Tillage system effect on the epidemic of soybean brown spot.

Six field experiments were carried out during the summers of 1997 to 2003 to evaluate disease progress of soybean brown spot caused by Septoria glycines considering two tillage systems, conventional and no-tillage (wheat was cultivated during winter in all plots). Two logistic models were fitted to the disease progress data: (i) a logistic model with constant disease progress rate r [y=1/(1+(1/y0-1)exp(-rt)), where y0 is the initial disease level at time t=0] and (ii) a logistic model with an exponentially increasing progress rate r(t)=r0 ebt [y=1/(1+(1/y0-1) exp(r0/b(1-ebt))), where r0 is the initial progress rate at time t=0, b the rate increasing parameter, and y0 again the initial disease level]. The logistic model with constant rate underestimated disease incidence on the first disease assessment in nine out of twelve epidemics. The logistic model with an increasing rate gave a better fit to all disease progress curves (R2 between 0.90 and 0.99; no pattern in the residuals). According to this model, y0 was in most cases (four out of six) significantly smaller in the plots with no-tillage compared with conventional tillage, but no differences were detected in the initial rate parameter r0 and the rate increasing parameter b (with one exception). It is proposed that these results are due to higher susceptibility of old leaflets compared with young leaflets, as demonstrated by artificial inoculation: In four laboratory experiments the mean brown spot severity was 16.7 % on old leaflets but only 3.9 % on young leaflets

Disease progress of soybean bud blight and spatial pattern of diseased plants.

Fourteen epidemics of soybean bud blight, caused by the tobacco streak virus, were monitored in two soybean cultivars ('IAC-4' and 'FT-10') for two years (1987 and 1988) at Arapoti County, state of Parana, southern Brazil. Disease incidence, based on different sowing dates, were analyzed for statistical fitness in several growth models. Ten epidemics were better described by the monomolecular model, while four epidemics were better described by the Gompertz model. The highest final level of disease incidence was y((f))= 0.6 in 1987 and 0.9 in 1988. There were no significant differences between the cultivars in relation to susceptibility. Early-crops always presented more disease than late-crops. The spatial patterns of diseased plants also changed according to sowing dates as determined by ordinary runs analysis. Diseased plants in early sowings had a random pattern, at first, later becoming clustered (Z below -1.64). In late sowings, diseased plants always had a random pattern

Análise espacial e temporal de epidemias de patógenos de solo.

A epidemia é um sistema e como tal comporta-se como um todo em resposta a estímulos dirigidos a qualquer parte. Obviamente o patógeno, o hospedeiro e a doença constituem-se em subsistemas do sistema epidemia. Dependendo do interesse do observador, qualquer um dos três subsistemas pode ser elevado à categoria de sistema e estudado isoladamente: isto é fácil de ser visualizado no caso do hospedeiro, que não depende nem do patógeno nem da doença para sua existência, mas também é possível no caso do patógeno, seja quando se considera sua sobrevivência na ausência do hospedeiro, seja quando se considera seu crescimento saprofítico em meio outro que a planta. A doença é a interação entre patógeno e hospedeiro, sob a influência do ambiente e do homem. A análise epidemiológica temporal e espacial, apresentada a seguir, aplica-se indistintamente tanto a epidemias causadas por patógenos aéreos quanto àquelas causadas por patógenos veiculados pelo solo.Acesso em: 23 out. 2009. Disponível em: http://www.cpatu.embrapa.br/livro-fitosanidade/LIVRO-FITO-PARTE-1.pd