Blackspot testing

I was thinking. I live in Fort Collins Colorado. I have not seen blackspot the five or six years I have lived here. But lets say I could get infected blackspot leaves. How feasiable would it be to paint rose seedlings leaves with blackspot and test the rate of infection on the leaves. Secoundly how exactly would one do this? Thirdly if one where to do this in an area like mine would it even take effect? I do not know how much climate effects this dissease. Plus I don’t want to introduce a disease if it is not in my area. Lastly I do not know if this is even legal to do.

I don’t know answers to most of your questions, but here’s something to think about…

I seem to remember reading something about William Radler (who hybridized ‘Knockout’ and ‘Carefree Sunshine’). If I’m remembering correctly he would pulverize diseased leaves (in a blender maybe??) and dust that powder onto a population of rose seedlings. Then I think he maintained a wet environment (with overhead sprinklers??) to provide the necessary conditions for the diseases to flourish.

Maybe just providing good conditions for disease growth would be enough to bring out your local diseases?

Eliminating air circulation is probably the fastest route to disease.

But you’d have to keep the humidity up long enough for the spores to germinate and infect leaves, not cotyledons.

Yeah, he’d have to provide some type of consistent moisture for his climate. I don’t have that issue in Oregon. I actually crowd my seedlings on purpose to see the weakest links the quickest, lol.

I crowd mine too Jadae and cull out the poor performers over time.

I’ve innoculated roses with blackspot for a number of controlled experiments. An easy way to do it would be to take blackspotted leaves and put them in a container of water in a frigerator overnight. The cold helps to induce and release spores. Filter out the leaf pieces and then use that “blackspot tea” to spray on the roses. If you want to be really nasty, spray your roses and then cover them in sheets of white poly (white to reduce heat; try to innoculate when temps are below 80F, ~70F is perfect) for a couple days misting periodically to keep free moisture available underneath. That should help get a really good rate of infection. I remember counting 80+ small infection points 4-5 days after innoculation on one leaflet in one past experiment.

I’m going to do a series of detached leaf assays this winter surveying different cultivars for resistance to 3 well characterized and preserved blackspot isolates representing different races as part of some of the Earthkind research I’m doing. I’m excited for that and just got another box of roses in the mail today to gather the cultivars we need for it.



An academic question. What rose was Diplocarpon rosae first described on?

Do fungal species specify substrates?

Is there a holotype of symptoms for the original race of D. rosae?

There are old garden roses that get fungal problems that don’t look like the Black Spot on modern roses and it would be interesting to take a known race of BS and infect some of the old ones, just to see what would emerge (or not).

Thanks David. I think I will try it. Probably not this year but when I get some seedlings of some size. My first priority is powdery mildew since it is my main problem. I never see black spot here. So I will have to find some leaves that are infected first before I can even experiment on this. At least powdery mildew will be easy to test for. No matter how disease resistant I have seen powdery mildew on it at one point or another. For example it surprised me when I saw powdery mildew on Baby Love, Bonica, and Homerun. All three don’t show it until the last month or so of the season, which is far better than most. The only roses I have not seen it on is Gallicacandy, but this is due to the fact that it lost all of its leaves in early August due to the heat. I worried about this last year when I had several roses do this. But most of them came back in spring. I think it because it is a young plant. It is deffenietly not a characteristic that I like, but it is no worst then having all the other roses showing powdery mildew at this time of year. But I think breeding something totally immune is out of the question considering what roses get infected but if my roses can make it to late august without infection I will be happy with that.

Hi Ann,

Wow, those are great questions, most of which I don’t know and would love to learn. For the substrate, yes it seems like some fungi prefer different medias. For blackspot I think my friend Vance just grows his cultures on potato dextrose agar media. Powdery mildew is a bear to work with because it seems to need to live on a host to survive. Perhaps people are tweeking ways to grow it better in culture.

Blackspot grows very slowly in culture and Vance cycles it through a host from time to time to make sure it doesn’t lose virulence.

I was walking the 400+ rose collection at a public garden last week with my plant pathologist colleage Michelle. It was great to get her perspective and learn from her. We rated each rose for blackspot and other pathogens out there. It was amazing to see all of the stem canker. The site is very open and windy and stressful and the canker gets in through stressed tissue.

Like you mentioned the blackspot was quite variable on different roses. Many of the gallica and other old garden roses had what we nicknamed sheet blackspot. They just resist turning yellow and falling. The leaves persist for as long as possible allowing them to continue to photosynthesize and contribute carbs to the plant. Vance included ‘Charles de Mills’ in his race differential looking at the infection of 14 blackspot isolates on 12 different rose hosts including modern roses and also some seedling selections and a clone of R. spinosissima. For most isolates the leaf area with symptoms were relatively large for CdM compared to other roses. If the leaves were watched over a longer period of time I suspect that “sheet” blackspot would eventually appear, while for most other roses the leaves would have been sensitive to the ethylene emmission and fell off.

It would be interesting to compare different isolates of blackspot specifically common on ogr’s like CdM and see if they are just the same as those in the natural garden setting as on nearby typical modern roses.

I think there may be more specificity to blackspot able to infect rugosas. None of Vance’s 14 isolates infected Hansa. He isolated blackspot off of Hansa and I’m not sure how far he is in determining if that isolate infects his other more typical modern roses.

I really enjoy working with and learning from Vance. He has seedling populations among different parents with race specific resistance and is following segregation in the offspring to determine how many genes are involved and hopefully will find molecular markers linked to the resistance. Vance’s work also points out that what appears to be horizontal resistance in some instances (limited infection and reproduction of the fungus) actually is controlled by few or a single gene and have some race specificity as well for degree of limitation on the fungus.

While walking the roses with Michelle I was surprised how prevelent Cercospora leaf spot was on many of the kordesii types as well as other roses. Many were being defoliated by that and were blackspot free. There are spots with typically purple margins and light dead centers that typically do not fall out like spot anthracnose. There was also a lot of canker and some roses seemed much more susceptible to it such as Morden Sunrise. The many plants of MS were just a foot or so tall and so compromized from barely surviving the winter (common here for it), blackspot, and then new shoots dying off from canker. Rainbow Knockout was hammered with cercospora, but the other knockouts were looking good.

It seems hard to keep up breeding against all diseases and disorders and I don’t think we can. I guess the best we can do is let our climate help in the selection of those things common where we live and then try to get help from others in other areas or do controlled innoculations for the other things we think are important.