Poor flower production on rose bushes if often caused by problems that occur when flowers are in the bud stage. The problems may be unnoticeable until the bud opens and shows deformity or browning. However, many times the rose buds themselves turn brown and may never open. If they do manage to open, the flowers are unattractive.
Thrips (Anaphothrips obscurus) are tiny insects that feed on new rose buds and blooms. The damage is noticeable when the buds are beginning to open. The rose petals may be brown or spotted and in severe infestations, the buds may be entirely destroyed. If thrips are suspected, remove the buds from the plant and remove it from the garden to prevent further infestations.
Most rose bushes are dormant in the winter until warm spring temperatures arrive. However, in warmer climates, some rose varieties may begin to set buds before the last frost after several warm days in a row. If exposed to freezing temperatures, rose buds may be damaged and turn brown. Also, if freezing temperatures arrive in the fall while the rose bush is still flowering, the rose buds may be damaged and turn brown.
Roses grow best in full sun locations that receive at least six hours of sun each day. However, the soil must be high in organic matter, such as compost, as well as moist, and well drained. When there is not enough moisture available to keep the rose bush healthy and hydrated, the stems, flowers and rose buds wilt. Without sufficient moisture, the entire plant begins to turn brown, including the buds, which fail to open.
Botrytis blight, or grey mould, grows on rose buds that are located in cool areas with high humidity and poor air circulation. The buds have a noticeable mildew-like growth on them and may be distorted, yellowed or brown. They usually fail to open properly and may appear glued shut. Botryits blight often appears in spring. Control with a fungicide listed for botrytis blight on roses. Remove infected buds and plant material from the rose bushes.