Late Spring Rains Result in an Increase in Foliar Pathogens

By Thembi Borras

In the past several months, the madrone tree outside my window has become increasingly unhealthy looking. More than 75% of the surface area on 50% of the leaves have turned brown.

So I called Jack Marshall at the California Department of Forestry and Fire Protection and asked him, why? Late spring rains in 2005 created a prime environment, high moisture during a warmer time of the year, for the growth of foliar pathogens including the native foliar pathogen causing the brown spots on the madrone outside my window. The consequence will likely be slowed growth, given that the photosynthetic area of the leaves has been obstructed, but the madrone will not likely die and will slowly recover.

A little more than a month ago, Mr. Marshall started observing the end of branches on interior live oak trees were dead or dying. The cause is Discula quercina, a native branch canker fungus that kills branch tips and buds. Similar to the madrone foliar pathogen it was accelerated by the environmental conditions set forth by the late spring rains.

New Sudden Oak Death (SOD) occurrences may manifest as a result of the late spring rains. Phytophthora ramorum, a water mold fungus that many believe to be nonnative, causes Sudden Oak Death in some of its hosts and occurs as a foliar pathogen in other hosts, such as California bay laurel. Bay laurels are thought to be very important in spreading the disease as the fungus readily produces spores on moist bay leaves. The spores of Phytophthora ramorum are then ready for transport, by mechanisms such as wind and water, to new hosts. According to Brock Dolman of the Occidental Arts and Ecology Center, "This past summer SOD finally really hit Sonoma County due to our late wet spring…" To get a thorough grasp on SOD and its movement go to the California Oak Mortality Task Force web site at www.suddenoakdeath.org.

The complexity of the environment that surrounds us never ceases to amaze me.

A portion of this production was gleaned from the book Diseases of Tree and Shrubs by Sinclair, Lyon and Johnson and a conversation with Jack Marshall, Forest Pathologist at the California Department of Forestry and Fire Protection. If you have a local disease or insect question for Jack Marshall, he can be reached at Howard Forest in Willits at 707-459-7448.