Benignly Ugly Tree Disorders: Oak Galls, Powdery Mildew, Sooty Mold, Tar Spot

Summary: Andrew Koeser of the University of Illinois in Urbana-Champaign diagnoses benignly ugly tree disorders: tree galls, powdery mildew, sooty mold and tar spot.

oak apple galls, created by California gall wasp (Andricus quercuscalifornicus), on Brewer's Oak (Quercus garryana var. breweri), Jackson County, southwestern Oregon: Jim Conrad, Public Domain, via Wikimedia Commons

Serious afflictions and unsightly appearances are not givens, according to Born to be Mild: Identification and Management of Highly Visible, Relatively Harmless, Tree Disorders in the April 2011 issue of Arborist News.

Andrew Koeser, Board-Certified Master Arborist and Ph.D. candidate and teacher at the University of Illinois in Urbana-Champaign, bases his observations on four mostly harmless tree disorders. He covers cosmetic nuisances and ecological curiosities called oak galls, fungal diseases described as powdery mildews and tar spots, and fungal disorders known as sooty molds. He deems them all cosmetic, foliar and temporary occurrences since the leaves tend to be unsightly, not unhealthy, because of an abnormally cool, damp growing season.

Damage ensues when benignly ugly tree disorders accord with a larger complex of multiple stress agents, affect many plant organs or are constant, extensive or extreme.

Members of the gall-making wasp family Cynipidae fit their eggs onto oak bud, leaf and stem surfaces where plant tissues form protective balls around hatched larvae. Larvae and pupae grow inside the irregular, rough or rounded globes from which diminutive, mature cynipid wasps emerge for mysterious, short-lived life cycles and natural histories.

Unlike tar spots, galls have uses in hemorrhoid-relieving pastes of crushed galls and lard and in iron gall ink from iron salts, macerated galls and water.

Tar spot is not so recyclable since dark, shiny lesions dot a leaf's upper sides in such Acer family members as Norway, red and silver maples. Arborists, master gardeners and master naturalists judge bagging or burning fallen leaves preferable to implementing "timely, frequent and thorough" fungicide schedules against benignly ugly tree disorders.

Like tar spot, powdery mildew keeps plant health care programs busy bagging or burning chalk-coated fallen leaves and stems against fungi overwintering benignly ugly tree disorders. At worst, powdery mildew limits flowering and fruiting and lowers aesthetic market values on spring- and summer-flowering plants while tar spot just lets leaves drop early. Buckeye, catalpa, cherry, crapemyrtle, dogwood, euonymus, honeysuckle, horsechestnut, lilac, oak, pecan, privet, roses, serviceberry, silver maple, soapberry, sycamore, tulip tree, walnut and willow must be monitored.

Unlike powdery mildew, sooty mold narrows the total surface area over which photosynthesis can occur by black powder coating leaves, stems and trunks that retain honeydew. Plant surfaces obtain the hazy, sticky, "sugary growing media" known as honeydew from the secretions of such sap-sipping arthropods as aphids, psyllids, scale insects and whiteflies.

Black knot, Graphiola leaf spot, hackberry gall makers, leaf miners, oak leaf blister and twig girdlers provide examples of similarly "visually distinctive, but largely diminutive" occurrences.

Such occurrences qualify for the appropriate response process of "systematically assessing plant health and client needs to determine what course of action, if any, is recommended." Prevention requires airy, unshaded locations while treatment rests upon botanical pesticides, horticultural oils and insecticidal soaps against girdlers, miners and sap-sippers and upon fallen leaf disposal. Moistened cloths sponge away honeydew, mildew, mold and spot even though "Washing sooty mold off the foliage of a large tree is a largely hopeless endeavor."

Just 28.35 grams of preventative siting take less resources and time than 0.45 kilograms of curative contact, systemic or translaminar chemicals against benignly ugly tree disorders.

tar spots on sycamore maple (Acer pseudoplatanus), Cadier en Keer, Limburg province, southeastern Netherlands; Friday, Sep. 2, 2005: Annabel, CC BY SA 3.0 Unported, via Wikimedia Commons

Acknowledgment

My special thanks to:
talented artists and photographers/concerned organizations who make their fine images available on the internet;
University of Illinois at Urbana-Champaign for superior on-campus and on-line resources.

Image credits:

oak apple galls, created by Andricus quercuscalifornicus, on Brewer's Oak (Quercus garryana var. breweri), Jackson County, southwestern Oregon: Jim Conrad, Public Domain, via Wikimedia Commons @ https://commons.wikimedia.org/wiki/File:Brewer%27s_Oak_apple.jpg

tar spots on sycamore maple (Acer pseudoplatanus), Cadier en Keer, Limburg province, southeastern Netherlands; Friday, Sep. 2, 2005: Annabel, CC BY SA 3.0 Unported, via Wikimedia Commons @ https://commons.wikimedia.org/wiki/File:RhytismaAcerinumDetailU.jpg

For further information:

Gilman, Ed. 2011. An Illustrated Guide to Pruning. Third Edition. Boston MA: Cengage.

Hartman, John R.; Thomas P. Pirone; Mary Ann Sall. 2000. Pirone's Tree Maintenance. Seventh edition. New York NY: Oxford University Press.

Hayes, Ed. 2001. Evaluating Tree Defects. Revised, Special Edition. Rochester MN: Safe Trees.

Koeser, Andrew. April 2011. "Born to be Mild: Identification and Management of Highly Visible, Relatively Harmless, Tree Disorders." Arborist News 20(2): 12 - 17.

Marriner, Derdriu. 12 February 2011. “Tree Load Can Turn Tree Health Into Tree Failure or Tree Fatigue.” Earth and Space News. Saturday.
Available @ https://earth-and-space-news.blogspot.com/2011/02/tree-load-can-turn-tree-health-into.html

Marriner, Derdriu. 11 December 2010. “Tree Electrical Safety Knowledge, Precautions, Risks and Standards.” Earth and Space News. Saturday.
Available @ https://earth-and-space-news.blogspot.com/2010/12/tree-electrical-safety-knowledge.html