Thoughts On Powdery Mildew

henryinctApril 28, 2014

When we moved to Pasadena from Connecticut in 2012 I was thrilled at the thought of never having to deal with black spot again but I soon discovered that SoCal has its' own scourge and that is powdery mildew. Most roses are susceptible to PM ranging from hardly at all to very vulnerable and in extreme cases the rose shrivels up. It can be as bad a problem as BS was back east.

How to deal with it? Here is what I have found out. First, I noticed that some areas of the gardens had a lot of PM and others none at all and that air circulation and amount of sun were probably the most important factors. The roses are bigger and thicker this year than last so there is more shade and less air circulation.

I also noticed that PM spreads from one rose to the next first appearing as spots on new growth and quickly covering infected areas. No single rose had it that didn't at least partially infect its' neighbors. I also saw that certain vulnerable roses were free of it in some areas and had it bad in others. For example, Oklahoma in one area was entirely free of it but Mister Lincoln which should be similarly vulnerable had it bad.

I've sprayed with Green Cure which is potassium bicarbonate and is absolutely safe and recommended for PM but it really doesn't help if the case is severe. It says on the label to spray at the first sign of PM which I didn't do but will in the future. If you are going to spray for PM you probably have to do it as a preventative as you have to for BS. Like BS, I doubt if you can do much about a severe infection of PM that is already present.

And then I found the answer (I think). I noticed that roses that were being doused every other day by the lawn sprinkler system had no sign of PM. Since the roses all are on drip systems most of them don't ever get wet except from the morning dew that doesn't always get dried off by the sun giving the PM spores a chance to take hold. Rain would wash them off but of course it almost never rains here. The spores which unlike BS seem to do their damage only on the outside of the rose spread from rose to rose needing only the moisture provided by the morning dew.

So the answer may be as simple as hosing roses down frequently something I know they enjoy anyway.. I'm hoping so. So far so good.

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seil zone 6b MI

I've heard a lot lately about giving your roses a good hard spray to wash the leaves of spores. I've been doing it myself the last couple of years and I do think it helps keep down some of the fungal diseases.

    Bookmark   April 28, 2014 at 5:52PM
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ordphien(9)

It really does work. To keep pm in control in my yard I spray the leaves with a house every morning.

    Bookmark   April 28, 2014 at 6:04PM
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socks

I've heard of that before and am not surprised it works.

I ignore the PM because after June, it goes away. I just cannot be bothered to spray. My worst bush by far was Tropicana, a horrible bush tortured with PM. I hacked it to the ground hoping to kill it, but it grew back fine and has had a lot less PM.

    Bookmark   April 28, 2014 at 6:15PM
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roseseek

There are several facets to the hosing off, Henry. First, anything which provides a coating to the leaf surface when the spores are in contact with that tissue and conditions are right for them to germinate will help prevent germination. Second, you may actually be "rinsing them off" the foliage. Just as importantly, drip irrigation is not sufficient for most ornamentals now the winds are back; it's as arid as it is; the sun is as intense as it is now and the temps are rapidly becoming extreme. Water stressed roses (and many other plants) are forced to mildew, and many are forced to rust. Their immune systems are impaired by the water stress and the foliage contracts the diseases. So, yes, hosing them off can greatly help. I used to water my old Newhall garden of 1200+ bushes via overhead oscillating sprinklers. Zero diseases. Zero bugs. As long as I "rained" on them with the oscillating sprinklers they were free from diseases and bugs. They grew like weeds and flowered amazingly. Unfortunately, those 'good old days' of cheap, plentiful water are long gone. Water remains the best fertilizer, fungicide and pesticide you can use. Kim

    Bookmark   April 29, 2014 at 12:24AM
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nikthegreek(9b/10a E of Athens, Greece)

Ideal time to hose down is very early in the morning. Avoid doing so at dusk or when the sun is too strong. Yes it helps, no it won't stop the PM magnets from getting it. Careful with the half open buds of many-petalled varieties. Many roses grow out of it (in the sense that they either stop getting it or they get very little of it) when they are mature and content. Some should be taken to the dust bin..
Nik

    Bookmark   April 29, 2014 at 12:36AM
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bayarea_girl(NorCA 9)

[NorCA 9b]
@henryinct, thank you for your thoughts on powdery mildew. I also use Green Cure and remove any leaves that have powdery mildew. However, I have been looking for a better way of control PM. I have been using your method of washing down the roses in the morning for two weeks and give the roses a bit more water than normal to help them fight the PM and the result has been promising.

@Kim, thank you for your detail explanation. I have learned so much from you and others. Please share your thought on rust if you can.

I understand why people want to shovel prune their roses. However, I have learned some roses need more extra care than others and if you have the time and effort to give that extra care for those special roses, they often behave well. One of the examples I have is Angel Face. It's not a strong rose and last year it always had PM. Because of sentimental reason I didn't remove it but had a hard time with it. This year, I give it more attention than other roses by making sure it has enough water and fertilizer, remove any diseased leaves right away. I'm so glad I didn't shovel prune it. It's still not the best rose in my garden. However, I think it won the war with PM and performs very well. This is just my opinion for my small and new (3 year or younger roses) garden in Silicon Valley 9b and may not apply to other areas.

This post was edited by bayarea-girl on Sun, May 4, 14 at 14:06

    Bookmark   May 4, 2014 at 6:25AM
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roseseek

You're welcome Bayarea-girl. The choice whether to retain a particularly "needy" plant is completely personal. I coddled Angel Face for years because of the color, scent and beauty it COULD provide, as long as I catered to its needs. I completely understand why someone would want to provide it the extra care. I have done that for many years to one of the most intractable roses ever introduced - Grey Pearl.

Everything in Nature exists for a purpose. Rust (and black spot) are "geriatric diseases" as they primarily affect aging foliage. Mildew is a juvenile disease as it affects new growth first. Rust is actually used by Nature to trigger many species to stop their juvenile stage, cease flowering (ovulating) and trying to reproduce. R. Arkansana utilizes the increasing sunlight and warmth of spring to put the melting snow and winter/spring rains to use, triggering its "reproductive stage". It begins pushing new growth and flower buds along the previous year's growth in its effort to set seed and reproduce itself. Reproduction and perpetuation of the species are the "Prime Directive" in Nature. As long as those light, temperature and moisture levels continue, it (and many other once flowering types) continue pushing new growth, new flowers, in pursuit of that goal.

As the year progresses, the sun rises higher in the sky. Heat intensifies and water begins to run short, making maintaining that growth and flowering push more of a strain on the plant. If it continued using its dwindling resources growing and flowering without preparing for the harsher winter conditions, it would either completely use them up prematurely and die, or enter the extremes of the winter weather where it is indigenous, unprepared for hard freezes and be killed by them.

As the heat increases and water becomes increasingly scarce, it appears the plant's immune system is impaired and the species contracts rust to a rather heavy degree. The fungi impairs the plant's ability to feed itself, triggering it to begin "hardening off", altering the sap to conserve the moisture and nutrients it has absorbed and generated when conditions were better for it to push its growth. Hardening off conditions it to withstand greatly reduced light levels and much greater cold. Nature uses rust (and some other triggers) to "tell" Arkansana to stop expanding, stop reproducing, ripen its seeds and prepare to shut down for winter. You will see the same mechanisms at work with many weeds and other desirable ornamentals. Petty Spurge, a common "weed" member of the Euphorbia family, as well as many Oxalis follow the same cycle of growing until triggered to rust badly and shut down until better conditions arrive.

It is entirely possible the contraction of rust by other types might be used similarly and be caused by similar triggers. I attempted to grow R. Arkansana in a large pot here in my garden. It suckers vigorously. I didn't want a yard full of the plant, nor did I want to have to irrigate a large stand of one type. I knew Arkansana's mature foliage rusts in fall, but I didn't expect the brand new foliage to rust immediately after unfurling, particularly in spring when nothing else was rusting. I theorized that it contracts the disease "in the wild" due to decreasing water, so I increased the water to that pot. The old, rusty foliage fell and was replaced by new, healthy foliage all over the plant. As long as I continued providing heavier water to it, the plant grew and flowered without contracting rust. Long before anything else in the garden showed any signs of rust, I cut back the water to my Arkansana. It began rusting almost immediately. Increasing the water again resulted in new, healthy foliage pushing all over the plant. As soon as I cut back the water again, even the new foliage developed rust. Arkansana has gone on to someone else's garden where it can enjoy greater water, more room and conditions closer to its natural habitat. I've grown a number of the Canadian cold hardy Arkansana hybrids. All but one performed the same way. As long as I provided the extra water and never permitted them to experience water stress, they grew and flowered well and remained healthy. As soon as they began stressing for water, all but Morden Blush rusted badly, very quickly. Morden Blush remained completely healthy in this climate. I really loved the whimsical veining and stippling in Morden Ruby's flowers, but I could not keep the foliage rust free unless I chemically treated it, something I refuse to do for a variety of personal reasons. So, that one is no longer in my garden. Eliminating water stress in this climate and my soil is virtually impossible. Varieties which can't endure that stress without quickly contracting rust simply aren't suitable for the variables I garden under.

One of my favorite miniatures to use for breeding is Cal Poly. It is extremely fertile, thornless, accepts a wide variety of pollen, generates many very viable seeds and produces many thornless offspring. I grow them potted to make protecting their hips from squirrels and other rodents easier and to make access to their flowers for pollination more convenient. I quickly learned not to allow their pots to dry out too far or they rusted quickly. Grown in the ground, they've never rusted for me anywhere I've grown them. When I have rust (or other disease issues) on seedling roses I raise, I experiment with water levels in hopes of discovering whether that might make a difference in their health. Most often, it does.

Of course, each variety, each seedling, varies quite a bit in its natural resistance to disease. Particular garden conditions as well as climate differences play enormous parts in the probability a variety may contract a disease. One of the easiest to control in your efforts to diagnose the cause of the infection is how much water, when and how the plant receives it. I would bet starting with that would help figure out whether the plant is suitable for where you are, the conditions you can't change and your preferences whether to chemically treat the infection or not. It has streamlined that determination for me greatly. Kim

    Bookmark   May 4, 2014 at 12:34PM
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bayarea_girl(NorCA 9)

Thank you Kim. It makes a lot of sense now. I will save this post in my clipping list. I'm not sure if you have the time or not but if you do would you please give more of your thoughts on other rose diseases? It is greatly appreciated. Thank you again Kim :-D

    Bookmark   May 4, 2014 at 2:21PM
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deervssteve(9)

My old garden roses, climbers, Olympiad and Julia child are clean.. Peace has rust and it has spread to PJP II.
Chrysler Imperial is a mildew magnet and it has spread to Just Joey, PJP II and to a lesser extent fire fighter.
St. Patrick has lost a lot of leaves but the canes are really healthy. I fertilized, watered and applied fungicide.
Last year I had virtually no fungus. It's always something.

    Bookmark   May 4, 2014 at 2:39PM
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henryinct

I've found here that rust is a symptom of water stress. It is more prevalent on most of the roses that are most susceptible to PM and roses can recover from it (like from BS) by producing new foliage. Strip any badly infected foliage and water enough to allow the rose to recover. Also, roses drop the infected foliage so you have to clean it up and it is a good opportunity to prune the stripped canes as well. My roses that got rust last year got it in the fall when I was probably not paying as close attention as I should have been. Those roses that had it last year show no signs of it so far this year.

    Bookmark   May 4, 2014 at 2:52PM
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deervssteve(9)

henryinct: All good ideas. My older roses are pretty maintenance free. I have nine young HT and I need to keep up with them. Today I deadheaded, weeded, fertilized, sprayed and watered and that was enough. I need to prune and remove diseased leaves. I am retiring in two months and will have the time to give them proper care. I haven't had any HT for over 25 years because of he deer. I wanted cut flowers and now I have to spend some time on them.

    Bookmark   May 4, 2014 at 3:08PM
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roseseek

You're welcome! The three main diseases, black spot, rust and mildew, all basically function similarly. They help prevent any one species from becoming dominant in their habitat. All three depend upon suitable conditions to grow and susceptible tissues to infect. The best control is to eliminate the susceptible varieties (to the best of your ability) then control the suitable conditions, again, to the best of your ability. There are many you simply can not control, but making sure there is decent air circulation; sufficient sun exposure (without over or under exposure); sufficient drainage with appropriate amounts and timing of water; appropriate nutrition at the appropriate timing and in appropriate amounts are more easily controllable. That leaves making sure what you choose to grow is suitable for your climate, conditions and gardening "style", how much time, effort, energy and money you choose to provide.

I know certain classes of roses simply won't flourish in my conditions. They may limp along and they may supply the begruding flower, but they will never perform as they are meant to, no matter how much money, time or energy I spend on them, so why waste precious real estate and other resources on them?

Once you begin really studying the plants, I think you will discover many issues are easily mitigated by making sure light, drainage and water are properly applied. Other measures are there in case those first three don't do the trick, or when attempting to bend an unsuitable type to your climate and will. Kim

    Bookmark   May 4, 2014 at 3:34PM
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