Question
My Mom, who lives in Maryland, has had a problem the last couple of years with her peonies.  The leaves develop a covering of white "dust" that makes them look a greenish grey.  How can she treat this problem with the leaves?  What is this?  Laura

Answer
Sounds like a common Powdery Mildew attack to me on your mama's Peonies, possibly aggravated by Spider Mites.  Take a close look with a Magnifying Glass to see if you can find any under the leaves.  Alternately you can hold a white sheet of paper and tap the leaves; a few red specks of dust may land on it, to be confirmed by you that they are, or are not, those tiny little beasts that fluorish in dry heat and low humidity.

Orchards in Ohio were having trouble with Spider Mites and Mildew a few years ago.  The State Coop Extension stepped up to the plate to help them.  Sulfur was suggested.  It's 'Organic' to the extent that it is necessary for plants and a deficiency will cause problems.  But there's a downside.

Noted the Ohio Cooperative Extension Service: 'Growers should note that Sulfur is lethal to some beneficial Insects, Spiders and Mites.  These beneficial insects are natural predators of harmful insects and Mites that affect fruit crops.  Killing these beneficial insects may increase certain pest problems, especially Mites.'

Now, you are probably thinking that in your case, 'beneficial insects' means 'Ants' because of the nice things they do for the Peony flowers.  Ants are not really as important to Peony buds as we give them credit for.  But it is true that some are predators of Bad Insects, and Ants are active participants in the breakdown of Organic Matter in garden soil.  So we don't want to annihilate the Ants if we can help it with measures that amount to the Hydrogen Bombing of the food chain.  We must spare the Ants.  We must spoil the Spider Mites and the Mildew spores.

Sulfur will destroy Ants and beneficial insects, even as it destroys Mildew spores.  Insecticidal Soap will repel Ants and beneficials, which is preferable to killing everything.

Home Harvest Garden Supply, which specializes in beneficial insects, has this observation: 'The most common Spider Mite, the twospotted Spider Mite (Tetranychus urticae), has been recorded on more than 300 species of plants, including all of the tree fruit crops, as well as small fruits, vegetables, and ornamentals.'

Your mama's Peonies are the kinds of 'ornamentals' they're talking about.

Unfortunately, while Home Harvest does sell Phytoseiulus persimilis, Neoseiulus californicus, and Mesoseiulus longipes -- insects that devour Spider Mites with gusto -- these beneficials are strictly for greenhouses.  If you do have a Spider Mites situation, opt for the least toxic control, which is probably going to be Insecticidal Soap.

Now, what about the Mildew?

A product called 'Messenger' is being used to address Mildew problems on many garden shrubs.  The product is based on something called Harpin Protein -- HrpN -- identified at Cornell University in 1991 and approved for use by the EPA nine years later.  Harpin Protein is generated by most plants under what sounds to me like stress situations, activating certain repair and defense responses.  Harpin Protein also promotes more efficient cell division, so a treated plant grows faster and better.

Cornell licensed its discovery to Eden Bioscience Corp.  Eden packages and sells it as 'Messenger' in garden centers around the country.

Here's what the EPA website says about Messenger:

'Harpin alpha-beta protein is a harpin-based active ingredient similar in chemical structure and biological activity to Harpin protein (Messenger) registered in April, 2000.  It acts by initiating a complex defense mechanism in plants, analogous in some ways to a broad spectrum immune response in animals.  This protective response makes the plant resistant to a wide range of fungal, bacterial, and viral diseases during crop growth and post-harvest.'

Translation: Harpin Protein makes plants stronger and healthier.

So I am pretty positive about what Messenger can do for your Mildew problem.  It will prevent it next year; sprayed over leaves now, it will prevent spreading.  Spray it instead of watering for best results.  This is also a good product to use if you transplant or grow from seed.  When the patent runs out a few years from now, hopefully others will be making it, and it will be cheaper.  That is, if we still have gardeners with gardens.

Keep me posted.  Your followups welcome.

THE LONG ISLAND GARDENER