I left two seedlings from last year in the greenhouse. They got mildew but I left them because I wanted to see the blooms before I threw them out. Unwise. Now the seedlings from this year, which are still very small (about two true leaves), are showing mildew infection! I think they are too young to be judging them on disease resistance, and I’m afraid the mildew will kill some of them.
I guess I should intervene, but am not exactly sure how to proceed. I sprayed some of the infected seedlings with potassium silicate (an organic anti-mildew spray) that works a bit, but also damages the seedlings a bit.
If I spray them with a real fungicide, shall I use the regular dose, or half of it? The seedlings are really small, some just germinated so I expect them to be sensitive.
What product should I use? I have tebuconazole, which is systemic and labeled for mildew, but maybe there is a better class of fungicides that I should use? It should be curative now.
Another option to prevent further infection is placing the seedlings outside, where there is more air circulation. We normally don’t have mildew on roses outside, it only affects seedlings in the green house.
Any helpful ideas?
Rob
By all means use the tebuconazole.
In the old days (1960s-1979s) we’d have used Actidione PM to cure powdery mildew. Cycloheximide, a form of Streptomycin, is the ingredient in Actidione PM, if I remember correctly. The Actidione PM did a good job if the temperature were high enough and sunshine were available. Please note that I’m alive to tell about it–and Actidione PM is much more toxic than tebuconazole.
I suspect that the tebuconazole will do a better job. It is licensed for use on food crops, so you can be sure it will take at least 3 weeks to kill you. 
Take sensible safety precautions, and if you don’t eat more than one spoon of the concentrated stuff on your ice cream and you don’t breathe it, you’ll live long and prosper.
Peter
I’m yanking most anything with mildew even as a seedling unless it has a killer pedigree.
Regarding tebuconazole. The link below gives a State of New York, March 23, 2005, “Denial of Application to Register Bayer Advanced Garden Disease Control for Roses, Flowers, and Shrubs Concentrate (EPA Reg. No. 72155-14). Chemical Code: 128997” for homeowner use on ornamental plantings.
http://pmep.cce.cornell.edu/profiles/fung-nemat/tcmtb-ziram/tebuconazole/tebuconazole_let_305.html
Link: pmep.cce.cornell.edu/profiles/fung-nemat/tcmtb-ziram/tebuconazole/tebuconazole_let_305.html
Rob, I agree with Robert. It is way better to do away with seedlings that are very mildew prone and only keep those that have parentage that you want to carry farther on. Though some roses are less susceptible to pm when mature, the seedlings that are completely resistant to pm are worth finding. But, you will only find them by not spraying.
When I finally stopped spraying my seedlings for fungal diseases I finally started having much cleaner seedling selections and ultimately, cleaner parent stock to use for further breeding.
Jim Sproul
Jim Sproul
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A lot of young children grow out of childhood respiratory (and other) diseases. Most of us don’t deny them medication when they need it.
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A lot of good looking adults were not so good looking when they were babies.
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If we don’t let the seedling grow enough to develop some sort of plant and to bloom, what do we learn about the cross?
Since Rob doesn’t live in New York and he already has the tebuconazole, I think he should use it (if he hasn’t already) to save his seedlings so he can see what he has. From Rob’s question, I gather that he has not decided that this cross yields only losers.
I think Rob has also learned a very important lesson about greenhouse maintenance: If you bring in plants from outdoors, you will almost always have problems–whether with spider mites or aphids or something else such as powdery mildew. A greenhouse is not a natural environment, and in a greenhouse things happen that would not happen under natural conditions outdoors.
And sometimes patience and careful observation beat profligate pollinating. Like people, not all roses develop at the same rate.
Peter
Thanks for the ideas! I have no problem using fungicides. I thought many hybridizers use them to protect very young seedlings, whose resistance may show up when they are grown. However, now that some of you state they have better results without using any sprays I decided not to spray.
Instead, I removed the big mildewed seedlings and other plants with mildew from the greenhouse and opened the windows on one side and the door on the other to get airflow in. I also only water in the morning now. I see no further spread of mildew, so I guess only some susceptible seedlings were affected and the rest is ok. Hopefully it will not spread further.
Rob
I think it also depends on how many seedlings you have. If you have many, or have the goal of especially disease free roses, culling early and vigorously makes sense. If you have a smaller seedling number, and you want to coddle each one, then by all means do so. My impression is that hybridizers are spraying less and less now as disease resistance is becoming more of a selling point for roses.
It is that coupled with the newer aggressive bans on pesticides for those without a RUP license. (restricted use pesticides). I notice a ton less pesticides being sold in retail stores now. Heck, I was thrilled when they came out with the new “pet friendly” slug killer pellets
Personnaly I do not breed for PM or other funghi resistance at the seedling stage. I fully support Peter arguments.
By the way I have very few PM on seedlings actually. May be it is because they get as much air movement as possible being close to the greenhouse day and night open end in a rather windy climate.
Our Rose Society recieved samples of Green Cure Fungicide developed by Dr. ken Horst, Cornell University plant pathologist. It is organic, kills powdery mildew and other plant diseases on contact, with up to 2 weeks residual protection. The active ingredient is potassium bicarbonate with patented spreader-sticker ingredients that enhance the fungicidal properties. It was selected as a top 10 BEST OF THE MUST- HAVES for gardening, and endorsed by Stan V. Griep, award winning rosarian and photographer. Info. is available at: info@greencure.net
That stuff looks nice. I wonder if they tried mixing the bicarbonate with neem oil? Neem oil is the only fungicide I’ll even get close to now, but this Green Cure looks okay too. It’s just not worth risking human damage to see some pretty blooms.
Nice marketing Isn’t that what we used to know as the Cornell mixture? Another organic alternative to prevent (and treat?) mildew is wettable sulphur. However, this can damage adult rose foliage so I am reluctant to experiment with it on very small seedlings.
Also, check out Serenade. It’s organic and does not hurt the seedlings even when sprayed in full sun. Neem has killed seedlings for me. I hear that Jojoba (Organic Gardening Since 1991 | Planet Natural Garden Supply) is terrific but I haven’t tried it yet.
Personally, I lose most of my seedlings to mildew when they go outside, but then my climate is different than most, being in the desert.
There is a tremendous amount of mildew in my area which is very much a desert climate.
I hybridize and germinate everything outside.
If I was going to use any intervention it would be Serenade as Judy suggests.
I prefer to see what has a propensity for mildew early on and cull accordingly.
I’m having thoughts of constructing a small greenhouse but only to protect against wind and predators such as birds.