John Pinkerton
Understanding the availability and movement of A. anomala ascospores is important to determine when and where the disease is likely to spread. Discharge of ascospores starts with the fall rains and continues through the following spring. Early research concluded that ascospores were dispersed in splash droplets from rain hitting on the cankers. However, ascospores are transported only short distances with splash and this mechanism can not explain the long distance disease spread observed.
Ascospores of A. anomala are released when stromata on the surface of hazelnut branches are wet from rain, but not from dew. Release of ascospores stopped after branch surfaces dried. The duration of free moisture on branch surfaces regulates the initiation and rate of ascospore release, but no significant effects of temperature, relative humidity, wind, or light on ascospore release were apparent. Most (>90%) ascospores were captured during precipitation events that exceeded 20 hours in duration. This represented about 10% of the total precipitation events each season.
Hourly Rate
With the onset of precipitation, the hourly rate of ascospore capture increased until the 5th hour of rain. It remained relatively constant between the 5th and 12th hours, and then declined gradually. The likelihood and rates of ascospore release associated with precipitation were highest at bud break and then declined through April and May until early June when the reserve of ascospores in the perithecia was depleted. Large numbers of ascospores were captured in special spore traps indicating that ascospores may be commonly dispersed long distances on air currents, as well as locally by splash dispersal within the canopy as reported previously.
Maturation
Spore maturation begins in late summer; by January, > 90% of spores are morphologically mature. Similarly, both the number of mature ascospores per perithecium and the proportion of ascospores that germinated, increased through autumn. After January, the number of spores per perithecium declined until May, when few viable spores remained.
Patterns of Release
Three patterns for the seasonal release of A. anomala ascospores have been observed: in the 1988-1989 season, > 80% of the seasonal ascospore release occurred between September and January; in 1989-1990, 32 to 42% of the seasonal ascospore release occurred after mid-April; and in the other 4 years, monthly releases of ascospores were relatively uniform over the 9-mo, seasonal period. Timing and amount of precipitation were the most important variables accounting for the differences among the yearly patterns of ascospore release. Over all years and sites, the cumulative proportion of total ascospores collected in each orchard was highly correlated with cumulative precipitation. This relationship was confirmed in mist chamber experiments.
Surveys of hazelnut orchards showed that trees infected with eastern filbert blight typically were to the north or northeast from the point of disease introduction. Studies in which 1-year-old Ennis trees were placed at 33 foot intervals to the north and south of diseased orchards confirmed that disease spread to the south is uncommon. The incidence of cankers on trap trees did not differ with distance from the orchard. In other studies, ascospores of A. anomala moved rapidly up and away from the tree canopy. These data suggest that ascospores can be carried long distances by wind.
The directional spread of eastern filbert blight to the north can be explained by the prevailing wind patterns during major rainstorms when most of the ascospores are released. Hourly weather records from Portland and Salem were evaluated for wind patterns. The majority of major rainstorms in the Willamette Valley were accompanied by winds from south or southwest. Major rainstorms rarely moved from the north to the south. In the Portland area, storms from the east also were common which explained the establishment of eastern filbert blight in the hills south of the Tualatin Valley early in the epidemic.