In vitro expression of resistance responses to Seiridium species in micropropagated shoots of Cupressus sempervirens and Chamaecyparis lawsoniana

Wounded and nonwounded micropropagated shoots of Cupressus sempervirens and Chamaecyparis lawsoniana were inoculated in vitro with the canker-causing pathogens Seiridium cardinale (Wag.) Sutton & Gibson, Seiridium cupressi (Guba) Boeswinkel and Seiridium unicorne (Cke & Ell.) Sutton. Seiridium cardinale was significantly more pathogenic on Cupressus sempervirens than on Chamaecyparis lawsoniana (Murr.) Parlatore, irrespective of the presence of wounds on the shoots. On wounded shoots, both S. cupressi and S. unicorne caused significantly larger lesions on Chamaecyparis lawsoniana than on Cupressus sempervirens by 20 days after inoculation. Superficial wounding of shoots prior to inoculation caused a significant increase in the lengths of lesions and numbers of shoots girdled by the pathogens on both hosts. These results broadly correlate with known virulence of the three pathogens on these two host tree species in field and glasshouse tests. Using histological methods, penetration of fungal hyphae through stomatal pores of both shoots and leaves into the substomatal cavity and the mesophyll space was observed. Penetration directly through the cuticle was also seen. Defence-related responses, including accumulation of oxidized polyphenolic compounds and deposition of lignin and suberin in cell walls, were detected in inoculated tissues. These responses occurred predominantly in the epidermis, including stomatal guard cells, and the hypodermis and were particularly marked in Chamaecyparis lawsoniana inoculated with S. cardinale. The possible utility of these methods in the study and detection of host genotypes resistant to Seiridium spp. is discussed.