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  This week one of my new clients pointed in despair at her rose bush and cried: “What is that nasty white stuff all over my roses???”

What’s all over her roses is a fungus called Powdery Mildew. Powdery Mildew is common in the Bay Area, especially when dry hot days are followed by fog that leaves moisture on the foliage. The first signs of powdery mildew appear on young leaves, which hold their color but begin to crinkle. Then small patches of mold appear that develop into spore-bearing fungal filaments on foliage, stems and all other parts of the rose, even the buds (looks like a thin, white powdery substance sitting on growth, which steadily becomes deformed with the spread of the disease). It spreads in white strands, which anchor themselves to the foliage. From there the fungus will draw on the moisture and nutrients within the leaves. As soon as you see the crinkling of young rose leaves be on the watch; the sooner mildew is arrested the better. Mildew can spread throughout your garden rapidly, attacking other plants as well as your beloved roses.

Powdery mildew disease is worst during hot, dry weather with cool, moist nights. Spores are dormant until they get the moisture required to germinate. Once the leaf surface is moist and remains moist (often from overhead sprinkling as well as fog) for about 3 hours, the mildew spores will begin to germinate. This can occur in damp, shady gardens where air circulation is poor, or where plants are stressed due to overcrowding or dehydration (plants insufficiently watered at the root level are often attacked by mildew). Plants grown in areas with not enough sunlight will produce thinner leaves making them more prone to infection. Also plants deficient in nutrients (especially calcium) have weaker leaf tissue and that makes them more vulnerable to disease.

It is important to keep roses well watered at the root level during hot days and prevent water from getting onto the leaves, especially before nightfall. Providing good air circulation between roses is vital. This helps dry up the roses more quickly. Good pruning methods and generous spacing between rose bushes when planting (generally 3 feet between hybrid teas and 4 feet between larger rose bushes is recommended) will also help.

Once mildew is noticed (keep an eye out for it in the early stages), spray the rose bushes with fungicidal soap or wetable sulphur (both products are readily available at the local nursery). Try to remove any diseased parts and dispose of them rather than composting them (if you put them in the composter they will come back to infect your plants later).

Another product that appears to yield good results is Seranade, which is available online and at some garden centers and nurseries. Serenade is an organic fungicide. The active ingredient is Bacillus subtilis QST 713, a naturally occurring soil microorganism. It works in several ways; first it creates an ‘inhibition zone’ on the leaf preventing pathogens from attaching. It also stops pathogen growth by competing for nutrients and space on the leaf, and lastly, it’s bacterial cells and lipopeptides destroy germ tubes and mycelium membranes. In addition to it’s use on roses, Serenade can be used up until day of harvest for Bacterial Leaf Spot (escallonia), Powdery Mildew, Rust, Leaf Blight, Black Spot and Fire Blight on melons, tomatoes, apples, and other edibles.

http://berkeleygardencoach.com/2011/06/19/what-is-that-nasty-white-stuff-all-over-my-roses/