Water isn't what induces black spot. Humidity in the proper temperature range is what provides the proper germination conditions for the spores. Anything which provides a layer on the leaf surfaces to prevent the spores from coming in contact with the leaf tissue can act as a fungicide. That can be anti transpirants, oils, chemical fungicides (which can also actually kill the fungi spores or, in the case of bacterials, interrupt their germination cycles), even water.

Often, it's suggested to increase air circulation to help prevent black spot break outs. Reducing the humidity, often by increasing air flow, can help reduce the infections. What can you do about your lawn watering to reduce the humidity at the warmer parts of the days?

Perhaps selecting those more resistant to the infections in your areas might help provide you with fewer fungal issues?

In my own garden, I don't have lawn irrigation to contend with as there is no lawn. I do have coastal marine influence which can result in higher humidity. What has worked best for me is to shovel prune those which cause the most problems. There are plenty of roses out there which have the disease resistance you seek, just waiting for good homes. Kim

Kim's advice works well in Oklahoma too.

Sammy

Thanks, Sammy. With the plethora of roses out there, even now in the beginning of the Dark Ages where selection is diminishing rapidly, there are still many which perform well in so many climate types, it's still possible to find ones which are happy where you are without having to resort to extreme measures. Happy Holidays. Kim

It helps if you move to a place that has very hot summers--too hot for BS to grow! But of course all that heat can pose other problems too.

I have found that by restricting myself to roses that, by and large, are known for their disease-resistance, I have eliminated about 3/4s of the BS problems I used to have. There are plenty of choices in the disease-resistance categories. One just has to spend more time researching the roses in order to determine how resistant they are--but that is what snowy winters were invented for--since we can't get in the garden anyway. : )

Kate

Wouldn't it be lovely to force the chemical manufacturers to produce more beneficial, less toxic products instead of garden chemicals, by selecting plants which don't require as many of them? Instead of outlawing them as has been done elsewhere, make them unprofitable to produce. I guaranty they'll find other products to create ASAP! Kim

Blackspot spores must be wet continuously for 7-12 hours in order to germinate. (Google diplocarpon continuously wet.) That's water on the leaves, not humidity. High humidity induces germination of powdery mildew spores but not blackspot.

So in summer, germination will occur if there is even a spatter of rain in the late afternoon or before midnight. In cooler weather, germination occurs on drippy overcast days with highs in the 60s and 70s.

This is why blackspot is rare in Southern California. Although nighttime humidity is high near the coast, it's virtually a desert for actual rainfall.

No cultural practices other than spraying a good fungicide will prevent blackspot. Overhead watering has been shown to reduce blackspot if it is done at a time the foliage will dry rapidly. Our Karl has experience with this. Picking off spotted leaves reduces the number of spores available. Very severe pruning in spring postpones the onset of blackspot for that season, because lesions on the green canes are the main source of blackspot carried over the winter. Dead leaves on the mulch are a secondary source.

I have personally observed the effect of severe pruning. In the 70s and 80s, my HTs had to be pruned nearly to the ground. Then I saw little or no blackspot until the beginning of the first flush. Our climate has warmed so much that I now have lots of surviving cane, and now--if I don't spray-- I may see BS as soon as the leaves expand on high-pruned HTs as well as on shrub roses.

I refuse to spray. I always try to plant roses that have excellent disease resistence. Even than some of these roses will still come down with BS. These are culled out of my garden. I usually try the vareties that are disease free in the NY Botanical Garden. That garden is about 15 miles from my house. Yet some of those roses, when planted in my garden, will come down with BS. The disease free rose across the road might not be disease free in your yard. This only proves that all rose gardening is local. Most of the roses that are listed as having excellent disease resistence will be fine in my garden. Some will still come down with BS. However this shouldn't discourage the rose gardener as there are more disease free roses coming on the market every year. You have to be willing to experiment and not waste space on a sickly rose.

For years my Mother would set out soakers (pre-drip era) and hand water only at the bases and be so careful not to get any of the leaves wet, etc., etc., etc...and she had terrible BS all the time. We're in Michigan. We live in a fish bowl surrounded by copious amounts of water. Any humidity reading less than 60% is positively arid here unless it's January and then it might go to 40% only because it's all frozen solid. The BS spores are out there everywhere, in the soil, in the air, in the water, everywhere! I overhead water all the time. I even wash my roses off with a hard spray from time to time. And on really HOT days I put the sprinkler on them to cool them off. I think sometimes I can actually hear them sigh in relief. I know it sure feels good to me when I run through the sprinkler trying to get it set up on those hot days. And with all that I have no more, and IMHO, I think I even have less BS than Mom had. I do know I have fewer problems with insects than she did. She always had problems with aphids and in the last 2 years I don't think I've seen any on my roses.

BS is out there. Battle it if you must but try and do it safely and sanely.

You probably grow more resistant types than your mom did, too, Seil. She primarily had products of the chemical generations to choose from. Virtually no one selected seedlings for health without chemical intervention as everyone expected to have to spray. You've culled many of those types from your garden over the years and have tremendously greater choices available of no to little spray types. Interesting we've come full circle back to "Natural Selection" for disease resistance, isn't it?

I frequently left oscillating sprinklers on the roses in the old garden on high nineties to triple digit days and they were incredible for it. HUGE leaves, LOTS of foliage, no insects nor diseases and remarkably few weeds. Huge plants which densely shade the ground prevent a ton of weed growth. It was so arid in that canyon, there was very seldom any fungal issues other than very early or very late in the year. Kim

Thanks for the responses, everyone. Beyond the obvious of trying to grow resistant varieties, so far I've also gotten:
1. Overhead watering during warm parts of the day
2. Prune worst offenders down to ground each spring (what if it's a climber?)
3. Maybe I should change my lawn watering to early morning instead of overnight...

I wish I could shovel prune, but my worst offender is Double Delight, which I love. It completely defoliated this year after the first flush. Its BS spread to immediate neighbors and defoliated them too...the neighbors being climbers Princess Margarite and Teasing Georgia, along with April In Paris and Aromatherapy. Several other Austins about 10 feet away in the same bed also showed BS infestation--those being Jude, Spirit of Freedom, Abraham Darby, and Brother Cadfael. That whole bed was a major mess. My other rose beds, about 30 and 50 feet away, showed little BS on individual roses, certainly not enough to defoliate.

I blame it on myself. In 2010, the BS was bad, and I didn't think to remove the foliage that had settled on the ground that winter before mulching with leaves. In the spring of 2011, I made it worse. Not only did I not prune to the ground, I also left the canes that I did prune cut up into small pieces on the floor of the bed. Yeah, I don't know what I was thinking either. As I have learned since, BS lives on in the leaf litter, and if it lives on in the canes as well, then that's probably why that bed was such a BS hotspot this year. That's a new cultural practice to note:

4. REMOVE all pruned foliage every year, like you would for peonies, to prevent spread of disease.

So how do I turn this bed around next year? I had my husband blow most of the 2011 diseased leaf litter out of this bed before winter protecting. But I'm sure there's still tons of BS left in the ground waiting to reinfest next spring. What to do? Is there something that can be sprayed once in the spring while dormant to kill the BS spores left on the canes and ground?

gz4,

Overwintering BS is not in the soil, but in spots on the canes and in dead spotted leaves on the mulch. If you cover the fallen leaves with a thick layer of fresh mulch in spring before the new leaves expand, it should reduce the number of spores that reach the new leaves.

I suggested hard pruning could help with hybrid teas, but it isn't appropriate for shrubs, much less climbers. If winter damage forces you to prune these severely, blackspot will be postponed, but you will get far fewer flowers in the first flush. Even with HTs, you get somewhat fewer flowers.

Unfortunately there are still very, very few repeat-blooming roses that are resistant to blackspot and cercospora spot diseases in the worst areas of the eastern US. In some areas, including my own and Olga's, if you don't want to spray and don't want to look at leafless plants, it's best to plant mostly once-blooming gallicas, albas, and species roses.

In zone 4 these once-bloomers will make a much better spring show than typical repeaters, because they are cane hardy and don't have to be cut way back.

Good point, Kim. Disease resistance is definitely a higher priority for breeders now than it was in Mom's day. But I still think the hose blasts help too. Anything that keeps the foliage clean, in theory, should be a deterrent to fungal infections.

Gardenerzone4, you don't need to prune to the ground each spring. Yes, there are BS spores on the canes that can overwinter. But they're also everywhere else in the garden so I can't see any point in cutting off good, healthy green cane for no other reason. All you end up doing is removing a lot of the energy that rose has stored in those canes to regenerate itself with in the spring. The spores will still be in the soil and air and cutting them off won't stop you from getting it. In the early spring when they first start to bud out, but before they're completely leafed out and you can't see the structure of the plant, prune them back from the top down in small amounts to the first point where you have good, healthy, green cane with a clean white center. If the center is brown or dark tan or if there is a dark streak running down the cane anywhere prune a little more until you reach a white pith and good green cane. The rose will tell you how far down you need to go that way. Take out any dead wood (removing dead wood can be done at any time of the year) and remove canes that cross or rub. You can also open up the center of the rose by removing small side canes that grow inward to allow better air flow through the rose which can help deter fungal infections.

As I've said the spores are already out there everywhere. Picking off the spotted leaves and cleaning up the debris will make your bed look neater but will not stop you from getting black spot. In recent studies they've found that even spotted leaves that are still green will continue to supply food to the plant until they yellow and fall off. So don't strip those leaves. You end up depriving your rose of it's food source when it needs it most to support it's immune system. If it's very hot and sunny you also remove it's shade and can cause dehydration and cane burn. As long as the leaf is still green I leave it on the plant to feed it. As for the leaves on the ground, once they've yellowed, fallen and died THE SPORES DIE TOO. They are not the cause of new infection. I do pick mine up but only because I hate the messy look of them on the ground.

Getting roses that are more disease resistant in your area is the best way to deal with BS. But, like you, I have some that I truly love regardless so they stay and I've learned to just deal with it. And finding ones that are resistant is a matter of trial and error because it's very location specific. What may be bullet proof for me could be a disaster in your garden and vice versa.

Hmmm, we seem to have some disagreement here. I've never before heard that when the leaves die, the spores die, too!

My own tack is to scrupulously pick up every BS-spotted leaf that falls, and to prune away whole branches if they get really spotted. Over the past 4-5 years (out of 35), this has really made a big difference. Probably along with the newer, more resistant plants. (I try to spray only once, after pruning in the late winter/early spring.)

Bottom line, as one of the Follow-Uppers said is, all rose growing is local.

Happy New Year, everyone!

You said you couldn't shovel prune your worst BS rose, Double Delight. Maybe you have a large enough lot to do what a rose friend of mine did with his BS problem roses. His rose beds were around his house so he moved his problem roses to a spot on the far side of his 2nd garage/workshop thus quarantining them.

Black spot is not very mobile. The spores are relatively heavy and sticky and not airborne. The typical mode of spread is by splashing rainwater carrying the spores a few inches at a time. Given that fact, the gardener's hands or gloves could play a big role in transporting spores around the garden or throughout the plant.

I believe the single biggest factor contributing to blackspot promotion is also the one I have zero control over--weather patterns between spring and mid summer.

One thing we know in Central Illinois is that most T-storm watches/warnings occur in the late afternoon, as the sun's heating provides energy to forming systems. We tend to get severe weather late in the day followed by lingering showers throughout the early evening into sunset.

Instead of a lovely shower that rinses off hot, dry foliage and then evaporates in the day's sunlight, we have humidity-soaked plants that get a thorough drenching and remain wet during the falling temps in the evening.

Where would you expect moldy, icky stuff to grow better--a dry, sunlit place that gets a regular rinsing, or a dark and cooler area that has moisture buildup with no chance to evaporate?

Bebba, that information came from a rosarian who is also a retired microbiologist and has studied black spot. Picking up the leaves does nothing to prevent BS spread and taking off entire branches will only set the rose back further. By removing cane you reduce the roses ability to produce food it needs to keep it's immune system functioning. And spotted but still green leaves continue to produce food for the plant until they yellow and drop.

I think Michael hit the nail on the head. We probably transfer more spores around the garden ourselves than anything else does. A good reason to keep our hands, gloves and tools clean.

And Flauara's point is also good. We can't control the weather. There isn't a whole lot I can do when the temps and humidities are at that perfect growth point for fungal spores. And spring and fall in the mid-west is perfect BS growing season.

Below is a link to a 2008 Vintage Roses newsletter, read p.5 where a UC-Berkley study on BS is cited.

Here is a link that might be useful: Vintage Roses newsletter

What an interesting newsletter! I'm interested in reading more about the UC Berkeley study. Is there a research publication that describes the research and results in detail?

The article on pruning is also very eye-opening. So what happens if a grandiflora or HT reaches 6' tall and you don't cut it down, and it doesn't get winter killed? Does it keep growing taller past 6'? Or does it stay at that height naturally but start expanding outward by adding canes at the edges of the plant?

Gardnerzone4--I experimented with non-pruning this past year and got some funny results. I had a couple of HT's that had very little die back over last winter, so I did minimal pruning. I started the spring with plants around the 4 or 5 foot mark. I ended up with a couple of beasts.

Blue Girl grows upright for me, and continued upright, to about the 9 foot mark by the end of the year. Yes she was big & impressive, but 80% of her blooms were over my head (and I'm 6') for most of the summer. This coming spring I'll take her back down to a more reasonable level. She looks downright silly next to her younger siblings.

Garden Party grows outward--into a more rounded & full shape. By the end of summer, it was about 6 feet wide and 7 feet tall. It looked like it would reach out & grab small children as they walked by & we started calling her Audrey. I loved the way it looked--a big, fat globe of a bush with beatiful fat, smelly roses.

Where I live, I think it depends on the growth habit of the rose & the amount of winter kill. I prune mainly to shape and to take off dead pieces, but I imagine things are much different in Chiswell Green than where I live. I have a friend in that region of England and she says it's much milder there in the winter than here in Central IL.

Yes, it will continue to get taller. Because most HTs are stiff caned they hold them selves up very well at these heights. My Memorial Day and Folklore routinely get over 6 feet by the end of the season. I do stake them to keep the canes from snapping in high winds but otherwise they do quite well and are lovely.

Yes, the reason most HTs need to be cut back every year is their bolt-upright growth with flowers only at the top. Flowers grow out of reach and canes may break over in the rain and wind. And yes, 'Garden Party' does have relatively bushy growth as well as unusual winter hardiness.

If anyone has a link or citation for the blackspot study mentioned by Seil and Vintage, I'd love to have it.

How much of the bolt-upright growth is innate to the HT rose, and how much of it is influenced by being grafted v. own-root?

I garden in a particularly dry part of Sweden and don't do anything to prevent the little blackspot I have in most years. I don't have time to pick the leaves from 250 roses. There are no efficient chemical anti-fungal products for home gardeners in Sweden, only for farmers.

Many other gardeners use trichoderma fungi to fight blackspot. It comes in various forms and under various brand names, for spraying the foliage or for dusting the soil. I am not impressed by the results.

gz4,

The bolt-upright growth habit is bred into HTs, especially from ca. 1910 ('Ophelia' for ex.) forward.

It appears to have been a trait specifically selected for as they've become more bolt upright each decade. Probably something demanded by those who sought the high-centered flower for their gardens as it makes growing them in "rose gardens" easier to maintain. Budding won't make a lax bush stiff and upright, but it can intensify the trait on one which already possesses it. Kim