Hot, humid summer weather is just around the corner. That means fungal diseases cannot be far behind, especially when a sudden rainfall event is added to the mix. Here are a few tips to follow to help keep your plants free of the blights and needle-cast diseases that can wreck the summer for your trees and shrubs.
Proper Watering– Fungi need moisture to germinate and infect plant material. Wet leaf and shoot surfaces are all they need to get things going. So apply water to soil to avoid wetting these plant surfaces. Also, prune densely foliated plants. This will improve air circulation and sunlight penetration. Both will help to dry plant foliage.
Sanitation– Disease causing agents often overwinter in shedding leaves, flowers and twigs. Gather and dispose of these shedding plant parts as soon as possible. This will reduce the threat of re-infections from one year to the next.
Keep Plants Healthy– Healthy plants are less likely to succumb to diseases. Keep them well-watered and mulched during the summer. This will maintain their vigor giving them a decisive edge against opportunistic fungi that often prey on water-stressed trees and shrubs.
Disinfect Hand Tools– It’s easy to spread disease organisms via hand tools because their spores are so pervasive in summer. Always remember to disinfect them with 70% alcohol or a 10% bleach solution, especially after pruning plants. This will reduce the chance of spreading diseases from one plant to the next.
Longitudinal splitting of bark along the trunk of young trees is a common problem during winter. The problem is more severe on the south and west side of saplings and is caused by the sun-beams striking the tender bark. It sets up a huge temperature difference between internal sapwood under the bark (hot) and the external environment (cold).
The big change causes the sapwood to desiccate, shrink and die. The outer bark eventually cracks along the fault line of the dead sapwood. As the picture shows, the best way to prevent winter bark splitting or sun-scald is to place light-colored tubing around the trunk of the trees. The white tubing will prevent the heat build-up by reflecting the rays of the sun away from the tree. You can also use white latex paint as an alternative prevention.
Trees need good structure. Good structure will neutralize the harmful effects posed by wind, rain and gravity. And the basis for good structure in trees is to prevent structural defects from cropping up on the trunk and lateral branches. The worst of these are co-dominate stems, over-sized branches and cluster branches.
They are commonly found associated with young, fast-growing saplings. The goal is to nip them in the bud before they get bigger and more costly to eliminate later. Structural pruning should start the year following plant installation and be repeated every 4 years over the next 25 years. Use the following illustrations and definitions to help locate and manage by pruning any major structural defects on your young trees.
1 Co-dominant stems: Two or more equal-sized stems competing for the leadership role at the top of the tree is a no-no. Like an orchestra guided by a sole conductor, only one stem is needed. Maintain a single central stem/leader by lopping off the weaker of the two. Maybe, if necessary, even remove one or two shoots located directly below the new leader to more strongly assert the latter’s dominance at the top of the tree.
2 Over-sized branches: Stems fitting this description are easy to find. They are much larger in diameter than their brethren. Here’s a rule of thumb to help you to identify them. For sound structure, no stem should be larger than 50% of the diameter of the stem at the point of attachment. For example, if a 2 diameter-inch stem is connected to a 6 diameter inch- trunk, all is well. However, if a 4 diameter-inch stem is connected to the same 6 diameter- inch trunk, that’s a no-no. It’s growing too fast at the expense of the rest of the branches. You have two choices for correcting this problem. The first is to remove the obese branch entirely. The second option is to remove a portion of the offending limb, usually about a third of it. This will slow its growth by denying it the extra carbohydrate (energy source) found in the lopped off portion of the branch. Eventually the shortened branch will come into compliance with the stem diameter ratio mentioned earlier. TIP: The second option is usually selected when total stem removal will leave a huge gap along the trunk of the tree.
3 Cluster branching: Two objects cannot occupy the same space at the same time. This law of physics also applies to trees. Unfortunately, this condition is a common problem on young trees, especially those with opposite branching patterns (maple, dogwood, catalpa, etc.,). Here’s a tip leading toward a solution. Alternate branching is far superior structurally to opposite branching. Strive to get young trees into this zig-zag branching arrangement through the routine removal of offending branches. Remember, as they increase in diameter, branches located opposite each other will eventually place a lot of stress at that point along the trunk. The stress can often lead to breaks. This is a big problem with the Callery Pear (Pyrus calleryana). It inherently produces this type of structural defect and shouldn’t be planted without a future commitment to training.
The following is an excerpt of the lecture presented by RI Tree Council’s Technical Advisor, John T Campanini, Jr at the RI Federation of Garden Clubs Annual Education Day Program- Roger Williams Park Casino, November 4, 2019. The lecture’s title was “Trending Pests and Weather Conditions Impacting Plants.” In this article, I abridge my earlier lecture to examine specific pests which loom as future threats to woody ornamentals.
PLANT PESTS- There are five plant pests that should be on the radar of all plant stewards in 2020. Four are insects while the last is a disease causing fungus. They are itemized below:
Asian Long-horned Beetle (ALB- Anoplophora glabripennis)
Spotted Lanternfly (SLF- Lycorma delicatula)
Emerald Ash Borer (EAB- Agrilus plannipennis)
Bagworm (Thyridopteryx ephemeraeformis)
Boxwood Blight Disease
The ALB is by the #1 pest threat of hardwood trees including maple, birch, willow and elm in the Northeast according to the US-Forest Service. ALB kills trees by tunneling through the wood. Once the tree is infested it must be destroyed. The bittersweet news is while this pest has yet to be found in Rhode Island it is everywhere around us. It was discovered attacking trees in Worcester, MA in 2008 and in Boston in 2016. By imposing a quarantine on moving wood products and by inspecting vulnerable species, the Commonwealth has done a super job of containing the spread of this destructive pest into other New England States. Early detection is paramount to its management. In winter, the most obvious sign of this pest infesting host plants is the presence of numerous dime-sized, round holes on the trunk and on branches. For more information about ALB, visit http://www.na.fs.fed.us/spfo/alb.
Photo 1: The Asian Lon-horned Beetle adult is about ¾ to 1 ¼” long with mottled spots on the back. Note the dime-sized exit hole made by adult beetles as they emerge from the tree.
The Emerald Ash Borer is an exotic beetle from Asia discovered in 2002 feeding on ash trees in Michigan. Since then this small, bright green beetle has been responsible for the loss of millions of ash trees in North America. In July 2017, adult beetles were discovered in bait traps set out by RIDEM plant health inspectors in Westerly, Providence, Lincoln and Woonsocket, RI. These findings show the EAB is here, and that ash species are at risk. Plant damage and death is caused by the worm-like larval stage of EAB. The larva feed on the inner bark and phloem, creating “S” –shaped galleries. The feeding activity destroys the plant’s ability to transport water and nutrients to other parts of the tree, killing it. Unlike the ALB, there are options for protecting ash trees from EAB damage. But these options require comprehensive planning and should be directed with the assistance of a certified arborist. Learn about these options and more on EAB at www.emeraldashborer.info.
Photo 2: EAB adult beetles are smaller than a penny. They emerge in July to feed on ash leaves before laying their eggs between layers of bark and in bark crevices.
The Spotted Lanternfly is the latest exotic pest discovered in the United States with the potential to become a major plant problem. This sap-feeding plant-hopper was discovered attacking fruit trees, woody ornamentals and timber crops in Pennsylvania in 2014. New York and Connecticut authorities have also reported finding adults feeding on nursery stock in the past two years. SLF nymphs and adults are very colorful making them easy to detect. Young nymphs are “tick-like” with black and white spots. As they age, the nymphs develop red patches and grow to ½” long. The adults have two sets of wings. The forewing is gray with black spots. The hindwings are bright red at the base. Both nymphs and adults cause damage by feeding in huge populations (swarming) on the tissues under the bark of the trunk and branches. Plants literally starve to death from nutrient depletion from this type of voracious feeding. To learn more on the SLF, visit: www.uri.edu/biocontrol/spotted-lanternfly.
Photo 3: The important life stages of the Spotted Lantern-fly: eggs, wingless nymphs and the adult with wings for flight.
The bagworm is a native insect that, until recently, was mostly found in Pennsylvania and south. In the past decade, however, it has migrated to Northern areas through infested nursery stock. For years the bagworm was considered just a nuisance in RI with scattered outbreaks here and there. But, as the winters have gotten milder, this pest has learned to adapt and thrive and is today much more common. The bagworm prefers attacking evergreens. They include Arborvitae, Juniper, Leyland Cypress, Spruce and Pine. Certain deciduous species such as Black locust, Honeylocust and American Sycamore are also known host plants. The pest damage is done by the worm-like larvae. They feed on needles and leaves from late May through July. This causes the plant foliage to discolor yellow, turn brown before eventually dying. Plant injury can be very severe and entire plants can be killed in a single season. Bagworm infestations generally go undetected until damage is complete, and the large bags constructed by this pest is conspicuous. The bags provide over-wintering shelter to the egg-laying female adults inside. A simple control measure is to remove and destroy these bags as they appear. The other is to apply Captain Jack’s Dead-bug Spray (i.e., Spinosad) to the plant from early to mid-June while the larvae are small. For more bagworm details, visit www.info.psu.edu/extension/factsheets/bagworm.
Photo 4: The tell-tale sign of bagworm infestation is the dangling of inch long, oval-shaped bags from the twigs of plants.
The last pest problem we will discuss is one that is causing great harm to the nursery industry as well as public and private landscapes and gardens where boxwood (Buxus sp.) is grown. The culprit is the fungus, Calonectria pseudonavicilata. It causes the fungal disease called Boxwood Blight. Boxwood Blight was confirmed for the first time in North America in 2011 on samples collected in North Carolina and Connecticut. Today, this disease is found in more than 25 states, including Rhode Island. Hosts of the fungus include plants in the Buxaceae family. American, English and Korean boxwoods are all susceptible. Disease symptoms begin as dark leaf spots that coalesce to form brown blotches. Rapid defoliation usually follows, starting on the lower branches and moving upward in the canopy. As the disease progresses, black cankers girdle the stems, killing them. There is no easy way to control this disease. Fungicides are effective at protecting plants from boxwood blight infection, but do not cure plants with the disease. For now, the best advice to help prevent introducing the boxwood blight fungus is to avoid introducing new boxwood plants to landscapes that already have established boxwoods. Find out more about boxwood blight at www.ext.vt.edu/agriculture/commercial-horticulture/boxwood-blight.html.
Photo 5: Boxwood blight has become a serious threat to nursery production and to boxwoods in the landscape.
Finally, the RI Tree Council has staff available to present lectures or workshops on many topics from plant selection to plant health care. The Tree Council also publishes a monthly newsletter, ROOT TIPS, during the annual plant growing season (April to October) providing invaluable information on the weather, pest alerts and tree and shrubbery management. To avail yourself of these or other offerings such as the Tree Council’s signature Tree Stewards Education Course, contact us by phone 401-764-5885 by e-mail ritree@ritree.org, or visit the website www.ritree.org.
1 creates vertical tunnels to improve light penetration and air movement throughout the shrub, 2 rids the plant of its weakest, unproductive stems and 3 promotes the new growth (stems) at the stem base from latent buds. Eventually, the new stems will grow taller, add girth and fill the spot left vacant by the removal of the original stem. The rule of thumb is to eliminate approximately 1/3 of the oldest stems each year. Using this approach, the entire plant will become ‘younger’ by replenishing most of the oldest stems with new ones in three years. Let’s use the illustrations below to demonstrate the benefits of renewal pruning on a sheared shrub with dense growth on top and a “leggy” plant, in general.
1 Sheared shrub with dense growth on top and many old, “leggy” stems. Renewal cuts shown in red slash line.
2 The shrub showing the creation of vertical tunnels inside the canopy following the removal of the largest stems by pruning.
3 Shrub grows ‘younger’ as healthy, thrifty stems grow from latent buds the next spring following renewal pruning.
