A Killer Lurks Below

The Delonix tree above fell over during the rainstorms earlier this month. The culprit?

The culprit is plainly visible looking at the crown of the tree. Notice the whitish almost tan growth on the edge of the broken crown of the tree. That is most probably Armillaria mellea, the honey fungus, whose large clusters of mushroom fruiting bodies are visible throughout the Arboretum after the first, soaking cold rain of the season.

Armillaria fungus on the underground portion of the crown. 

Below you can see the fruiting bodies (mushrooms) of the Armillaria fungus pushing up near a Brachychiton tree in the Australian section. 

Armillaria mushrooms fruiting in the Australia section. 

Besides 'Honey Mushroom' the other common name for this fungus is the 'Oak Root' fungus. It can be found in areas that have or have previously had native oak trees. Most of the Arboretum was, in the past, oak woodland. Two species, Quercus engelmannii the Engelmann or 'Pasadena' oak and Quercus agrifolia, the Coast live oak dominated the landscape here.

Indeed, the Honey fungus seems to have an almost symbiotic relationship with the Coast live oak and the Engelmann oak. Healthy trees seem to thrive despite being infested with the fungus. Indeed the oaks and the fungus might have what you might call a semi-beneficial relationship. The fungus destroys woody trees and shrubs that might compete with the oak, and the oak might even provide water and minimal nutrition for the fungus. In fact, it has been shown that an Armillaria fungus has a symbiotic relationship with the Chinese medicinal orchid Gastrodia elata and has been reported as having a mycorrhizal relationship with at least two oak species. 

Although the oaks relationship with the fungus may be beneficial at times, if an oak tree is under any kind of stress (especially soil compaction and over-watering during its dormant period in the late summer) the fungus will aggressively go after the tree, sometimes killing it in one to three weeks time. 

So how to deal with this underground killer? Unfortunately there is no chemical method of control. The main method of control is watering your landscape correctly to prevent activating the fungus (in particular by encouraging water loving organisms like the root rotting Phytophthora fungus that damage plant roots, in turn causing them to attract Armillaria that is present) and removing any large pieces of wood that are 2" or more in diameter that may be buried in the ground.

Mushrooms Mass on Local Lawn

Question:  My front lawn has been inundated by mushrooms.  I just picked 1/2 pail this morning and now I have another 1/2 pail out on the lawn. What can I do?

Answer: Mushrooms can pop up in a lawn anytime the conditions are right. Some come during winter rains, others during summer humidity, and still others when there is enough moisture to precipitate dew out on the lawn. Even lawns that are watered only once a week can produce mushrooms. Besides in some cases producing an unsightly band of darker and then lighter colored grass, few mushrooms contribute to lawn diseases.  Many people feel that mushrooms are unattractive and they look for a way to kill the fungus.  The actual mushroom is simply the fruit of a fungus.  Killing the mushroom does not remove or-kill-the mycelia, or fungi, causing their growth.

Still some may consider these unattractive or a nuisance. What can be done to help lessen their occurrence? Below are some options that can help reduce the number and occurrences of mushrooms:

1.  Reduce amount of moisture in the area:  Mushrooms thrive in moist soils, refrain from, or slow down, the amount of irrigating or water you do to the location. This will not get rid of all the mushrooms, but can in some cases decrease their numbers.

2.  Eliminate decaying elements that enhance the growth of mushrooms:

• ·         Remove grass clippings after mowing, raking them with a rake or using a bag grass catcher on the mower.
• ·         De-thatch your lawn. Some lawns, especially tropical grasses like Bermuda and Saint Augustine, build up a layer of dead stems and roots that encourages mushroom growth.
• ·         Have tree stumps ground and removed. Leaving tree stumps in the ground can encourage fungi that will infest other trees and cause heart rot. Also, buried wood over two inches thick from trees that were infested with Oak root fungus (Armellaria mellea) can encourage infestations of the fungus in trees whose roots come in contact with it, as well as producing large clusters of honey colored mushrooms.

3.  Increase the amount of light in the area mushrooms grow in; this can work for some mushrooms that do not need the sun to stimulate their fruiting, but does not work for all mushrooms.

4. Change your brand of mulch or compost: Some mulches and composts are very friendly to mushroom growth. If your using wood mulch make sure that that it is composed mostly of bark –under composted wood chips are great chow for mushroom producing fungi. If you’re using compost or amendment that is high in under-composted steer or horse manure you are likewise treating mushroom producing fungi to a very nutritious meal.

4. Learn to enjoy the fungi. In most cases mushroom producing fungi is not harmful to your lawn.

6. Have your mushrooms identified. You can bring your mushrooms to a local mycological society meeting and have them identified. This is very good idea if you feel that your dog or neighbor children might eat the fungi off of your lawn

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7. Replace you lawn with ground cover and landscaping less susceptible to mushroom growth. Low water use landscape is less likely to harbor mushroom mycelia, and the period of mushroom production is much more likely to be shorter than that of a lawn.