Crapemyrtle woes

An infestation of Crapemyrtle Bark Scale on a crapemyrtle trunk. Sooty mold is darkening some of the outer bark layers. Photo: Jim Robbins, Univ. of Ark. CES, Bugwood.org

Q:  My crapemyrtle has white stuff on the bark that I’ve never noticed before, though the foliage looks unaffected, if a bit dull lately. I thought these plants were pretty pest-free, so what might this be?

A:  We’ve had a lot of inquiries about this lately. Your plant has Crapemyrtle Bark Scale (CMBS), a non-native insect pest that was discovered in Texas in 2004 and confirmed in Maryland in 2020. Incidentally, crapemyrtles also can host Crapemyrtle Aphid, and it’s possible for the two to be infesting a plant simultaneously; their impacts on the plant are similar.

While CMBS could potentially feed on other host plants, so far they seem to strongly prefer crapemyrtle. The aphid sticks to crapemyrtle. Both secrete honeydew, the sugar-water waste common to sap-feeding insects, which is dulling the leaf appearance and probably cultivating a bit of sooty mold.

Since mid-Atlantic gardeners have embraced crapemyrtle to such an extreme that it’s everywhere you look, that’s a big buffet enabling this pest proliferation. We really need to diversify our landscapes.

Scale insects lead relatively sedentary lives, generally only moving about to any notable degree as newborns, appropriately called crawlers. After roaming to find a feeding site, crawlers settle down and stay put, using their straw-like mouthparts to feed on plant juices. Layers of protective wax, in this case felt-like and white, cover their bodies as they mature. For our purposes, this also means they are harder to treat with contact-type insecticides like oils or soaps because that shell prevents the pesticide from reaching them. Crawlers, running around shell-less for that brief window of time, are the most vulnerable life stage any treatment should focus on.

The problem is, this pest is so new to our area that we are still collecting data on when those crawlers appear. Insect development is dependent on temperature, so while we can make predictions based on how CMBS behaves to our south, we’re still refining our knowledge for Maryland. Complicating matters is the likelihood of several generations per year, and they might overlap.

For such a tiny thing with limited mobility, you may wonder how it got there in the first place. Like plant mites, crawlers can blow around on the wind, and might also disperse by hitching a ride on other animals, like birds. CMBS arrived in our area the way many plant pests do – accidental introduction on plants with undetected infestations shipped-in from out of the area.

Management of an established scale population, usually booming by the time we notice them, takes time. Don’t expect one or two treatments to resolve the issue quickly, and you’ll probably need to employ the services of a certified pesticide applicator. Manually scrubbing scale off while not wounding bark is difficult and not highly effective, given the nooks and crannies they can wedge themselves into that you cannot reach.

Not only should certified applicators treat trees too high to reach, but they will have more effective equipment and the ability to apply chemicals the general public cannot due to the Maryland pollinator protection law. Overlapping the use of more than one type of pesticide may be needed, and re-treatment might occur for over a year. Dead scale won’t fall off right away, though treatments for scale will probably suppress aphids at the same time.

While we usually suggest trying other methods to suppress pests, once scale are numerous, there is little recourse than resorting to pesticide treatment. Certain species of lady beetle larvae will consume these scale and could knock-down their numbers somewhat, so avoiding contact-type pesticide use or enthusiastic scale-squishing attempts does at least spare them. Drastically cutting back a large crapemyrtle is not recommended since that can ruin its branching structure, though you could try it with dwarf shrubby varieties since otherwise-healthy plants should regrow over future seasons. With proper application timing to avoid impacts on scale predators and other insects (something well-trained pesticide applicators and pest scouts know how to do), treatments can be done with minimal risk to the biodiversity in your landscape. Or…just plant something different and rely on other plant species to provide summer color.

By Miri Talabac, Horticulturist, University of Maryland Extension Home & Garden Information Center. Miri writes the Garden Q&A for The Baltimore Sun. Read more by Miri.

Have a plant or insect question? University of Maryland Extension has answers! Send your questions and photos to Ask Extension.

Q&A: Why are lilac leaves brown and curling?

Irregular brown spots and blotches appear on lilac leaves, followed by leaf curling and defoliation in late summer. Photo: UME Ask Extension

Q: My lilacs look like death-warmed-over this time of year. Do you know what’s wrong, and is there anything I can do at this point?

A: Lilacs are sadly not very well-suited to our mid-Atlantic conditions. We’re at the southern edge of their heat tolerance, so while they weren’t among the best flowering shrub choices to begin with, climate change is only going to worsen their prognosis. Several types of leaf-spotting fungi and bacteria, plus general heat stress (which also increases their vulnerability to borers), results in foliage that looks quite beat-up by late summer. Brown spots, crispy leaf edges, and bare stems from premature leaf drop are all typical. You can explore lilac ailments and their management on our lilac diagnostic page.

No fungicide will reverse these symptoms once they appear, and while they might work as a preventative if applied before bud-break (and re-applied repeatedly well into the summer), it’s simpler to just grow something else if a plant is going to be that much of a hassle. This is especially true if the treatments don’t work and the plant still winds up looking horrible. Fungicides also carry the risk of harming other organisms.

For now, you can rake up and dispose of any fallen leaves, though this isn’t a foolproof way of removing a source of infectious spores. Cut down the oldest, thickest stems this winter (they tend not to bloom well at that age anyway) and open up the canopy by selectively removing some stems that contribute to foliage crowding. You can do this thinning after bloom next spring.

For anyone really wanting to grow lilac despite these challenges, try cultivars with above-average disease resistance and heat tolerance. While not immune to problems, they perform much better, even if they don’t look exactly the same or have blooms as large or heavily perfumed. ‘Miss Kim’ is a round, compact-growing cultivar with pale lavender-purple flowers that’s been around for decades. Other varieties are now available with pink or deeper purple blooms, some of which even rebloom a bit, sporadically producing flowers into summer and early autumn, though high heat could still hamper that.

dwarf lilac with lavender flowers
Dwarf lilac species and hybrids handle Maryland conditions much better than the traditional varieties. Some recent introductions will also re-bloom sporadically later in the summer. Photo: M. Talabac

All lilacs, but especially the traditional, classic “French” types, should be planted in a location with great air circulation (so, not up against a fence or wall) so wet leaves dry quickly after rain, dew, or irrigation. Wet foliage is more easily infected by pathogens.

The main perk of growing lilacs is fragrance, so if you want a scented replacement, consider: Winterhazel (Corylopsis), Koreanspice Viburnum (Viburnum carlesii) and its hybrids, Summersweet (Clethra), Seven-son Flower (Heptacodium), Carolina Allspice (Calycanthus), various deciduous Azaleas (Rhododendron viscosum and several others), Mockorange (Philadelphus), and Fragrant Abelia (Abelia mosanensis). Their scent characteristics, flower colors, mature sizes, and preferred growing conditions may differ from lilac, but nothing is going to be an exact substitute. Plus, several of these species will offer the additional bonus of showy autumn foliage or (for the native ones) better wildlife value. These are just some shrub ideas; there are also fragrant perennials and, if you have the room, several fragrant trees.

By Miri Talabac, Horticulturist, University of Maryland Extension Home & Garden Information Center. Miri writes the Garden Q&A for The Baltimore Sun. Read more posts by Miri.

Have a plant or insect question? University of Maryland Extension has answers! Send your questions and photos to Ask Extension.

Celebrate National Moth Week

A moth feeding on nectar of a purple  verbena flower
Hummingbird Clearwing. Photo: M. Talabac

The last full week of July is National Moth Week, and I encourage everyone to take a closer look at the vast diversity of moths that fill our natural world. Butterflies and moths belong to the same insect group, but moths far outnumber butterflies in species diversity. Since many moths have muted colors or fly at night, we’re largely unaware of this bounty. Let’s take a whirlwind appreciation tour of the group to illustrate the amazing, bizarre, and quirky features of this major insect order.

Moth adults come in all shapes and sizes, and like butterflies, wings are their most prominent feature. There are “micro-moths” whose wingspans are less than an inch, and giant “silk moths” up to 6 inches, making them the largest moths in North America. (Our native silk moths are not closely related to true silk moths, but they got the name because people thought they could be farmed for silk.) The wings of some moths look like mere slivers, seemingly insufficient for flight, while others are tucked around their body so they look fairly cylindrical. Some lay so flat at rest with their wings spread that you’d swear they were two-dimensional.

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Q&A: What is a good summer-blooming plant that’s deer resistant?

Yellow flowers of St. Johnswort plant
St. Johnswort flowers. Photo: M. Talabac

Q:  What can I use as a summer-blooming shrub, especially if this part of the garden is sunny and somewhat dry? I also sometimes have deer problems.

A:  I think St. Johnsworts (Hypericum) are underused, and several species are native here in Maryland, though those might be harder to source. Some of the commonly-grown forms are non-native hybrids, though well-behaved ecologically. (The only locally invasive species, Hypericum perforatum, is fortunately not likely to be sold at a nursery.)

St. Johnsworts bloom anywhere between June and September, prefer direct sun, generally tolerate drought well, and are distasteful to deer. Blooms are nearly always an intense yellow, and some species or cultivars have colorful summer or autumn foliage. A few cultivars have berry-like seeds that ripen by fall and make good bouquet accents. I love the bark on native Hypericum densiflorum – peeling with a smooth underlayer that’s a rich, warm-toned cinnamon-brown that’s especially showy during dormancy.

You’ll find St. Johnsworts sold as both perennials and shrubs, because some species stay low, sprawl like a groundcover, and have stems that aren’t very woody, occasionally dying back in winter as other perennials do. Other species have woody stems and grow to about three or four feet tall and wide. Flowers are loaded with pollen, but no nectar, so butterflies will probably detour while bees and flower flies (predators we like to keep in the garden) will visit. Don’t deadhead developing seed capsules if you want to support Gray Hairstreak butterfly caterpillars, which can use Hypericum as a host plant (among a huge variety of other plants).

For more plant ideas, visit the Home & Garden Information Center’s pages on Plant Selection and Deer-Resistant Native Plants.

By Miri Talabac, Horticulturist, University of Maryland Extension Home & Garden Information Center. Miri writes the Garden Q&A for The Baltimore Sun. Read more by Miri.


Questions about home gardening? Send them to Ask Extension

Q&A: Why do my cucumber and zucchini plants wilt?

One Spotted and multiple Striped Cucumber Beetles, feeding on overripe pumpkin. Photo: Whitney Cranshaw, Colorado State University, Bugwood.org

Q: Last summer I had cucumbers and zucchini wilting and dying even though I’m pretty certain I didn’t have root rot or squash vine borer. What should I try this year so I can hopefully get a harvest?

A: Bacterial wilt disease, transmitted by cucumber beetles is the prime suspect for crop failure in this instance. Both of these garden pests – Striped Cucumber Beetle and Spotted Cucumber Beetle – are native to North America and can cause serious damage to vegetables in the squash/cucumber family, though they can also feed on unrelated fruits, vegetables, and ornamental plants.

Although their feeding causes direct plant damage, the main issue comes from their introduction of one or more plant pathogens. These beetles can transmit diseases like bacterial wilt and viruses, none of which are curable.

Delaying the planting of squash and cucumber transplants until mid-June may evade the host-seeking adults. Until they bloom, cover plants with insect netting or row cover (the former is ideal as it doesn’t trap heat). Bees will need to reach the flowers for pollination, but once the fruits start to develop, plants tend to be less susceptible to infection. Since more than one beetle generation can occur per year, clean-up veggie garden debris in autumn to deny remaining adults overwintering shelter.

For now, ‘County Fair’ is the only available variety resistant to bacterial wilt. This pickling cucumber is parthenocarpic– it produces mostly female flowers that don’t require pollination to set fruit. The Cucumber Beetles page at the Home & Garden Information Center has more information about these insects and their management.

By Miri Talabac, Horticulturist, University of Maryland Extension Home & Garden Information Center. Miri writes the Garden Q&A for The Baltimore Sun. Read more by Miri.

Send home gardening questions to Ask Extension at the Home & Garden Information Center

Q&A: Some insects have a lot of gall

Maple Eyespot Gall, caused by a native midge (tiny fly) that occurs throughout the state. Photo: M. Talabac

Q: What on Earth is going on with this maple leaf? I saw it on a wild tree while taking a walk down a neighborhood path, but wonder if it’s something that can spread to nearby gardens.

A:  This is a great example of a gall, which is a tissue deformity on a plant caused by either insects, mites, fungi, bacteria, or nematodes. Usually galls cause swelling or weird projections on leaves or plant stems, but sometimes the more obvious feature is a color change like this.

The activities of the organism responsible creates chemical changes in the leaf tissue, redirecting tissue formation to suit its needs. For instance, insect-made galls give the larvae their own little house to feed in while being protected from most predators or harsh weather. (Impressively, tiny parasitoid wasps, little bigger than a dash on this page, still find their prey inside these structures and interrupt their life cycle. Isn’t that amazing?)

Despite how drastic galls may look to us, they don’t cause much harm to their host plants, which can be trees, shrubs, or perennials. Oak trees are renowned for harboring many kinds of eye-catching galls, some of which become most noticeable when they fall out of the canopy onto our lawns or gardens. See if you can find anything living inside those swollen red or brown lumps or balloon-like pockets on leaves. A wise bird or other insect may have beaten you to it, though, or the culprit is long gone and already flew away as an adult before the plant jettisoned the injured leaf.

If an eyesore, you can clip off heaviest infestations of leaf galls on witchhazel (caused by insects), azaleas (fungus), oak saplings (usually insects), and any other easy-to-reach plant. Keep in mind that the unaffected portions of those leaves are still functioning to feed the plant, so don’t remove too much growth. Otherwise, I suggest you leave them alone and just marvel at the intricacies of the natural world. Gall-forming insects can feed songbirds and don’t risk the health of the plant. As with any organism, populations wax and wane over time and galls might be prevalent one year and nearly absent the next.

At the Home and Garden Information Center, we have several web pages with more information about galls, including Shade Tree Galls and Eyespot Galls on Trees.

By Miri Talabac, Horticulturist, University of Maryland Extension Home & Garden Information Center. Miri writes the Garden Q&A for The Baltimore Sun. Read more posts by Miri.

Have a plant or insect question? University of Maryland Extension has answers! Send your questions and photos to Ask Extension.

Q&A: Fruit tree considerations for the home garden

a grove of peach trees
Peach trees. Photo M. Talabac

Q: I’d like to eventually grow some of my own fruit. What’s a good starting point and what do I need to consider?

A: It’s fun to try growing your own food, though fruit trees require the greatest amount of commitment and patience to be rewarding. Ill-prepared first attempts can easily end in failure. We suggest inexperienced gardeners or anyone short on time try small-fruit (berry) cultivation first before diving into fruit trees – they’re much simpler to grow, need little to no spraying, and take up less space – but if you do enough research to know what to expect, you can certainly start small so it’s not overwhelming.

Easier crops for a novice grower to start with include fig, persimmon, and some of the more esoteric fruits like jujube, serviceberry, and pawpaw. Ironically, the most popular fruits – apple, pear, peach, plum, cherry – are the hardest to grow well in our mid-Atlantic conditions. This is not due to temperature hardiness but rather disease and pest pressures.

Plan as much as possible first: where do you have enough space and the best conditions, how will they be cared for year-round, what problems should you anticipate, and how will you process the perishable harvest? Even when grown organically, there’s a lot of intervention and preventative treatments that will typically be needed to produce a useful harvest and to keep the tree healthy. After a problem arises, curative options are few, so knowing ahead of time what to look for and when is important in avoiding plant damage or a ruined crop for that year.

“Location, location, location” applies in gardening too. Fruit-bearing plants usually require full sun (6+ hours daily in summer) and well-drained soil to perform well. A site with good air circulation reduces disease, and proper pruning and training will make harvesting easier. Choose an area where you have enough space to avoid crowding and competition between plants. Different varieties mature at different sizes, and training style will also impact how much room trees use.

Most fruit trees are propagated by grafting. That means the variety you want is joined to a rootstock of a related variety for the purposes of improving hardiness, disease resistance, and/or dwarfing the plant’s stature for ease of maintenance and harvest. Terms like semi-dwarf, dwarf, and miniature refer to the overall growth habit compared to a full-size tree, not the fruit size itself.

Check which varieties need cross-pollination to fruit well, as some will not produce fruit if planted alone. Self-fruitful groups include figs, peaches, nectarines, and apricots, plus some varieties of apple, cherry, pear, persimmon, and plum. Keep like with like when you can, because cross-compatibility may not occur; for instance, don’t rely on pollination between an Asian and a European pear, or an early-blooming apple with a late-blooming apple. Web-based suppliers sometimes provide cross-pollination charts to illustrate compatible pairings. Avoid multi-graft plants (trees with several varieties grafted onto a shared trunk) unless you’re quite experienced because they run the risk of increased maintenance headaches or the loss of an entire variety’s crop.

ripening figs on a fig tree
Fig fruits.

Good inherent disease resistance goes a long way to reducing pesticide use and lowering the risk of bad outbreaks. No varieties are immune to problems, but those with noted resistance aid your efforts tremendously. It may surprise you to learn that some of the most popular varieties found at supermarkets are not the more disease-resistant options or even easy to grow overall. Ideally, narrow your options down by resistance traits first, desired flavor and other traits second.

Our website has a lot of information about growing fruit. You can search for a specific fruit type and find information on plant selection and good care practices.

By Miri Talabac, Horticulturist, University of Maryland Extension Home & Garden Information Center. Miri writes the Garden Q&A for The Baltimore Sun. Read more by Miri.