White fungi in rose

nhq2006August 18, 2013

Dear All:

I live in Santa Ana, Southern California. My roses which are grown in pots and flower bed got some kind of white fungi on the leaves (photo). I am told that this disease may be cause by over watering. At the moment, I water the roses on every 2 days basis, at 6 AM.

How can I cure this fungi disease simply and inexpensively? It is happening to many roses of mine.

One more thing is that how I can make the rose leaves always green and healthy after I get rid of this fungi

Your help is much appreciated.

Thank you

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dublinbay z6 (KS)

Mildew. My understanding is that if a plant suffers from mildew, it needs more water. In fact, give it a spray "bath."

I think you can also spray the leaves with the same thing you use to spray for blackspot. In my case, that would be Bayer Advanced Disease Control for Roses, Flowers, and Shrubs (available at Lowes or online). Perhaps other posters can suggest something more "organic" to use.

Mildew is a common problem. I'm sure you will get lots of advice from posters.


    Bookmark   August 18, 2013 at 3:39PM
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It does appear your roses have some mildew, a white, powdery appearing fungus infection. There are several issues at work. The first is the varieties of roses you appear to be growing are not resistant to mildew. Replacing them with more resistant varieties over time will "cure" much of the issue.

Mildew is usually due to cool evening temps with high humidity. It can also EASILY be caused by water stress. It's been hot here in SoCal and that stresses all plants. Yours are in pots so the soil is heating up tremendously compared to what the temperature of the in ground soil is. Even watering them daily, this can cook all the water out of the soil ball rather quickly. It also accelerates the water use of the plant, leading to water stress. It is very easy to have roses wilting from extreme heat and sun while standing in a pool of water in soggy ground. Water stress impairs whatever immune system the plant has, making it more susceptible to fungal infections.

Then, there is the reflected, radiated heat issue from the block walls and any other stone or concrete surfaces near your pots. You're probably painfully aware how hot your patio and walks remain after the sun moves off them. That extra heat can quickly, easily overheat the pots and plants many extra degrees, increasing their water use (transpiration, their version of "sweating"), creating water stress, leading to increased mildew.

My first suggestion is to eventually replace the less resistant varieties with more resistant ones. If you choose to retain the ones you have, you're left with reducing the increased heat they endure. Planting them in the ground will help a bit by keeping the roots cooler, but it does nothing to address the extra radiated, reflected heat from the surrounding hard scape. Moving them from those walls and any walks or patios will help with that issue.

If you can't move them away from those heat sources, or choose not to, you're left with chemical intervention. BUT, applying any chemicals (including inorganic fertilizers) to plants subjected to those kinds of temperatures and artificially induced increased water stress can be very dangerous. Anything sprayed on the foliage can burn in brilliant sun and high heat, so spraying is a definite "NO" until these conditions change. Treatments which are soil applied must be used VERY carefully as they can also burn the plant in those conditions. Bayer Systemic Rose Care containing soil applied fungicide is highly toxic in extreme heat. I would advise against using any chemical means to control the mildew with those roses in those conditions. NO "oils" of ANY kind, period. All WILL fry the foliage in those conditions. NO sulfur at all as it, too, will burn. You honestly need to change the heat and water stress inducing conditions they endure first. Much of the mildew may easily "cure" itself once they aren't being cooked. For those which don't clear up after the conditions are improved, I would suggest replacing them with varieties known to be more mildew resistant. AFTER altering the heat conditions and when the weather is appropriate for their use, you may consider either sprays or soil applied systemics. Good luck with whichever you choose. Kim

    Bookmark   August 18, 2013 at 3:49PM
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jerijen(Zone 10)

Think twice before embarking on a spray program. Everything has two sides, and remember that "cide" in Pesticide means that it kills something. In any case, they will not eradicate the existing patches of fungus. DO NOT use oil-based sprays. Those will burn your leaves to a crisp in your summer weather.

Powdery mildew is a fungal disease, and many of the fragrant red roses are particularly subject to it. They have inherited a weakness that gives the fungus an advantage.

You are in Santa Ana? I'm further North, but if I am remembering weather news correctly, your area is also having a far cooler, occasionally foggy summer. That sort of weather is encouraging to mildew. When temps return to scorching hot, things will be better.

Because we have pets here, we do not spray. Rather, we select roses that are less-susceptible to mildew, and don't require a chemical bath to look good.

Have you had these roses in the pots for a long time? Because in general, resistance to mildew increases as the plants mature. But they will always perform best in the ground, rather than in a pot.

If you have not fed them this year, you can give them some time-release fertilizer. But keep your dosage to the recommendations on the package. Use rates for roses in containers will be lower than roses in the ground. More is NOT better.

Finally, I would recommend that you want the roses off with a spray of water every morning. This will retard the spread of fungal organisms.

OH -- and there are some very GOOD and active American Rose Society local Societies in your area. Folks there will tell you which roses will excel for you. Find them listed at the website of the American Rose Society.


    Bookmark   August 18, 2013 at 3:53PM
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hoovb zone 9 sunset 23

Keep in mind the mildew may look bad, but it won't kill the rose.

Try washing off the foliage every morning for a while. The plant should have time to dry before it gets dark. It can make a remarkable difference, and the only costs are a little time and a little water, with no toxins at all.

    Bookmark   August 18, 2013 at 6:15PM
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I saw this while researching for ways to treat blackspot. Hope this will help you.

Solution #1 - The Cornell Baking Soda Formula
(for Powdery Mildew AND Blackspot)
4 teaspoons baking soda
2 tablespoons horticultural oil such as SunSpray brand UltraFine Year-Round Pesticidal Oil
1 gallon water

Mix well then pour into a no-clog type hose-end sprayer. If you only have a couple of roses to treat use a hand-held spray bottle. Thoroughly soak the entire plant making sure to get the undersides of the leaves. Apply in the morning, approximately every 7 days as needed.

    Bookmark   August 21, 2013 at 10:58AM
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jerijen(Zone 10)


You won't have to worry about fungi if you do, because you won't have any leaves. They will all burn to a crisp.

I have learned by doing that I cannot depend on hort oils if the highs for the season are over about 75 deg., and in intense, dry weather, I've damaged foliage at 75 deg.

This may not be true in other parts of the country, but it is true in Southern California.

Just hose the roses off daily, and try to mitigate the effects of heat bouncing off walls, and cooking roots.


    Bookmark   August 21, 2013 at 1:37PM
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Thanks Jeri. I surely do not mean to cause your roses any harm, nhq2006. I have to keep this in mind that different zones and climates require different treatments. Sorry about that.

    Bookmark   August 22, 2013 at 10:36AM
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jerijen(Zone 10)

That's OK, Kousa. I do the same thing. I think we all do.

Folks ask for a recommendation, and I race to suggest something I love -- forgetting that I live in a foggy niche along the Southern California coast. Our temperatures are so much gentler than those in most of SoCal.

I'm trying hard to train myself, tho -- to look at where folks are gardening, before jumping in. :-)

Years ago, tho, some friends who were "bigtime" rose exhibitors heard about one of these baking soda/dish detergent "recipes." They sprayed EVERYTHING with it, and lost their entire spring flush. Boy! Was that guy TICKED! So, we do have to consider location, when adopting new ideas. (He should have sprayed ONE bush!)


    Bookmark   August 22, 2013 at 11:23AM
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