Wednesday, July 15, 2009

It's Not Just Gardeners That Wilt in the Heat

    From the garden mailbag, Monica writes: "I have a maple tree that has been in the ground about 12 years. This year it got green leaves but now they are turning brown and falling off. I don’t see bugs or holes on the bark. It gets watered in the summer three times a week. Help!"

    Monica, you are not alone. Several varieties of trees appear to be dying this time of year. Leaves turn yellow or brown, and fall off. Sometimes, it happens to just one side of the tree. The possible culprit? A soil-dwelling fungus, Verticillium Wilt.

    There could be other causes for your maple's demise. Maples can also lose leaves due to hot dry winds, root rots and crown rots, which can be helped by improving the drainage and watering correctly. 
The presence of Verticillium Wilt, though, is being reported throughout the area this summer. This fungus resides in the soil, and multiplies in the cool weather of spring, attaching to tree roots, according to the excellent University of California book, "Pests of Landscape Trees and Shrubs". As the weather warms in late spring and early summer, this fungus travels upwards through the tree's vascular system, blocking the plant's ability to utilize food and water. Among the trees that Verticillium Wilt can infect here include ash, camphor, Chinese pistache, maple, olive and the pepper tree.

    This happened in our own yard, where a beautiful Chinese pistache lost half of its leaves, primarily on the west side of the tree. Leaves then began to yellow on the other side, not a good sign. 
     Peeling back the bark of an infected branch confirmed the culprit. Many tree species will show dark streaks following the wood grain if it is infected with Verticillium Wilt. The tree died about a month after the first symptoms were noticed.

    Verticillium Wilt can dwell in the soil for years, and only become noticeable when conditions are right. It spreads slowly in trees; it may take several years for the tree to die. The good news: the tree may recover if conditions become favorable for plant growth and poor for disease development. 

    So, what is a gardener to do? Proper irrigation and the application of small amounts of a slow release fertilizer can help the tree put on new growth. Watering once or twice a week with a soaker hose or drip irrigation system, throughout the plant's root zone, is a great way to apply water slowly. Slow release fertilizers include liquids such as fish/kelp meal or a granular organic fertilizer.
Why an organic fertilizer? Many synthetic chemical fertilizers are salt-based, a detriment to soil-dwelling mycorrhizae. These beneficial bacteria and fungi act as nature's waiters and waitresses, delivering the nutrients from the soil to the plants. The more mycorrhizae there are, the better the chances for your suffering plant to get the nutrients it needs. 
An excellent book on the subject is "Teaming with Microbes" by Jeff Lowenfels and Wayne Lewis. After reading it, you will look at your garden very differently.
     If branches die (they will snap, not bend), prune them out. Watch out for infestations of insects, which prefer to attack weakened trees. 
Painting the trunk of the tree with a 50-50 mix of interior white latex paint and water can help protect the suddenly exposed trunk from sunscald.

    If the tree does not recover, consider removing it and planting a tree species
that may be resistant to Verticillium Wilt (according to the University of California):  crabapples, birch, citrus, cedar, dogwood, figs, oaks, pines, pears, sycamore or walnut. 
For added insurance, covering the infected area formerly occupied by the tree with a sheet of clear plastic for six weeks during summer can solarize the soil, killing off this pathogen to a depth of a few inches. It's not a cure-all before planting a new tree, but it can help. 
Check the UC Integrated Pest Management Website for more information about Verticillium Wilt.

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