Update on Tomato and Impatiens Blights; & Just What Makes a Plant Disease a "Blight"?

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Question. Last year our impatiens did very poorly, and we heard there was some problem with either the weather or a blight. So many of our plants died that we don't know if we should try again this year. Is there a known problem or was it just a bad year?

---Michael and Myra in Allentown PA

Answer. To misquote a popular Sinatra song lyric: "It was a very bad year…for shade loving flowers and tomato plants…"

Yes, an impatiens "blight" wiped out huge numbers of the shade loving flowers last year. They looked like they were getting too much sun one day, the next day they were leafless and the next day they were gone. Done. Kaputski. The correct common name for the affliction is 'impatiens downy mildew, explains Dr. Margaret Tuttle McGrath (no relation, we think/guess/maybe) from the Cornell University Department of Plant Pathology; Long Island Horticultural Research & Extension Center in Riverhead, NY.

A different but equally deadly downy mildew has also started attacking basil plants, she adds. Both 'blights', she explains, "seemed to come out of nowhere. There had been a few reports of these pathogens, but nothing really significant. Then suddenly we had widespread late blight on tomatoes one season and devastating impatiens and basil blights the next."

The infamous late blight on tomatoes is not a downy mildew, but a pathogen so powerful and notorious it was given a formal name that roughly translates to "the destroyer of plants". (And destroyer of people as well; it's the same pathogen that led to the starvation of so many of my ancestors during the Irish Potato Famines of the 1800s.)

So what exactly qualifies each of these diseases as a 'blight'? What IS a blight, anyway?

"The plants just get wiped out," explains Dr. Meg; "And they get wiped out really fast. The word 'blight' isn't so much a strict horticultural term as it is societal; Biblical—'a blight upon you'. It's a terrible thing that happens to plants, happens fast, and then people suffer."

Right now the only way experts can think of to try and stop the impatiens blight is to not plant impatiens for a few years. That's why horticulturists—and reporters like me—are urging gardeners to grow different shade loving flowers for the next few seasons; things like New Guinea impatiens and "Sunpatiens" (which are not affected by the blight), annual begonias, coleus and Torenia, 'the wishbone plant'—an under-used and very attractive shade-loving annual.

Question. Last September, my tomatoes were blighted. Despite growing in raised beds, being supported by nice sturdy cages and fed no chemical fertilizers, the bottom leaves began browning, then the stems; and then the fruits developed "bad spots", which spread rapidly. It all happened at a pretty fast clip. The papers and blogs were all abuzz about "late blight", so I pulled out the plants and threw them in the trash. I would like to avoid this blight next year. I already plan to rotate my tomatoes to a new bed. Should I also use a bleach solution on the cages to eliminate carryover? Thanks.

---Mike in Green Lane, PA

Answer. No bleach, please! Bleach is incredibly toxic and dangerous. It was used as a weapon of mass destruction in World War I, and has no place in a nice organic garden. If you feel you must disinfect something, use white vinegar instead.

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As in previous years, the disease has already been detected down in Florida, and will work its way up the coast, traveling on the wind. It spreads most rapidly when humidity is high, but temps are relatively cool. (The pathogen goes dormant in really hot weather.) It attacks chemical and organic plots equally.

Your best protections, explains Dr. Meg, are to grow resistant varieties (better ones are coming on the market every year, she notes), to check purchased transplants carefully before planting (dark stems are a bad sign; take a pass on such plants or return them) and to provide the best possible growing conditions—that means morning sun, great airflow in and around the plants and watering only at the base. "You can't affect the actual outdoor humidity," she notes, "but you can keep your growing area as dry as possible."

You can track the spread of the disease at https://usablight.org/; and learn the symptoms of late blight at Dr. Meg's diagnostic and informational web pages, which we will link up to, as well as her treatment recommendations for organic growers.

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