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Disease progression plot monitoring and plotless evaluation of Phytophthora ramorum incidence in different forest types in coastal California. (03XU020)

Program

Exotic Pests and Diseases Research Program

Principal
investigators

R.B. Standiford, Center for Forestry, IHRMP, UC Berkeley
N.M. Kelly, Division of Ecosystem Science, UC Berkeley

Host/habitat

Coast Live Oak; Tanoak; Shreve Oak

Pest

Sudden Oak Death Phytophthora ramorum

Discipline

Plant Pathology

Review
panel

Urban Systems

Start year (duration)

2003 (Three Years)

Objectives

Evaluate incidence of Phytophthora ramorum in a different forest types in coastal California.

Maintain disease progression plots in Marin County and develop additional plots incorporating Shreve oak in Santa Cruz County.

Maintain center-point-quarter, plotless monitoring in Marin County, while expanding this approach to tanoak-dominated forests in Marin and Santa Cruz Counties.

Project
Summary

A combination of long-term disease progression plots and plotless, transect-based monitoring will be employed to continue evaluating the impacts of sudden oak death syndrome, both on individual tree species and forest ecosystems. Plots that have been in place since March, 2000, are the basis for much of our present understanding of the effects of this disease on coast live oaks, California black oaks, and tanoaks. Continuing this monitoring will allow trees to be followed from first bleeding symptoms through death. If Phytophthora ramorum becomes established in Eastern forests, the knowledge base developed using these plots and transects will direct actions to mitigate its effects on those forested ecosystems.

Final report

Disease progression: Two of the ecologically important hardwood species in Marin County forests, coast live oak and tanoak, have been severely impacted by Phytophthora ramorum. Infections in coast live oaks increased from 26% in 2000 to 30% in 2006, a trend that underestimates the true effect of the disease, as the percentage of dead trees increased from 9% to 34%. Infected tanoaks increased from 35% to 70% and mortality from 7% to 40% during this period. Black oaks showed similar responses, with 26% infected and 23% dead through 2006. Larger diameter coast live oaks and tanoaks were more likely than smaller trees to develop infections, and larger infected trees were more likely to be attacked by beetles. Beetle attacks were strongly correlated with accelerated death of infected trees and structural failure of living trees. For both coast live oaks and tanoaks, estimated survival of infected trees dropped from nearly 9 years to less than 3 years following beetle attacks.

Landscape-scale plotless disease evaluation: In China Camp State Park (Marin County), the infections in coast live oaks fluctuated between 15% and 18% from 2001 through 2006, while mortality increased from 9% to 17%. Black oak infections increased from 23% to 29%, and mortality from 14% to 23%. Analysis of the distribution of infected coast live oaks in 2001 detected a spatial clustering at a 300 m scale. By 2004, the scale detected was 700 m, a result that may indicate a wave of new infections, leading to a pulse of increased mortality in coming years. In the redwood-tanoak dominated Soquel Demonstration State Forest (Santa Cruz County), tanoak infections increased from 13% in 2001 to 17% by 2005, as mortality increased from less than 2% to 16%. Shreve oak infections increased from 2% in 2001 to 10% by 2005, and mortality increased from 1% to 12%.

Third-year
progress

Phytophthora ramorum: From 2000 to 2005, 26 to 23% of coast live oaks were symptomatic, whereas tanoaks increased from 35 to 74%. Mortality increased from 9 to 28% for coast live oaks and 7 to 28% for tanoaks. Survival models for both species in different stages of disease progression provided estimates of approximately 8 years for symptomatic trees, and approximately 3 years after ambrosia and bark beetle attacks.

Larger diameter was a significant predictor of infection in both oaks and tanoaks. For three infected coast live oaks, larger stem diameter was also a significant predictor of beetle attack.

New infections after 2000 increased in the order: tanoaks greater than black oaks greater than coast live oaks. This suggests that the most susceptible coast live oaks were infected early in the epidemic, leading to a peak in mortality in the late 1990s through 2001, leaving the more resistant trees. Tanoaks appear to have much less resistance than oaks to P. ramorum.

Plotless disease evaluation: Landscape-scale assessment in Marin County from 2001 to 2004 found that the percentage of infected coast live oaks decreased from 23% to 13% and increased from 23% to 28% for black oaks. In Santa Cruz County, redwood dominated forests, from 2001 to 2005 infection increased in tanoaks, 14 to 29% and Shreve oaks 2% to 11%. We have isolated P. ramorum from 40% of bleeding oaks. To date, no other Phytophthoraspp. have been confirmed.

Second-year
progress

Disease progression: Statistical survival models were used to estimate expected life spans for coast live oaks and tanoaks in different stages of disease progression caused by Phytophthora ramorum. Estimated survival for both species was approximately seven to eight years for symptomatic (bleeding) trees not colonized by ambrosia and bark beetles, which dropped to approximately three years after beetle colonization. Tree diameter at breast height (dbh) was a statistically significant predictor of infection for coast live oaks and California black oaks. Larger stem diameter was a statistically significant predictor of beetle colonization for bleeding coast live oaks. New infections after 2000 increased in the order: tanoaks are greater than black oaks, and black oaks are greater than coast live oaks. This suggests that the most susceptible coast live oaks were infected early in the epidemic, leading to a peak in mortality in the late 1990s through 2001, leaving the more resistant trees. Tanoaks appear in this assessment to have much less resistance to P. ramorum than the true oaks.

Plotless disease evaluation: Unbiased landscape-scale disease assessment in China Camp State Park from 2001 to 2004 found that the percentage of bleeding coast live oaks decreased from 23% to 13%, but increased from 23% to 28% for black oaks. The combined total of bleeding and dead trees was stable for coast live oaks at 30 to 31%, and increased from 33% to 46% for black oak. In collaboration with David Rizzo (UC Davis), we have isolated P. ramorum from 40% of bleeding coast live oaks and black oaks (species combined). To date, no other Phytophthora spp. has been confirmed.

First-year
progress

Effects of sudden oak death (SOD) on trees: Since 2000, we have followed the fate of more than 1000 coast live oaks, black oaks, and tanoaks in Marin County forests where SOD is established. By 2003, 24% of the coast live oaks were bleeding and 17% were dead. For black oaks, 25% were bleeding and 9% were dead. Tanoaks showed 62% bleeding and 22% mortality by 2003. The rate of newly bleeding coast live oaks was 3% to 5% per year. For tanoaks, this rate was 10% to 12% per year. Shreve oaks show much lower infection levels in Santa Cruz County forests than other host oaks, although these numbers may be increasing. Tanoak appears to be the species most susceptible to SOD. Disease progression: We have established that the order of sign and symptom development in coast live oaks is bleeding, followed by beetle attacks in about eight months, then fruiting of the fungus Hypoxylon thouarsianum six to nine months later. Death may take five or sometimes fewer years.

Insect associations: Bark and ambrosia beetles attacked 42% to 58% of bleeding coast live oaks from 2000 to 2003. These beetles colonized every tree that died with bleeding while it was still alive. More than 73% of beetle-colonized bleeding coast live oaks in 2000 had died by 2003. Structural failure was observed in 86 bleeding trees, >41% of which were alive when they failed.

Bleeding coast live oaks and black oaks have significantly larger mean trunk diameters than asymptomatic trees.

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