Influência de práticas culturais e da resistência genética na intensidade do cancro da haste e produção de soja no Cerrado

Os efeitos de práticas culturais e resistência genética na intensidade ao cancro-da-haste da soja (Diaporthe phaseolorum f.sp. meridionalis - Dpm) foram examinados na safra 1995-96 em áreas de produção comercial, que haviam sido severamente atacadas na safra anterior (1994-95). Um experimento investigou os efeitos de cultivo mínimo (MT) e do plantio direto (NT) no desenvolvimento da doença e produtividade das cultivares FT-Cristalina e FT-Seriema. Em outro experimento, semeado em plantio direto, estudou-se o efeito da densidade de plantas (8, 15, 21 e 36 plantas/m) no desenvolvimento da doença e produtividade das cvs. FT-Cristalina (suscetivel), FT- 101 (moderadamente resistente) e FT-104 (resistente). Incidência e severidade da doença foram menores em NT do que em MT. A produtividade aumentou no sistema NT (23% para ‘FT-Cristalina’ e 14% para ‘FT-Seriema’), comparada com as produtividades em MT. As curvas de progresso da doença foram melhor descritas pelos mo- delos de Gompertz e logístico. A severidade do cancro aumentou pro- porcionalmente ao aumento das densidades de plantio nas cvs. suscetível e moderadamente resistente. No final do ciclo, 100% das plantas da cv. FT-Cristalina estavam infetadas por Dpm, em todas as densidades de plantio. Níveis intermediários de doença foram observa- dos na cv. FT-101, enquanto apenas níveis muito baixos de doença ocorreram na cv. FT-104. Estabeleceu-se uma correlação negativa entre severidade de cancro-da-haste e produção. Algumas das práticas estudadas demonstraram potential para aplicação direta no controle da doença, e poderiam ser combinadas considerando-se seus efeitos aditivos.Field experiments were conducted in the 1995-96 soybean (Glycine max) growing season to evaluate the effects of cultural practices and host genetic resistance on the intensity of soybean stem canker, caused by Diaporthe phaseolorum f.sp. meridionalis (Dpm). Experiments were conducted in a commercial field severely infected in the previous (1994-95) season. In one study, minimum tillage (MT) and no-tillage (NT) cropping systems were investigated for their effects on disease development and on plant yields in cvs. FT- Cristalina (susceptible) and FT-Seriema (moderately resistant). Another study evaluated the effects of plant densities (8, 15, 21 and 36 plants/m) on disease development in cvs. FT-Cristalina, FT-101 (moderately resistant) and FT-104 (resistant). Disease incidence and severity were consistently lower in NT than in MT, and plant yields were increased by 23% and 14% in the NT system for the susceptible and moderately resistant cultivars, respectively, compared to the yields in the MT system. The Gompertz and Logistic models described well the disease progress curves in all situations. For both susceptible and moderately resistant cultivars, disease severity increased proportionately to the increase in plant densities. At the end of the season, 100% of the plants of cv. FT-Cristalina were infected by Dpm, at all plant densities. Disease levels on cv. FT-101 were intermediate while only very low disease levels were recorded on cv. FT-104. There was a consistent negative correlation between stem canker severity and yield. Some practices demonstrated potential for direct application in disease control, and could be combined considering their additive effects

Transmission and overseasoning of Diaporthe adunca on Plantago lanceolata.

To obtain a better understanding of the onset and the progress of epidemics caused by Diaporthe adunca on Plantago lanceolata we investigated the role of the asexual phase of the fungus not only in the transmission of the disease from season to season, but also in the spread of the fungus within the same season. Splash dispersal of conidia during rain episodes proved to be an effective method for the spread of the fungus. Conidial production by pycnidia present on scapes left on the ground during winter was delayed in comparison to that of pycnidia produced on greenhouse infected scapes, indicating that the fungus overwinters with immature pycnidia. Latent infections occurred in 8.3% of the seedlings which emerged from seeds originating from infected spikes. A scenario for the transmission and spread of the fungus is proposed. [KEYWORDS: Pseudocercosporella-herpotrichoides spores; infected wheat straw; aerial dispersal; phomopsis; splash; communities; pathogens; helianthi; inoculum; nodorum

Biology of the weevil Trichosirocalus troglodytes and impact on its host Plantago lanceolata

We tried to measure the effects that a native parasite may cause to its native host by studying some biological properties of the specialist weevil Trichosirocalus troglodytes in relation to the impact on its host Plantago lanceolata. Density and activity of the weevil were estimated in four different ways: Tullgren, sweep net, capture-recapture and counting feeding holes in the leaves. The sweep net gave reliable information on relative densities and on presence of the weevil on the spikes, but the results were much influenced by the diurnal and seasonal activities of the weevil. Most eggs were laid during spring in the oldest leaves and most larvae were found in the caudex of the plant. Weevils of the new generation appeared from July onwards and developed flight muscles. The impact of 0, 3, 6 and 12 larvae on the spikes and the rosette of different P. lanceolata genotypes was investigated. The number of seeds per spike decreased linearly with the number of larvae added to the spikes. No effect on plant performance was found when the larvae were added to the rosette. The weevil spreads its risks by using different plant parts as oviposition sites during a prolonged period, hereby softening the ''pain'' put upon the plant. [KEYWORDS: insect herbivore; impact; weevil; Plantago lanceolata; density; oviposition; flight muscles Spatial autocorrelation; generalist caterpillars; behavior; curculionidae; evolutionary; coleoptera; preference; specialist; goldenrod; insects

The role of weevils and of mowing in epidemics of Diaporthe adunca in naturally occurring populations of Plantago lanceolata

The aim of this paper is to report on how a complex pathosystem, consisting of the plant (Plantago lanceolata), the weevil (Trichosirocalus troglodytes) and the fungus (Diaporthe adunca) functions in natural populations. In laboratory and garden experiments the weevil has previously been shown to transmit the fungus to the spikes of P. lanceolata, where infection usually starts. However, the interactions of this pathosystem in natural situations were unknown, for example, if the weevil and the fungus occur at the same place and time. To study this as well as the preference of the weevil and the fungus for certain environmental conditions, transects at 23 locations in the Netherlands were laid out. The effect of one of the environmental factors namely the mowing regime on epidemics of the fungus was experimentally tested in a hayfield. A Principal Component Analysis (PCA) showed that weevil and fungus preferred the same habitat. However, the data obtained during two years in 13 permanent transects, showed that the weevil was only active in spring and that the epidemic reached its maximum in autumn. These findings together with additional, circumstantial evidence presented suggest that the weevil does not play an important role in the transmission of the fungus in natural situations. The experiment with the mowing regimes showed that active management can exert an important effect on epidemics of the fungus. The later the vegetation is mown, the lower the level of disease at the end of the season. Late mowing can be seen as having a sanitary effect and might have negative consequences for disease occurrence in the long run. [KEYWORDS: insect herbivore; pathogen; epidemic; PCA; Plantago lanceolata; vector Phomopsis-subordinaria; lawn weeds; major l; disease;variability; genecology