantin, 1994), although alternative suggestions have been made (Cervelatti et al., 2002). The first symptoms appear as chlorotic leaf spots later developing into straw-colored elliptical lesions.

Leaves with typical PLS lesions from 1 to 10 mm were examined (Fig. 1a & b), from different sites in Rio Grande do Sul (RS) and Goiás (GO) States in 2001, 2002 and 2003. Disinfected samples (1 min in 70% ethanol, 2 min in 1% NaOCl) were placed in a humid chamber for 96 h. Fungal sporulating structures were picked off and placed on potato dextrose agar (PDA) with 0.02% streptomycin sulphate. Typical colonies were sent to CABI Bioscience for species identification, which was done based on colony, pycnidia and spore morphology. Phoma sorghina (Fig. 1d & e), a minor pathogen recorded on sorghum in Brazil (Pinto, 2002), was found in 70% of all samples. Phaeosphaeria maydis was obtained from only 7% of the samples from RS.

Conidial suspensions (adjusted to 1x10⁶ conidia per ml in sterile distilled water) of Phoma sorghina and Phaeosphaeria maydis were sprayed onto three maize plants each of two cultivars: AS-3466 (resistant to PLS) and AS-32 (a susceptible cultivar). Plants were kept under a plastic cover for 72 h after inoculation. Control plants were sprayed with sterile distilled water alone. Plants inoculated with Phoma sorghina were kept in a greenhouse at 70% relative humidity (RH) at an average temperature of 25°C. Phaeosphaeria maydis treated plants were kept in a growth chamber at 80% RH, 21°C and a 12 h photoperiod.

Plants inoculated with Phaeosphaeria maydis or Phoma sorghina developed similar symptoms (Fig. 1c). Both sets of treated plants showed chlorotic lesions after 10 days on AS32 and in 15 days on AS3466; both becoming necrotic and straw coloured after 20 days. These lesions were smaller than the ones observed in the field. Both Phaeosphaeria maydis and Phoma sorghina were reisolated from these lesions, confirming their pathogenicity on maize. The pathogenicity tests were repeated with the same results. This is the first report of Phoma sorghina as aetiological agent of leaf spot on maize in Brazil.