Answer: Roses are potentially subject to so many problems, it is difficult to make a guess as to what is affecting yours. There are several common reasons for this to occur. A heavy aphid infestation can ruin the buds. The aphids can be knocked down with a spray of water from the garden hose or with insecticidal soap, or, if lady bugs or other natural predators have appeared you may not have to do anything to control the problem. They usually also subside once the summer turns hot. Thrips may damage buds, and in fact white roses seem to be preferred by thrips. Insecticidal soap may also control these along with rose midges which might also attack buds. Occasionally you may find caterpillars inside the buds. These can be controlled by removing the buds and treating the plant with applications of Bt (Bacillus thuringiensis). The buds could also be infected by botrytis, a fungus. This can be controlled to some extent by ensuring good air circulation and possibly by the use of a product containing neem. The fungal infection is made worse by cooler wet weather, too. Bacterial leaf spot (you will see brown sunken spots on the leaves and stems) can also infect the buds; cut away, remove and destroy affected stems. With some careful observation and a little detective work perhaps you can figure out what is happening to your rose.
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