A ferrugem da folha e a helmintosporiose são as principais doenças fúngicas foliares de aveia branca. Em cultivares suscetíveis, o controle químico não considera o limiar de dano econômico (LDE). O objetivo deste trabalho foi obter funções de dano, por modelos de ponto crítico, ao relacionar o dano pela ocorrência simultânea da ferrugem da folha e a helmintosporiose com o rendimento de grãos nas safras de 2006 e 2007, em Lages, Santa Catarina (SC). O gradiente da intensidade das doenças foi gerado por número de aplicações e doses de fungicidas nas cultivares 'UPFA 20' e 'UPFA 22'. Os ensaios consistiram de 10 tratamentos, distribuídos em blocos casualizados, com quatro repetições. As doenças foram avaliadas nos estádios de alongamento, emborrachamento e florescimento. Os modelos de ponto crítico foram obtidos por regressão linear entre rendimento de grãos e incidência (I) e severidade (S). Em 2006, funções significativas foram obtidas para 'UPFA 20' no emborrachamento (R=6.345-22,07I), enquanto para a 'UPFA 22' foi obtida no alongamento (R=5.726-17,96I; R=5.160-541,5S), emborrachamento (R=5.961-21,85I; R=5.433-966,8S) e florescimento (R=6.217-36,41I; R=5.539-1.185S). Em 2007, para 'UPFA 20', no alongamento (R=6.369-55,61I; R=4.965-2.911S), emborrachamento (R=6.620-40,91I; R=5.181-1.702S) e florescimento (R=6.129-32,34I; R=5.252-638,6S), e para 'UPFA 22', no alongamento (R=5.201-41,96I; R=4.977-657,8S), emborrachamento (R=5.329-60,51I; R=4.238-798,1S) e florescimento (R=5.604-32,83I; R=4.680-588,5S). A produção na cultivar 'UPFA 22' foi mais afetada pela ocorrência das doenças, e a intensidade das doenças no estádio de florescimento apresentou maior relação com o rendimento de grãos. As funções geradas permitem obter o coeficiente de dano para uso no cálculo do LDE em patossistema múltiplo ferrugem da folha e helmintosporiose.

The leaf rust and helminthosporium blight are the main foliar diseases of white oat. The chemical control in susceptible cultivars does not consider the economic damage threshold (EDT). The objective of this research was to obtain the damage functions by critical-point models to relate damage by simultaneous occurrence of white oat leaf rust and helminthosporium blight with yield grains during 2006/07 crop seasons in Lages, SC, Brazil. The gradient of diseases intensity was generated by number of applications and rates fungicides on cultivars 'UPFA 20' and 'UPFA 22'. The trials consisted by 10 treatments distributed in randomly blocks with four replications. Diseases evaluations were done in the elongation, booting and flowering stages. The critical-point models were obtained by linear regression between grain yield and leaf diseases incidence and severity. In 2006, significant function was obtained for 'UPFA 20' in booting stage (R=6.345-22.07I), with 'UPFA 22'. The significant functions were obtained in elongation (R=5.726-17.96I and R=5.160-541.5S), booting (R=5.961-21.85I; R=5.433-966.8S) and flowering stages (R=6.217-36.41I; R=5.539-1.185S). In 2007, the significant functions were obtained in all growth stages evaluated. To 'UPFA 20', the significant functions were in the elongation (R=6.369-55.61I; R=4.965-2.911S), booting (R=6.620-49.91I; R=5.181-1.702S) and flowering (R=6.129-32.34I; R=5.252-638.6S), while in the 'UPFA 22', were in the elongation (R=5.201-41.96I; R=4.977-657.8S), booting (R=5.329-60.51I; R=4.238-798.1S) and flowering (R=5.604-32.83I; R=4.680-588.5S). The 'UPFA 22' cultivar production was more affected by diseases occurrence and the diseases intensity in the flowering stage showed greater relation with the yield grain. The generated functions allowed obtaining the damage coefficient to use in the EDT in the multiple pathosystem leaf rust and helminthosporium.