Black-Stain Root Disease

Black-stain root disease is a vascular wilt restricted to conifers of western North America. It infects roots and can move some distance up the base of the stem. It has become increasingly damaging since its discovery, likely because it is favored by forest disturbance.  It is the only vascular wilt known among conifers.

Hosts

Various western conifers in three distinct host groups are susceptible: piñons, hard pines, and Douglas-fir (see table). In most areas, other tree species are seldom infected ​[15]​.  However, in British Columbia, the hard pine variant frequently infects Pinus monticola, Picea glauca, and P. engelmannii, which are generally minor components in stands of the primary host, Pinus contorta ​[26]​.

Pathogen

The pathogen is Leptographium wageneri, an ascomycete closely related to Ophiostoma stain fungi. Thus far no sexual fruiting has been found with certainty.

Do not confuse this pathogen with blue-stain fungi.  Yes, it is closely related to many blue-stain fungi, and like them, it has dark hyphae and is vectored by insects. But unlike them, it grows almost exclusively in tracheids rather than in ray parenchyma and epithelial cells of resin canals as blue-stain fungi do ​[2, 30]​.  This results in completely different patterns of stain development in hosts, and very different types of pathogenesis.  See Pathogenesis below for more details.

The pathogen is almost certainly insect vectored for long-distance dispersal, but it grows root-to-root for secondary spread as many root pathogens do.

Three types have been described as varieties or subspecies. The hard pine variety differs in cultural and conidiophore morphology from the other two ​[11, 30]​. The piñon and Douglas-fir varieties differ in colony morphology and allozymes ​[11]​. But most importantly, the three varieties are host-specialized ​[2, 9–11]​.

Environment

Site and stand factors

For the piñon variant, a clear association of disease with high site quality, deep soils, relatively cool and moist environment, relatively high elevation, and high stand density was established ​[23]​.  These characteristics are generally associated with extensive root development, which would enhance likelihood of secondary spread.  They may also be related to conditions that are too hot and dry on poor sites.

Also in piñon, initiation of waves of disease is associated with well-distributed summer rains, maintaining good soil moisture levels through the season ​[30]​.  Such conditions could favor disease by reducing soil temperature, increasing vector activity, or both.

Similarly, greenhouse inoculations of P. ponderosa at 5 moisture levels resulted in a clear trend of infection, which was greatest at the highest moisture level ​[5]​.  Field studies also suggested highest levels of infection associated with wet or poorly drained soils and high organic matter ​[2]​.  Inoculation success with the hard pine variant was best at 16 C ​[10, 29]​.  21 C resulted in less success, and higher temperatures prevented infection altogether.  The other variants were somewhat more tolerant of high temperatures.

Disturbance

The apparent increase in disease incidence since the disease was recognized is likely related in part to the role of disturbance in favoring disease.  Disturbance effects may be especially strong for the Douglas-fir variant, but probably exist for all.

Black-stain root disease in Douglas-fir was highly concentrated along roads in comparison to the forest at large ​[7]​.  In an extensive survey of Oregon and Washington, 80% of infection centers occurred in disturbed stands ​[6]​.  Soil disturbance alone can have the effect, but tree wounding and cutting also contribute. Precommercial thinning often leads to disease because vectors are attracted to fresh stumps and soil disturbance ​[12, 13, 16, 33]​.

Vectors are most active in May.  Thinning after the flight period results in the least stimulation; thinning before or during appears to attract the most vectors ​[8, 33]​.

Similar observations have been made in ponderosa pine, but the lower number of new centers makes comparisons difficult ​[2]​.

Symptoms

Tree growth decreases progressively for up to 8 years. As their end nears, trees develop thin, chlorotic crowns, make no growth, and are attacked by bark beetles or borers.

If you cut into roots and the lower stem of a dead tree or one with a symptomatic crown, you will be thrilled to find dramatic streaks of stain. The color varies depending on moisture content, resin soaking, and whether the cambium is alive. It may be intense black, dull or grey-black, to brown or purple-brown ​[30]​.  A diagnostic feature is that stain tends to be crescent shaped in cross-section, following the annual rings.

In large Pseudotsuga menziesii, and depending on temperatures and how long the tree can survive, stain may reach up to 15 m in the stem above ground ​[2]​.  In hard pines, stain may reach 3-5 m above ground.  In the piñons, it generally reaches less than 1 m ​[30]​.

The pathogen can be transmitted to neighboring trees at root contacts or grafts. In the case of Douglas-fir, weevils may also vector the disease locally. Trees thus die in expanding disease centers showing a chronosequence of mortality, as many root diseases do.

Disease Cycle

Primary infection, that which initiates a disease center, generally begins with an insect carrying the fungus (although this has not yet been demonstrated for the piñon variety).  Vectoring is treated in detail under Vectors below.

Because vectors may be attracted to a local area by disturbance and oleoresin components, it is not unlikely that a group of trees becomes infected via vectors at about the same time.  Even if not, secondary infections will often result in a growing disease center.  Transmission takes place by growth of the pathogen across root contacts or grafts ​[2, 23, 30]​, or even short distances through soil ​[3, 17]​.  Evidence suggests that minor wounds in fine roots are the primary infection court for secondary infection.

In this way, a disease center develops and expands.  Most active centers grow radially about 1 m yr^-1 or more.  Estimates include:

• 4 plots in Pinus monophylla or P. edulis over 6 yr, radial growth average 2.0 m yr^-1 ​​[30]​​
• 30 centers in P. edulis, by dendrochronology, average 1.1 m yr^-1 ​​[22]​​
• 52 P. ponderosa centers over 11-15 yr, radial growth 0 to 7 m yr^-1, average 1.0 m yr^-1 ​​[2]​​
• 10-30 yr-old Pseudotsuga menziesii over 5 yr, average 1.2 m yr^-1 ​​[8]​​
• 20-yr-old P. menziesii (measured in roots), average 2.2 m yr^-1 ​​[18]​​

Most centers become inactive at some point, in some cases after only a few trees are killed.  Meanwhile, vectors are reproducing in diseased trees, resulting in infested brood ready to initiate new centers.

There is conflicting evidence about survival of the pathogen in dead roots.  Several studies indicated that the pathogen can no longer be isolated a few months to a year after the host dies ​[3, 20, 29, 30]​.  However, in another study the pathogen was regularly isolated from piñon roots dead for 5–8 years and once from a root dead for 16 years ​[22]​. Moreover, seedlings are often killed around stumps of infected trees.  A study of planted Douglas-fir seedlings showed mortality at least 4 years after logging, and the fungus could be isolated after 3 years .

Pathogenesis

The key to understanding how unique this disease is, and how it attacks the host, is the anatomy of pathogen growth and the resulting pattern of stain.  It was recognized early that the disease differed markedly from blue-stain in that the pathogen develops exclusively in the axial tracheids.  These cells have no cytoplasm, no living contents.  They provide the dual functions of structural strength and water conduction.  Since L. wageneri is not a wood-decay fungus and cannot degrade the tracheid walls, one can immediately hypothesize that the disease involves the other function: water conduction.

Contrast this with blue-stain fungi, or similar fungi in any genus ​[2]​.  Such fungi invariable colonize the cells in wood that are alive.  In conifers this means ray parenchyma and the epithelial cells that make the walls of resin canals.  Those cells are colonized and killed, killing the sapwood and preventing any active response to invasion.

These distinct growth patterns result in distinct patterns of resulting stain.  Since it grows in axial tracheids, the black-stain pathogen grows most rapidly in the axial direction, can also spread tangentially along the annual ring, and grows most slowly in the radial direction (crossing between annual rings.  The result is, when you make tangential cuts along the lower stem or a root, you see long axial streaks of stain that are relatively narrow.  In cross section, you see arcs of stain following annual rings, and only occasionally crossing them.  This is the opposite of blue-stain fungi, which grow rapidly along the rays, forming a wedge-shaped stain pattern in cross-section.

Black-stain root disease is considered a wilt disease ​[2]​ because the pathogen:

• colonizes only actively conducting xylem tracheids
• penetrates bordered pit-pairs and does not decompose or penetrate mature cell walls
• induces occasional tyloses that protrude from parenchyma cells of an adjacent ray
• has large, sheathed hyphae that almost fill the tracheid lumina and give the appearance of dark occlusions in the outer growth rings
• in seedlings and saplings, it can become systemic even to the point of invading xylem of young needles ​​[29]​​.

Within a month of inoculation, P. ponderosa seedlings experience dramatic decreases in transpiration, closure of stomata, and decreases in photosynthesis ​[14]​.  Living host cells are not killed ​[19]​.  Toxins may add to the effect of physical occlusion of water conduction ​[2]​, as do gums produced by the host ​[19]​.

Distribution and Damage

The piñon variant is generally distributed in the range of piñons in the southwestern United States.  This is primarily southern California, Nevada, Utah and Colorado ​[25]​, with some also in southern Idaho and northeastern Arizona ​[30]​ and northwestern New Mexico ​[31]​.  In Colorado, it is restricted to the western side of the Continental Divide, although there are quite similar piñon-juniper stands on the east side ​[23]​.

The Douglas-fir variant is mostly an issue in California and western Oregon, but the range extends north through western Washington ​[25]​ into southern coastal British Columbia, including all of Vancouver Island ​[26]​.  More recently it was found also scattered through the southern interior of British Columbia ​[21]​.  It also occurs in eastern Oregon, northern Idaho and western Montana ​[25]​, and apparently in one known location each in the Front Range of Colorado ​[11]​ and in New Mexico ​[25]​.

The hard pine variant is most abundant in California and eastern Oregon, but it also occurs in southern Nevada, western Oregon, the northern Rocky Mountains in northern Idaho and Montana ​[25]​, extending north to south-central interior British Columbia ​[21, 26]​.

Other Issues

Although the disease was first discovered in 1938, it was not reported until 1960 (on Pinus contorta and P. strobus in Montana) and 1961 (on P. ponderosa, P. jeffreyi, and P. monophylla in California and on P. edulis in Colorado and Arizona) ​[2, 24, 30]​.  The relatively recent discovery and apparent increase in disease incidence often suggests an introduced, non-native pathogen, but that is not the case here ​[2]​.  The disease, or anything like it, has not been reported outside western North America.  Furthermore, if it were non-native, one would have to posit the separate introduction of all three varieties.

Vectors

The spores of L. wageneri are produced in a sticky drop under the bark, mostly in roots.  Sticky drops are often an adaptation for dispersal by insects. It is very unlikely that these spores could be dispersed effectively without a vector.

Hylastes spp., root feeding bark beetles, appear to be the most important vectors ​[4, 12, 27, 32]​.

The following features demonstrate vectoring and enhance vector efficiency ​[8, 32]​. Vectors:

• create galleries in diseased roots.
• acquire inoculum before leaving their parent gallery when they brush against sticky spore droplets projected on conidiophores into the gallery. They then carry that inoculum when they disperse.
• visit healthy trees under conditions suitable for transmission (maturation feeding).
• transmit the fungus when feeding on healthy trees.  Maturation feeding often wounds the outer xylem, facilitating infection.

Evidence suggests that overland vectoring, resulting in primary infection, is much less common in the hard pine variant than in the Douglas-fir variant ​​[2]​​. This implies that the more effective vectoring of the Douglas-fir variant, combined with the attraction of vectors to soil disturbance, tree wounds, and cutting, makes it a much greater threat to forest management.

In Pseudotsuga menziesii, the root-feeding bark beetle Hylastes nigrinus is an important long-distance vector ​[8, 12, 32]​.  Two weevils are also known vectors, though one does not fly and can only transmit the disease locally, and the other is uncommon ​[32]​.  All three insects make galleries in diseased Douglas-fir, wound and create suitable infection courts in susceptible trees, carry inoculum in the field, and transmit the pathogen to hosts under laboratory conditions ​[32]​.

In Pinus ponderosa and P. jeffreyi, Hylastes macer is the most important vector known.  Conidiophores of the pathogen are only common in galleries of this insect, fungal colonies and stain develop in association with galleries, and the insect’s behavior is consistent with vectoring ​[4]​.  In a survey of bark beetles, H. macer carried spores of L. wageneri far more often than did other beetle species ​[27]​.

Vectoring has not been demonstrated for the piñon variant, but it is likely.  Candidate vectors have been identified in Colorado ​[1]​.

Vectors are attracted to fresh wounds and stumps, so thinning initiates disease in some stands. In pines, damage is associated with slightly wetter conditions, flats and drainages.

Management

Current approaches to management of this disease are probably not fully effective or satisfactory.  A major problem is that disturbance, including vegetation treatments, tends to increase disease incidence.

For the Douglas-fir variant, the following approaches are recommended ​[8, 15, 16, 33]​:

1. Favor nonhost tree species during any partial cutting in areas with existing infection centers, and when planting in high-risk areas.
2. Minimize soil disturbance and tree injury
3. Where feasible, use high-lead or skyline logging instead of tractor logging.
4. Where tractor logging is necessary, arrange skid trails and season of operation to avoid soil compaction.
5. Avoiding road building or other actions that result in tree injuries in plantations.
6. Where roads are necessary, remove host trees along the roads to pre-empt new centers.
7. Conduct precommercial thinning between late June and late August.

For the hard pine variant, the following approaches have been recommended ​[2, 15]​:

1. Maintain and promote a mixture of species, avoiding pure pine.
2. Minimize site and stand disturbances during partial harvests.
3. Avoid wet areas

For the piñon variant, three measures for arresting expansion of disease centers were studied in southwestern Colorado ​[28]​:  a) felling all dead and diseased trees plus the first healthy trees on the perimeter;  b) soil fumigation with Vapam, and;  c) insertion of a polyethylene barrier into a trench between healthy and diseased trees.  None were effective.  This is likely not because the measures themselves are ineffective, but because asymptomatic but infected trees were left outside the treatment area.  Trees may be infected for some time before symptoms develop or stain is even visible at the root collar ​[23]​.  Clearly a wider margin must be given to the center when attempting such control, but how wide is unknown.