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Summary of current research on pitch canker at the University of California

Prepared by : Thomas R. Gordon (Principal Investigator: UC Davis), David L. Wood (Principal Investigator, UC Berkeley) and Andrew J. Storer (UC Berkeley)

We are working closely with CDF to address the priorities for research identified in SB 1712. Major activities may be grouped in the following categories:

I. Determining the current distribution of pitch canker in California

II. Characterizing the risk of disease spread to other parts of the state

III. Management of pitch canker and genetic conservation of Monterey pine

IV. Management of infested material within the zone of infestation

I. Distribution of pitch canker in California

CDF is taking the lead role in this effort by conducting surveys and collecting samples of suspect material. We are providing confirmation through isolation and identification of the pathogen. We also identify the specific strains that are recovered, thereby gaining information on their location and movement within the state.

In addition to statewide surveys of disease distribution, we are monitoring disease development in permanent plots, in cooperation with the U.S. Forest Service. These plots provide a measure of how pitch canker will progress in native stands of species where the disease has not yet become a serious problem: knobcone, ponderosa and bishop pines. We also continue to monitor disease development in Monterey pine stands that have suffered a range of impacts from the disease. This work is supported by the U.S. Forest Service and the Pebble Beach Company.

II Risk of disease to conifers in currently uninfested parts of the state

The risk of pitch canker in forested areas where it does not already occur is a function of the potential for the pathogen to reach those areas and the prospects for establishment once it arrives.

A. Risk of disease spread

Work in this area is focused on survival of the pathogen in the various substrates on which it might be transported.

1) Potential for movement of the pathogen on infected seedlings

Our research has shown that pitch canker can kill seedlings but also that non-lethal infections may occur and remain cryptic for over one year and perhaps longer. Thus apparently healthy seedlings constitute a risk for movement of the pitch canker pathogen. On-going studies will assess the upper limits of survival on symptomless trees and the potential for these infections to subsequently become active.

2) Potential for movement of the pathogen in infested soil

Studies already completed show the pitch canker pathogen can survive for at least several months in wet soil and at least 18 months in dry soil. This was somewhat unexpected because this fungus does not produce the kind of survival structures usually associated with persistence in soil. Experiments are planned to determine whether or not the pathogen has alternative modes of growth in soil or simply survives as inactive spores.

3) Potential for movement of the pathogen on seed

Our previous work has shown that Monterey pine seed collected in infested areas has a high probability of carrying the pathogen, even if the branch from which the cones were collected is free of disease. In most cases the infestation is superficial and can be eliminated by soaking in 20% bleach. However, the pathogen can be borne internally and no methods are presently available to effectively treat seeds in this condition.

B. Risk of disease establishment

Pitch canker is a serious problem in coastal areas of California for the following reasons: 1) highly susceptible hosts are widely available, 2) the weather is conducive to infection throughout the year, and 3) insect vectors are abundant. The extent to which similar conditions prevail in other parts of the state will largely determine the risk of pitch canker becoming established in those areas. Our research will quantify the relevant factors, ultimately providing the basis for a statewide risk assessment model.

1) The susceptibility of the low elevation conifers:

ponderosa pine

gray pine

knobcone pine

Bishop pine

Douglas-fir

shore pine

In order to provide a comprehensive view of each species, we have obtained seed from multiple provenances, effectively covering their statewide ranges. These seed have been sown and trees will be inoculated when they are 1-2 years old. We have also conducted tests on older trees of each species that were available through the nursery trade. Results from these tests indicate that Douglas-fir is relatively resistant to pitch canker, but all of the pines are clearly susceptible. Ponderosa pine is the most variable of these, with some trees being highly susceptible and others fairly resistant. These tests also included pinyon pine, which appeared relatively resistant and coulter pine, which was intermediate in susceptibility.

2) Climatic limitations on survival and infection by the pitch canker pathogen

Research already completed has shown that the pathogen is extremely tolerant of drying. It can survive in wood chips at 35 C (95 F) and 0% relative humidity for at least four weeks (longer time intervals are now being tested). These findings indicate that the arid Central Valley is not likely to pose a barrier to short term survival of the pathogen. However, warm dry conditions probably will limit the infection process. Studies are underway to determine the minimum levels of atmospheric moisture required for an infection to become established. Results to date indicate that infections can occur under fairly dry conditions if a sufficiently deep wound is provided. Because insects appear to be the most important wounding agents, we need to better characterize the manner in which twig beetles and other vectors introduce the pathogen into the tree. These studies are also in progress.

Also of interest is the effect of low temperature on the pathogen. Studies already completed have shown that the fungus cannot grow or cause infections below 10 C (50 F), and that it grows only very slowly at 14 C (57 F). As a result, the pathogen will encounter progressively less favorable conditions as it moves northward in latitude or upward in elevation. However, as with warm dry conditions, the fungus can survive very low temperatures.

Further insight into the effects of climate on disease development will be gained from the continued monitoring of permanent plots. These include urban plantings of Monterey pine on an east-west transect running from the coast, inland to the Diablo Valley near Walnut Creek. Permanent plots have also been established in native Monterey pine forests on the Monterey Peninsula. To date, results have shown that disease develops most rapidly in plots closest to the coast.

3) Host range of insect vectors

The insects that vector pitch canker are native associates of California conifers and many have wide host ranges. However, many of these insects may have strong preferences for certain host species. This could limit the movement of disease from, for example, infected Monterey pine, to a nearby susceptible ponderosa pine. Studies are in progress to characterize the frequency with which twig beetles, and some of their associated species such as predators, move between pine species.

III Management of pitch canker on Monterey pine

Monterey pine is presently the most important host for pitch canker and the rapid development of the disease in native populations is of particular concern. Our work in this area may be divided into two categories: genetic resistance and regeneration.

A) Genetic resistance in Monterey pine

Previous work has shown that individual trees in native forests and in urban plantings show great variation in their susceptibility to pitch canker. This includes both highly susceptible and highly resistant trees, with most falling between these two extremes. Current work on genetic resistance is intended to: 1) advance efforts to develop and deploy resistant trees in areas heavily impacted by pitch canker, 2) understand the genetics of inheritance of resistance, 3) determine the extent to which untested strains of the pathogen or environmental conditions may overcome resistance to pitch canker in Monterey pine, 4) determine whether or not ecotypic variation exists within Monterey pine populations that would dictate ecological limits for use of resistant genotypes in native forests, and 5) to better understand the nature of induced resistance to pitch canker.

1) Development of resistant trees

We are working with CDF, the U.S. Forest Service, the Pebble Beach Company and the City of Carmel to identify and propagate genetically resistant trees. This includes the development of clonal hedges for producing rooted cuttings of known resistant individuals, and determining the frequency of resistance in seedlings originating from seed produced by resistant trees. We are also testing the potential for insects to overcome resistance indicated by mechanical inoculation.

2) Inheritance of resistance to pitch canker

Utilizing both standing trees and seed from various sources, experiments have been designed to reveal the genetic basis of resistance to pitch canker. The results of these studies will reveal whether or not resistance is conferred by multiple genes, as available data suggests, and/or if major gene resistance is involved. We will also gain insight into possible differences between native populations with respect to the frequency of resistance.

3) Effects of exotic strains and environmental conditions on resistance

Preliminary testing under growth chamber conditions has shown that strains originating from other parts of the world, such as Florida and Mexico can be more virulent than strains already resident in California. Further testing of such strains is planned to determine the extent of this variation and what limits it may place on the durability of genetic resistance to pitch canker under California conditions. Work already completed has shown that temperature is an important influence on the rate of pathogen development on susceptible trees. Experiments are planned to determine how this factor may influence the utility of resistant genotypes in landscape settings.

4) Monterey pine ecotypes

In the interest of maintaining the native genetic structure of Monterey forests, reforestation with resistant trees should use appropriate genetic stock. Unfortunately, only very limited information is available on the nature of the genetic variation within individual populations of Monterey pine, making it difficult to define limits for the use of a given genotype. For this reason we have undertaken a study of the distribution of genetic variation on the Monterey Peninsula by collecting seed from trees on all the geomorphic surfaces which support Monterey pine stands. Representatives from all of these collections (280 parent trees) have been established in five common garden sites on the Monterey Peninsula. The results of this study will provide guidelines for the deployment of pitch canker resistant trees in the native forest. The California Department of Transportation is providing funding for this work and the Pebble Beach Company and the Del Monte Forest Foundation have donated land for the experimental plantings.

5) Induced resistance to pitch canker

Our research has shown that trees repeatedly inoculated with the pitch canker pathogen tend to become less susceptible over time. Furthermore, recent observations in long term monitoring plots show evidence of disease remission, which may reflect natural occurrence of the same phenomenon. Experiments are in progress to determine whether or not induced resistance can be manipulated to provide a means of protecting susceptible trees. In these experiments, both planted and native trees are being treated to determine if their susceptibility to pitch canker can be significantly reduced. Possible applications of this technique would include protection of valuable landscape trees and disease management in Christmas tree farm operations.

B) Regeneration

Because naturally occurring genetic resistance is found in the native populations of Monterey pine, susceptible trees should eventually be replaced by those that are more resistant. Of course, this will require regeneration, something that fire suppression has rendered problematic in many areas. To address this concern, studies have been initiated to test the effects of prescribed burning and other forms of removing under-story vegetation on seedling establishment and the incidence of pitch canker-induced seedling mortality.

IV Management of infested material

The extensive tree mortality caused by pitch canker in several coastal counties has generated a great volume of infested material, which must be processed in a way that does not spread the disease. Our research has focused on the handling of two classes of material: branches of various size classes that are suitable for chipping and tree trunks that could be cut for firewood or other uses. For branches we have collected several years of data, which documents that chipping immediately eliminates most of the insects associated with this material. The pathogen however survives for over a year in chipped wood. Thus it remains prudent to uses chipped wood only in an already infested area. Our research has shown that composting is an effective method for eliminating the pathogen from chipped wood. Continuous exposure to temperatures at or above 50 C (122 F) for ten days is sufficient for this purpose.

For logs we have studied the time course of insect departure after cutting and survival of the pathogen. As with the chipped wood, the pathogen survives very well, being recoverable for over one year after the tree is cut. Large numbers of insects emerge from logs during the first four months after they are cut but some emergence continues for up to one year. These studies are continuing to determine the end point of insect emergence and pathogen survival. Presently we regard one year as a preferred minimum period for a log to remain in-place before movement out of the zone of infestation. To minimize the risk of the pathogen on the surface of logs being dispersed when the wood is moved, any of several registered fungicidal materials can be applied. Our tests show that some of these materials are effective, in that the fungus is not recoverable from the surface of the logs after the treatment. Debarking can be beneficial as well but fungicidal treatment is still required to eliminate the pathogen on the surface of the logs.

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