Arboriculture & Urban Forestry 36(1): January 2010 barrier line. Pathogen distribution in the root system of trees in or near disease centers cannot be determined reliably based on symptom development above ground, so “primary” lines are in- stalled to contain the pathogen effectively and “secondary” lines are placed within the primary line in an attempt to save healthy oaks in close proximity to wilting ones (French and Juzwik 1999). Systemic fungicides such as propiconazole have been used to protect oaks in situations where, for example, a vibratory plow cannot be used to mechanically disrupt root grafts (Appel 2001). Propiconazole applications also provide additional protection for high value asymptomatic trees within root graft barrier lines (Juz- wik et al. 2004). However, propiconazole has not been shown to prevent root graft transmission; rather it is believed to prevent disease development through an, as yet, undetermined means. Management of oak wilt via control of root graft transmission of C. fagacearum has the potential to be more efficacious should managers achieve a more thorough understanding of pathogen movement into and colonization of the root system, because treatments are directed at preventing infection of healthy trees that generally do not show symptoms immediately following root infection. These latent infections are a substantial obstacle to oak wilt management. They can lead to costly treatment fail- ures, for example, in situations where the pathogen has already spread beyond root graft barrier lines at the time of treatment or when disease development occurs in chemically-treated oaks following fungicide degradation, and can put additional trees at risk that are normally not considered in management regimes. The spatial distribution of C. fagacearum in the near-surface roots of wilted or wilting northern red oaks and proximal asymp- tomatic northern red oaks was investigated in east-central Minne- sota. Field studies involving pairs of oak wilt symptomatic and as- ymptomatic trees were conducted in 2005 and 2006 to determine: 1) the presence of the pathogen in primary roots (roots originat- ing directly from the root collar) in wilting trees, 2) the frequency of inter-tree grafts and self grafts (grafts between two roots of the same tree) within 1 m (3.3 ft) of the soil surface, and 3) the pres- ence of the pathogen in roots joined in functional root grafts. A pre- liminary report has been published (Blaedow and Juzwik 2007). MATERIALS AND METHODS Experimental Design The field study was conducted at 12 sites, each with an active disease center, within the Carlos Avery Wildlife Refuge in Chis- ago County in east-central Minnesota. All sites were located on Zimmerman loamy sand or Sartell fine sand, and northern red stage (percent wilt) Asymptomatic 20 - 40 41 - 85 86 - 99 Dead TOTAL Number of trees sampled 12 2 4 4 2 24 taken at 0.3 m (1 ft) intervals from the root collar. y Number of primary roots z y Assayed 48 8 16 16 8 96 4 6 4 2 18 z Four primary roots were sampled from each northern red oak, and the incidence of C. fagacearum was determined by isolation from four 10 cm (3.9 in) long sub-samples The number of primary roots yielding C. fagacearum was similar for all crown wilt stages (P = 0.2874). ©2010 International Society of Arboriculture 29 oak was the predominant species. Wilt development of individual northern red oaks in disease centers was monitored between 2004 and 2006. A pair of trees consisting of one diseased tree and the nearest asymptomatic tree were selected from each disease center during this time period. The diseased trees were suspected to have been infected via root graft transmission of the pathogen because of their close proximity (< 5 m; 16.4 ft) to actively wilting or re- cently wilted trees. The extent of pathogen distribution in the root system of wilting trees and the incidence of root graft transmission to the neighboring asymptomatic tree was expected to increase with increasing symptom progression. Therefore a minimum of two diseased trees from each of the following stages of crown wilt (percent of crown displaying symptomatic foliage) were select- ed for this study: 1) 20%–40% crown wilt, 2) 41%–85% crown wilt, 3) 86%–99% crown wilt, and 4) 100% crown wilt for 12–14 months (dead). The diameter at breast height (dbh) of selected trees ranged from 12.7–63.5 cm (5–25 in); average dbh was 31.2 cm (12.3 in) (Table 1). Distance between trees within a pair ranged from 0.4–4.6 m (1.3–15.1 ft); average distance was 2.6 m (8.5 ft). Root Excavation and Sampling Partial root system excavation of each of the 24 selected trees (12 pairs of diseased and proximal asymptomatic trees) was con- ducted with an air excavation tool (Airspade® , Guardair Corp, Chicopee, MA). After removing approximately the upper 15 cm (6 in) of soil containing fine roots and understory plants, the un- derlying mineral soil was removed while leaving the root system of study trees intact and undamaged. At the selected field sites, the roots of the red oaks had frequently grown at a high angle of descent, beyond one meter below grade. However, the maxi- mum excavation depth that could be efficiently achieved in the sandy soil with the excavation tool was approximately one meter. One primary root in each quadrant of the four cardinal direc- tions was sampled to investigate the spatial distribution of the pathogen in each tree. Root segments approximately 10 cm (3.9 in) in length were removed from each primary root at 0.3, 0.6, 0.9, and 1.2 m (1, 2, 3, and 3.9 ft) from the root collar. The inci- dence of root grafting between pairs of trees was also examined at each site. All soil was removed from an initial search area 3 m (9.8 ft) wide that spanned the distance between the wilting and asymptomatic tree to a depth of 0.6 m (2 ft). Each root exposed in the initial search area was examined beginning at the root col- lar of the originating tree until one of the following conditions were met: 1) the root grafted to the root of a neighboring tree, 2) the root descended to a depth greater than 1 m below grade, 3) the root diameter tapered to less than 1 cm (0.4 in), or 4) the Table 1. Numbers of primary roots yielding Ceratocystis fagacearum by crown condition of sampled northern red oak. Crown wilt Yielding fungus 2 Number of primary roots per tree yielding Ceratocystis fagacearum Mean 0.2 Maximum 2 2 1.5 1 1 2 4 2 2
January 2010
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