Understanding and Managing Sudden Oak Death in California

1.2.2.1. California bay–oak distance and disease risk—

SOD incidence, severity, and mortality rates increase as the distance from oak trunk to California bay foliage decreases (fig. 1-5) (Swiecki and Bernhardt 2007, 2008b). Oaks with California bay foliage directly over or within 1.5 m (5 ft) of the trunk have the highest risk of infection and mortality (fig. 1-5). Disease risk also increases as the total amount of California bay cover within 2.5 to 5 m (8 to 16 ft) of the oak trunk increases.

Proximity of California bay to oaks strongly influences disease risk because few P. ramorum spores are moved more than 5 m horizontally by rain splash. Spores in water drops splashing from leaves higher in the canopy are more influenced by wind, and may travel greater horizontal distances. Spores present on the soil surface and in low foliage (less than about 0.5 m above the ground) are only dispersed about 1 m by rain splash alone (Tjosvold and others 2006).

Figure 1-5—Relationship between minimum bay foliage-oak trunk distance and SOD incidence and mortality in coast live oak. Bay foliage-oak trunk distance is defined as the shortest distance between the main stem of an oak and a vertical line dropped from the nearest bay foliage (fig. 1-6). Data shown were collected in 2004 from coast live oaks in Marin and Napa Counties (Swiecki and Bernhardt 2007, 2008b). Note: 2.5 m = 8.2 ft.

 

Other factors may also influence the spore deposition on oaks from infected California bays (Swiecki and Bernhardt 2007, 2008b).

• Height of the California bay canopy relative to the oak. Especially under windy conditions, average spore dispersal distances increases with foliage height . Tall California bay canopy, and canopy located upslope from oaks, can disperse spores greater distances than low understory canopy. In dry locations with widely spaced trees, this effect may be less important because fewer leaf infections are present in the tops of California bay canopies.
• Prevailing storm wind direction. Since most spore dispersal occurs during rain events, greater numbers of spores are dispersed downwind. This effect is more important when the spore source is extensive and high (tall canopy or upslope) relative to the target oaks.
• Oak canopy size. Large oak canopies that have many large branches may serve to collect and concentrate P. ramorum spores. Large trees with dominant canopies are typically among the first trees to develop severe SOD infections.
• Interception (screening) by nonsusceptible species. If nonsusceptible species, especially those with dense foliage, are located between infected California bay foliage and an oak, many spores will be intercepted and fewer will deposit on the oak . Similarly, the spread of foliar infections throughout a stand containing California bay may be slowed if individual California bays are widely separated from each other by nonsusceptible species

Figure 1-6—Example of minimum distance between California bay foliage and oak trunk (white arrow).