Screening for resistance to Seiridium canker in the Cupressaceae and vegetative propagation of cypresses
Artificial inoculations in the Cupressaceae proved that Seiridium cardinale was more virulent than S.cupressi and S.unicorne. Cupressus macrocarpa was found to be highly susceptible to Seiridium canker, C.sempervirens very susceptible, while C.torulosa and C.arizonica were moderately susceptible. Chamaecyparis lawsoniana was highly resistant to S.cardinale, but very susceptible to S.unicorne and moderately susceptible to S.cupressi. Intraspecific variation in susceptibility to S.cardinale was found in C.sempervirens. S.cupressi was more pathogenic than S.unicorne on C.macrocarpa, C.arizonica and C.torulosa, whereas it was less pathogenic on C.sempervirens. Mature bark proved to be more resistant to Seiridium canker than young bark. Low variability in pathogenicity of S.cardinale was found, with only one isolate out of eight proving to be a weaker pathogen. Histological examination of bark of cypress seedlings following infection with S.cardinale revealed the formation of strong necrophylactic periderm as an important resistance mechanism against Seiridium attack, and was particularly marked in C.lawsoniana. Strong necrophylactic periderms were detected in resistant and tolerant clones of C.sempervirens, whereas weak or a series of easily re-invaded ones were found in susceptible clones. Variations in pathogenicity of Seiridium in in vitro inoculations of micropropagated cypress shoots paralleled results found in the glasshouse. Wounding of micropropagated shoots significantly increased the size of lesions caused by all three Seiridium spp. In axenic conditions, hyphae of Seiridium spp. penetrated host tissues through stomatal apertures or directly through the cuticle. Under these conditions, infected tissues of C.lawsoniana formed ligno-suberized barriers as a result of fungal invasion, whereas those of C.sempervirens did not.