Q: Can you tell me what is causing the brown spots on my Knockout rose stalks?
A: At first I thought it was rose cane canker but pathologist Jean Williams-Woodward says:
“Cytospora canker tends to have a tanner center and the canker girdles the stem. These look more like black spot infection of the stems. The fungus overwinters on fallen leaf litter and on stem infections (cankers, lesions, whatever you want to call it).
“The stem infections will not girdle the stem as other canker causing fungi would.
“Knockouts are not immune to black spot; they just get it less than more susceptible rose varieties. They are not “disease-free” as touted. The difference in disease development is dependent upon environmental growing conditions.
“Where I see more black spot in landscapes is where the plants are hit by lawn sprinklers and may be planted in mass. Knockouts don’t drop their leaves as easily when infected, so black spot may go unnoticed.”
You should start a black spot spray control program immediately. Prune out affected rose canes. Be sure to rake up fallen leaves each October/November.
black spot on leaf
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