Influence of previous crops and fungicide seed treatment in the incidence and control of damping-off caused by Rhizoctonia solani in cotton seedlings under greenhouse conditions

ABSTRACT The aim of this study was to verify the influence of previous crops and fungicide seed treatment in the incidence and control of damping-off caused by Rhizoctonia solani in cotton seedlings under greenhouse conditions. This experiment was carried out during two years at Embrapa Western Agriculture, in Dourados, Mato Grosso do Sul State, Brazil. In addition to cotton (treated and untreated seeds) and fallow, the following cover crops were tested as previous crops: black oats, millet, corn, forage sorghum, soybean, common beans, crotalaria (Crotalaria juncea), brachiaria (Urochloa ruziziensis) and brachiaria (Urochloa ruziziensis) + crotalaria (Crotalaria juncea). The fungicide mixture used as treatment to cotton seeds was triadimenol + pencycuron + tolylfluanid (50 + 50 + 30 g a.i./100 kg seeds). Seeds from cotton and previous crops were sown in soil contained in plastic trays and pots; the seeds were placed in individual and equidistant 3cm-deep wells. Inoculation of R. solani was obtained by homogeneously distributing the fungal inoculum onto the substrate surface (2.5g/tray and 0.34g/pot). The fungus was grown for 35 days on autoclaved black oat seeds subsequently ground to powder using a mill (1mm). Damping-off was daily evaluated from the seventh day after sowing. There was a significant effect of the interaction previous crops x fungicide treatment (P<0.05). The fungicide seed treatment was efficient in controlling seedling damping-off caused by R. solani and its effect was potentiated when grasses were the previous crops. Use of grasses such as brachiaria (Urochloa ruziziensis), black oats, millet, corn and forage sorghum as previous crops, besides fallow, significantly contributed to a smaller R. solani population in the soil, which resulted in lower rates of cotton seedling damping-off. On the other hand, using cotton continuously, as well as the legumes soybeans, beans, crotalaria (Crotalaria juncea), and brachiaria (Urochloa ruziziensis) + crotalaria (Crotalaria juncea) as previous crops to cotton, was consistently associated with higher rates of seedling damping-off, contributing to the increase or at least the maintenance of R. solani inoculum in the soil. The highest damping-off percentages were observed in plots under continuous cotton cultivation without fungicide seed treatment. The present results reinforce the need of improving damping-off control in cotton seedlings by adopting integrated management programs in areas infested with R. solani.

Controle químico de brusone e mancha parda na cultura do arroz irrigado

RESUMO As principais doenças fúngicas que acometem a cultura do arroz na região Sul do Brasil são a brusone e a mancha parda, responsáveis por perdas significativas de produtividade e qualidade de grãos. Com o objetivo de disponibilizar alternativas de controle químico para essas doenças, foram avaliados, durante três safras agrícolas, dez tratamentos fungicidas, aplicados nos estádios R2 (emborrachamento) e R4 (pleno florescimento). Para o controle da mancha parda, os tratamentos ciproconazol + picoxistrobina, epoxiconazol + cresoxim-metílico, tebuconazol + trifloxistrobina e triciclazol + tebuconazol foram os mais consistentes nos três anos avaliados. Para o controle da brusone, a mistura triciclazol + tebuconazol e o tratamento somente com triciclazol resultaram na menor incidência da doença, combinada com rendimento de grãos estatisticamente superior aos outros tratamentos. A mistura de triciclazol + tebuconazol apresentou a melhor combinação entre eficiência de controle de ambas as doenças e rendimento de grãos.

Phakopsora pachyrhizi is the major pathogen associated with soybean rust in Colombia

ABSTRACT Soybean crops grown in the plains of eastern Colombia have been affected by the incidence of rust. This disease, still officially regarded as American soybean rust (caused by Phakopsora meibomiae) by the Colombian Department of Agriculture (ICA), causes serious damage to soybean growing areas. Symptoms of rust, such as reddish-brown lesions, have been observed since 2004 in the upper half of plants during vegetative stages. In 2005 and 2018, infected leaf tissues and uredinospores were collected from an experimental area and from commercial soybean fields. Once morphological identification and Koch’s postulates confirmed the presence of Phakopsora spp., molecular characterization was performed to identify the pathogen associated with the disease to species level. This was based on standard procedures using internal transcribed spacer (ITS) regions of rDNA and PCR amplification with specific primers for Phakopsora meibomiae and P. pachyrhizi. The obtained sequences were BLASTed against GenBank/NCBI data bank. Results indicated that P. pachyrhizi Sydow is in fact the causative agent of soybean rust in Colombia, considering the samples collected in 2005 and 2018. The ITS-rDNA sequences of P. pachyrhizi were deposited at GenBank under the accession numbers MK933723 to MK933731. This finding was reported to ICA, so that they could officially update the phytosanitary status of this soybean pathogen in Colombia.

Influence of temperature and daily leaf wetness duration on the severity of bacterial leaf blight of garlic

ABSTRACT In the present study, climate control chamber conditions were adopted to investigate the influence of temperature (10, 15, 20, 25 and 30°C) and leaf wetness duration (6, 12, 24 and 48 hours) on the severity of bacterial leaf blight of garlic, caused by Pseudomonas marginalis pv. marginalis. The relative density of lesions was influenced by temperature and leaf wetness duration (P<0.05). The disease was more severe at 20°C. The obtained data underwent non-linear regression analysis. Generalized beta function was used to fit the data on severity and temperature, while a logistic function was chosen to represent the effect of leaf wetness duration on the severity of bacterial blight. The response surface resulting of the product of those two functions was expressed as ES = 0.019419 * (((x-5)0.5893) * ((35-x)0.5474)) * (0.51754/(1+23.59597* exp (-0.145695*y))), where: ES represents the estimated severity value (0.1); x, the temperature (ºC), and y, the daily leaf wetness duration (hours). This model shall be validated under field conditions to assess its use as a forecast system for bacterial leaf blight of garlic.

Qualidades fisiológica e sanitária de sementes de pinhão-manso submetidas a tratamentos alternativos e químico, e ao armazenamento

RESUMO Resumo: Objetivou-se estudar o efeito de tratamentos químico e alternativos e do ambiente de armazenamento na qualidade de sementes de pinhão-manso. As sementes foram submetidas aos seguintes tratamentos: produtos fungicidas, ambiente de armazenamento e tempo de armazenamento. Foram avaliadas as qualidades fisiológica e sanitária das sementes. Sementes tratadas com Carbendazin+Thiram apresentaram menor infestação de patógenos e germinação e vigor superiores àquelas tratadas com produtos alternativos ou sem tratamento, quando armazenadas por 12 meses, em câmara fria. Em câmara fria, os tratamentos alternativos que tiveram melhor desempenho foram alecrim, canela e cravo. Armazenadas em laboratório, aos oito meses, os melhores desempenhos das sementes, quanto à germinação e ao vigor, ocorreram quando foram tratadas com Carbendazin+Thiram, alecrim, canela e cravo; nessas condições, após 12 meses, houve queda drástica na qualidade fisiológica das sementes de todos os tratamentos. Os tratamentos alternativos mais promissores foram com alecrim, canela e cravo.

Fatores de riscos e dinâmica espaço-temporal da meleira do mamoeiro no extremo sul do estado da Bahia

RESUMO A meleira do mamoeiro é considerada um dos maiores problemas fitossanitários da cultura do mamoeiro, mas diversos aspectos da sua epidemiologia ainda são desconhecidos. O objetivo deste trabalho foi determinar o risco e o padrão espaço-temporal da meleira nas condições de cultivo da região extremo sul do estado da Bahia. Foi utilizada a regressão logística para identificar os fatores de riscos associados à ocorrência da meleira na região do extremo sul da Bahia. Para o estudo da distribuição espacial, foram aplicadas as seguintes análises: sequências ordinárias; teste t (student) e áreas isópatas. Os resultados da regressão logística mostraram que o risco de um pomar apresentar meleira sendo consorciado ou consorciado com a cultura do café é maior do que quando estes fatores estão ausentes. Em geral, a meleira evoluiu lentamente do primeiro até o sexto mês de avaliação, com média de até 17,2% de plantas infectadas no sexto mês e chegando até 88% das plantas infectadas em campo ao final da epidemia. Agregação de plantas doentes foi observada em menos da metade das áreas avaliadas. A análise de áreas isópatas indicou uma tendência para início das epidemias a partir das bordas dos pomares e a presença de focos secundários e isolados da doença.

Elaboração e validação de uma escala diagramática para a queima bacteriana do alho

RESUMO A queima bacteriana do alho causada por Pseudomonas marginalis pv. marginalis é a principal bacteriose foliar na cultura. Pouco se conhece sobre a epidemiologia dessa doença e para sua avaliação não existe uma escala ilustrativa que descreva o quantitativo de severidade nas folhas. Com base nisso, esse trabalho teve como objetivo elaborar e validar uma escala diagramática para a doença. O trabalho foi realizado através da digitalização de 108 folhas de alho coletadas no Instituto Federal Catarinense - Campus de Rio do Sul com diferentes graus da doença. As imagens foram processadas para verificar o percentual de tecido lesionado presente na folha. A validação foi realizada por oito avaliadores sem e posteriormente com auxílio da escala diagramática com 30 folhas de alho com diferentes níveis de severidade. Obtiveram-se os valores representativos de 21, 37, 45, 56, 63 e 85% de severidade para compor a escala diagramática O uso da escala diagramática proporcionou maior acurácia e precisão das estimativas visuais para a avaliação da queima bacteriana do alho.

Development parameters of the latent period as a response of rubber tree resistance to South American leaf blight

ABSTRACT The major phytosanitary problem for rubber tree cultivation in Brazil is the disease known as South American leaf blight, caused by the fungus Microcyclus ulei. Its symptoms manifest in young leaves and cause intense defoliation, resulting in reduced latex production and even the death of susceptible plants. Thus, this disease consists in a constant threat to East Asian plantations. As worldwide traditional breeding programs have evolved, interspecific hybrid clones have currently been used for planting. They are more productive and show better resistance to pathogens. However, traditional breeding programs have not led to significant progress in resistance to South American leaf blight since the selection is directed to clones with complete resistance. In this pathosystem, horizontal or partial resistance (HR) and vertical or complete resistance (VR) can act simultaneously, evidencing their complexity and difficult quantification. This study aimed to: characterize the foliar lesion type in Hevea sp. X M. ulei pathosystem; verify the clonal susceptibility to pathogens; analyze the infection frequency for resistance quantification in Hevea sp. x M. ulei pathosystem, and recommend differentiating rubber tree clones to quantify M. ulei races in Brazil. The monocyclic parameters (latent period and lesion diameter) are applicable for resistance quantification in Hevea sp. x M. ulei pathosystem. Latent period had slight variation among clones. Lesion diameter had wide variation among clones and was a discriminating parameter for horizontal resistance and vertical resistance.