Downy Mildew Question

After reading the newsletter I have a few questions on Downy Mildew. First off it says that it can be harbored in stem tissue of roses waiting for the right conditions. Is it possible that DM can be passed from the seed parent to the seedlings form the seeds?

Secondly what is the best way to tell the difference between DM and powdery mildew?

Finally if you have a bunch of seedling that are five two seven inches tall and all of the leaves fall off would this point to dm? Because this happens very often to me? I am positive I am not over watering and nothing seems to be wrong with them. Some seem extremely healthy until this happens? Sometimes they do have some white spores on their leaves. I have always taken it to be PM but after reading the article I am not sure?

Adam,

One of the questions you can answer for yourself is "was the ‘powdery spore stuff’ on the underside of the leaf? Another is could you have possibly over fertilized? That would cause leaf drop pretty fast and over a large portion. The spores of DM are on the underside, and they usually do not show up in large areas, although I had some on Gemini that almost covered the whole underside–that shrub does everything on a large scale. But if the spores are on the tops of the leaves and around buds, it is probably powdery mildew. And are the spores very fine? If they are somewhat course or almost granular, maybe they are Botrytis. I have come to the conclusion that the reason the DM hit my shrubs and seedlings so hard and was so difficult for me to eradicate is they had both a flaming outbreak of DM and Botrytis at the same time. I am still fighting both but much more successfully, although the botrytis is proving to be the more difficult to eradicate with the cool, moist evenings, the fog, rain and sometimes 3-5 days of overcast at a time. All my seeds are started and grown outside-at the mercy of what ever the weather chooses to be.

As for the question about whether the ‘fungal spores’ can be seed transmitted, I have only last yrs. seeds to judge by and this sure is not scientific, but either the botrytis or DM sure looks to be transmitted, because the seedlings barely get out of their seed coats and instantly show symptoms. I have taken a small number this year, that showed extreme red spotting on the cotyledons, and placed them in a very warm sheltered area, and the cotyledons did return to a solid green and continue to grow normally, which suggests to me that the red/purple discoloration was not a natural coloration. And in trying to be much more observant of all the minutia involved in the first few weeks of growth, I have noticed that quite a number (20%) of my new seedlings either got symptoms of DM or Botrytis when that young, but most of the ones that have died seemed to be attacked by what I am judging to be Botrytis, where fungal spores attacked the 2-5 week old seedlings very close to the soil line, or just above it, and the stems either showed a scar, or an open split (just like a tomato that receives to much water after the fruit has semi matured,) or quickly turned black and killed the whole plant. This year I have approx. 500 seedlings, but last year I had close to 900. Last year I lost approx. 750 of the seedlings, and this year I have lost about 100. This has been a forced learning opportunity. And whoever said that this raising roses from seeds isn’t going to be quite like raising beans (or whatever) surely should get the understatement award of the year.

And whoever said that this raising roses from seeds isn’t going to be quite like raising beans (or whatever) surely should get the understatement award of the year.

Certainly true! I keep on learning by messing up.

The ones that have spores the spores are always on the top. I kept them around so that they can spread it to others. So that I can see who resistant or nearly resistant against the ones that totally suck.

I did fertilize. I thought I made it pretty weak however but I guess it was stronger than I thought. So if it is the fertilizer will these seedlings bounce back or have a chance to bounce back?

Thanks for the reply Jackie

I posted photos of DM last year:

http://www.rosehybridizers.org/forum/message.php?topid=20333#20344

Powdery mildew is very different and typically the leaves do not drop. Usually the seedlings get variable amounts of “frosted” leaves and stems - and can cause leaf and stems to curl, but typically doesn’t kill the seedlings.

For leaf drop in new seedlings, I would think DM or black spot. If you think that you have DM in the seedling, I would definitely spray. Very few roses seem to have good DM resistance, and I suspect that even those when they were seedlings, would have been susceptible.

I would doubt that DM inhabits the seeds, but I suppose that anything is possible. It is more likely that it is just in the environment since you have had it in the past.

Jim Sproul

Link: www.rosehybridizers.org/forum/message.php?topid=20333#20344

It is definitely not DM. I think I over fertilized.

Downy is quite distinct; you won’t see spore bodies on the upper leaf surfaces. Downy produces spores on the undersides, and they are very difficult to see even when you know what you are looking for. Jim’s photos illustrate the disease brilliantly, although it can take on a different look depending on the cultivar/class it is affecting. Still, the purplish markings and rapid death of foliage are most characteristic.

I think some kind of physiological stressor has caused the leaf drop, and powdery mildew is probably one contributor. Does the soil mix tend to hold water maybe a bit too well? I have found that young seedlings can easily suffer root death and subsequent leaf drop if the soil stays wet and cold for extended periods.

Paul After unpotting some handful of seedlings. They all seem to have good root. The root tips are white just like you would expect.