Fungal Diseases On Roses And Lavender.
Fungal diseases on Lavender and roses – as a summer control for the fungal disease I would recommend using Bordeaux Fungicide more details can be found at the following address.
http://gardenadvice.co.uk/advisor/gardening-tips/bordeaux-fungicide/
As for the black spot try spraying with a tar wash whilst the plants are dormant,clear up all the dead leaves from last year and remove from the site and then to stop last years spores from being splashed onto the new springs growth by the spring rains you need to mulch the ground with well rotted farmyard manure or spent mushroom compost or another such material.This mulching is slightly dependent on the type of soil you have.