Joseph's Coat Roses are susceptible to black spot, as well as orange rust and mildew. The best way to avoid these fungal diseases is through careful watering. Always water your roses at the base, and early in the day so that they have plenty of time to dry before dew settles for the night. Use fungicide on your plants as soon as you spot any troublesome black spots or orange dust, and dispose of infected plant stems and leaves properly.