Autumn updates: 3 plant diseases and a pest

healthy impatiens by Stofko
Healthy impatiens. Photo by Connie Oswald Stofko

by Connie Oswald Stofko

Here are updates on three diseases and one pest: downy mildew on impatiens, tar spot on maple leaves, late blight on tomatoes and potatoes, and the brown marmorated stink bug.

Maybe you’ve been dealing with these issues for years, or you might be noticing one of these for the first time. John Farfaglia, extension educator with Cornell Cooperative Extension in Niagara County, gives us updates.


There’s good news and bad news about impatiens (Impatiens walleriana). The bad news is that this popular flower for shade is still affected by downy mildew. (See an update from 2016 here.)

The good news is that it didn’t hit some plants until the fourth week of September, according to Farfaglia. It usually affects plants in August, and he got a call as recently as last week.

“I thought to myself, ‘It’s almost October,'” Farfaglia said. “That’s pretty late in the year. It’s a lot better than having it show up in August.”

Research is continuing on creating new strains of impatiens that are resistant to downy mildew, he said.

In the meantime, there are many alternative plants you can choose for shade:

Tar spot

tar spot on maple leaves in Western New York 2014
Photo by Connie Oswald Stofko

Farfaglia has noticed more calls about tar spot, which shows up as black circles on maple leaves.

“That makes sense because we had a wet spring,” he said.

There’s good news and bad news with tar spot. The bad news is that it is very ugly.

The good news is that it’s a cosmetic issue, but not a health issue for the maple tree, Farfaglia said.

See more about tar spot here.

symptoms of late blight on tomatoes
Here are symptoms of late blight on tomatoes. Photo courtesy Cornell Vegetable Program

Late blight

Late blight has been making its way across Western New, and it was confirmed a couple weeks ago in Niagara County.

“It might not be in every single garden, but it’s out there,” Farfaglia said.

Late blight is a serious disease that can kill tomato and potato plants in just one week. It’s best known for causing the Irish potato famine.

Read more about late blight here and what steps you should take to decrease the chances of it spreading.

brown marmorated stink bug Buffalo area
Here’s a closeup of a brown marmorated stink bug on my porch. Photo by Connie Oswald Stofko

Brown marmorated stink bug

Farfaglia said he hasn’t gotten as many questions about brown marmorated stink bugs this year.

“Maybe people are getting used to them,” he said.

It may also be possible that they haven’t been out in full force yet. Last year it was the middle of October when I saw them in large numbers at my house. Right now, we’re seeing just one or two at a time.

Brown marmorated stink bugs aren’t dangerous to people; they’re just ugly and annoying in your house. They make a loud noise when they fly and thump when they land. When you squish them, they stink.

They enter homes in the fall looking for a place to hang out during the winter. They don’t do any damage in your house, and in the spring they go outside to mate.

Brown marmorated stink bugs might be more of a problem in your garden because they feed on a wide range of plants. See a list of plants here that they can feed on.

There doesn’t seem to be an increase in agricultural damage from brown marmorated stink bugs, Farfaglia said.

“That might be a good sign,” Farfaglia said, “or it might be that it will just take more time” until their numbers grow to a level where they will cause damage. “We’ll find out.”


13 Comments on “Autumn updates: 3 plant diseases and a pest

  1. Earlier I had an infestation of 1/2 inch shiny black worms eating the leaves of both hibiscus plants. Now i’m SEEING THEM ON THE COTONEASTER LEAVES. i’VE TRIED TO SPRAY THEM WITH MITICIDE AND OTHER ANTI-WORM STUFF, BUT THEY HAVE SURVIVED. Any idea what they are? Thanks for replies, Judy W. in South Wales

  2. These are most likely sawfly larvae. A good blast of water knocks them off the leaves. It is unwise to use sprays that are intended for other pests and which can damage pollinators.

  3. Perhaps placing marigold or other plants known to repel bugs might help.

    I had infestation of two different bugs. Moved the potted marigolds nearby and the bugs reduced considerably.

  4. The stinkbugs are beginning to come in the house. They are able to fit in between the triple track storm windows – like to nestle into the bottom groove of the upper storm. I’ve sprinkled diatomaceous earth (DE) – a safe, white powdery substance – on the windowsills in the southeast corner bedroom, which is their primary entry point. Each night, I first put on disposable gloves. Then I check the windows, pull out the screens and storms, pick up and place each bug into the DE to coat it (DE pierces their exoskeleton and they end up dying), then throw it out the window. Interestingly, this seems to be reducing the number seeking shelter there – I had two nights with no bugs. Will continue to test this method.

  5. Pam, I wonder if the weather might have affected the number of stinkbugs coming into the house on a particular day. Please keep us posted on what you find.

  6. Will do Connie – I had similar thoughts about different conditions affecting their behavior, so will keep a log to share.

  7. I have a mature maple tree that is dropping its leaves early. There appears to be a black spot on the bark that is about 7-10 ” wide that is about 5′ long . It is only on the South side of the bark. I would appreciate any information as to the nature of this spot and any treatment which may be appropriate. I do not want to lose the tree.

  8. Stinkbug update: Since last Wednesday, I had found just one or two stinkbugs hanging on the curtains (we have heavy ivory lace curtains, so they are easy to spot). Still have not seen them collect in the groove on the storm/screens. Then on Saturday, suddenly saw well over a dozen throughout the day, again hanging around on the curtains. Early Sunday morning, I heard one fly over me as I was still in bed… eventually it came to land on my pillow. One by one, I picked them up, doused them with DE, and threw them outside. Interestingly, two of those I caught looked like they had white streaks on them (different from their natural markings) – may have been returnees. The DE doesn’t kill right away, takes a few days.

    My theory is that the DE does act as a deterrent, so I’m not seeing them congregate as they did at first. I also wonder if they might communicate danger to each other.

    I think those I caught over the weekend had come inside and crawled into the space where the ropes that raise/lower the double hung windows are housed. One of ropes is broken, and hangs down – I found at least two crawling there, and others close by. The weather warmed up on Saturday, and they seemed to be active mostly during the early/late afternoon. Maybe that’s their behavior – crawl into a “safe” place and hide for a few days, then become active for a short while.

    They hibernate over the winter – two years ago we had a much worse infestation – first time – and I found them everywhere in the bedroom – nestled in clothes in drawers and such. I had to pull everything apart and wash and clean in the spring.

    This morning, there were none to be found. More updates to follow!

  9. Pam, thanks for the update. If the diatomaceous earth acts as a deterrent, it could help if you know where they come in. I haven’t seen nearly as many stinkbugs this year as I did last year, so maybe the population is down. I’ve got my fingers crossed.

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