A monthly e-newsletter from UMass Extension for landscapers, arborists, and other Green Industry professionals.
To read individual sections of the message, click on the section headings below to expand the content:
Hot Topics
Pest Alert: Leaf Damage to Oaks caused by the Oak Shothole Leafminer and Oak Anthracnose
A dramatic and possibly widespread outbreak of the oak shothole leafminer (Japanagromyza viridula synonym Agromyza viridula) and oak anthracnose (Apiognomonia errabunda) has occurred this season. To date, samples and observations of the outbreak have been made in western and eastern Massachusetts, coastal Rhode Island, southern New Hampshire, eastern New York and eastern Pennsylvania. The oak shothole leafminer is a small fly in the family Agromyzidae. Not much is known about this particular species, although very short-lived outbreaks of this insect on ornamental oaks have been recorded in New England in the past. The oak anthracnose pathogen appears to be readily colonizing foliage damaged by the leafminer. The anthracnose damage appears mostly minor to moderate in severity (leaf spots and blotches). However, for some trees the disease has been far more damaging (leaf wilting, death and premature shedding). Infected leaves may have tan to brown-colored spots and blotches or appear blackened and wilted. For more information, go to https://ag.umass.edu/landscape/news/leaf-damage-to-oaks-caused-by-oak-shothole-leafminer-oak-anthracnose.
Tawny Simisky, UMass Extension Entomologist, and Nick Brazee, UMass Extension Plant Pathologist
Questions & Answers
Q: Why do American sycamores look so terrible right now?
A: American sycamore (Platanus occidentalis) and London planetree (P. × acerifolia) can be seriously defoliated by sycamore anthracnose early in the growing season. Sycamore anthracnose is caused by the fungal pathogen Apiognomonia veneta. The most common symptom of the disease is the slow development or inability of trees to leaf out in the spring. Typically, this is associated with wet spring weather, which facilitates spore dispersal and infection of newly developing shoots and leaves. Even minor rain events (e.g. <0.3″) can be enough to promote disease development. This past April was extremely wet, with almost twice the average monthly rainfall recorded at many weather stations in the region. While May was comparatively drier, there were still numerous rain events during the month that promoted the growth of the fungus. Despite regular infection and high disease severity during certain years, trees are usually able to flush new growth once conditions become warmer and drier as they are now. A good barometer for sycamore health is how they appear by late June to early July. Despite their highly conspicuous early season struggles, many trees will have a full, robust canopy by mid-summer.
Visible symptoms of sycamore anthracnose include angular leaf spots and blotches on the foliage (especially along the midrib), shoot and bud blight, and splitting stem cankers on twigs and small branches. In fact, the characteristic right-angle branching pattern typical of American sycamore is the result of repeated anthracnose infections. The terminal bud is killed allowing the lateral buds to flush, creating the irregular branching pattern. There are three distinct but often overlapping stages of sycamore anthracnose: (i) dormant twig/branch cankering and bud blight; (ii) shoot blight; and (iii) foliar blight. The fungus is active in twigs and branches during mild weather when the tree is dormant in autumn, winter and early spring. Active fungal growth within one-year-old twigs kills the tips, leading to bud blight. In addition, the fungus often grows into older branches below the dead twigs, creating perennial cankers. During wet spring weather, fruiting structures mature in discarded leaves from the previous season and in the bark of blighted twigs and cankered branches to disperse spores via wind and rain splash. These spores infect emerging shoots and developing leaves, which rapidly wilt and die. Because of the pathogen’s ability to overwinter in twig and branch cankers, it can readily produce spores that easily infect newly developing tissues nearby in the canopy.
Because Platanus become so large, active management can be a major challenge. Furthermore, because many trees grow out of the damage by mid-summer, management may not be warranted in many cases. Yet, for recently transplanted trees and those weakened by other stresses, fungicides may be useful. Fungicide injection into the base of the tree can be one of the most practical means of delivery, given the systemic nature of the pathogen. Fungicides applied during the spring when shoots and foliage are developing will help protect these sensitive plant parts. Many anthracnose fungi are most active in the spring during wet and mild weather, only to go dormant during the mid-summer period when conditions are warmer and drier.
Nick Brazee, UMass Extension Plant Pathologist
Trouble Maker of the Month
Iris Leaf Spot
Iris leaf spot is one of the most common diseases of irises in the landscape. It is caused by the fungus Cladosporium (Heterosporium) iridis, formerly known as Mycosphaerella (Didymellina) macrospora. The disease has also been reported on Narcissus, Gladiolus, Freesia, and Belamcanda. The cool, wet weather this spring was highly conducive to disease development, and symptoms were observed locally beginning in late April.
Iris leaf spot produces tan, oval lesions with margins that may appear water-soaked at first, but then turn a reddish brown. Lesions first appear at the leaf tips and may eventually coalesce, resulting in a burned or blighted appearance. The dark spores of the fungus may be visible in the middle of lesions with the aid of a hand lens. Spores are spread by wind and splashing water, and germinate at 50-77oF. The disease can progress throughout the growing season as long as the environmental conditions are right.
Iris leaf spot does not kill plants, but can make the foliage unsightly, and chronic infection may eventually weaken plants. The fungus overwinters in dead leaves and produces new spores in the spring, so strict sanitation is critical for disease management. Improve air circulation through proper plant spacing and avoid overhead watering. Bearded (German) iris is highly susceptible, but Siberian iris is largely immune to this disease, and it has not been reported on Japanese iris. Non-susceptible perennials may be planted in front of irises to shield the foliage from view later in the season.
Angela Madeiras, UMass Extension Plant Pathologist
Plant of the Month
Hydrangea quercifolia, oakleaf hydrangea
Although less planted than Hydrangea macrophylla and H. paniculata, Hydrangea quercifolia provides multiple seasons of interest and is a great addition to the landscape. Native to the southeastern United States, Hydrangea quercifolia is a multi-stemmed, mounded shrub reaching 4-8’ tall (although plants can reach 12’). The common name, oakleaf hydrangea, comes from the oak-like leaf shape. Leaves are 3-8” long, 3-7 lobed, and have serrated margins. The coarse foliage is dark green above with a white to tan underside. Fall color is an attractive blend of red, red-purple, and sometimes orange with leaves holding late into fall. Flowers are upright, pyramidal clusters of white flowers. Similar to other Hydrangea species, panicles contain small fertile flowers along with showy sterile flowers near the base of the clusters. Peak bloom is early July, with flowers turning pink as the age in late summer. Older stems have exfoliating bark that provides winter interest.
Plants are best grown in a well-drained, moist soil in full sun to part shade. Mulching helps to retain moisture. Pruning is not often needed, but plants should be pruned immediately after flowering when needed. Hydrangea quercifolia can suffer from twig kill and flower bud injury in zone 5, so plants should be protected in winter. Bark, flower buds, and leaves are poisonous. Hydrangea quercifolia can be planted as a specimen or can be used en masse or to make an informal hedge. Best planted where the exfoliating bark can be enjoyed in the winter months. Oakleaf hydrangea has no serious insect of disease problems.
Cultivars include:
- ‘Alice’ – large, arching flower panicles, improved fall color
- ‘Snow Queen’ – larger, more numerous sterile flowers that are more upright
- ‘Queen of Hearts’ – flowers later than most cultivars; hybrid of Snow Queen and Pee Wee
- ‘Pee Wee’ – compact cultivar, smaller size, leaves, and flowers
- ‘Ruby Slippers’ – hybrid of Pee Wee and Snow Queen, smaller size; flowers mature to ruby red
- ‘Little Honey’ – leaves emerge golden yellow in spring; change from yellow to chartreuse to green by early fall before turning red
Mandy Bayer, Extension Assistant Professor of Sustianable Landscape Horticulture
Upcoming Events
Plant Camp!
June 20 & 27, 2019
Just starting out in the green industry, taking the MCH exam, or looking to expand your plant palette? The UMass Extension Plant Camp can help! In 2019, the first two workshops in this ongoing series are The Basics (Day 1) and New Introducations (Day 2). Additional topics in new locations will be covered in coming years. Attendees can choose to take either or both sessions.
- The Basics (Day 1) - June 20, 2019, 9:00 am to 3:30 pm
Get started with “The Basics”. This workshop is for those new to plant material or those who are looking for a refresher on common plants. Attendees will learn to identify and use some of the most common landscape plants in Massachusetts. This workshop will be a combination of a morning classroom session for learning identification skills and an afternoon plant walk to look at identifying features in person. (Note: The schedule may be altered to accommodate the weather.) Cost: $95 ($150 if registering for both days). Location: UMass Amherst - New Introductions (Day 2) - June 27, 2019, 1:00 to 4:00 pm
Let’s talk cultivars. With the plethora of cultivars on the market, it can be good to step back and compare. This workshop will very briefly talk about identification skills but will mostly focus on ornamental attributes. Build on the knowledge you gained in The Basics first or take this as a stand-only workshop if you're just looking to learn about new introductions. Cost: $75 ($150 if registering for both days). Location: Weston Nurseries, Hopkinton, MA
Registration and Additional Information.
Other Upcoming Events:
- 6/19: Weed Walkabout
- 6/20: Plant Camp 1: The Basics
- 6/25: Landscape and Turf Weed Topics - You pick!
- 6/27: Plant Camp 2: New introductons
- 7/17: Turf Field Day
- 9/14: Landscape and Forest Tree and Shrub Disease Workshop
- 10/9: Tick Webinar - 2019 Tick Updates from the UMass Laboratory of Medical Zoology
- 10/17, 10/31 & 11/14: Invasive Insect Certification series
For more information and registration for any of these events, go to the UMass Extension Landscape, Nursery, and Urban Forestry Program Upcoming Events Page.
Additional Resources
For detailed reports on growing conditions and pest activity – Check out the Landscape Message
For commercial growers of greenhouse crops and flowers - Check out the New England Greenhouse Update website
For professional turf managers - Check out Turf Management Updates
For home gardeners and garden retailers - Check out home lawn and garden resources. UMass Extension also has a Twitter feed that provides timely, daily gardening tips, sunrise and sunset times to home gardeners, see https://twitter.com/UMassGardenClip
Diagnostic Services
A UMass Laboratory Diagnoses Landscape and Turf Problems - The UMass Extension Plant Diagnostic Lab is available to serve commercial landscape contractors, turf managers, arborists, nurseries and other green industry professionals. It provides woody plant and turf disease analysis, woody plant and turf insect identification, turfgrass identification, weed identification, and offers a report of pest management strategies that are research based, economically sound and environmentally appropriate for the situation. Accurate diagnosis for a turf or landscape problem can often eliminate or reduce the need for pesticide use. For sampling procedures, detailed submission instructions and a list of fees, see Plant Diagnostics Laboratory
Soil and Plant Nutrient Testing - The University of Massachusetts Soil and Plant Nutrient Testing Laboratory is located on the campus of The University of Massachusetts at Amherst. Testing services are available to all. The function of the Soil and Plant Nutrient Testing Laboratory is to provide test results and recommendations that lead to the wise and economical use of soils and soil amendments. For complete information, visit the UMass Soil and Plant Nutrient Testing Laboratory web site. Alternatively, call the lab at (413) 545-2311.
