Gisele Barata da Silva

An Unprecedented Outbreak of Rice Blast on a Newly Released Cultivar BRS Colosso in Brazil

Rice blast occurred in epidemic proportions on a newly released upland rice cultivar BRS Colosso, in the rice growing season 2004/2005. Twenty isolates were collected from the affected panicles of the cultivar BRS Colosso from three different States, two pathotypes IB-1 and IB-17 were identified. They were classified into thirteen Brazilian pathotypes based on the reaction pattern on eight upland rice cultivars, utilized as local differentials. Differences in aggressiveness of the isolates on the cultivar BRS Colosso were evident. Ten highly aggressive isolates were used to determine the partial resistance to leaf blast in the BRS Colosso and BRS Bonanca. There was no significant isolate x cultivar interaction for partial resistance. The mean leaf blast severity was significantly higher in BRS Colosso than in BRS Bonanca. Inoculation of culms with the same ten isolates showed cultivar x isolate interaction. Some isolates were more aggressive showing severe culm blast. There was no correlation among the aggressiveness of the isolates on leaf and culm. The disease outbreak in BRS Colosso could be attributed to the absence of adequate degree of partial resistance, the preexistence of compatible pathotypes IB-1 and IB-17 and their dissemination through seed.

Morphoanatomical and Biochemical Changes in the Roots of Rice Plants Induced by Plant Growth-Promoting Microorganisms

The goal of the present study was to characterize anatomical and biochemical changes in rice plant roots in response to seed treatment with rhizobacteria (Burkholderia pyrrocinia (R-46)

Increased enzymatic activity in rice leaf blast suppression by crude extract of Epicoccum sp

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.

Induction of leaf blast resistance in rice by an avirulent isolate of Magnaporthe oryzae

The induction of disease resistance can be activated in plants with biotic and abiotic agents such as avirulent isolates of pathogens. The objective of this investigation was to determine the concentration of inducer isolate and the time required between the inoculation with inducer (avirulent isolate) and virulent isolate of Magnaporthe oryzae, for the induction of resistance in rice leaves. Eighteen-day-old plants of rice cultivars Metica-1 and Cica-8 were sprayed in a greenhouse with an avirulent isolate utilized as inducer, at the concentrations of 0, 105, 3x105 and 6x105 conidia mL-1; sprayed 24, 48, and 72 hours prior to inoculation with virulent isolate. The induction of resistance was manifested in the reduction of leaf area affected and lesion type. The degree of induced resistance was greater in cultivar Metica-1 than in cultivar Cica-8 in relation to respective controls. The induction of resistance was superior in Cica-8 in treatments where inducers were applied 48 hours prior to inoculation with the virulent isolate, at concentrations of 6x105 and 3x105 conidia mL-1. On the other hand, the induction in Metica-1 was significantly higher in relation to control, at all concentrations and all periods of application of the inducer, and did not show differences between treatments.

Resistência de somaclones da cultivar de arroz IAC 47 a Monographella albescens

Leaf scald caused by Monographella albescens is one of the major rice (Oryza sativa) diseases in Brazil. Somaclones derived from immature panicles of IAC 47 were assessed for genetic resistance to leaf scald under natural field conditions of infection and artificial inoculation tests in the greenhouse. The differences among somaclones in relation to incidence and severity were significant in the field. The disease severities ranged from 12.4% to 32.03% for somaclones SCIA14 and SCIA28, respectively. The relation between incidence and severity was linear and positive (r = 0.87; P £ 0.01). The lesion length on the leaves of somaclones inoculated with mycelial discs positively correlated with severity (r = 0.93; P £ 0.01) and incidence in the field (r = 0.88; P £ 0.01). The relation between flag leaf width and leaf scald susceptibility of somaclones was linear and positive. Considering disease assessment, both in field and greenhouse, 19 somaclones showed higher degree of resistance compared to the cultivar IAC 47.

CONTROLE QUÍMICO DA BRUSONE EM ARROZ DE TERRAS ALTAS: EFEITOS NOS FUNGOS NÃO ALVOS DO FILOPLANO

Upland rice blast significantly affects yield, requiring one or two fungicides applications to control it. However, the effect of fungicides on beneficial fungi and on fungi not targeted by rice phylloplane are unknown. This study aimed to evaluate the effect of fungicides on panicle blast control, on fungi not targeted by rice phylloplane, and to study the antibiosis of fungi not targeted by M. oryzae . A field experiment was conducted by using two cultivars (Primavera and Bonanca), four fungicides (trifloxistrobina + propiconazol, azoxistrobina, tebuconazol, and triciclazol) and a control, with a randomized blocks design, in a split-plot scheme, and four replications. The panicle blast severity, leaf chlorophyll content, number of colony-forming units cm - ² of leaf area, and antibiosis were assessed. The Primavera cultivar showed higher panicle blast severity than Bonanca. The fungicides azoxistrobina and trifloxistrobina + propiconazol significantly differed from the control, showing the lowest panicle blast severities. The fungicides did not affect the chlorophyll content, but differences were noticed between the cultivars. The fungicides tebuconazol, trifloxistrobina + propiconazol, and azoxistrobina significantly reduced the phylloplane fungi, concerning the control. The triciclazol did not differ from the control, showing the highest number of colony-forming units cm - ² of leaf area. Among the four fungi tested, only Epicoccum sp. showed antagonism to M. oryzae .

Influência da adubação nitrogenada, época de plantio e aerosporos sobre a severidade da mancha de grãos em arroz de terras altas

Grain discoloration (GD) is ranked second among the most economically important rice diseases. The influence of nitrogen levels, planting date and air spora concentration on the disease severity was studied in the field. GD severity in the rice cultivar BRS Bonanca was assessed on two planting dates (30/11/2006 and 21/12/2006), using five nitrogen levels (0, 30, 60, 120 and 240 kg of N.ha-1) and a randomized split plot block design with three replicates. The effect of N level on GD severity was not significant. The correlation between GD severity and empty spikelets was positive and significant. The fungus population in the air (air spora) was quantified using the volumetric trap Rotorod Sampler from panicle emergence to maturation. Grain discoloration linearly increased with time (r = 0.98; P < 0.01), but the same did not occur with the total number of fungi, which varied from 0.23 to 2.97 spores/liter of air/minute. The fungi present in the air in decreasing order of spore concentration were Nigrospora sp., Pyricularia oryzae, Pithomyces sp., Alternaria sp., Cercospora sp., Fusarium sp., Curvularia sp. and Bipolaris sp. These fungi, Phoma sp. and others were also detected in the seed health testing. The correlation between the quantity of spores of P. oryzae and other fungi was linear and positive (r=0.80, P < 0.01). The number of spores increased with the increase in relative humidity and exponentially decreased with the increase in maximum temperature.

Progresso da brusone nas folhas no plantio direto e convencional de arroz de terras altas

The disease progress of leaf blast (Pyricularia grisea) and chlorophyll leaf content in upland rice (Oryza sativa) were studied under direct drilling (DD) and conventional drilling (CD), in field experiments during 1998/1999 and 1999/2000. The apparent infection rates of leaf blast for CD and DD were b = 0.099 (r = 0,97**) and b = 0.666 (r = 0,90**) respectively, in the first growing season. In the second season, the rates for CD and DD were b = 0.054 (r = 0,99**) and b = 0.05 (r = 0,99**). However, there was no significant difference in the rates of leaf blast progress in these two tillage systems. A linear, positive and significant relationship was observed between the chlorophyll content of the leaf and leaf blast severity.

Manejo integrado da brusone em arroz no plantio direto e convencional

The objective of this work was to develop adequate measures for rice blast (Pyricularia grisea) management integrating cultivar resistance, cultural practices and chemical control. Two field experiments were carried out, one under direct drilling and the other one under conventional tillage, during two consecutive rice growing seasons, 1998/1999 and 1999/2000. The treatments totaling 16, in a factorial scheme 24, included two cultivars, Carajas and Primavera, two levels of N, 30 and 60 kg ha-1, nontreated seed and seed treated with pyroquilon, plots nonsprayed and sprayed with two applications of fungicide mixture benomyl and difenoconazole. The incidence and severity of leaf and panicle blast were significantly lower under direct drilling as compared to the conventional tillage. The cultivar Primavera showed greater leaf blast susceptibility, independently of the tillage system. The application of 60 kg ha-1 of N increased leaf blast in the second year. Two applications of fungicide reduced panicle blast severity in both tillage systems. The grain yield under conventional tillage was higher than under direct drilling. The cultivar Carajas was superior to Primavera.

Morphoanatomy and histochemistry analyses of cassava roots do not discriminate resistant from susceptible genotypes to soft root rot

A mandioca e uma cultura importante no Brasil e no Norte do pais, e a podridao mole da raiz tem afetado a producao. O objetivo deste trabalho foi identificar caracteres morfoanatomicos e histoquimicos da raiz associados a resistencia a podridao da raiz. Em areas sem ocorrencia da doenca, nove genotipos de mandioca foram testados, quatro resistentes e cinco suscetiveis. A colheita da raiz foi realizada doze meses apos o plantio e medidas de espessura do suber, suber e cortex e xilema secundario foram realizadas. Alem disso, foram analisados a textura, as camadas de celula do suber, o conteudo de lignina e os monomeros de lignina da porcao suber e cortex. Ainda, testes histoquimicos e microscopia eletronica de varredura (MEV) das raizes foram realizados. Os resultados foram submetidos a analise de variância e as medias foram comparadas pelo teste de Scott-Knott (p≤0.05). Os caracteres: espessura de suber e cortex nas porcoes mediana e proximal, espessura do xilema secundario nas porcoes mediana e apical, numero de camadas de celulas do suber, espessura do suber e textura da raiz apresentaram diferencas entre os genotipos, mas nao associadas a resistencia ou suscetibilidade. Nao houve diferencas para: elementos de vaso, lignina e monomeros de lignina para os testes histoquimicos com Sudan IV e Lugol. Imagens de MEV apresentaram diferencas entre os genotipos, mas nao associadas a resistencia ou suscetibilidade. Assim, os caracteres avaliados nao podem ser usados na selecao de mandioca com resistencia a podridao mole da raiz em programa de melhoramento genetico.