Roses are beautiful, but notoriously difficult to grow. Even under expert care, rose bushes are susceptible to a number of diseases, pests and fungi. And any number of these afflictions can cause your rose's petals to brown. Most of these problems are relatively easy to fix with a few cultural changes or sprays. But if the problem crops up season after season, it may be a sign that you should plant a more resistant variety of rose.
Avoid overhead watering. When water sits on your roses or their leaves for more than eight hours, any number of fungi can develop. And, according to the website Rose Gardening Made Easy, some of these fungi can turn your rose petals brown. Instead, water your roses at their base and avoid wetting the foliage altogether.
Check your rose bushes for insects periodically. According to the University of Kentucky, several insects can cause rose petals to brown. Thrips (tiny brown winged insects), for example, feed on rose buds which causes the petals to start their lives with brown edges if the rose opens at all. As soon as you notice an infestation, correctly identify the culprit by collecting and bagging a specimen then taking it to your local county extension office or garden center for professional identification. Then spray your roses with insecticidal soap or an insecticide prescribed for use on that particular bug.
Protect your roses from heat and drought stress which can turn petals brown. When temperatures climb over 90 degrees, mist your roses early in the morning to keep them moist. Check the soil frequently to make sure that it does not dry out. Container grown roses may need to be watered up to twice daily.
Prune your rose bushes to encourage air circulation and light penetration in their interior. According to All-America Rose Selections, this will help to keep the bush's foliage dry and prevent the development of fungus which can cause petals to brown.