The root insect-- black-stain root disease association in Douglas-fir : vector relationships and implications for forest management

by Witcosky, Jeffrey John

Verticicladiella wageneri Kendrick is a vascular wilt

pathogen of conifers, causing a black staining of

colonized sapwood of roots and lower stem. In Douglas-fir,

V. wageneri is intimately associated with insects.

Hylastes nigrinus, Pissodes fasciatus, and Steremnuis

carinatus are commonly associated with diseased hosts,

carry inoculum of V. wageneri in the field, successfully

transmit the pathogen to seedlings under laboratory

conditions, and create suitable infection courts in

susceptible hosts. Furthermore, insect-mediated

transmission of V. wageneri has been documented tor the

first time.

Stand density management, such as precommercial

thinning, results in elevated activity of H. nigrinu, P.

fasciatus, and S. carinatus in disturbed stands. Insects

colonize roots and the root collar region of cut trees;

these hosts are susceptible to infection by V. waqeneri.

Also, crop trees are wounded on the roots and root collar

region by H. nigrinus for one to two years following

precommercial thinning. Some of these wounds penetrate to

the xylem and are, therefore, suitable infection courts

for V. wageneri. Time of precommercial thinning can be

manipulated to significantly reduce immigration of

vectors, i.e., by thinning plantations during early summer

after the peak flight of H. nigrinus.

H. nigrinius and S. carinatus are attracted to alpha-pinene,

a major constituent of Douglas-fir oleoresin.

Forest management activities that injure hosts, and hence

cause release of alpha-pinene, may attract vectors of V.

wageneri. H. niqrinus and S. carinatus also are attracted

to ethanol. In addition, root sections infected with V.

wageneri are more attractive to H. nigrinus and S.

carinatus than uninfected roots. Aspects of injury and

stress to hosts leading to the release of host attractants

are discussed.

A crop production/pest management system structure is

developed which links pest management activities for

black-stain root disease prevention with normal intensive

forest management. Pest management should be addressed at

all stages of forest management: the harvest-establishment, annual, precommercial, and commercial

phases of crop production.