Black Spot

• This is a fungus, and causes black spots to develop on the leaves. These spots grow in size and a yellowing occurs. A rapidly spreading disease, it usually leads to severe defoliation. Heavy infections can cause leaf buds and stems to die. It is encouraged by a potash shortage and warm, wet summers. Infected leaves should be removed and destroyed.
  
  

Purple Spot

• Not to be confused with black spot, the symptoms are similar, although markings are slightly smaller. Spots tend to be purple and more irregular. This is not a fungal disease but is usually the result of poor growing conditions. Improving drainage and applying a peat mulch will help.
  
  

Rust

• This is not so common but is often fatal when it does strike. An orange speckling can be found on the under side of the leaves, which can also turn black. This causes the top side of the leaves to take on a yellowy tinge. This is a wind-borne fungus, but can stay dormant in the soil. Therefore, any infected, dropped leaves should be collected immediately and burnt.