Spider mites are relatives of ticks, in the class arachnidae, that like to suck the juice out of rose leaves. They look like very tiny red dots on the bottom side of rose leaves. First the infected rose leaves turn brown. Then they whither and drop off the rose bush. The spider mites can be washed off the leaves with a hose that sprays water hard. Another way to get rid of spider mites is to apply a miticide preparation that you buy from a local nursery. You should treat the rose bushes once each week during the summer. If you prefer a more natural treatment, buy some ladybirds because they eat spider mites.
Another reason that the leaves on a rose bush turn brown is black spot, which is a mycelium, a fungus. First the disease makes the leaves develop brown or black spots. Next the leaves turn yellow and fall off the rose bush. Treat the rose bush with a fertiliser after removing the infected leaves and canes.
When it is too hot or too sunny and you fertilise the rose bushes with Miracle Grow or another fertiliser, the leaves can get brown. This can also be the result of salt, sun or wind burns. To fix this problem, give the rose bushes more water. Another reason that rose leaves may turn brown is if the bush is deficient in potassium. Before the leaves turn brown due to a shortage of potassium, though, they will turn yellow. After they turn brown, they may become purple. Many fertilisers include potassium supplements which should resolve the problem.