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Diagnosis - Black Walnut Problems on Leaves

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Anthracnose

Anthracnose or Gnomonia leptosyla

Anthracnose is a common fungal disease of shade trees that results in leaf spots, cupping or curling of leaves, and early leaf drop. In Minnesota, anthracnose is most common in cool, wet spring weather. It is not a significant threat to the health of the tree and doesn’t require treatment in most cases. Circular brown lesions form on both sides of leaflets

Lesions are smaller than a pencil tip and surrounded by a yellow area
Infected leaves turn yellow, then brown on the margins, and eventually fall off
Smaller but similar spots form on nut husks, interfering with nut development and causing a premature drop

Photo by: M. Grabowski, University of Minnesota

Photo by: Andrej Kunca, National Forest Centre - Slovakia, Bugwood.org

How does anthracnose survive and spread?

Anthracnose fungi survive winter in buds, twigs, fruit, fallen leaves or petioles (the stem that joins a leaf to a branch) depending on which types of trees and fungi are involved. In spring, spores are splashed short distances by water or carried long distances by wind to newly forming leaves. If weather conditions remain cool and wet, spores will form within the leaf spots and spread throughout the tree canopy. These spores will form new leaf spots. This cycle continues as long as cool, wet weather is present. Once the weather becomes dry and the leaves mature, spread of the disease will end and the tree will replace lost leaves with new growth.

For ash, maple and oak trees, young leaves and shoots are most likely to be infected. Mature, fully expanded leaves are largely resistant and only become infected through minor wounds like damage from insect pests.

Anthracnose can continue to progress through summer months on trees like walnut and hornbeam. Anthracnose can occasionally occur on any tree in the summer if cool, wet weather occurs when the tree is producing a new flush of young leaves.

How to Manage Anthracnose

Tolerate Anthracnose
In most cases, anthracnose does not cause permanent damage to established trees. Leaf spotting and leaf distortion have very little effect on the health of the tree. No action needs to be taken to help the tree recover from this minor stress.

Keep your shade trees healthy

Reduce stresses on trees by adequate watering throughout the growing season.
Wet conditions promote disease so redirect sprinklers that splash water on the tree’s leaves.
Do not fertilize trees unless a soil test shows the need for it.
Rake up and destroy fallen leaves before the first snowfall. This will get rid of locations where the fungus can survive to re-infect the tree the following spring.
Prune to remove infected twigs, increase light penetration and improve air circulation throughout the tree canopy.

When are fungicides necessary?

Fungicides are not necessary unless a tree has lost most or all of its leaves several years in a row because of anthracnose. Fungicides are protective and need to be applied before symptoms appear on the leaves. Proper timing of fungicide applications can vary widely from growing season to growing season and can be difficult to predict.

For large trees, high-pressure spraying equipment is needed to get complete coverage.

Hire a professional arborist who can safely operate all necessary equipment.

Chemical treatments include products with the following active ingredients:

Propiconazole
Thiophanate methyl
Copper containing fungicides
Mancozeb
Chlorothalonil

CAUTION: Mention of a pesticide or use of a pesticide label is for educational purposes only. Always follow the pesticide label directions attached to the pesticide container you are using. Remember, the label is the law.

Information source: University of Minnesota Extension

Leaf Spot

Leaf spot or Mycosphaerella juglandis

Leaf spot diseases weaken trees and shrubs by interrupting photosynthesis. Most leaf spot diseases affect only a small percentage of the tree’s overall leaf area, and are a minor stress on the health of the tree. Leaf spot diseases should be taken seriously if they result in moderate to complete leaf loss two to four years in a row. Leaf loss during several consecutive growing seasons can result in reduced growth and increased susceptibility to pests and other diseases.

Small black spots scattered across leaf
Dark brown veins and leaf tips are common due to many spots forming on leaf veins and leaf tips
As the disease progresses, leaves yellow and fall off
In mid-summer leaves appear yellow from a distance
Occurs on black walnut and butternut

Photo by: USDA Forest Service, North Central Forest Experiment Stations

Managing leaf spot diseases

Leaf spot diseases will not seriously harm your plants, but there are things you can do that, when done together, can reduce the disease on the tree in the following years.

Rake up and destroy fallen leaves before the first snowfall to eliminate locations where diseases can survive to re-infect the plant the following growing season.
Do not overcrowd plants — use size at maturity as a spacing guide when planting.
Prune trees or shrubs to increase light penetration and improve air circulation throughout the canopy.
Wet conditions promote disease, so water trees at the base and be careful not to splash water on leaves. A drip or soaker hose works best for this. Avoid sprinklers.
Reduce stress to your tree. Water your tree throughout the growing season so that the top 6 to 8 inches of the soil is moist, especially during dry summer periods. Soil should be allowed to dry before watering again. Maintain a 3- to 4-inch-deep layer of mulch around your tree. Do not mound the mulch around the trunk of the tree but lay a flat layer with at least a 2-inch space between the mulch and stem to allow for air movement. Annually reapply mulch and inspect to ensure levels are maintained.
Do not fertilize trees and shrubs suffering from leaf spot diseases, unless it is recommended by a soil test to correct a nutrient deficiency.
Fungicides are not necessary unless a tree has lost all of its leaves several years in a row.
Fungicides are protective and need to be applied before symptoms appear on the leaves. Proper timing of fungicide applications can vary depending on the biology of the disease. High-pressure spraying equipment is needed in order to get complete coverage of the canopy of large trees.

Call Ostvig Tree to treat leaf spot diseases in large trees.

Information source: University of Minnesota Extension

Walnut Lacebug

Walnut lacebug or Corythucha juglandis

Adult and immature lace bugs feed on the undersides of leaves of deciduous trees and shrubs. Lace bug feeding can discolor leaves, but does not normally affect the health of woody plants. Management is normally not necessary. Pesticides can be effective in reducing lace bug numbers if treatment is necessary.

Feeds on undersides of leaves with needle-like mouthparts causing white or yellow speckled or pinprick discolorations on leaf surface
Black dots or specks (droppings) are on the underside of leaves
Heavy feeding can cause severe discoloration and leaf drop
Lace bugs are 1/8 to 1/4 inch long; light colored bodies; intricate, lacy wings
Damage is most noticeable in mid to late summer

Photo by: John Wiedhass, Virginia Polythechnic Institute and State Univ., Bugwood.org

Photo by: David Cappaert, Michigan State University, Bugwood.org

Photo by: J. Hahn, University of Minnesota

How to protect trees and shrubs from lace bugs

Check for lace bugs in late spring. Check for lace bugs on susceptible trees and shrubs, starting in late spring or early summer. Examine susceptible plants about once every two weeks. Pay close attention to plants that have had a history of infestation.

Tolerate their presence. Lace bugs generally do not affect plant health, and the best option in most cases is to tolerate and ignore their feeding. Many natural enemies, such as assassin bugs, lady beetles, green lacewings and other predators feed on lace bug eggs, nymphs and adults to help limit lace bug feeding.

Use a high pressure water spray. A high pressure water spray from a garden hose acts like a heavy rain, knocking nymphs off of small plants. Nymphs lack wings and cannot return to plants. Target water sprays at the undersides of the leaves where lace bugs are feeding.

Use pesticides. When using pesticides, be sure to spray the undersides of the leaves where lace bugs are found. Insecticidal soaps and horticultural oils, low risk products, are effective against lace bugs and have low impact on natural enemies. For good results:

The product should come in direct contact with lace bugs.
Repeat applications may be necessary.

Broad-spectrum pesticides are effective, but they will also kill natural enemies. Use these pesticides carefully:

Bifenthrin, cyfluthrin, lambda cyhalothrin, permethrin and other pyrethroids (typically ending in –thrin)
Acephate
Carbaryl

Systemic insecticides, like imidacloprid and dinotefuran, are effective. These products move through the plant to the leaves.

CAUTION: These products are very toxic to bees. Avoid treating plants when possible. If making an application, wait until after the trees or shrubs have finished flowering to reduce pesticide exposure to bees. Do not apply systemic insecticides to the soil when bee-attractive flowers are planted directly adjacent to trees.

Contact Ostvig Tree Care, who is a licensed landscape pesticide applicator, if you would like a professional to treat your trees or shrubs.

Information source: University of Minnesota Extension

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