Leptographium procerum (white pine root decline)

White pine root decline was first reported in eastern USA and L. procerum was later found to be consistently isolated from diseased trees (Kendrick, 1962; Dochinger, 1967). However, the role of the fungus in causing this disease has been a matter of considerable debate (Lackner and Alexander, 1982; Harrington and Cobb, 1983; Wingfield, 1983, 1986). The disease is commonly referred to as a root decline but in most cases manifests itself as a wilt (Lackner and Alexander, 1982). The symptoms of the disease are similar to those caused by infestation by the root-collar weevil (Hylobius radicis) (Wingfield, 1983), for which it can be easily mistaken.In inoculation studies using Leptographium isolated from western white pine (Pinus monticola), symptoms appeared about 25-35 days after inoculation and tree death occurred 35-40 days after inoculation (Hubert, 1953). One of the first symptoms to appear, in both inoculation studies and natural infections, is a reduction in height growth, followed by crown discoloration. A dark stain can be observed in the roots of the diseased trees, which rapidly progresses upwards in the tree (Leaphart, 1960; Alexander et al., 1988). In severe infections, marked resin exudation is observed (Alexander et al., 1988). Symptoms associated with white pine root decline include extended periods of bud break, retardation of shoot elongation, crooking of growing shoots, loss of turgour in older needles, wilting of younger needles (Hubert, 1953; Dochinger, 1967; Halambek, 1976; Pest Alert, 1977; Smith, 1991), retention of needles, needle wilt, browning of needles, and resin-soaked, black-streaked wood at the base of stems and basal cankers (Houston, 1969; Pest Alert, 1977; Towers, 1977; Anderson and Alexander, 1979; Mackenzie and Dick, 1984; Alexander et al., 1988; Smith, 1991; Otrosina et al., 1997). The basal cankers are at first circular to irregular, reddish, sunken areas, which in some cases enlarge and coalesce to form larger flattened cankers (Houston, 1969). Colonized roots are resin-soaked and cross-sections of the stems reveal prominent wedges of blue-stained wood. Discoloration of the sapwood is consistent with the patterns and physiology of blue-stain fungi (Alexander et al., 1988). In some cases the roots show diffuse black streaking (Livingston and Wingfield, 1982).L. procerum acts in a similar way to wilt pathogens. It occurs in the vascular system, where it erodes the cell walls and travels from cell to cell through the pits (Kilbertus et al., 1980). White pine infected with L. procerum showed greater vascular occlusion, lower moisture content and reduced hydraulic conductivity than non-infected trees (Butnor et al., 2000). The water in the stem is reduced, leading to desiccation of the sapwood and foliage and ultimately premature death (Butnor et al., 2000).Reduced water potential in symptomatic trees supports the notion that this root disease is associated with xylem dysfunction (Horner et al., 1987). Tree death occurs when the xylem is blocked by resin (Alexander et al., 1988). Symptoms were found to vary between Pinus strobus and P. sylvestris; it takes longer for symptoms to appear on P. sylvestris (Horner and Alexander, 1983b).Red pine decline disease is characterized by circular regions of dead and dying trees that expand gradually. The root systems of these trees show high levels of mortality and several fungi (L. terebrantis, L. procerum and Ophiostoma ips) can be isolated from stained areas (Klepzig et al., 1995; Erbilgin and Raffa, 2002).

Procerum root disease can be controlled by planting trees on sites suitable for the species, controlling weevils and bark beetles, removing slash in and around the plantation, and controlling weeds (Alexander et al., 1988). It is advisable to allow sites to lie fallow for 1 year or to consider planting non-susceptible trees (Lewis, 1985). Salmon et al. (1996) found that some natural compounds (e.g. R (-) carvone, S (-) carvone and limonin) could inhibit the germination of L. procerum conidia. However, these compounds only suppressed germination of the conidia and did not affect the growth of the fungus (Salmon et al., 1996).

White pine root decline is now known to occur in various parts of the world in a number of ecosystems, and is not restricted to forest trees (Livingston and Wingfield, 1982; Morelet, 1986; Alexander et al., 1988; Morrison and Hunt, 1988; Smith, 1991). The extent of damage associated with the disease has not been fully assessed (Towers, 1977; Meyer et al., 1983).