DANGEROUS INTERNATIONAL FOREST TREE DISEASES 87 



out the crown. Generally, bronzing is less pronounced and defoliation 

 is reduced in diseased white oaks. Streaking commonly occurs in the 

 outer sapwood vessels of white oaks, but is less frequently found in red 

 oaks. 



The oak wilt fungus produces abundant endoconidia in gray or tan 

 mats formed beneath the bark of infected dying or dead trees. Mat 

 production varies widely within the range of the disease and is more 

 abundant on red than on white oaks. Endoconidia are 1-celled, hya- 

 line, cylindrical, 2-4.5 X 4-22/u, and are produced in slightly tapering 

 hyaline to brown conidiophores. Cushion-shaped structures, called 

 pressure pads, are usually formed in mat centers and exert sufficient 

 pressure to raise and often split the bark, exposing the fruiting surface 

 of the mat. 



Since this fungus is heterothallic, perithelia are produced only when 

 exposed mats are spermatized with an opposite compatibility type. 

 Spermatization is accomplished by insects which happen to visit both 

 types and accidentally transfer conidia of one type to the mat of an- 

 other. Both compatibility types, A and B, are common in nature, but 

 do not often occur in the same tree. 



The principal features of the perfect stage, adapted from the origi- 

 nal description are as follows: Perithecia flask-shaped, black with 

 spheroidal base, 240-380/a in diameter, largely embedded in the 

 subiculum, appearing in 7 to 10 days in culture inoculated with com- 

 patible isolates: walls membranous to leathery; usually 1, rarely 2, 

 erect beaks, 250-450/u long, black, terminated by a fringe of hyaline 

 filaments; asci 8-spored, evanescent; ascospores hyaline, 1-celled, 

 elliptical and slightly curved, 2-3 X 5-10/x, collecting at the ostiole in 

 a sticky, creamy- white mass; hermaphroditic, self -sterile but cross- 

 fertile. 



Approximately one out of every two infections spreads locally, while 

 the remaining infections, usually single trees, die without further 

 spread. Some of the local spread is known to occur through root grafts 

 connecting diseased and healthy trees. Other local spread, as well as 

 long distance spread, occurs overland. Overland spread probably 

 takes place by means of insects, but evidence is lacking on the relative 

 importance of specific vectors. However, the mats produced on wilt- 

 killed trees have a fruity odor and are attractive to insects. 



Sap-feeding nitidulid beetles, which commonly visit these trees, be- 

 come heavily contaminated with spores and can transmit the fungus 

 when they feed on the sap of fresh wounds on healthy trees. Also, bark 

 beetles (Pseudopityophthorus spp.) and borers (Agrilus bUineatus 

 and Graphisurus fasciatus) reared on wilt -killed trees on which no 

 mats occurred were found to be contaminated with spores. In limited 

 tests under caged conditions, contaminated bark beetles were further 

 shown to transmit the disease to healthy trees through their feeding 

 wounds. However, the role these insects play in the natural spread of 

 the disease is, as yet, unknown. The fungus does not appear to be 

 ideally suited to wind dissemination. 



Based on the slow rate of spread and the generally negligible losses 

 experienced since the disease was identified almost 20 years ago, 

 there is hope that the disease will never develop epiphytotic propor- 

 tions. Nevertheless, oak wilt is well established in the natural range 

 of oak, and has caused extensive damage in a few localized areas. 

 Furthermore, oaks are species of high value, comprising one-third of 



