I regularly hear gardeners terribly concerned about foliar diseases: powdery mildew, black spot, rust, etc. Their main concern seems to be that the disease will spread from the first plant infected to everything they grow. Hence the belief that they have to react quickly to treat the diseased plant or even to eliminate it before the disease spreads.
This is actually fairly true if you grow other plants of the same species: a disease that affects one potato plant may well affect other potato plants… but it’s very unlikely to infest fuchsias, apple trees, marigolds, and other unrelated plants.
That’s because most foliar diseases are specific: they have a host plant and live only on this plant or perhaps on a few others that are genetically similar.
Take powdery mildew as an example.
It’s a common disease found on a wide range of plants… but in actual fact, it’s not one disease, but many: a series of fungal diseases in a wide range of genera: Oidium, Erysiphe, Sphaerotheca, Uncinula, Podosphaera, Leveillula, etc. Each of these genera contains several to many species and each of these species is further divided into strains. Each strain—and there are thousands!—has its preferred host.
What these diseases have in common is that, at some point in their development, the plant’s foliage will be covered in what appears to be a white powder (actually sporangia, the final stage of the disease), giving them all a shared common name: powdery mildew. But despite this common symptom, in fact, powdery mildews are in fact different diseases.
That means that when your phlox suffers from powdery mildew, you may have to worry that the disease might spread to other phlox plants, but not to bee balms, lilacs, tomatoes, squash, etc. Each of these plants has its own strain of powdery mildew, one it only shares with its closest relatives.
To Treat or Not to Treat?
Knowing this detail can greatly influence your response.
If you cultivate only one ninebark (Physocarpus), for example, and it shows symptoms of powdery mildew, with some leaves showing white marbling or even turning entirely white, it may not even be worthwhile reacting.
The disease is not going to spread to your other plants and is, in fact, fairly harmless even to the ninebark: it certainly doesn’t undermine its overall health. It’s one of those diseases that can be said to be mostly esthetic. So what if your ninebark turns a bit white at end of the season? Does that keep you from sleeping at night? Is it important enough for you to get out of your hammock and spray a potentially toxic fungicide to try and control it?
I learned long ago that no garden is perfect and the occasional leaf disease simply isn’t worth fighting. I just turn a blind eye to it and go on with my life.
You still want to try and control powdery mildew? Here are a few home remedies you could try.
Powdery mildew is probably the most common plant disease, but there are many others. However, the information above applies to almost all foliar diseases. Black spot (Marssonina rosae) attacks only roses, for example; tomato late blight (Phytophthora infestans) only tomatoes (but there is another strain of Phytophthora infestans that infests potatoes), apple scab (Venturia inaequalis) would be a thing of the past if there were no apple trees… and so on.
Grow Resistant Varieties
At any rate, the single best treatment for leaf diseases is…. to plant resistant varieties. There are powdery mildew-resistant varieties of phlox, ninebark, squash and bee balm, rust-resistant forms of hollyhock, black spot-resistant roses, late blight-resistant tomatoes, scab-resistant apples… and the list goes on and on. If you’ve had a problem with a plant disease in the past, chances are that there is a disease-resistant variety you could easily substitute.
You’ll be amazed at home many gardening problems you can solve simply by planting the right plants!
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Pear blight happens to infect both Pyrus calleryana and Pyrus kawakamii. It would not be such a problem for the deciduous Pyrus calleryana, since it must start over in new foliage annually. It is worse for the semi-evergreen Pyrus kawakamii, which does not shed old foliage until new foliage develops. New foliage is infected directly from old foliage. What is worse is if the two species are mixed in a landscape. The Pyrus calleryana is not so resilient to the disease as the Pyrus kawakamii is, so the new foliage gets damaged early in the season, and remains that way until autumn.