Critical yield-point model to estimate damage caused by brown spot and powdery mildew in barley/Modelo do ponto critico para estimar danos causados pela mancha-marrom e oidio em cevada

Barley (Hordeum vulgaris L.) is the second most important winter crop in Southern Brazil The excessive rainfall in this region during the crop-growing season increases the frequency and intensity of foliar fungal diseases. The research aimed to determine the damage function equations (DFE) for the multiple pathosystem of barley brown spot and powdery mildew based on the relationship between grain yield and diseases intensity at different BRS Caue' cultivar growth stages (GS) during 2009 and 2010 growing seasons in Southern Brazil. The experiments were arranged in a randomized complete block design with nine treatments and four replicates. The disease gradients were generated by strobilurins and triazols fungicides rates and number of applications on barley cv. Caue. The fungicide applications and disease incidence and severity assessments were performed at the 22, 31, 39, 45 and 56 plant GS. The DFE were obtained by variance analysis and linear regression between grain yield and diseases intensity. Significant and negative DFE were obtained and the damage coefficients (DC) varied from 29.48 to 100.08 (2009) and from 36.08 to 113.57kg [ha.sup.-1] (2010) for incidence, and from 219.5 to 6,276.6 (2009) and 102.3 to 5,292.5kg [ha.sup.-1] (2010) for severity. The largest damage coefficients were obtained when diseases assessments were made on GS 22 and 31 on both growing seasons evaluated. DFE were used to calculate the economic damage threshold (EDT) as a criterion to indicate the fungicide application moment to control the diseases in cultivars similar to 'BRS Caue ' in Southern Brazil Key words: Bipolar is sorokiniana, Blumeria graminis f. sp. hordei, Hordeum vulgare, multiple pathosystem, economic damage threshold. A cevada (Hordeum vulgaris L.) e a segunda mais importante cultura de inverno no Sul do Brasil. Nessa regiao, o excesso de chuvas durante a estacao de crescimento da cultura favorece o aumento na frequencia e intensidade de doencas foliares. O trabalho objetivou determinar as equacoes de funcao de dano (EFD) para o patossistema multiplo mancha-marrom e oidio da cevada pela relacao entre rendimento de graos e intensidade foliar das doencas em diferentes estadios fenologicos da cultivar de cevada 'BRS Caue', nas safras 2009 e 2010 no sul do Brasil. O delineamento foi em blocos casualizados com nove tratamentos e quatro repeticoes. Os tratamentos foram constituidos de diferentes doses e numero de aplicacoes dos fungicidas triazois e estrobilurinas p