Posted: 09/01/2014 at 16:00
The black patches in Photinia leaves are caused by Entomosporium maculatum, a fungal leaf spot disease. It's is very common and is not harmful to the shrub. The main reason of these spots occurring, is the leaves remaining wet over a long period. It doesnt look very nice though.
Plants in a sheltered spot are more prone to leaf spots, as well as new foliage. Pruning during the growing season will encourage new growth. Mature leaves are more resistant to leaf spot.
- Rake up and discard fallen leaves, and remove infected plant material. Apply fresh mulch around plants to cover any leaves that were missed. You could also try underplanting with low growing perennials, such as catmint.
- Provide sufficient air circulation. This often means thinning out a few plants in a hedge.
- Avoid wetting the leaves when watering. Splashing water spreads the fungus.
- Avoid summer fertilization that will promote new growth late in the season.
If nothing seems to work, you can try spraying with a fungicide just before and during new foliage is emerging. If that doesn't make any difference, I would be inclined to remove Red Robin althogether.
I hope this will help.