I have Black Spot on one of my rose bushes and wondered what to do to treat this?
Black Spot is a fungal infection that thrives in rainy or humid conditions. Once a rose is infected it is very hard to completely get rid of the fungus but by following the guidelines below, you may be able to control the spread and keep your rose bush blooming. A healthy rose will have a much better chance of fighting off the fungus than a rose that is in a weakened state.
Cut out any of the diseased stems and clean up any fallen leaf debris is the first step. Also, scrape the top few inches of soil around the base of the plant and remove it along with any decaying leaves that are incorporated into it. This will clean away fungal spores that may be on old leaves that are not yet completely decomposed. These should all be placed out for collection by the city/municipality and not put into a home compost pile. You can do all this now and then continue to keep the debris clean through until all the leaves have dropped in the late fall. In the spring, any leaves that were missed in the fall should be cleaned up and discarded.
Make sure to sterilize cutting/pruning tools between each cut to avoid spreading the black spot to healthy plant tissue. A weak bleach and water solution will do the job nicely.
Good quality compost or well rotted manure should be incorporated into the soil around the base of the plant making sure not to cover any graft between rootstock and scion. This soil can then be mulched in order to prevent weed growth and to assist with water retention.
Watering of the rose should be done at the soil level if possible, in order to avoid wetting the leaves. This will help control fungal spread. Roses are thirsty so should be watered really well and less often in order to force good root growth. Roses are also heavy feeders and should be fed with a lower nitrogen fertilizer such as a 6-12-6 every 4 weeks up until mid-August when the feeding should stop.
Pruning in the spring is very important in order to not only cut away diseased, dead or dangerous branches but also to open up the plant so that air can freely circulate and sun can penetrate the centre of the bush. Think of creating an open vase shape. Cutting back or moving other surrounding plants will also help with air circulation around the rose. Moving air will keep leaves dry which leads to less chance of fungal spread.
Mounding the base of the rose bush for winter with clean material will help protect the graft from freeze/thaw cycles which are common during Canadian winters.
If Black Spot continues to be a problem and can’t be controlled, then I would suggest digging up the infected rose bush, disgarding it and planting something else instead. You may find that you have different spot in your garden for another rose bush as there are many species and cultivar roses to be had that are Black Spot resistant. Look at it as an opportunity to try something new and change the garden a bit.