Images of Libertyville in the Fall
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Maple Leaf Tar Spot
Tar spot is caused by a fungus in the genus Rhytisma.1 I first became aware of tar spot when I noticed that my newly mowed lawn was covered the following day with dropped maple leaves…in August! Upon closer inspection of the leaves on the lawn and on the tree, I noticed the leaves contained varying numbers of the black spots shown in the images above. As it turned out, several other trees in my neighborhood also had the same black spots.
Tar spots begin as small yellow-colored spots on the upper leaf surface that turn into black raised structures resembling tar. The fungus that causes tar spots overwinters on infected leaves and then releases its spores from the stroma (black spot) the following spring. The spores are spread by the wind onto new leaves that are beginning to unfold. The spots where the fungus colonizes the leaves become visible 1-2 months later, during the summer. Fortunately, tar spot is not usually detrimental to the tree—it is a cosmetic concern that can be managed by raking up the infected leaves and destroying them in the fall.2
- Hudelson B. Tar spot. 8/13/2012. Available at: http://hort.uwex.edu/articles/tar-spot (accessed 12 December 2013).
- Healy R. Minor leaf spot diseases of maple: tar spot and leaf blister. 8/8/2007. Available at: http://www.ipm.iastate.edu/ipm/hortnews/2007/8-8/maplespot.html (accessed 12 December 2013).
Pin Oak Tree: Magnificent Marcescent Leaves
Trees may be grouped into two types based on the duration of time with leaves: evergreen and deciduous. Evergreen trees have leaves or needles year round and the leaves or needles stay green (due to the presence of chlorophyll). Deciduous trees, on the other hand, have leaves for only part of the year and the leaves may change color (due to the disappearance of chlorophyll).
Interestingly, some deciduous trees like the pin oak tree pictured above have leaves that change color but the leaves do not drop until spring (this photo was taken in late November). This characteristic is referred to as marcescence, from the Latin marcescent – beginning to wither. Botanically speaking, the term refers to leaves or fronds that wither but remain attached to the stem.1 Beech, sycamore, and several species of oak have marcescent leaves.
What causes leaves to fall from trees? The leaves of deciduous trees have an abscission zone or abscission layer that forms where the leaf meets the stem. In response to changes in the length of the dark period each day, the cells in the abscission layer start to block the transport of nutrients between the leaf and the stem. Eventually, the cells weaken or are digested and the leaf separates from the tree. In marcescent leaves, the abscission zone is not present or does not fully develop until the start of the next growing season, when buds begin to emerge. The result is a deciduous tree that keeps its brown leaves all winter! The articles by Andrew Hipp2 and Ted Griess3 provide good overviews of this process. The US National Arboretum also has a nice summary.4
Not sure what type of tree you have? Try the searchable factsheets at the Department of Forest Resources and Environmental Conservation at Virginia Tech.
- Oxford Dictionaries. http://www.oxforddictionaries.com/definition/american_english/marcescent
- Hipp A. When oak leaves fail to fall. Tag Along. Vol 6 #2. Spring 2007. Available at: http://systematics.mortonarb.org/lab/publications/Hipp2007_TagAlong6_marcescence.pdf
- Griess T. Yard and Garden. 11-02-2013. Available at: http://buffalo.unl.edu/c/document_library/get_file?uuid=9419c5cc-3be5-45da-a972-f36d2022d071&groupId=134881&.pdf
- The Science of Color in Autumn Leaves. http://www.usna.usda.gov/PhotoGallery/FallFoliage/ScienceFallColor.html (accessed 11 December 2013)
Leaf Gall
Hackberry nipple gall
Hackberry nipple gall – top of leaf
Spindle gall
Spindle gall close-up
Spindle gall viewed from underside of leaf
I found these leaves on trees growing in my neighborhood. The photos were taken in early November. The bumps on the leaves are known as leaf galls. According to the American Heritage® Science Dictionary, a gall is defined as “an abnormal swelling of plant tissue, caused by injury or by parasitic organisms such as insects, mites, nematodes, and bacteria. Parasites stimulate the production of galls by secreting chemical irritants on or in the plant tissue. Galls stimulated by egg-laying parasites typically provide a protective environment in which the eggs can hatch and the pupae develop, and they usually do only minor damage to the host plant. Gall-stimulating fungi and microorganisms, such as the bacterium that causes crown gall, are generally considered to be plant diseases.”1
What I find interesting about galls is that they look very different from the leaf, but they are formed by normal leaf cells that have gone awry. The parasite releases chemicals that cause the leaf cells to grow aberrantly and form the gall. In turn, the gall may provide the parasite with a source of nutrients and protection from harsh environmental conditions. Galls may also protect the parasites from natural enemies, produce antimicrobial agents,2 and release volatile compounds that deter herbivorous mammals.3
There are hundreds of gall-forming organisms. The photos shown above are just two examples of leaf galls. Despite their unusual appearance, galls generally are not thought to be harmful to the plant. More information on leaf galls and other types of galls may be obtained from a local university extension. I found the publication by Mark Shour and colleagues from the Iowa State University Extension to be particularly informative.4
- Gall. Dictionary.com. The American Heritage® Science Dictionary. Houghton Mifflin Company. http://dictionary.reference.com/browse/gall (accessed: November 27, 2013).
- Gerchman Y, Inbar M. Distinct antimicrobial activities in aphid galls on Pistacia atlantica. Plant Signal Behav. 2011 Dec;6(12):2008-12. Available at: http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3337195/
- Rostás M, Maag D, Ikegami M, Inbar M. Gall volatiles defend aphids against a browsing mammal. BMC Evol Biol. 2013 Sep 11;13:193. Available at: http://www.biomedcentral.com/1471-2148/13/193.
- Shour M, Jesse L, Lewis D. Insect galls on trees and shrubs. Available at: http://www.extension.iastate.edu/publications/ic417.pdf