By Jim Downer, Ph.D
By now everyone has heard the hue and cry of drought and the peril we Californians all face from prolonged snow pack depletion and lack of rain. Those who study climate change suggest that some places will get wetter and others will dry out. We seem to be one of those others.
Despite some modest spring rains, we are still in a drought of historic proportion. Reservoirs around the state have never looked so low, even in some cases since their initial filling. Drought has a cocktail of effects on landscape plants and trees, all of which can ultimately be adverse lead to disease development.
For several years now the amount of evaporation and transpiration (ET) have exceeded the amount of precipitation or rain that falls in many parts of California. To make up the difference, we irrigate with well or surface waters as irrigation.
This water (unlike rainfall) contains salt and when this water is used by trees and the soil is depleted, the salt remains behind. After years of low rainfall and irrigation with salt filled water, soils become increasingly saline. Salts in soil are carried by mass flow in water into trees and translocate to leaves where they are left behind when water evaporates during transpiration. This can result in burning (salt burn) and is a significant abiotic disease in its own right. Salinity and associated drought are also predisposing factors for some biotic pathogens like the canker fungi such as Botryosphaeria and the root rot diseases caused by Phytophthora spp.
Botryosphaeria spp. are fungi that cause canker and blight diseases of many trees. Major outbreaks occur in both landscapes and native plant communities after prolonged periods of drought. Drying predisposes many woody plants to infection during the occasional wet periods when spores can splash.
These canker fungi often are called Dothiorella or Fusicoccum because these “imperfect” stages are asexual reproductive stage of the fungus (Botryosphaeria). These organisms cause blight on madrone, Ceanothous, Ficus microcarpa, Liquidambar, maple, avocado, and many other landscape trees and shrubs. They affect weakened or stressed trees, sometimes leading to extensive dieback, cankering of branches or blighting of foliage. When deep soaking or leaching rains return, many trees grow out of the disease.
Phytophthora spp. such as P. cinnamomi (avocado root rot) or P. ramorum (sudden oak death) cause extensive damage to trees and other woody plants in landscapes, in agriculture, and wildlands. While drought does not directly promote these diseases (they need wet conditions to infect), the increased soil salinity causes roots to become “leaky” and root exudates from salt stressed plants are more attractive to root rot pathogens when water does saturate soil, especially from irrigation. Deep leaching rains of pure water help plants avoid this kind of predisposing stress that makes root rot pathogens so infective.
Another way that drought can affect diseases of trees is through their relationships with insects. While drought does not favor leaf consuming or sucking insects (since foliage production is reduced) it does attract boring insects, especially beetles.
As wood of trees dries down during extreme moisture depletion in soil, it begins to emit more volatile chemicals. Some of these serve as location-signaling molecules to insects that a tree is stressed and its wood is dry enough for that insect to successfully enter and make galleries for its own reproduction. Pines, eucalyptus and oaks all have a number of insects that are capable of this.
A certain class of beetles called the ambrosia beetles are actually symbionts with fungi that they carry in their mouth parts. When ambrosia beetles make a gallery in their tree host, they infect it with a fungus. They can then feed on that same fungus.
The new invasive pest in Southern California, the Polyphagous Shot Hole Borer (PSHB) operates in a similar fashion. While some fungus-carrying beetles such as the vector of Thousand Cankers disease of walnut are very host-specific (walnuts), others such as PSHB attack a wide number of trees.
Beetles with a wide host range may not necessarily cause disease in all the trees they bore. This is because different trees have different levels and types of host defense responses that allow them to fight the fungal infection. There are many beetles and even a few moths with infection capabilities. For example, clear winged moths that bore willow carry Cytospora and the cypress bark moth carries spores of cypress canker disease. So if we can prevent insect infestations then trees can be free of some of these diseases.
What then is the best defense against these drought-motivated pathogens? Of course fresh rain water that leaches salts is best. This is largely out our control. Irrigation of about an inch of applied water per dry/hot month under the and beyond the canopy is recommended. An occasional deeper soak to move salts to a lower level in soil is helpful during long dry periods.
What if you have too many trees to water them all? This is a time to think about your firewood supplies and perhaps remove some of the less important trees so that others can have the available water resources still left in soil. If you don’t thin dense stands of trees, nature will with its fungus-wielding beetles.
Jim Downer, PhD is an horticulture specialist with the University of California Cooperative Extension in Ventura County.