A collection of Gaeumannomyces graminis var. graminis isolates obtained from symptomatic rice and grass plants in central and northern Brazil were studied in regard to pathogenicity on rice and cultural characteristics. For all isolates, only one type of lobed hyphopodia was observed both in the field and artificially inoculated plants. Perithecia were formed on artificial media and inoculated leaf sheaths. Hyphopodia were formed from ascosporic germ tubes. The hyphae under moist stress conditions produced dark brown chlamydospores that were initially hyaline. Large variation in the number of chlamydospores produced and duration of perithecia formation was observed among the isolates. Pathogenicity assays showed that 60-day old rice plants were more susceptible than 35-day old plants. The isolates from rice and grasses varied significantly in regard to disease severity on both rice seedlings and adult rice plants. In general, the isolates from rice were more aggressive than the isolates from grasses. Spontaneous infection of rice plants by ascosporic inoculum from perithecia on rice stubbles was observed in the greenhouse, suggesting their role as a source of primary inoculum in the field, which deserves further investigation.

Epicoccum sp. showed in vitro antagonism to the rice pathogens Magnaporthe oryzae, Rhizoctonia solani, Sarocladium oryzae, Monographella albescens and Cochliobolus miyabeanus in dual culture. The colony growth of the rice blast fungus, M. oryzae, was reduced by 42.50%. The lethal doses (LD50) determined based on probit-log dosage response curves at 3 and 6 hours after germination were 393.0 and 326.6 ppm, respectively. The crude extract (600 ppm) reduced appressorial formation by 95.68%. A greenhouse experiment comparing the relative efficiency of conidial suspension and crude extract of Epicoccum sp. in the suppression of leaf blast showed no statistical difference between both application methods. However, the crude extract of Epicoccum sp. (4000 ppm) 48 hours before the application of M. oryzae induced resistance and suppressed leaf blast by 97.6%. Scanning electron microscopy of rice leaves inoculated with crude extract of Epicoccum sp. and challenged with M. oryzae showed appressorial deformation on penetration phase. Peroxidase and β-1,3-glucanase activities increased in plants sprayed with crude extract of Epicoccum sp., 24 hours after the application of the challenger. PAL as well as chitinase activities increased 72 hours after challenge inoculation. Epicoccum sp. was shown to be a potential antagonist and inducer of resistance against M. oryzae.

A brusone (Magnaporthe oryzae) é um dos principais fatores limitantes da produtividade do arroz de terras altas no Brasil. A época de plantio e o nível de adubação nitrogenada determinam o grau de severidade da brusone nas panículas. Foi realizado um experimento de campo com a cultivar BRS Bonança, visando determinar a relação entre o conteúdo de clorofila nas folhas superiores e a severidade de brusone nas panículas. O delineamento experimental foi blocos ao acaso em esquema de parcelas subdivididas com três repetições. Os tratamentos consistiram de 12 épocas de plantios em intervalos semanais e cinco níveis de nitrogênio (0, 30, 60, 120 e 240 kg ha-1 de N). As leituras do teor de clorofila foram feitas na folha bandeira com um clorofilômetro. A brusone nas panículas aumentou significativamente com o aumento dos níveis de N. Houve correlação significativa e positiva entre os níveis de N e o conteúdo de clorofila na folha bandeira (r=0,98; p < 0,01). A relação entre a severidade de brusone nas panículas e o teor de clorofila foi linear e positiva (r= 0,99; p < 0,01). Estes resultados mostram que o teor de clorofila pode ser incluído como variável independente no desenvolvimento de modelo matemático para a previsão de brusone nas panículas.

Rice blast (Magnaporthe oryzae) is the most important yield limiting factor of upland rice in Brazil. The nitrogen application rate and planting date determine the level of panicle blast severity. A field experiment was conducted with the cultivar BRS Bonança to establish the relationship between chlorophyll content in leaves and panicle blast severity. The layout was a randomized split plot design with three replications. The treatments included 12 planting dates at weekly intervals and five rates of nitrogen (0, 30, 60, 120 and 240 kg ha-1 of N). The chlorophyll content was measured on flag leaf with a chlorophyll meter. Panicle blast severity increased significantly with the increase in nitrogen fertilization. The correlation between nitrogen rate and chlorophyll content in flag leaf was positive and significant (r=0.98; p < 0.01). The relationship between panicle blast severity and chlorophyll content was linear and positive (r= 0.99; p < 0.01). The chlorophyll content in the flag leaf can be included as independent variable in the development of a mathematical model for panicle blast forecasting.