Fungal Diseases

• Black spot is a fungal disease that causes black spots on leaves, surrounded by an overall yellowing of the other leaf tissue, before leaves fall off the plant. It occurs most readily in wet, warm weather. Rust is another fungal disease common in rose bushes. Orange spots on leaves and stems identify this disease, but leaves also turn yellow or brown and drop off in severe cases.
  
  

Insects

• Insect damage from mites and thrips also causes yellowing of the leaves on rose bushes. Mites cause speckling of leaves as they suck sap from the rose bush. Leaves sometimes drop and heavy infestations result in visible webbing over the plant. Thrips also suck plant sap, causing distorted, yellow-flecked leaves.
  
  

Nutrient Deficiencies

• Yellowing leaves appear as a symptom of several nutrient deficiencies. These include iron, nitrogen and potassium deficiencies, with iron deficiency being the most common. Other deficiencies, such as one of phosphorous, do not result in yellowing leaves, though leaves may turn other colors or drop prematurely with no warning.
  
  

Prevention and Control

• Fungicides protect rose bushes from fungal diseases. Cultural controls include avoiding wetting the leaves during watering, pruning off infected branches and removing fallen leaves from infected bushes to avoid infecting other nearby plants. Spraying plants down with a strong stream of water from the hose removes many mites and eggs, though a miticide may prove necessary in extreme cases. Thrips are difficult to control, requiring insecticide sprays or soaps applied at five- to seven-day intervals. Specialized fertilizers for rose bushes prevent nutrient deficiencies. Maintaining a 6 to 6.5 soil pH helps optimize nutrient absorption.