MELeaf: A Newsletter From the Horticulture Program, July 14, 2020

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MELeaf: A Newsletter From the Horticulture Program, July 14, 2020

In this issue:

Lookout Maine: Pests to Watch

Luckily for Mainers, many destructive invasive pests are often found elsewhere first. By the time an invasive problem gets to Maine, other states have been combating it or managing it for many years (e.g., emerald ash borer). We are lucky to learn from others what works and what doesn’t. In all cases, awareness and early detection provide essential advantages when dealing with the inevitable. Here are a few invasive pests found in other states that may come to Maine sooner than later. Please report any suspect sightings of these pests to Photos or samples are requested to help make identifications.

Asian longhorned beetle (Anoplophora glabripennis). The Asian longhorned beetle is a woodboring beetle that spends up to two years inside its host tree as a larva. It prefers to attack maple trees but will also attack and kill other deciduous trees. ALB was first confirmed in the U.S. in 1996 in Brooklyn, NY. Currently there are quarantines and active eradication efforts in effect for Worcester, MA, Central Long Island, NY, and Clermont Co, OH to prevent ALB from spreading. It has successfully been eradicated from IL, NJ, Boston, MA and Brooklyn, NY, 23 years after its discovery. In June 2020, ALB was found in South Carolina. When ALB is detected early and the populations are small, it can be successfully eradicated. The discovery of ALB in the US led to strict treatment rules for solid wood packing material used in international shipments to help prevent further invasive woodborer introductions. Learn more:

Asian longhorned beetle

Spotted lanternfly (Lycorma delicatula). The spotted lanternfly is a large planthopper from Asia that was first found in Pennsylvania in 2014. At the time, populations were contained to a few counties and PA was trying to eradicate it. However, spotted lanternfly quickly spread as the overwintering egg masses were difficult to detect and contain. SLF is now found in six states. SLF feeds on a variety of woody host material by inserting its proboscis and sucking out nutrients. This feeding leads to sooty mold growth on the excreted honeydew and causes plants to lose vigor. Some economically important hosts are apple, grape, hops, and maple. SLF either prefers or needs to feed on tree-of-heaven (Ailanthus altissima) so reports of this plant in Maine are greatly appreciated also. Learn more about spotted lanternfly

Spotted lanternfly

Beech leaf disease. Beech leaf disease (BLD) was first discovered in Ohio in 2012 and has since been found in Pennsylvania, New York, Connecticut, and this year in Massachusetts and Rhode Island. Thought to be caused by a foliar nematode, it impacts American, European, and Oriental beech species. Early symptoms include dark-green striped bands between lateral veins of leaves, leaf curling, and reduced leaf size. As the disease progresses, leaves turn yellow and drop prematurely, ultimately resulting in death of sapling-sized trees within 2-5 years and of large trees within 6 years. In areas where the disease is established, the proportion of symptomatic trees can reach more than 90%. Learn more about beech leaf disease 

Beech leaf disease

Oak wilt (Bretziella fagacearum). Oak wilt is a vascular disease of oak trees. All oaks are susceptible to the fungus, but the red oak group often die much faster than white oaks. Oak wilt spreads above ground via sap beetles transferring fungal spores and below ground through root grafting, which is more common and is responsible for more than 85% of new infections every year. Symptoms include browning leaves starting at edges, branch dieback starting from the top, and leaves suddenly wilting and dropping in spring/summer. Oak wilt was first discovered in Wisconsin in 1944, but its origin is still unknown. It is currently found throughout the midwest, Texas, and New York, and kills thousands of trees a year in the U.S. Oak wilt has not been found in Maine. Learn more about oak wilt

Oak wilt

Tomato Brown Rugose Fruit Virus (ToBRFV). ToBRFV is a viral disease primarily of tomatoes and peppers that are grown indoors. ToBRFV greatly affects the yield and quality of the fruit produced from infected plants. First identified in Jordan in 2015, ToBRFV has since spread to many other countries, including a find in a California greenhouse in 2018. The disease has since been eradicated from the California greenhouse, but little is known if there may be other areas in the US where this disease may occur. ToBRFV is difficult to control and easily spread on seeds, plants, fruit, hands and equipment. Symptoms include yellow/green patterns and rough brown wrinkled patterns on fruit and yellow mottled or wrinkled leaves. If you observe symptoms in your greenhouse tomato crop contact horticulture@maine.govLearn more about ToBRFV

Tomato brown rugose fruit virus

Notes From the Field: Observations From Nursery and Greenhouse Inspections


July is the time of year that Japanese beetle, Asiatic garden beetle and Oriental beetle adults typically start emerging in Maine and causing damage on common hosts such as rose, grape, raspberry, fruit trees, elm, birch and maple. Grubs of these beetles damage turf by chewing on the roots, often causing brown areas in lawns. While most people are aware of Japanese beetle, you may get questions from customers about any of these three pests.

  • Japanese beetle - Inspectors have not observed any large populations of this pest yet this year. Remember if you see a white dot on the thorax of a Japanese beetle, don’t kill that insect. That is the egg of a parasitic fly that will hatch and kill the beetle and continue the parasite population.
  • Asiatic Garden Beetle - This copper/brown colored beetle typically feeds at night and hides during the day, sometimes making it a mystery as to what is causing chewing damage on leaves of vegetables and herbs. If you suspect this beetle, a night hunt with a flashlight may be your best detection method.
  • Oriental Beetle - Often the lesser known of the three, this beetle is usually dark brown with a white or cream-colored mottled pattern. It also has distinctive 3-pronged antenna. Inspectors have observed this beetle this year in landscaped settings.

We have also observed a few common pests on unusual host plants this summer. One nursery had a customer submit a sample of lettuce that was covered with and damaged by lacebug. This pest is typically found creating stippling damage on woody plants such as Rhododendron, Azalea, Pieris, and Amelanchier.

At another nursery, a significant population of aphids was seen on Echinops and several varieties of daylily. The presence of white cast skins of the aphids was apparent on the daylily leaves, but the actual insects were found down on the new growth at the center of the fans.

Also on daylilies, we observed daylily leafminer. This pest is becoming a common occurrence in daylily plants offered for sale. Look for long pale tunnels in leaves created by the larvae as they feed inside the leaf. Daylily leafminer populations can be reduced by removing and destroying affected leaves.

While not yet observed in nurseries, daylily gall midge was reported by a homeowner in Maine last year. Not previously known to occur in the eastern part of the US, feeding damage by the maggots of this midge cause flower buds to become distorted and fail to develop properly. If daylily flowering is reduced, especially in early flowering varieties, it may be worth dissecting a bud or two and looking for small pale maggots inside.


At this time last year, we observed symptoms of chrysanthemum white rust (CWR) on perennial mums at a Maine business. CWR is a federally regulated disease; plants infected with this disease must be destroyed to prevent further spread. Symptoms to look for include light green to yellow spots on the upper surface of chrysanthemum leaves with corresponding beige to pink pustules that form on the underside of the leaf. Any symptoms that are observed should be reported to Like most plant diseases, CWR requires the right environmental conditions to develop and spread. Preventative sprays can help prevent infection on fall mum crops. Chrysanthemum White Rust Spray Recommendations from NYS   

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