Botrytis is easily controlled using normal fungicidal treatments. And like most fungi, it more easily prevented than cured. So be on the lookout for any signs of disease and treat immediately.
Lack of good drainage - caused by improperly preparing the soil to an adequate depth for good drainage around the rose roots. This is the most serious problem in my part of the country - where soil is heavy in clay content. Rose roots need air. And since water displaces air in the soil, the lack of good drainage means the roots are not receive sufficient air. The result is brown-edged blooms, weak spindly stems, and "sad" leaves.
Improving drainage is best done before planting the rose. Double digging may be required. If drainage cannot be improved, then consider planting the roses in elevated beds or in containers.
Calcium deficiency - may reflect a calcium/boron deficiency or soil imbalance. This is common in roses where high nitrogen and high phosphorous sources (like those from water soluble, synthetic rose fertilizers) are used during periods of heat stress and in soils with high salt indices. A soil test is a must. If calcium is deficient, add dolomitic lime. Chances are, the problem is caused by the fertilizer used on the roses. Consider changing from a water soluble, high nitrate/high phosphorus fertilizer to an organic, slow release blend.
Phytotoxic reaction - common in roses sprayed with fungicides or insecticides during the summer. The chemicals (or their inert ingredients like surfactants) can cause browning of petal edges if applied during or just before the heat of the day. Spraying, if necessary, should be done very early in the morning.
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