I do a lot of garden visits to clients houses and also a lot talks to all types of groups and the biggest rose question is of course how do I stop black spot and mildew on my roses. Trouble is it’s a not a short answer!
First of all what exactly are we talking about when we talk about Rose leaf problems, so let’s have a look in a little detail before we work out how to deal with it
Black spot. This is a horrible fungus called Diplocarpon rosae. It starts off with black spots (hence the name!) sometimes they maybe small dots on the leaves, other times, it will be large black spots, all depends on the hybrid being grown and also the weather conditions. The leaves may slowly turn yellow (but not all times) and then fall off! The trouble with this fungus is that it is always changing, with new strains emerging at all times, the plant that wasn’t effected last year may not be safe for this year, same with roses bred with blackspot resilience. It’s quite often, the more modern roses like hybrid teas, florabundas, bourbons, hybrid perpetuals that suffer the most. Weather conditions are also a major play a major part with this fungus, the spores themselves are spread by water splashing, so it’s worse in a wet summer, of course we never get a wet summer do we!
The mildews! There are 2 that can effect roses, one is more common than the other one, so let’s first of all look at the worst problem I think that faces roses and that’s
Powdery Mildew, Podosphaera pannosa. As the name suggests, it forms a white powdery fungus over the buds, leaves and new growth. This can cause the infected flower buds to fail and shrivel up and die, new growth can be deformed and have the vigour reduced, which in the case of rambling roses, where the new growth is so important for the plant flowering next year, can cause major problems. Unlike blackspot, it prefers a humid hot summer, with the plants being more susceptible during periods of dry spells
Downy Mildew, peronospora sparsa, is not commonly found on garden plants more within a nursery system where the plants are grown so close together. Unlike powdery mildew it prefers a hot humid conditions with 85% humidity and temperatures over 80f, when it does happen however it’s the worse out of the lot, totally defoliating the plants, biggest problem is that it looks like blackspot. Biggest diffenence is the speed of defoliation on the plants and the spots from downy mildew are more squared than the more circular spots of black spot. Just thankfull it isn’t a major problem in the uk
Lastly there is rust, Phragmidium tuberculatum, it can appear from early spring and be around all summer, it will not kill the plant as it can only live on live material. It forms orange pustules on affected growth and mottling on the leaves, they turn black by late summer, it is probably the least worst of all the fungus we have talked about.
Treating the above problems isn’t just a simple one of mixing up a cocktail of chemicals and spraying them but is indeed a combination of many things that all put together, can reduce the impact of the fungus attacks. First thing is good husbandry, in the autumn, pick up all the fallen leaves and either burn them or add to the green waste bags, second part has to be the main prune during the later winter months. This main pruning does quite a few things to help disease control, it should open up the roses well and encourage good air flow through the plant, this helps to stop the spores resting on the foliage and stems, it should also encourage stronger more healthier shoots from the plant and remove any badly infected wood. It also reduces the amount of wood within the plant again reducing the amount of wood the spores can rest on.
The third thing is the feeding and mulching of the roses in the spring, a mix of vitax Q4 and a good feed mulch like composted green waste, home produced compost and well rotted manure. Please see my old blog on doing this. this again helps to produce good healthy soil, with all the friendly fungus, bacteria in the soil, a good healthy soil will produce a healthy and strong growing rose and a healthy strong growing rose is once more resistant to fungal attacks. The mulch helps to keep the moisture in the soil, which helps to stop plants drying out too much and once more being understress, that again can allow fungus to gain a hold once more. Spreading the feed and mulch over the whole bed rather than just around the base of the rose, also encourages the roots of the Rose to spread over the whole bed, meaning the plant is more able to have a wider pick up are for water and nutrients. After the main flush of flowers in June, I will then feed the soil once more with blood fish and bonemeal this time, this helps the plants produce their second flush of leaves and new growth as well.
The fourth and final part has to be the regular spraying and treatment of the foliage of the Rose during the rose season. This should be ideally carried out from April-September every 10-14 days. Firstly only really rust, black spot and powdery mildew can be treated this way, Downy mildew has its own problem and shall deal with that lastly. Now what to spray? For the domestic garden the choice is a little more limited to the following, tebuconazole (Bayer Fungus Fighter Concentrate), tebuconazole with trifloxystrobin (Bayer Fungus Fighter Plus), and triticonazole (Scotts Fungus Clear Ultra), there are a few others that are used for spraying roses but I don’t like to use insecticide within the garden, the damage too non problematic insects is too great for me personally. The trick is to use each one every other time so the fungus doesn’t have time to build up a resistance to the spray. At the same time it’s worth adding a foliage feed like potassium phosphite (uncle toms Rose tonic) and liquid seaweed. These help to feed the leaves and in the case of potassium phosphite, encourage strong healthy growth on the plant that’s a lot tougher and again much harder for the fungus to get hold onto the plant. Indeed over the past few years, I have dropped using fungicides on the roses and just use both the uncle toms Rose tonic and liquid seaweed. The results have been very good, with clients saying they haven’t seeen their roses looking so healthy, yes there’s a little bit of blackspot sometimes but it’s only a small amount. Mildew can be a problem of ramblers that put on a lot of growth in the summer, so I do tend to carry on with the fungicides with them. There are other products that I am trying at home to see the results before recommending them. Only one that’s a problem is downy mildew and sadly there’s no real treatment available to the public to buy, but it is so rare, so tends not to be a problem. Ideally this should be sprayed with a low pressure sprayer with a fine mist and sprayed until the leaves are throughly soaked until it ‘runs off’ both the surface and underneath of the leaves should be sprayed. It’s worth remembering to use the right safety equipment when spraying roses regularly just to help you avoid prolonged exposure to any chemicals, organic or non organic and if spraying climbers, please wear some form of eye protection. One other thing I think helps is underplanting the roses with other plants. I feel that they provide both a small ‘wind ‘ break between the roses which helps with the dieseases that’s spread via the wind and also with blackspot that is spread by water splashes, it acts as a deflector for the rain, both as it falls and also as it bounces once it hits the ground, both of which helps to cut down the area the water droplets will tend to hit.
Other methods have been used to treat powdery mildew, include mixing oil, milk into water, this tends to coat the leaves of the roses in a thin layer of dried oil or milk and this stops the spores resting on the leaves of the plant, sulphur is another old treatment that works well on black spot. I have tried them in the past, have found the results ok but not as good as my current methods. Other things like choosing a more resistant form of Rose can help, but please be aware than it can still become infected by one disease or another at sometime.
One small note, I am not too worried what the plant looks like after it finishes its main flower time in late June early July, it has a 2-3 week spell of looking pretty aweful, nothing you can do about it, they are just worn out after putting so much effort into producing beautiful flowers for us to enjoy, they just need a little break, within a couple of weeks, new growth will push though and the Rose will start to look beautiful once more .
I hope this helps you to produce healthy roses this summer
I would also like to say a big thank you to Annie Irving for the use of the rose rust picture!