Effect of Alternaria and Septoria spot on sunflower yield.

Leaf diseases affect crop yields. In sunflower crops, leaf spot severity can reach 100%, but the magnitude of the yield loss caused by the disease is not known. This study aimed to evaluate the effect of Alternaria and Septoria leaf spot severity on sunflower yield across different years in a humid subtropical climate. We conducted 37 experiments in Santa Maria, RS, Brazil, over 7 years. The hybrids Embrapa 122, Helio 358, Aguará 03, and Altis 99 were sowed and managed according to national crop recommendations. Severity assessments for Alternaria and Septoria spots were performed at 2- to 7-day intervals using a diagrammatic scale. We evaluated the effects of Alternaria and Septoria leaf spot severity on crop yield using upper limit graphs. The 37 experiments comprised 13 normal season crops (August to October) and 24 late season crops (November to February). The results were also classified according to the contemporaneous phases of the ENSO (El Niño Southern Oscillation): El Niño, La Niña, and Neutral. In normal season crops, severities of up to 24% do not result in yield decrease. After this, each 1% increment in disease severity produces a decrease of 66 kg ha-1 on sunflower yield. In late season crops, the reduction in productivity occurs at severities greater than 34%, with a decrease of 50 kg ha-1 for each 1% increase in combined disease severity. The highest severity values and lowest yields, both in the normal and late season crops, occurred in El Niño years.

Plastocrono e número final de nós de cultivares de soja em função da época de semeadura / Plastochron and final node number of soybean cultivars as a function of sowing date

O plastocrono, intervalo de tempo entre o aparecimento de dois nós sucessivos na haste principal da planta, e o número final de nós (NFN) são importantes variáveis do desenvolvimento de plantas. O objetivo deste trabalho foi determinar o plastocrono e o NFN de cultivares de soja (Glycine max (L.) Merrill) semeadas em diferentes épocas em um local de clima subtropical. Um experimento de campo foi realizado em Santa Maria, RS, no ano agrícola de 2005/2006, com três épocas de semeadura (09/11/2005, 09/12/2005 e 28/01/2006). Foram utilizadas 15 cultivares de soja recomendadas para as regiões Norte e Centro da Argentina, e Sul e Sudeste do Brasil. As variáveis medidas foram o número acumulado de nós (NN) e o NFN na haste principal. O plastocrono foi estimado pelo inverso do coeficiente angular da regressão linear entre o NN e a soma térmica acumulada a partir de emergência (temperatura base = 10°C). Semeaduras tardias resultaram em menor plastocrono e menor NFN, provavelmente devido ao menor fotoperíodo a que as plantas foram submetidas durante a fase de emissão de nós, uma resposta característica de plantas de dia curto.

The plastochron, which is the time interval between the appearance of two successive nodes and the final node number (FNN) are important variables of the vegetative development in soybean. The objective of this study was to determine the plastochron and the FNN of soybean (Glycine max (L.) Merrill) cultivars sown in different dates under irrigated conditions in a subtropical location. A field experiment was carried out in Santa Maria, RS, Brazil during 2005/2006 growing season, with three sowing dates (09/11/2005, 09/12/2005 and 28/01/2006). It was used 15 soybean cultivars recommended for the Central and North Argentina, and South and Southeast Brazil. The variables measured were the number of nodes (NN) and the (FNN) on the main stem. The plastochron was estimated by the inverse of the slope of the linear regression between NN and the thermal time (base temperature = 10°C) accumulated since plant emergence. The late sowing resulted in lower plastochron and FNN, probably because of the lower photoperiod under which the plants were submitted in the node phase, a typical response of short day plants.

Controle da requeima em batata cv. 'Asterix' como base para modelos de previsão da doença / Controlling potato cv. 'Asterix' late blight base on forecast systems

O controle da requeima da batata requer aplicação freqüente de fungicidas, o que encarece a produção, impactando de modo desnecessário o ambiente. A utilização de modelos de previsão dessa doença permitiria reduzir as aplicações sem afetar a produção. Neste trabalho, objetivou-se avaliar os modelos "Blitecast e Prophy" como referência para o controle da requeima por fungicidas. Os experimentos foram conduzidos na primavera de 2004 e no outono de 2005, em Santa Maria, RS. Os dados meteorológicos foram medidos no centro da área experimental, a 0,10 e a 1,50 m acima da superfície do solo. Utilizaram-se diferentes valores de severidade (VS) acumulada, calculada pelos modelos "Blitecast" (VS= 18, 24, 30, 36 e 42) e "Prophy" (VS= 15, 20, 25, 30 e 35) que se constituíram os tratamentos, adicionando-se o tratamento com aplicação semanal e a testemunha, sem aplicação. O delineamento foi inteiramente casualizado com quatro repetições, sendo cada parcela composta de quatro fileiras de plantas com 5 m de comprimento. Avaliou-se a severidade da requeima por parcela a cada três a cinco dias. Verificou-se que o uso do modelo "Blitecast" com 18 valores de severidade acumulados, incrementou, em pelo menos, 42,6 por cento a produtividade de tubérculos comerciais em relação às áreas sem aplicação de fungicida e reduziu o número de aplicações em 25 por cento nos períodos úmidos e, em 70 por cento nos períodos secos, em relação às aplicações semanais. A eficiência de controle da requeima foi similar à obtida com aplicações semanais de fungicida nos tratamentos Bli18 e Pro15. O uso do modelo "Prophy" requer maior número de aplicações do que o "Blitecast" e não resultou em melhor controle.

The control potato late blight needs a great number of fungicide sprayings. These increase the costs of cropping and cause undesirable environmental impacts. The use of forecast systems to predict disease has the potential of reducing fungicide applications without reducing yield. The objective of this study was to evaluate the performance of Blitecast and Prophy systems as a reference model for predicting late blight potato and support decision of spray fungicides. Experiments were carried out during Spring 2004 and Autumn 2005, in Santa Maria, RS, Brazil. Meteorological data were measured in the center of the experimental area at 0.10 and 1.50 m above soil surface. Different accumulated severity values (VS) were calculated with 'Blitecast' (VS = 18, 24, 30, 36 and 42) and 'Prophy' (VS = 15, 20, 25, 30 and 35) forecast systems. These values were used to perform. Two additional treatments were the weekly sprays and without fungicides. The experimental design was a completely randomized, with four replications. Each plot had four rows plants with 5 m length. Late blight severity was evaluated in each three to five days. The fungicide spraying schedule based on Blitecast system with 18 accumulated severity values increased tuber yield at least 42.6 percent compared to the control without fungicides treatment. It also reduced the number of sprayings by 25 percent during wet periods and 70 percent during dry periods compared to weekly sprayings. The efficiency of controlling late blight was similar to the weekly sprayings treatment in the treatments Bli18 e Pro15. The Prophy model predicted higher number of fungicide sprayings than the Blitecast system and did not improve disease control.