The most common reason that plants' leaves turn yellow is because of moisture stress, which can be from either over watering or under watering. If you have a plant that has yellow leaves, check the soil in the pot to see if the soil is dry.
What causes rose leaves to turn yellow with brown spots?
Black spot (blackspot) is one of the most common diseases of rose bushes; and, if left unchecked, it can cause quite a bit of damage to your rose garden. Caused by the fungus Diplocarpon rosae, black spot begins just as its name suggests, with black spots showing up on the surface of the leaves.
How often do you need to water roses?
Soil, temperature, and surrounding plants affect how much water a rose needs. In temperate climates, weekly watering is usually enough. Two inches of water a week (4 to 5 gallons) may be all that is needed. If the soil is sandy or the garden is hot, dry, or windy, more frequent watering may be necessary.