By this time of year, some plants are looking downright ragged, showing the the effects of age, wind, insects, and disease.
Don't begrudge fungi for the few yellowed leaves and black specks they cause, but there is a certain point where such damage gets out of hand. When that point is reached, spraying is an option, and one useful spray is an old-fashioned fungicide called Bordeaux mixture.
Bordeaux mixture's origin goes back to the 1870s, to a vineyard in Bordeaux, France, where grape farmers were having trouble with people stealing fruit from vines along roads. One farmer decided to discourage theft by making his vines look poisonous. He poured some powdered bluestone and lime into a bucket, added water, then swirled the mixture around and splattered it over the vines with a broom.
One day that season, a man named Millardet happened to be walking along the road bordered by those evil looking vines. Millardet was in Bordeaux studying downy mildew, a grape disease that had recently entered France by accident, and was now threatening the whole French wine industry.
Serendipity struck when Millardet noticed that the vines dowsed with that eerie, pale blue mixture had less disease than did other vines. Back in the laboratory, he fine-tuned the ratio of bluestone, lime, and water to the most effective proportions, and Bordeaux mixture was born.
Bluestone is copper sulfate, a naturally occurring mineral. The copper is what makes Bordeaux mix toxic to fungi (and some bacteria), but watch out: copper can also be toxic to plants. Bordeaux mixture has been known to delay fruit set of tomatoes, even to cause rose leaves to yellow and drop -- injury that could be mistaken for blackspot disease, for which the spray is sometimes used.
Bordeaux mixture is prepared in different proportions for different situations. These days, you do not have to mix up your own bluestone and lime, then splatter it on your plants with a broom. Just buy prepared Bordeaux mixture, mix it with water, then apply it with a sprayer.
Never use Bordeaux mixture -- or any pesticide -- with abandon. If you use Bordeaux, realize that it will not cure already damaged leaves, that copper can accumulate in soil, and that it can injure plants. Bordeaux mixture also can injure you, being moderately toxic.
Before using Bordeaux mixture, read the label to make sure it is effective against your particular plant disease problem, and then decide whether the amount of disease damage warrants spraying.