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The rice (Oryza sativa L.) cultivars Puntal and Thaibonnet were used in a field trial designed to investigate how temperature and relative humidity affect the airborne concentration of Pyricularia oryzae spores and the development of rice blast. A relative humidity of 95% and an average temperature of 26-27 deg C were optimum for infection and substantially favoured spore release. The first symptoms of infection were detected on the leaves when the crop was at the mid-tillering stage, the number of lesions increasing as the plant developed. After the onset of heading the number of leaf lesions decreased while the number of node and panicle base lesions increased. A temperature increase of 1 deg C in early August (mid-tillering stage) led to an increase in the mean intensity of disease. The presence of spores in the air from July 15th predicted the appearance of leaf lesions a few days later. The peak spore concentration in August could be used to forecast panicle blast. Assessing the airborne concentration of P. oryzae could help in understanding the population dynamics of this pathogen., Los cultivares de arroz Puntal y Thaibonnet fueron sembrados en campo para analizar el efecto de la temperatura y la humedad relativa media sobre el contenido de esporas de Pyricularia oryzae en el aire y el progreso de la enfermedad. Valores de humedad relativa del 95% y temperatura media entre 26-27 grados C propician condiciones óptimas para la infección del patógeno y favorecen sustancialmente la expulsión de esporas. Los primeros síntomas aparecieron sobre hojas, cuando el cultivo se encontraba en estado de ahijado medio. La cantidad de lesiones infecciosas en hoja incrementó conforme la planta aumentaba su desarrollo. Después del inicio de espigado estos valores decrecieron, mientras que los valores de infección en nudo y base de la panícula se incrementaron paulatinamente. Un incremento de temperatura de 1 grado C a principios de agosto (estado de ahijado medio) causó un incremento de la intensidad media de la enfermedad. La presencia de esporas en el aire desde el 15 de julio ayuda a predecir la aparición de manchas en hojas varios días más tarde. El pico de esporas registrado en agosto puede ser usado para predecir la infección en panícula. La evaluación del contenido de las esporas de P. oryzae en el aire ayuda a comprender la dinámica de la población del patógeno.

The susceptibility of 14 commercial rice cultivars to Pyricularia oryzae was tested under field conditions agronomically appropriate for natural infection to occur. The first symptoms of infection were detected on the leafs when the crops were at the medium tillering stage. Generally speaking, the incidence of infection for each particular cultivar was similar in the 2002 and 2003 seasons and at the two trial locations (the Sartenejales and Casudis estates in the Province of Seville). The pathogen infected 80-100% of "Maratelli", "Baixet", "Thainato" and "Galatxo" plants. Differences were seen among the cultivars with respect to the parts of the plant affected. "Baixet" and "Maratelli" were the most susceptible cultivars, followed by "Thainato". "Ullal" was the most tolerant, followed by "Marisma", "Puebla", "Maso", "L-202", "Susan", "Doongara", "Fonsa" and "Jacinto"; these last six cultivars showed medium disease severity according to the scale used in this work. Further studies on the interactions between different strains of P. oryzae and rice cultivars are underway., Se sembró en campo una colección de 14 cultivares comerciales de arroz bajo condiciones agronómicas apropiadas para que se produzca la infección natural por Pyricularia oryzae, a fin de estudiar su susceptibilidad al patógeno. Los primeros síntomas de la infección se detectaron en las hojas cuando el cultivo se encontraba en estado de ahijado medio. Generalmente, la incidencia de la enfermedad fue similar para cada cultivar en dos campañas (2002-2003) y localidades (Sartenejales y Casudis, Sevilla). El patógeno infectó el 80-100% de las plantas de "Maratelli", "Baixet", "Thainato" y "Galatxo". Se observaron diferencias entre los cultivares estudiados relativas a las partes de las plantas afectadas. Considerando el porcentaje de infección ponderado, "Baixet" y "Maratelli" fueron los cultivares más susceptibles al patógeno, seguidos de "Thainato". Por otro lado, "Ullal" fue el más resistente a P. oryzae, seguido de "Marisma", "Puebla", "Maso", "L202", "Susan", "Doongara", "Fonsa" y "Jacinto". Estos últimos seis cultivares mostraron una severidad de enfermedad media. Se están llevando a cabo estudios adicionales para conocer el espectro de razas de P. oryzae y su respuesta patogénica frente a una colección de cultivares diferenciales.

5 pages, 2 figures, 4 tables, Field studies were conducted at nine locations in southern Spain during 2 yr to develop models of nodding broomrape competition with sunflower and to establish economic thresholds. At each location, 30 to 35 small plots, each consisting of three sunflower plants, were chosen at random. The infection severity (BIS, no. of emerged broomrapes per sunflower plant) varied from 0 to 35. Plots were harvested at maturity to assess several sunflower and broomrape population variables. The percent sunflower yield reduction averaged over locations due to broomrape was estimated by the equation: % SYR= 1.7 × BIS (r² = 0.92). Crop yield loss per BIS unit increased with the expected yield and was estimated to be about 25, 50, and 75 kg ha-¹ for yields of 1000, 2000, and 3000 kg ha⁻¹, respectively. A consistent relationship could be established between broomrape-infected sunflower yield, crop and broomrape biomass, and BIS parameters: SSY1 = 0.2259 x PoBio/(1 + 0.0687 x BIS) (r2 = 0.7820). The BIS economic threshold was about 1.5 and 3.5 for control treatment cost of $ 40 ha-1 and potential yields of 2000 and 1000 kg ha-1, respectively., Europen Union (EEC XII-TS2*0124-C) and the Spanish Scientific and Technological Commission (CICYT Agro-Petri 0057/92).