Pest and Disease Spotlight: Black Spot, Mildew and Rust on Roses
March 30, 2023

This month, as part of their Pest and Disease Spotlight, Kate and Len look into how to protect your roses!

by Kate and Len Lucas

With a very long history, the rose has to be the most familiar of all plants in the garden and all those years of breeding and development has produced a list of varieties with hundreds, possibly thousands, of names on it. And yet, these most elegant of plants can still suffer from any one, or all, of these three diseases. They look like their name suggests – black spot appears as black spots. Mildew looks like the familiar mildew and rust, although less common, appears as small round bright yellow or orange blobs on the underside of the leaves.

In the distant past, when burning coal was common,  black spot on roses was almost unknown. As the years went by coal burning went down and down but black spot became more and more common, any ideas why?

If left untreated these diseases can totally defoliate the plant in the growing season leaving an unsightly mess and making you feel, ‘why did we bother?’

They like different weather conditions, so if you don’t get one then another will turn up and it might look like we just can’t win. But all is not lost, there are things that we can all do to limit the damage and our approach to when to start spraying your roses might help. 

First, is good housekeeping. During the colder months you will still see leaves on roses with familiar black or rust spots on them, so take them all off and remove them from the garden, and that also applies to fallen rose leaves on the ground. It can be hard work but it’s worth it. And when you do the spring pruning make sure to take off any old leaves on the remaining stems at the same time.

Second, be aware of the level of disease resistance of the rose you are about to buy or even the ones you already have. Not all roses get severe attacks, some roses are better at resisting these fungi than others. Some roses are very susceptible to them, for example, we have the old bourbon rose “Zephryn Drouhin” introduced in about 1868 which we grow because it has one the best scents ever, but we knew it would go down with an attack if we didn’t do something about it. This is in contrast to another rose we have called “Noisette Blush” which is less scented but hardly seems to get any disease at all. 

We are not experts in this field, but it is our experience that the older and the more scented the rose, and we mean when it was bred and not how long you have had it, the more likely it is to be susceptible, as in the 1868 rose named above. Rose breeders have worked hard to improve resistance and a lot of today’s varieties are indeed better. So a bit of homework should pay off. However, even if the label on the plant might lead you to believe that the rose is resistant, it doesn’t mean it will be 100% disease free.

Finally, having done all that, do we just wait for an attack before treating the roses? 

We wait until we see the first greenfly make its appearance on our roses which could be anytime in early Spring. We then can be fairly sure that these diseases will not be far behind so we spray all of the roses, even the ones that have no greenflies, with “Roseclear” in a ready-to-use trigger spray. This product will control these three diseases and a wide range of other pests. We will then repeat the spraying every three or four weeks, especially young new growth. For most of the year, our roses are pest and disease free but we still get outbreaks of mildew and some rust.

For more detailed information, head over to the The Royal Horticultural Society, to read more about the range of products available to amateur gardeners for the treatment of pests and diseases of roses.