Plant Problem Q&A [online event]

Watch The Recorded Talk, Plus Bonus Questions and Answers

Nate, Emma and Cheryl

In a recent episode of Granite State Gardening, Dr. Cheryl Smith and Emma Erler joined host Nate Bernitz for a live talk about plant problems they've seen from growers and gardeners in New Hampshire. Topics discussed included tomato diseases and other problems, powdery mildew, rusts, apple diseases, black knot, scorch and drought stress, caterpillars and slime molds. That talk can be viewed below.

We didn't get to all the questions viewers had for us, so we are sharing them here as a written Q&A. Enjoy!

Questions and Answers with Emma Erler

Jean from Fremont asks: What is eating my flower plant, and only selecting a few plants? How do I get rid of the pest?

Perennial insect feeding

Based on the black droppings on the foliage I would guess that a caterpillar has been feeding on some of your perennials. I actually wouldn’t recommend getting rid of it without identifying what it is. There are many native butterflies and moths that feed on garden plants without causing them any serious harm. This late in the season I would go ahead and let the insects have their fill. I’d also argue that having butterflies and moths in the garden is one of the best parts of gardening in general.

Ellen from Melrose, MA asks: Powdery mildew on lots of my plants, even with drip irrigation and native species. Probably from a wet April, although it didn’t show up until summer. Anything I can do to prevent this?

Unlike many other fungal diseases that grow best during wet weather, powdery mildew actually thrives when leaves are dry and the humidity is high. Humid weather with little rainfall, like we’ve had this summer, is perfect for powdery mildew infection and spread. Increasing airflow around plants through pruning and good spacing can help limit infection. Growing powdery mildew resistant varieties can also help. Powdery mildew overwinters in plant debris, so disposing of leaves and stems in the fall can help reduce infection next summer.

Ellen from Melrose, MA asks: Tomatoes have repeated seasons of early blight! Help!

Have you tried growing an early blight resistant variety? A few common varieties are Iron Lady, Mountain Magic and Verona. Seed catalogues will usually designate if a plant has resistance to any common disease issues. Other good practices that help avoid early blight are rotating where you plant tomatoes, mulching, staking, removing the lowermost leaves and using drip irrigation.

Betsy from Hancock asks: How do I know when to divide a peony? For background: Prior owners planted them between 2000-2004 or so. Some are huge but with few blooms.


Are the peonies located in full sun? Plants that get too much shade usually don’t bloom very well. Also, peonies that are planted too deeply often don’t flower as they should. Peonies don’t usually require division unless you’d like to create more plants. It may be that you need to transplant them to a new location, but not divide them.

Early fall is actually the best time to divide peonies, just in case you do want to try it. Cut the stems down and then dig around and beneath the plant to lift it from the ground. Next, use a knife or sharp shovel to divide the plant into smaller clumps. Each clump should have at least 3-5 buds (these should be fairly obvious at the top of the roots). When you go to replant, make sure the buds are no more than 1-2 inches below the surface. Planting too deep can result in poor bloom and growth.

Betsy from Hanock asks: Another peony question: some have started to mildew. Do I cut those back now or when the foliage dies, later in the fall?

I would wait until the foliage dies back in the fall. As long as the leaves are still green the plant is photosynthesizing and storing energy in its roots for next year’s bloom. As long as you remove the foliage from the garden before the winter you will be able to reduce the chance of infection next year.

May from Hope, ME asks: This is Yokatta-Na, an Asian green. It was very heathy, but then some plants started yellowing and then drying up. Would love to know what causes this.


These symptoms look like scorch to me, or in other words, leaf damage that results from exposure to heat and drought. Letting the soil dry out for just a day could be enough for leaf edges to start to die.

The holes in the leaves are probably from a caterpillar, such as cabbage looper or imported cabbage worm.

Anne from Smithfield, RI asks: Phlox-lower leaves turn yellow/brown. Have not tried any treatment-prefer to avoid chemicals. I have tons of phlox and am moving; was hoping to take one of each color with me to start my new garden. Any cure for this leaf issue, or should I leave them behind? Thank you!


The foliage dieback could be drought related, which there is no cure for at this point in the season. The symptoms don’t look disease related to me, so I would have no qualms about taking divisions from these plants and moving them to your new home.

Laura from Hudson asks: Lilacs, hydrangeas, Maples, Spirea all small shrubs suddenly (dying off) deforming, with research it looks like Witch Broom. I eliminated some of the most infested trees and shrubs however , everywhere I look on my property and that of those neighbors adjacent, are showing the same signs. Even Large maples above the effected shrubs... What should we do. I fear a mass devastation of the trees and shrubs in our neighborhood.

witches broom

Witches’ brooms are often caused by fungal, viral, or phytoplasma (bacteria-like organism) infections. Since you’re noticing symptoms on such a wide host range, I suspect that phytoplasmas might be the issue. Phytoplasmas are usually move from infected plants to healthy plants by insects with piercing-sucking mouthparts. Eliminating infected plants from the landscape can slow the disease spread. Once witches’ brooms are noticed they should be pruned out as soon as possible. However, the organisms that cause these growths may have spread throughout the plant so that pruning may not provide control and the plants need to be replaced. Regardless, my advice is to start by pruning out all distorted branches, and hopefully you’ll be able to get ahead of this issue before it spreads further.

Kim from Francestown asks: Vine borer ate up my zucchini and yellow squash. I still had blossoms. Was there any way I could have saved the plants at this point? I just ripped them out.

I’m wondering if you actually had squash bugs, not squash vine borers? Squash vine borers cause the vines to wilt and die very early in the season, while squash bugs will feed on the sap of leaves, causing yellow spots to form that later turn brown. If most of the foliage had died back then it was definitely too late to save the plants this year. The fruit won’t develop properly without lots of green leaves to support its growth. However, next year you’ll know what to look for and take early measures to control the squash bug population before it gets out of control. There are some good control recommendations listed in this fact sheet:

Additional comments from Nate: For squash vine borer, "surgery" to physically remove the larvae can be effective. According to Nebraska Extension, "Gardeners can use a sharp knife to cut a vertical slit in the stem and remove caterpillars by hand. Following removal, the stem should be squished back together and then soil should be mounded over the cut portion to encourage re-rooting." Learn more in their excellent fact sheet:

That all said, prevention through cultural practices and physical exclusion are the most important techniques. You will know to utilize row covers during windows of time when squash vine borer moths are flying to protect your squash and zucchini, as well as to rotate your veggies and thoroughly remove plant debris at the end of the season. If you practice tillage, tilling the area at the end of the season may also disrupt overwintering borers. Keep in mind that if you are utilizing row covers, which will also help exclude squash bugs, you may need to hand pollinate if you have male and female flowers blooming at the same time you need to have the row covers on your plants. The link above from Nebraska also discusses pesticide options that can be applied directly to the stems, but we recommend trying these other techniques before going down that road.


Lori asks: My tomatoes aren’t turning red ? They were planted the first week of June. Leaves are lush and tomatoes are big but have been the same size for about a month . They are “ big Boys” and another that are supposed to be red . I know sometimes hot temps can affect them turning but all of my neighbors are picking red . Please help

Are your neighbors growing the same variety of tomato? Big Boy takes approximately 78 days to mature, so it’s quite possible the fruit just haven’t been on the vine long enough yet. Drought conditions can also impede tomato ripening, so make sure your plants are getting plenty of water. You need to run the sprinkler or irrigation system long enough to wet the soil to a depth of six inches in order to get water to all of the roots. If you water the garden deeply, you may only need to water every few days.

Sheri from Hollis asks: We have had squash beetles, and who knows what else infesting our veggie beds. We are pulling up plants from.some of the beds. Is there anything we should do to the soil before planting fall greens? Also, can the infested plants go in the compost? Thank you!

The only thing you can do to limit the number of squash bugs in your garden next year is to remove as much plant debris as possible to reduce the number of overwintering sites for the insects. I don’t see any problem with composting the vines. Eggs and immature squash bugs won’t survive if they don’t have a food source, and adults will move someplace else to find food and spend the winter. Next year you might try to get ahead of squash bugs using some of the management recommendations in this fact sheet:

Cynthia from Sanbornton asks: Help me get rid of wisteria! I have bees, I don’t want to poison the earth! Mechanical or other non-toxic advice? Thank you!

Wisteria has deep roots, making it hard to eliminate once it has been established. Repeatedly cutting the vines to the ground will eventually exhaust the root system, though this could take a few years. Depending on the size of the vines you may also be able to dig up them up or use a weed wrench tool to pull them out.

Marion from Fitzwilliam asks: This is my Mandarin Lights azalea. I just planted it this Spring after it was purchased at a local nursery. It has been defoliated by, I am presuming azalea caterpillars. My question is, will it survive? I know I can pick off the critters, but I haven't seen any. Do they feed at night? Do you think it was in the soil at the nursery? What can you tell me about them? Thank you!


Yes, I definitely thing your azalea will survive, especially if you keep watering it through the fall. I’m curious why you think caterpillars were feeding on it. Did you notice small holes in the leaves? Or did entire leaves disappear overnight? Deer are known to feed on azaleas, and can defoliate plants in just a night or two.

Watch Previous Episodes of Granite State Gardening

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