By Colinlangton
United Kingdom 
I put in 150 yds of rosas rugosa hedge 2 yrs ago and its done v.well and is about 3ft high now . But suddenly its got black spot and rust on the leaves which are turning yellow and falling off.
I think it may have spread from 2 nearby large rambling roses which were infested with it.....ive since destroyed them.
Being Dartmoor the ground is acidic and ive read that spraying with diluted milk will alter the PH to alkaline and get rid of blackspot? To use a chemical spray on this much hedge would cost a bomb!
Would it also help to feed it now with say fish blood & bone fertiliser or is it better to wait till spring?
6 Sep, 2016
Answers
The milk remedy is 1 part cows milk to two parts water, sprayed weekly from spring onwards, spraying all parts,including under the leaves, till run off, so its more a preventative than truly effective treatment. Alternatively, Rose Clear Ultra sprayed fortnightly from spring onwards.
6 Sep, 2016
Previous question
Although some members will disagree, I would still go down the chemical remedy; perhaps try a horticultural suppliers for larger amounts. The blackspot spores will overwinter in the fallen leaves and it will be a mammoth task to pick all of them up. If you do decide to spray with which ever method you decide to use, you need to spray as soon as the leaves emerge in the spring and on a regular basis throughout the season. I wouldn't say that the soil type would affect the disease resistance.
6 Sep, 2016