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Defeating Leaf Spot and other Dread Diseases


Q. Mike: I have purplish leaf spots on my perennial rudbeckia. Can you venture a guess as to what's wrong? Thanks,
    ---Chris in Williams Township, PA
A. Whoa—you win the Black Plague Prize, Chris! Your "rude-Becky-a", a twenty-dollar way of telling people you're growing Black-eyed Susan or some other member of the cone flower family, is supposed to be one of the most disease-resistant, easy-care perennials! Nice work!

Anyway, you want a diagnosis for your spotted leaves? How about "leaf spot"? No applause, please. The common name for a number of patchy pathogenic plant problems, yours might be any one of a dozen or so, like Alternaria leaf spot, Septoria leaf spot or the much harder-to-pronounce one our next listener thinks SHE has…

Q. Mike: I bought an Endless Summer hydrangea a couple of years ago. It seemed to be growing perfectly, but now has black spots on its leaves. From checking the web, I think this is Cercospora hydrangeae, and maybe I need to use a fungicide. But listening to your show has made me more sensitive to the need for safe, non-toxic gardening—and our well is right next to the hydrangea. What's the best way to deal with this?
    ---Barbara; Washington Crossing, PA
A. You have named yet another of the 'leaf spots', Barb—and by following the disease name with "hydrangae" you reveal that you even know what plant it's on! Yes, that dramatic-sounding diagnosis for the spots on your hydrangea's leaves simply means that…well, your hydrangea has spots on its leaves.

Sorry, but I've noticed over the years that a lot of our listeners waste a lot of time searching for a specific name for what's going on, only to come up a Latin phrase for what you already know. So enough with all this diagnostic dithering; the dirty little secret of plant disease is that the specific identity of the ailment rarely matters.

Now, one of the reasons you two are having problems is that your plants had to suffer through a hugely warm and wet season the year you submitted your questions. But I am also growing Black-eyed Suzy and Endless Summer Hydrangeas, and was doing so in the very same tropical jungle of a season, and mine were (and are) unblemished and full of flowers.

My coneflowers are in a fairly reasonable position—a little crowded by a peach tree and a rose (do I know how to taunt disease or what?) but in a spot that gets some sun, which they absolutely require. However, the Red Cross keeps coming by to try and provide my hydrangeas with blankets and legal counsel; they're in a tight corner, right up against the side of the house and get about 40 minutes of sun a day. LATE in the day.

So—how can those plants possibly still be free of disease? Because they have never been subjected to chemical fertilizers, and get lots of compost. I firmly believe that chemical fertilizers like Miracle-Gro, Peter's and Osmocote are the biggest cause of plant disease. The fast, unnatural growth those concentrated chemical salts cause makes plants VERY attractive to pests (yes, disease is a pest; a little-bitty, teensy-tiny one, but a living pest nonetheless). Compost, on the other hand, contains teensy-tiny living creatures that prevent disease—three different ways.
  1. Some of the good guys in compost create an environment that impedes the growth of disease organisms.
  2. Some compete for the same food as disease organisms.
  3. And others—my favorites—actually eat disease organisms. ("I'll have the anthracnose with downy mildew and a mixed leaf spot salad, please.")
Stop using chemicals, feed your plants compost and you'll be able to get away with murder. Eh, let me rephrase that. You'll be able to grow disease-prone plants in dicey conditions with a minimum of problems.

Sanitation is also very important. Many diseases—including leaf spot and the dreaded black spot on roses that Roberta in Chicago just emailed about—get totally out of hand when diseased leaves fall to the ground and become illness incubators. All the books tell you to remove discolored leaves from the plant as soon as you see them—and that is good advice. But having a 'clean floor' is even more important.

So for now, just prune off any dead or diseased portions and clean up under your plants. Then clean up again in the Fall; don't prune! You should never prune anything in the fall; just clean. Then next Spring, prune off anything that's diseased and clean up like an old Polish lady scrubbing her stoop. Remove everything down there, including any old mulch, and put down an inch of the best compost you can find. If Spring and Summer are wet, hot, and humid, replace that compost with a fresh inch later in the season.

Yes, it is important to not use any kind of wood mulch; to position disease-prone plants where they get morning sun; and to make sure that ALL plants have good airflow. Prune crowded plants to open up the centers. And don't be afraid to just plain remove some plants from an over-crowded landscape; it'll improve the vigor of the survivors, and you'll actually be able to see what's out there!

But if you simply eschew chemicals, feed and mulch with compost and keep a clean floor, you'll rarely see more than a few discolored leaves.

And if that's still too much for you, go ahead and use a fungicide. But use it preventatively—BEFORE problems occur, or at the very first sign of trouble. And make sure it's a nice natural one; like that new player on the market that utilizes the plant disease fighting power of a naturally occurring soil organism named Bacillus subtilis; or neem oil, or compost tea, or a baking soda spray.

Chemical fungicides? They may (or more often, may not) cure your plant's diseases, but they'll certainly increase your changes of coming down with something dread later in life. And cleaning your floor will not help you then.

(NOTE: You'll find lots of details on compost tea and baking soda at the "black spot on roses" Question we linked to above).

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