Birch rust or Melampsoridium betulinum

• Angular yellow leaf spots turn brown in the center with age
• Raised yellow to orange pustules form on the lower surface of leaf spots midsummer and may release powdery orange spores
• Severe infection can cause leaves to turn brown and fall off prematurely
• Occurs in cool, damp weather

Symptoms of leaf rusts

• Bright yellow, orange or red leaf spots.
• Leaf spots can produce yellow, orange, red or brown powdery spores that can be easily rubbed off and seen on a tissue or paper towel.
• Spores may be in little blisters within leaf spots or may emerge from tiny cups or tubes on the lower surface of the leaf.
• In some hosts, leaf rust fungi also infect petioles, young green stems and fruit.

In Minnesota, leaf rust diseases are found on rose, currants and gooseberry, arrowwood or cranberry bush, apple and crabapple, ash, hawthorn and many other deciduous trees and shrubs.

Causes of leaf rust

• Leaf rust diseases are caused by a group of related fungi.
• Some rust fungi only need to infect one host plant to complete its lifecycle.
• Other rust fungi require two different host plants.
• The most common examples of rust fungi that require two hosts in Minnesota are cedar apple rust and other rusts caused by the Gymnosporangium fungus.

These rusts infect plants in the Rosaceae family (e.g., roses and apple trees) and also infect species of juniper.

Spores of rust fungi can be blown long distances and rust fungi need long periods when leaves remain wet to start an infection.

Management

See the general management practices listed above.

• Plant rust resistant varieties available for certain trees and shrubs.
• Don’t plant two different plants that are susceptible to the same rust fungus close to each other.

Information source: University of Minnesota Extension

Birch skeletonizer or Bucculatrix canadensisella

• Feeds on the underside of the leaf between the veins, giving the leaf
  a lacelike appearance
• Leaves brown in late summer and can fall prematurely
• Caterpillars are approximately ¼” long; whitish to yellowish
  in color
• Damage occurs when larvae are active in mid-summer
• Preferred hosts are paper birch

Information source: University of Minnesota Extension

Birch anthracnose or leaf spot or Cryptocline betularum, Discula betulina and others

• Leaf spots may be 1/8th to 3/8th inch brown spots to large brown blotches surrounded by yellow tissue (C. betularum)
• Leaf spots may be 1/8th inch wide brown rings with a light center surrounded by yellow tissue (D. betulina)
• Infected leaves fall from tree even if still partly green
• Leaves on lower branches most affected
• Common in years with wet rainy spring weather

There are many leaf spot diseases that occur on a wide range of native and ornamental trees and shrubs. Many leaf spot diseases have similar biology and therefore very similar management options.

Information about some of these diseases is included below along with management information applicable to all leaf spot diseases of shade trees and shrubs.

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 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.
  • Hire a professional arborist to treat leaf spot diseases in large trees.

Information source: University of Minnesota Extension

Powdery mildew or Phyllactinia guttata

• White or gray powdery mats, resembling talcum powder, occur
  in spots, blotches or coat leaves
• Leaves may become deformed and turn yellow and die in
  severe infestations
• Common on lower leaves or leaves that are shaded
• Commonly seen in spring or fall

How to identify powdery mildew

• White to gray powdery spots, blotches or felt-like mats form on leaves, green stems and buds. Infected plants may appear to be sprinkled with baby powder or covered in cobwebs.
• Young leaves, water sprouts and green shoots are often most severely affected.
• Infected leaves may be cupped or twisted at the site of the infection.
• When severely infected, leaves may turn yellow and fall prematurely during the growing season.
• In some plants, leaves turn purple to red around the infection.
• In late summer or early fall, tiny round orange to black balls form within white fungal mats.

How does powdery mildew survive and spread?

• Powdery mildew fungi create dark round resting structures that contain and protect spores through the winter.
• In spring, these resting structures break open, releasing spores that are spread by the wind. These spores start new infections on succulent, new growth.
• Some species of powdery mildew fungi survive the winter in infected buds.
  • In spring, the young shoots growing from infected buds are covered with velvet-like white growth of powdery mildew.
• The powdery mildew fungus grows into the plant to steal nutrients.
• Powdery spores are produced in leaf spots throughout the growing season.
  • Spores spread by wind and start new infections within the plant or in neighboring plants.
• Powdery mildew needs humid conditions to start new infections.
• Spread of the disease is reduced by rain or irrigation.
• Water on the leaves prevents the light airy spores from moving on the wind.

How to manage powdery mildew

• Tolerate powdery mildew. Powdery mildew does not significantly affect the health of the tree or shrub and does not require management.
• Powdery mildew resistant varieties are available for many ornamental shrubs. Choose disease resistant varieties for new plantings or as replacement plants.
• Do not overcrowd plants. Use size at maturity as a spacing guide when planting.
• Do not fertilize infected trees and shrubs unless it is recommended by a soil test to correct a nutrient deficiency. Fertilizer will cause the tree to produce young shoots which are highly susceptible to powdery mildew.

Information source: University of Minnesota Extension

Sooty mold

• Black, brown, or gray soot-like covering on leaf surfaces, or twigs
• Sticky, shiny secretions on leaves from sap-sucking insects (aphids, leaf hoppers, psyllids, etc.)
• Insects or signs of insect damage (distorted, pin-prick feeding marks, etc.) may be seen on leaves above the worst affected moldy areas

Frequently, tree limbs and leaves are covered by an unsightly, black, sooty growth called sooty mold. It may occur on any tree, shrub, or leafy plant.

Sooty mold is often found on or below plants infested with certain types of sap-sucking insects, especially aphids and soft scales, which produce a sugary secretion called honeydew. This honeydew drips down onto leaves and branches providing a food base on which the sooty mold fungi can grow.

• Sooty mold is caused by saprophytic fungi.
• It is not a disease and does not infect living plant tissue.
• Heavy growth by the fungus can reduce photosynthesis but does not harm the plant in any other way.
• Sooty mold may also grow on sap or resin associated with wounds.

We do not recommend control of the mold itself. However, the presence of sooty mold is often an indication of insect activity that has the potential for causing damage. Proper identification of the insect is necessary to determine if management is warranted.

Light coverings of the mold will gradually disappear during dry weather when its nutrient source is eliminated. Sooty mold can be physically washed off small plants if desired.

Information source: University of Minnesota Extension

Birch erineum or velvet gall

• Patches of reddish fuzz appearing on leaf surface
• Caused by eriophyid mites which are microscopic
• First formed in spring shortly after leaves expand

How galls are formed

Galls usually form during the accelerated growth period of new leaves, shoots and flowers in late spring.

• Insects or mites damage plants by chewing on them and their salivary secretions (spit) cause plants to increase production of normal plant growth hormones.
• Higher hormone production results in increased cell size or cell numbers. These abnormal cell growths are called galls.
• Mature plant tissues are usually not affected by gall-inducing organisms.
• The gall keeps growing as the gall-making insect feeds and grows inside the gall.
• If galls start to form, they continue to form even after the insects die.

Most galls remain on plants for more than one season since they become noticeable only after they are fully formed.

Leaf galls

• Formed on leaf blades or petioles.
• Most common galls.
• Appear as leaf curls, blisters, nipples or erineums (hairy, felt-like growths).
• On the upper or lower leaf surface.

Damage caused by galls

• Galls are growing plant parts and require nutrients just like other plant parts.
• Galls can steal vital nutrients from the plant and affect plant growth.
• Can be a problem when galls are numerous on very young plants.
• Damage may occur if there are many galls on branches or present for several years in a row.
• In most cases galls are not numerous enough to harm the plant.

How to protect your plants from galls

Most galls do not cause any severe damage to plant health. Control options usually are not needed. Gall numbers vary from year to year.

• Chemical applications often are not effective because the precise timing of sprays is critical.
  • To be effective, apply pesticide before gall formation begins, but when insects and mites are active.
• Once galls start to form, it is too late for treatment, as the galls protect the insects or mites.
• For insects or mites that spend the winter on the host plant, apply horticultural oil before insect activity begins in the spring.

Information source: University of Minnesota Extension