It seems that last week’s Question and Answer portion of this column created a bit of a stir.

You may remember that Maggali in Newport Beach wrote to me about how to best control powdery mildew on her roses. She also stated that she wanted to use an organic, earth-friendly product and wanted my advice.

I responded by passing along the recommendation of some very close rosarian friends of mine. These friends include past presidents of The American Rose Society and a rose breeder. Each of them grow hundreds of roses in their gardens, and I genuinely trust their expert advice.

So, in my answer to the powdery mildew question I spoke about a product that was just recently introduced to local gardeners. It is a spray made completely of all-natural soybean oil and rosemary oil. In fact, the oils are food grade, which, I suspect, is part of the reason that it works so well. My rose friends have been using this soybean/rosemary oil this spring and are ecstatic about the results. They swear that they can spray it on an infected rose and by the next morning the leaves appear to be free of mildew.

Why did this simple advice about a new product cause such a commotion? When I checked my e-mails the next day I had dozens of questions. Phone calls were coming into local nurseries and garden centers at an alarming rate asking about the product. By 10:00 AM my receptionist was already overwhelmed by the calls and asked me what it was I had suggested. I brought her a bottle to set at her desk and the rest of the weekend she headed off the barrage of inquiries.

It seems that in last week’s column I never mentioned the name of the product. Remember, I’m a gardener, not a writer; as I am certain you have determined if you have read this much. This new certified-organic fungicide is called Fungus Pharm. No, that’s not a misspelling.

Fungus Pharm is an all natural, 100% organic spray to be used on powdery mildew, rust, black spot and other fungal diseases on roses and other plants. In fact, it is so different from any other fungicide that is does not even need to be registered with the state of California as a pesticide – something I have never seen happen before. It is the first and only fungicide to be Certified Organic by the USDA.

Powdery mildew is one of the most persistent plant diseases in the world. Early Greek writer and gardener Theophrastus wrote of powdery mildew on roses in 300 B.C. Then, as now, different strains of the fungus attack different plants and the one on roses (Sphaerotheca pannosa var. rosae) is not the same as what you might see on your begonias, sweet peas or squash.

Contrary to popular belief, powdery mildew will not infect or spread on wet rose leaves. This belief, like rocks in the bottoms of pots, is a garden myth passed on from gardener to gardener, with no scientific basis. Overhead sprinkling may actually reduce the spread of powdery mildew because it washes spores off the plant and if spores land in water, they die.

Infection actually occurs on dry leaves, especially with warm daytime temperatures, cool nights and high humidity. Whenever possible, spraying the foliage of roses with water may kill mildew fungi and inhibit new outbreaks. Crowded roses with poor air circulation and growing in a bit of shade are especially susceptible to powdery mildew. A rose susceptibility to powdery mildew will also vary by variety; some being magnets for the disease, while others may be nearly immune.

To my readers I apologize for any anxiety I caused by leaving the name Fungus Pharm out of last week’s column. I especially apologize to the lady in Costa Mesa who called to tell me of her frantic trips to several supermarkets. She said she had no trouble getting the soybean oil, but the pure rosemary oil sent her into a tizzy.

Ron Vanderhoff is the Nursery Manager at Roger’s Gardens, Corona del Mar