I’ve grown a lot of Phacelia from seed and there is a lot of it growing throughout my yard. Some of it is gorgeous and healthy, growing into small stands nearly 2 feet tall. Some of it has seeded in the more shady parts of my yard, however, and can barely grow a foot tall. Every summer the plants in part shade are severely attacked by powdery mildew, to the point where I wonder if it will survive to the next season. Despite this, however, I do not treat it. I don’t treat any of the plants in my yard for disease, native or not.
Powdery mildew is a fungus that spreads rapidly through the air by tiny spores. It can complete an entire life cycle in a bit more than a week, going from spore to sporulating colony, making it practically impossible to exclude from the garden completely. It will always be present, but is usually only a problem for one reason: weak plants. Plants that are growing in a suitable environments generally suffer only very minor damage, if they suffer any at all. The fungus is ever-present and attacks weakened plants, usually those that are damaged or growing in sub-optimal areas (like a Phacelia in shade), often leaving them looking like they took a bath in a bag of flour and sometimes completely killing them.
Diseases like powdery mildew are not always fatal, however, and often simply kill the plant down for the winter. It is not clear, then, what a gardener might do in this situation. Even the presence of a disease like powdery mildew is an increase of biodiversity in the garden, and it is often a very neutral one. If powdery mildew is usually not a huge deal, then, and usually only kills already-weakened plants, why do so many people care?
I think people often care simply because these diseases cause the plant to become a bit less attractive. Humans don’t like this. We want to be able to grow what we want, where we want it. In crops like grape and squash powdery mildew is controlled through two primary means: chemical and cultural. Sulfur is one of the most common chemical treatments, with sulfur powders being mixed with water and applied to the plant leaves via sprayers. It is usually quite effective and can allow plants to grow in places they would otherwise not be able to survive in. It’s also not terribly dangerous to humans, though it can linger and create sulfites and strange flavors in wine. Synthetic and organic fungicides are also used and are sometimes persistent in the environment and can be extremely dangerous to aquatic life (most fungicides are not acutely dangerous to people or terrestrial animals, but sublethal effects are not well understood and that fact alone should scare you.)
Cultural controls are better because they are cheaper and safer. The issue, however, is that cultural controls are not always effective and generally cannot allow a plant to grow in a sub-par environment. Proper pruning can open up plant canopies and improve air flow, lessening the ability of mildew to take hold. Removing other plants that can be infected with powdery mildew reduces inoculum, but is still not entirely effective and definitely not desirable for those of us who want to maintain a diverse native garden. Even organic growers still end up using fungicides: there are plenty of ‘natural’ chemicals (including sulfur) that are permitted for organic use. Growing wine grapes in Oregon is nearly impossible without some form of powdery mildew treatment, organic or inorganic. They just don’t really belong here.
So back to my phacelia. In the sun it is glorious. In the shade it is decrepit. In the shade, I allow it to try to grow. I also allow it to die. Native plants grow best in environments that suit them. I mention this seemingly obvious fact because so many of us try to grow plants where they are not really meant to grow. If a native plant gets sick in your garden, first try to understand what disease it has. If it is something devastating and invasive, like sudden oak death, defer to experts and get rid of the plant to contain the spread of the disease.
But if it is something like mildew – ever present but only occasionally an issue – consider just doing nothing.