|Abstract:||Individual studies were done on two spring oat cultivars, Ogle(IL) and Noble(IN) in 1989 and 1990, to estimate crop loss due to Puccinia coronata f.sp. avenae (crown rust), in the presence or absence of barley yellow dwarf virus (BYDV-PAV-IL). Several levels of crown rust (CR) were established in all experiments by inoculation with CR or fungicide application. For experiments including BYDV, plots were infested with viruliferous aphids (Rhopalosiphum padi) and BYDV incidence was determined visually and with TAS-ELISA. CR disease severity was assessed several times, and yield and yield components were determined. Linear critical point regression models were developed for both cultivars, primarily from CR assessments made on the flag-1 leaves at Zadoks growth stage 75. Yield loss, for each unit increase in CR, was 56.7 kg/ha for Noble and 46.0 kg/ha for Ogle. Test weight decrease, for each unit increase in CR, was 1.2 kg/m$\sp3$ for Noble and 1.1 kg/m$\sp3$ for Ogle. Ogle exhibited tolerance to crop loss from CR. Yield loss, due to each unit increase in CR, in BYDV infected plots, was 19.2 kg/ha for Noble and 28.7 kg/ha for Ogle. Test weight decrease, for each unit increase in CR, was not different from zero for Noble and 0.7 kg/m$\sp3$ for Ogle. Noble exhibited tolerance to crop loss from CR in BYDV infected plots. The effect of BYDV, on yield and test weight loss due to CR, was not additive. Studies to develop an oat plant growth model were done in 1988, 1989, and 1990. Minimum and maximum daily temperatures, growing degree days (GDD), leaf area and several yield components were determined. Leaf area was evaluated as an empirical parameter for predicting potential oat yields. Leaf area had no predictable relationship to yield, test weight, spikelets/panicle, or, seeds/panicle, and no models to predict yield were developed. The relationship between leaf area and GDD was evaluated and linear models were developed to predict leaf area increase due to accumulated GDD. Linear components describing leaf area increase for each unit increase in GDD ranged from 0.84 to 1.04 cm$\sp3$ and quadratic components ranged from $-$0.0005 to $-$0.0009 cm$\sp3.$ Overall, approximately a one to one relationship existed between leaf area increase and accumulation of GDD.