Assessment of colonization in chrysanthemum grown under different photoperiods and infected with Verticillium dahliae

Abstract

The relationship between daylength and Verticillium dahliae infection in chrysanthemum was studied in glasshouse and growth room experiments. Plants inoculated at the rooted cutting stage showed a substantial and progressive reduction in leaf area and stem height after two weeks. Long day (16 h photoperiod) infected plants showed some diminution of symptoms but no major differences were recorded between these and short day (8 h) plants. Plant age at inoculation had little effect on disease severity up to 4 weeks. Conidia were more infective than bud cells derived from shake cultures below 1 × 10 4 cells ml −1 but above this concentration up to 1 × 10 7 cells ml −1 bud cells induced more severe symptoms. Colonization in stems and leaves was measured on split samples of identical tissue by the tissue maceration, colony count method and by chitosan glucosamine determinations. Close correlation was found between both methods in all but one experiment. Photoperiod had a large effect on colonization. Plants grown under short days (SD) had high levels of stem mycelium which reached a peak at flowering (3 weeks) and then declined sharply. Stem mycelium in long photoperiod (LD) plants remained consistently lower than those from SD plants. In leaves, colonization was unrelated to flowering. Hyphal growth increased up to 4 weeks after inoculation in both SD and non-flowering LD plants. SD leaves showed a small but consistent increase in fungal content over LD leaves. Estimates of dry weight mycelium per plant were unacceptably high based on glucosamine recovery experiments using V. dahliae mycelium. It was concluded that colonization was unrelated to symptom intensity and that infection resulted in a stimulation of host-bbund glucosamine synthesis. This synthesis was directly proportional to quantity of mycelium present in the host or of its extra cellular-inducing metabolites.