Knockout rose problem

Asked September 9, 2016, 1:31 PM EDT

I have 3 knockout roses in my front yard and one has been losing leaves while the other 2 look ok. Any idea what the cause is or what I can do?

Brazos County Texas

1 Response

The rose with yellowing leaves appears to have a bit of black spot - common fungal rose disease. It is caused by the fungus, Diplocarpon rosae and is most severe after long wet, warm periods in the spring commonly and sometimes the fall. Symptoms are circular, black spots on leaves surrounded by a yellow area. Infected leaves often drop from the plant. The immature wood of first year canes develops raised, purple-red irregular blotches. Plants become stunted and produce fewer, paler flowers. Knockout roses are "resistant" to the disease, meaning they can and do get it, but more often than not they tolerate it.

This rose may need some better drainage in that spot and probably better air flow. Exposure makes a difference as well, with morning sun the very best since it dries moisture off the leaves early in the day which reduces development of fungal disease. Given the wet weather we've had in the last few weeks, it has been difficult to manage this disease. If this area in your landscape is irrigated with spray heads - observe to see if leaves are getting wet at that time and try to adjust, as that exacerbates the problem. Remove infected leaves and canes. Then a refined horticultural oil can be sprayed onto foliage to help manage the disease. An alternative is to treat the rose with a fungicide spray according to label directions.