Blackspot on Rose Leaves
Blackspot on rose leaves - Photo by Irene Roth


Rose Fungus

        I receive a lot of questions about plant fungal diseases. Once the rainy season is over and folks begin to use their irrigation systems in earnest, fungus happens. Usually, these questions center around rose fungus, but fungal diseases occur on a variety of plants. Early summer is when fungus most commonly appears on roses and rose related plants like photinias, fruit trees and pyracantha. Non-rose family trees and shrubs to commonly get fungal diseases are magnolia, holly and viburnum.

        Many gardeners discover black or brown spots on the plant’s leaves. Usually, these spots are surrounded by a yellow or tan colored halo. On roses, this disease is called Blackspot. Although this is a specific pathogen to the Rose Family, the term and its identification can be equally applied to other trees and shrubs, as well.

        Other common fungal diseases include Powdery Mildew - a grey or white flocking appearing on new growth buds and stems. In coastal areas like the Pacific Northwest and southeast Atlantic states, rose Rust is also common. This disease appears as tiny rust colored dots and splotches along stems and the underside of leaves. And reports of Downy Mildew along the Gulf Coast have indicated this fungal pest may be destroying many gardens. Downy mildew appears similar to blackspot, but infected leaves develop irregular purplish-red spots. Eventually the leaves turn yellow. When defoliation occurs, leaf drop will start at the top of the shrub instead of the bottom as with blackspot

        By far, however, blackspot and powdery mildew are our most common fungal diseases. Optimal conditions for blackspot are 64° - 75°F (18° - 24°C) with relative humidities around 95%, and the spores must be wetted continuously for several hours. For powdery mildew, optimal conditions are 81°F (27°C) during the day and 61°F (16°C) during the night with relative humidities anywhere from 40% - 70% during the day and 95% - 99% during the night. Although these are optimal conditions, most experienced gardeners know that fungi can cause a lot of damage even in less than optimal conditions.

Garlic GP Rose Fungicide Concentrate

        The reason why fungal development is so prevalent this time of year is heavy rains and frequent irrigation soak the soil and surrounding mulch; wetting the fungal spores. The water droplets falling onto the soil also splatter dormant fungal spores back onto the plant. (Although some of the spores also hide in cracks and crevices on the shrub itself.) Once it quits raining or one quits irrigating, the leaves partially dry, the relative humidities increase as the moisture begins to evaporate, and the spore's haustorium (fungal "root") finds entry into the leaf’s stomata (leaf pore). Viola! Blackspot! Powdery Mildew! Downy Mildew! Rust!

Green Light 07924 Rose Defense II - 24 oz Spray

        Controlling fungus in the garden is easy. For non-food ornamental plants, shrubs and trees, I recommend using Rose Defense™ by Green Light Co. This whole neem oil product is not only rated for fungal diseases, it will control spider mites and pest insects, as well. And it will do all three chores without harming beneficial insects. For food crops, Green Light has come out with another whole neem oil product called Fruit, Nut & Vegetable Spray!

By Mark Whitelaw, Kindly Provided by Laura Whitelaw

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