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.
Rose fungus "Blackspot"
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.
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!
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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