Diseased Knock-Out Rose Bush

Powdery Mildew

• Powdery mildew is very common to roses and Knock Out roses can also be afflicted if they are sited in an area without good air circulation. Powdery mildew is caused by a fungus that spreads when days are warm and dry but nights are cool and damp.
  
  The mildew appears as a white powder on stems, which may start to look deformed. It also appears on the undersides of the leaves, which start to curl when affected.
  
  

Black Spot

• Raking up leaves can halt the spread of blackspot.Resting rake. image by bluefern from Fotolia.com
  
  Knock Out roses are susceptible to blackspot according to Louisiana State University's Agricultural Center, where a landscape trial of Knock Out roses is taking place. The black spots caused by the fungus are often seen on the leaves. There may also be some yellowing around the spots. Lower leaves are often infected first. If left untreated, the leaves may all fall off.
  
  Because the fungus overwinters in fallen leaves, tidying up the yard and discarding leaves may help prevent blackspot. Watering Knock Out roses from below and ensuring that the water does not splash on the leaves will also help to prevent the fungus.
  
  

Rust

• Water roses during the day.watering can image by Bartlomiej Nowak from Fotolia.com
  
  Yet another fungal disease that may attack Knock Out roses is rust. Rust appears as tiny orange spots on the underside of leaves and can slowly expand onto the stems as well. Rose leaves are susceptible to rust fungus when they remain wet for long periods of time.
  
  Watering Knock Out rose bushes should always be done during the day---never at night---so that the leaves have a chance to dry out. This may prevent rust from appearing on the leaves and stems.
  
  

Cankers

• Different varieties of fungi attack rose canes to cause cankers, and while Knock Out roses are disease-resistant they are not immune to cankers if proper garden hygiene is not maintained. Brown and black spots on the Knock Out rose canes indicate that they have been infected. If uncontrolled, the canker will girdle the stem and kill it.
  
  Prune out all diseased wood and destroy infected canes to control the spread of cankers.
  
  

Other Diseases

• Aphids spread rose mosaic virus.spider and baby aphid. image by mdb from Fotolia.com
  
  Other diseases that can affect Knock Out roses include rose mosaic virus, which is spread by aphids and shows up as wavy lines on leaves. Rosette and witch's broom produces thick stems that are very thorny, as well as small, deformed stems that look like a witch's broom. Crown gall is a disease characterized by bulbous growths where the stems meet the soil.