Garden Help: Your fall tree checkup guide

Fall is when trees are starting to slow down. Soon we will be raking leaves and dreaming about the pleasant weather just around the corner. While we are outside working around our landscape trees, there are a few things we may notice and wonder what they mean for our arboreal treasures.

Oak leaf blister

As you look up in your oak trees, or as you are raking up leaves you may notice that the leaves don’t look so good. Upon closer inspection of the leaves, it looks like they have little warts or blisters on them. Don’t worry, your tree is just infected with the disease called oak leaf blister.

Oak leaf blister is caused by the fungus Taphrina caerulescens. It is a very common disease in the humid climate of the South. It can affect many species of oaks. Members of the red oak group such as water oak (Quercus nigra) and laurel oak (Quercus laurifolia) are particularly susceptible to infection. Oak leaf blister can show up on live oaks as well, although it's not as common. The disease seems to be more prevalent when we have years with cool, wet springs such as this past spring.

The fungus survives the winter on twigs and bud scales. The leaf spots we are seeing now are from leaves being infected last spring. As the buds start to swell and expand, and leaves are formed, the fungal spores are washed and splashed onto the young tender foliage by rainfall and mist infecting the young oak leaves. As the pathogen develops within the leaf, it causes a disruption of normal development of leaf cells.

The infected tissues appear swollen or blister-like. Severe infections can cause the leaf to become curled or twisted. Over time the infected tissues die, leaving gray-brown areas scattered within the given leaf which is what we are noticing now. If large areas of the leaf are infected, the leaf may prematurely fall. This fall the disease seems more widespread.

The good news is that this is primarily an aesthetic problem, and the disease does no long-term harm to the tree. When the leaves drop this fall, just rake them up — don’t use them for mulch. Put the raked leaves out with the yard waste or properly compost them to reduce the amount of spores that will be around next spring. Hopefully next spring, when the leaves are emerging, the weather won’t be as conducive to oak leaf blister.

Mushrooms

As cooler, wetter weather rolls in with fall nor’easters, we may start to notice clumps of tan/honey-colored mushrooms in our turf, or near our trees. These may be the fruiting structures of the armillaria fungus. Armillaria can be identified by conspicuous clusters of small mushrooms appearing at the base of trees appearing in the late summer or fall.

Sometimes the mushrooms appear in middle of the yard. This is because the fungi are colonizing a stump or root that is underground. They are tan to brown, fleshy with gills beneath the cap. The mushrooms are only seen during the cooler wetter days in the fall. Also, the mushroom does not persist very long. It starts to turn black and fade within a week or so. The Armillaria fungi causes a disease in trees and woody shrubs commonly called mushroom root rot, or shoestring root rot.

The Armillaria root fungi can colonize the root system and persist in the tree unrecognized for years or decades in some cases. Bit by bit, year after year, the root fungi gradually compromise the trees’ ability to take up water. The symptoms of Armillaria infection in a tree or shrub include reduced growth, yellowed leaves, and branch and twig dieback. The foliage progressively becomes sparser with an increasing amount of branch dieback until it is obvious that the tree must be removed.

Mushroom root rot is often diagnosed only by the decline symptoms mentioned above. The Armillaria fungus is in the roots and do not always produce mushrooms. In the case where a mushroom or spore producing structure is spotted, diagnosis can be made based solely on the appearance of the mushroom.

Armillaria moves around in our landscape soil by producing strands or “shoestrings” called rhizomorphs. These rhizomorphs originate from a dead stump or root already colonized by Armillaria and fan out through the soil. The rhizomorph can infect a tree or woody shrub when it meets a damaged or diseased root while leaving healthy roots intact.

If you suspect mushroom root rot but there are no mushrooms present, you can remove a patch of bark on a root and look for a spot of bright white in between the bark and the wood of the root. This bright white tissue is the fungal mycelium colonizing the root tissue.

Unfortunately, there is no control for mushroom root rot. Collecting the mushrooms will not help in eliminating the fungus from your landscape. The mushrooms are just the fruiting structures of the fungus. Just removing the mushrooms would be like removing the flowers from a weed as an attempt to kill the weed.

Because Armillaria infects and damages the roots of trees, trees infected with Armillaria have a greater risk of falling and should be removed if they have the potential to cause harm to people or property. When you remove shrub or tree that has died from mushroom root rot, get rid of as much of the affected plant’s root system as you can to eliminate the food source for the fungus. If you do replant, make sure your new tree or shrub is well suited to the area, and is planted correctly.

The best way to prevent your trees from having a root disease problem is to keep them healthy and keeping root injuries to a minimum. If you notice the disease in shrubs, improve the rooting environment by pulling any accumulated mulch away from the base of the shrub. For more information on Armillaria you can go to http://edis.ifas.ufl.edu/pdffiles/EP/EP47800.pdf.

Mulch

Reports have been in the news lately of folks with yard debris sitting for weeks out at the curb before it is picked up . Fall is the season where leaves will soon be dropping, and we will be raking them up.

To alleviate some of the debris collection backlog, perhaps we may want to use some the leaves we are raking as mulch or put them into our compost pile. If the leaves are leaf-spot free (see above), this is a great opportunity to refresh your landscape beds with leaf mulch.

Leaves can be used as a supplement to other mulches and can be used in almost any garden bed. It is important to remember that mulch should only be 2-3 inches deep, so if you have a lot of leaves, perhaps you can use them in your compost pile. In addition to the leaves, add some green grass, or leaf clippings to recipe to create the perfect compost.

Yes, some of your yard chores may be winding down as the temperatures cool down. However, be diligent while looking for leaf spots and mushrooms, and be sustainable using leaves for mulch and compost.

Larry Figart is urban forestry extension agent from the University of Florida/IFAS.