|Benson, D. M. - N.C. STATE UNIV.|
|Grand, L. F. - N. C. STATE UNIV.|
|Vernia, C. S. - N. C. STATE UNIV.|
Submitted to: Plant Disease
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: September 15, 2005
Publication Date: May 24, 2006
Citation: Benson, D., Grand, L., Vernia, C., Gottwald, T.R. 2006. Temporal and spatial epidemiology of phytophthora root rot in fraser fir plantations. Plant Disease. 90:1171-1180. Interpretive Summary: Frasier Fir is an important holiday crop for the Carolinas however a soil fungus known as Phytophthora can cause root rot and kill fir trees, severely limit production and cause losses in fir plantations. Trees are grown for multiple years, and can become infected and die at any time up to the time they are cut for sale. This study examined the dynamics and mechanisms for spread of the disease in commercial plantations in an initial attempt to gain a better understanding of the disease. The dynamics were tied to both sources of the disease internally in the plots and from incursion of the disease from outside sources. The data and interpretation will be used to help develop future disease control and management strategies for Phytophthora disease in Frasier fir.
Technical Abstract: In 1999, 19 plots of Fraser fir (Abies fraseri) with a disease focus were established in commercial plantings grown for Christmas tree production in the mountains of five western North Carolina counties. Progress of Phytophthora root rot caused by Phytophthora cinnamomi as estimated by mortality was followed in each plot over 3 to 4 years in an attempt to understand dispersal of inoculum. Slope, aspect, and field production age at the time plots were established were recorded. Rainfall estimated from National Weather Service stations each growing season also was recorded. The relationship of site parameters and rainfall to dispersal and disease was investigated. Disease incidence and mortality were assessed in June and September each year for 3 or 4 yr depending on plot. Phytophthora root rot as estimated by mortality counts over time in a logistic regression model progressed in only five of 19 plots over 3 years. None of the site parameters correlated with mortality data, although slightly more disease was found in plots with a north aspect. Rainfall was below normal in the 3 yr of the study and did not correlate with mortality in any year. Lack of disease progress in the majority of plots was attributed to drought conditions in the region. In the five plots were mortality increased over time, spatial analysis suggested an aggregated pattern of diseased plants. Aggregation was apparent but not very strong among nearest neighbors, but was considerably stronger among groups of trees within a local area. This aggregation within groups was stronger when larger group sizes were examined by beta-binomial analysis. A spatial analysis by distance indices method (SADIE) indicated the presence of secondary clusters occurring several meters away from the main focus. A stochastic model also was employed that indicated a combination of spatial processes were likely involved, specifically a tendency toward spread within a local area, but not necessarily to nearest neighboring trees, combined with an influence of background inoculum that could not be accounted for within local areas and may have come from external sources. Thus, all sources of inoculum including infected planting stock, inoculum in soil, infected trees, and contaminated equipment were equally important in epidemics of Phytophthora root rot in Fraser fir and dispersal of P. cinnamomi.