Diseases in oats Powdery mildewPowdery mildew can affect both winter and spring sown oats. The disease, produces a white fluffy fungal growth on the leaf surface. This can cause leaves to turn yellow, leading to a loss in green leaf. Read more here. Oat crown rustCrown rust is one of the main diseases to affect oats, and can affect some grasses too. Notable from the orange coloured pustules on both sides fo the leaves. Read more here.