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October is the month when the sweet basil declines quickly: The gardener can’t stop it from flowering, its sweetness is stained with bitterness, and the cooling nights fleck the leaves with dark spots.

But even this seasonal demise has become nothing more than a fond memory for many gardeners who are seeing the once ubiquitous herb harder to grow and certainly harder to sustain this far into the growing season.

The problem is the resurgence of an old disease — basil downy mildew, which was thought consigned to history, not to mention another continent (Africa) — before it reemerged in Florida eight years ago. It spread up the Eastern Seaboard within two years and is now entrenched in at least three dozen states and the District.

Scientists view the disease as the single biggest threat to the commercially most important herb in the United States.

If you planted out your basil plants in May and by July the leaves were pretty much pale above, gray below, and on their way to blackening, it was probably the downy mildew.

Varieties of citrus and Asian basils are less susceptible. Unfortunately, the classic Genovese sweet basils, and especially the largest-leafed ones, are magnets for the pathogen, a fungus-like organism called an oomycete.

You will still find sweet basil at the supermarket and, in the spring, as seedlings or transplants at the garden center. But these have been raised under a spraying regime. Once your plants are planted out in the garden, the wandering spores of the pathogen are likely to find them.

Meg McGrath, a Cornell University scientist studying the disease, says she is amazed that the mildew will turn up even if the gardener is growing just one or two plants. “It documents how much inoculum must be out there,” she said.

McGrath and others have studied the efficacy of organic vs. synthetic fungicides and have concluded that the organics aren’t terribly effective — they don’t get into the leaf tissue as well as the conventional chemical ones. One of the reasons home gardeners grow their own herbs (and fruits and vegetables) is that they can control what is sprayed on an edible plant, or not. But growing your own sweet basil without spraying poisons seems nigh impossible since the disease arrived in 2007.

Last year was grim for recorded outbreaks of downy mildew, and this season “was another bad year,” McGrath said. The disease spreads most when leaves are kept wet or humidity is high. Leaves were wet in June, all right, with 12 inches of rain, but even during the dry spell that followed, the air felt like a swamp on some days in the garden.

Plant scientists in academic and commercial realms are working furiously to try to breed resistance into sweet basils. As part of the largest single effort in the United States, researchers at Rutgers University unveiled in August a handful of varieties that they hope to get to growers, consumers and gardeners in the next few years.

These include three or four sweet basil hybrids that should be available in 2017 or 2018, with two or three sweet ornamental basils and at least one Thai basil type to follow, said Jim Simon, Rutgers’s lead scientist.

He and his colleagues have grown six generations in their quest, and the handful of finalists are the cream of the crop. The challenge has been to find basils that shrug off the disease. To do this, they examined plants from other basil species, including citrus basil, holy basil and a shrubby species named Ocimum gratissimum. They also found a spicy basil more closely related to sweet basil named Mrihani that seemed genetically resistant to the disease, and they chose this as a breeding parent. Once they identified disease-resistant candidates, they had to hybridize that sweet anise flavor back into the plant.

Another task was to find the right look: the classic sweet basil has a smooth, robust and slightly convex upper leaf. A lot of their resistant stock has ruffled foliage.

Seed companies are doing their own work, and one has introduced a variety named Eleonora, which has moderate resistance and would still need good growing conditions to limit the disease. These include planting in an open location with lots of air movement, and not getting their leaves wet if possible. In other words, water their feet, not their heads, and avoid overhead irrigation.

McGrath says she tries to manage the disease by planting basil in pots, which she brings in at night when humidity levels climb. But she lives in Wading River, N.Y. South of the Mason-Dixon Line, I’m not sure such a strategy would help.

Local farmers I know have stopped or curtailed their basil crops for now. David Giusti is a young market gardener who tends his three-acre field near Purcellville. He grows edibles for most of the year and last week harvested his first broccoli of the fall season. But he farms organically and doesn’t want to raise basil that he’d have to spray with chemicals. “Many people have given up on it or just do an early planting,” he said.

Hiu Newcomb, of Potomac Vegetable Farms in Vienna, said she used to make eight or 10 cuttings of basil per season and now does only two or three before the crop has to be plowed under. “We are all waiting for a better variety of green Italian basil,” she said, echoing some pretty widespread sentiments.

More from Home and Garden:

In ‘The Martian,’ finally a botanist in space — but without seeds?

Asian vegetables enrich and extend the autumn garden

Three fruits you thought were tropical, but can grow in your back yard

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