There are five races of black spot in the US. Many overlap and you just may be seeing the effects of several types occurring at the opposite ends of your season. Perhaps you have one which hits early in the season, attacking only those which are most susceptible, with another (perhaps even multiple types) striking others which are susceptible to them later in fall. It's possible the early type is more virulent, striking less mature foliage of more susceptible types, while the later types require older, more "geriatric" foliage to take hold. It's entirely possible for some of your roses to be resistant to one or more of the races or strains at work, as well as some to be susceptible to all of them. It seems logical to me the worst would be the late season attacks. There is much more "geriatric foliage" available for the fungi to "eat", and Nature seems to use diseases to help "tell" plants it's time to slow down and begin hardening off for colder winter weather. Kim

Blackspot has been a major problem here (zone 6 KS) also. Normally mid- to late summer is so hot around here that even the BS gives up and leaves. However, this is the coolest and wettest (although there have not been heavy rains with big accumulated downfalls, just rather continuous light moisture or overcast skies) summer I've ever experienced in Kansas and I've been gardening here for 25-30 years. The BS is unbelievable, plus some other leaf diseases also, plus leaves just going limply yellow (no markings) and falling off.

And most of my roses are very disease-resistant (supposedly), but nearly everything has heavy-duty BS and other diseases. I'm at my wits end here--just have to live with it, I guess. I sprayed fungicide a couple days ago, and rose bushes all over the yard suddenly were sporting limp yellow leaves--and I mean, lots of them--that limply fall off! I just don't understand what is happening.

Kate

Your limp, yellow foliage sounds like water logging, the leaves "drown" because the plant has too much water. Because it's wetter and milder than any other season you've experienced, the disease pressures are higher than you've ever known before. Stress the plants with too much water and it's entirely possible their immune systems are impaired, permitting them to succumb to diseases they normally would shrug off. YOU know how to play the game, but NATURE changed the rules on you this year. Kim

zaphod, it may be that the plants that defoliated early beefed up their immune response and so got less blackspot later.

Another possibility is confusion between blackspot and cercospora or some other spot disease. One set of plants is more susceptible to BS and the other more susceptible to cercospora.

I have severe cercospora on some plants this very rainy season. This disease progresses more slowly than blackspot. Leaves develop dark spots and then weeks or months later you find large dead brown spots and the leaves turn yellow and fall.

I am not sure whether I didn't spray often enough or whether tebuconazole doesn't control cercospora well. Because of the slow progress of the disease, it is hard to tell.

Between the mild winter and above average rainfall -- often rain at night-- it's black spot heaven here in my South Carolina gardens.

My strategy this year:
Major Spray every 14 days, rotate fungicides (Banner Maxx, Heritage, Compass) and put Pentathlon in all spray mixes.

5 to 10 days after a major spray, do Supplementary sprays with Pentathlon. Do this either before or after a rain.

One of the best ways to stop a black spot outbreak is to spray every other day with Pentathlon over the course of a week.

I guess this borders on spray mania. I use this program on the gardens I tend professionally. In my home garden, I don't have as many HTs, and have more resistant varieties, so the spot is less of a problem.

Fungal diseases stink is all I have to say....

I was just looking out front at all our BS covered roses...
Out back Double Ko's are doing really good but everything else is too young to evaluate but no BS on them yet...

Deleted and put in a separate thread.

This post was edited by dublinbay on Sun, Aug 25, 13 at 6:44

This maybe a rather stupid question but being new to growing roses in pots, I wonder if roses can get water stress in pots if I water them everyday? Some of my potted roses are showing signs of blackspots as zaphod42 described. I water them only in the morning.

Yes. If the soil remains too water logged, it's like having poor drainage and either too much sprinkler water or too much rain. Too much water rinses the nitrogen from the soil, resulting in a malnourished plant. That kind of stress (too much water, water logged soil, nitrogen and other nutrient deficiencies) can impair the rose's immune system, permitting it to contract other bacterial and fungal issues. The problem is exacerbated by the use of saucers under the pots where water and the salts it contains, collects and evaporates as well as is reabsorbed into the pots, concentrating the salts until the soil becomes toxic to the plants.

If you don't have to water every day, I wouldn't. If you must, be aware of needing to increase your feeding schedule to compensate for the extra leaching out of the nutrients. If you have saucers under the pots, get rid of them or put bricks or gravel under the pots inside the saucers so that water evaporates and doesn't re enter the pots. Kim

Thank you very much for the advice Kim. I have to make some changes.

As Michael correctly stated, many people confuse cercospora and even anthracnose with black spot. Both of those diseases tend to produce "black spots" on leaves and information on them is not as widely publicized as is the information for black spot. Both diseases tend to appear in summer.

All that being said, in many parts of the US, true black spot can appear pretty much any time during the growing season depending on local weather conditions That is why it is important to establish a fungicide program for susceptible roses and STICK TO IT for the entire season.

As for different races of black spot, as has already been ,mentioned, there are at least five known pathogenic races of the fungus. Chances are there are more. So a rose that is resistant to one race may not be resistant to one or more of the others. However, all the races are controlled by widely available fungicides.

Roses - and plants in general - do not have the same immune system as is found in insects, birds, mammals, etc. In other words, specific plant individuals do not actually "develop" a resistance to a specific pathogen as they are exposed. - they either have the genetic resistance or they don't ( this is highly simplified, of course). In a practical sense, if your rose gets black spot, it won't become resistant to later infections - you will have to control the fungus.