32 Blaedow and Juzwik: Distribution of C. fagacearum in Roots and Root Grafts Slow movement of the pathogen into the root system during wilting or following tree death could be explained by the necessi- ty for spread via vegetative growth. In other cases the pathogen is found in roots more rapidly than could be accounted for by veg- etative growth (even at the earliest stages of incipient wilt); it has been suggested the negative water potential generated by healthy, actively transpiring trees grafted to the wilting tree may draw pathogen propagules from the diseased tree into the root system and across root grafts (Nair and Kuntz 1975). Rapid movement into a few grafted roots, as opposed to colonization of the entire root system via vegetative growth, could explain the sporadic dis- tribution of the pathogen in primary roots of diseased or dead oaks. The study authors did not attempt isolations from all primary roots, so it is possible pathogen distribution in the root system is more widespread than the results indicate. The distribution of C. fagacearum observed in this investigation suggests colonization of the root system in wilting or wilted trees is limited however. In dead trees, absence of the pathogen near the root collar may be due to displacement of C. fagacearum by more efficient sapro- phytes. Presence of the pathogen did increase with increasing dis- tance from the root collar in the dead trees, likely because moisture availability and temperature are more suitable for pathogen sur- vival with increasing distance from the soil surface (Tainter 1995). While there was no attempt to examine the entire root system of the study trees, the frequency of inter-tree grafting, particularly between trees in close proximity to one another (< 5 m), in sandy soils, was surprisingly low. Large portions of the root systems of most excavated trees appeared to be distributed well below 1 m of the soil surface (the maximum depth examined in this study). Inter-tree grafting in the upper 1 m of soil was a relatively infrequent phenomenon when compared to the incidence of self- grafting. However, inter-tree graft occurrence is likely frequent enough to account for the rate of expansion of disease centers observed in sandy soils. In North Carolina, U.S., Boyce (1960) was only able to locate three inter-tree root grafts between 42 wilting oaks and proximal wilt-killed trees, and only one of these grafts was shown to be the pathway for infection. Yet, the author noted that 96% of wilting oaks in the state were within 15 m (50 ft) of wilt-killed trees, suggesting that root graft transmission was the primary mode of infection. In the state of Missouri, a slightly higher percentage of black oaks (16%) and white oaks (20%) were found to be grafted to other trees (Jones and Par- tridge 1961). Parmeter et al. (1956) reported over 70% of northern pin oaks in stands on deep sand soils in central Wisconsin were grafted. Therefore, it is likely that inter-tree grafts on sites such as those used in this study may exist below the range of the excava- tion equipment, as well as that of vibratory plows and trenchers commonly used for the installation of root graft barrier lines. Self grafts, while rarely discussed in the literature, may play an important role in pathogen transmission in the root systems of oaks. In this investigation, the results of isolations from self grafts demonstrate that the pathogen is able to readily move from root to root through self grafts. Therefore, lateral spread of C. fagacear- um from one root to another may not require pathogen movement into the root collar zone, but could occur through networks of self grafts. Such a mechanism of pathogen movement could allow the fungus to spread throughout large portions of the root system while bypassing the root collar zone where systemic chemical treatments are applied. It was previously believed that transloca- tion of propiconazole into roots following injection is limited and ©2010 International Society of Arboriculture does not prevent infection via root grafts (Appel 2001). Recently however, high concentrations of propiconazole in primary roots following macro-infusion treatment have been observed (Blaedow 2009). Substantial acropetal transport of propiconazole from the point of injection into the upper portion of the tree seems to prevent wilt development in infected trees. If systemic fungicides such as propiconazole move downward into the roots (even short distanc- es) following injection, it could provide the added benefit of lim- iting latent root system colonization in infected trees because of the prevalence of self grafts within 1 m of the root collar. Xylem- mobile dyes have been found to move downward into xylem of woody roots of red oak and eight other tree species (Tattar 2009). CONCLUSIONS The results of this study reveal the sporadic and unpredictable distribution of C. fagacearum in the root systems of wilted and wilting trees. Based on these results, distribution of the oak wilt pathogen in the root systems of diseased or neighboring asymp- tomatic trees cannot reliably be determined by assessment of aboveground symptoms. Current control approaches assume the pathogen is present in all roots at the time of symptom appear- ance, and call for conservative placement of root graft barriers and preventative chemical treatments that account for latent in- fections are warranted. The frequent inability to isolate C. fa- gacearum from the root system of diseased trees, even from trees that had died the previous growing season, suggests pathogen movement into and subsequent colonization of the root system may take months or years following tree death in most cases. Even when efforts are made to prevent root graft transmis- sion, and connected root systems between diseased and asymp- tomatic trees are severed, it may be possible for latent infections in asymptomatic trees result in pathogen movement beyond vi- bratory plow lines or trenches, enabling disease development outside such zones. Chemical treatments, such as propiconazole, which do not prevent root graft transmission but prevent dis- ease development (Blaedow 2009), may allow latent coloniza- tion of root systems and even transmission to neighboring trees. Following the period of protection provided by chemical treat- ments, generally less than two years, disease development may be initiated as a result of masked infections. However, propicon- azole translocation into roots could limit pathogen colonization of large portions of the root system and spread to uninfected trees if distribution of the fungicidal compound is sufficient to prevent pathogen spread out of inter-tree grafted roots contain- ing the invading pathogen into self grafts and the root collar. Acknowledgments. This project was funded by several sources: Tree Research and Education Endowment Fund, Rainbow Treecare Scientific Advancements, the U.S. Forest Service Pesticide Impact and Assess- ment Program, and the U.S. Forest Service North Central Research Station. We thank Paul Castillo for excellent field support and Dr. Robert Blanchette, Dr. William Chaney, and Dr. Richard Zeyen for review of the manuscript. We also thank Dr. Joseph O’Brien for review and discussion on earlier versions of the manuscript. The paper is based on a portion of a Ph.D. Thesis by the first author.
January 2010
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