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Transmission of Fusarium graminearum (Schwabe) from maize seeds to seedlings.

Surface disinfected and non-surface disinfected sublots of seeds of a yellow dent maize (Zea mays L.) hybrid were used to study the transmission of F. graminearum seed-borne inoculum to determine whether the pathogen is seed transmitted, and to confirm pathogenicity through fulfilment of Koch's Postulates. Surface disinfected seeds plated on carnation leaf agar showed a F. graminearum (Gibberella zeae) infection level of 59%, while non-surface disinfected seeds tested by the deep freezing blotter method revealed 42% seed infection. Laboratory seed germination for each sublot was 92%. Seedling emergence and seedling infection were assessed after sowing seed in moist sterile sand under aseptic conditions in a glasshouse with mean daily temperatures of 14-17°C. F. graminearum had little effect on seedling emergence and survival but produced disease symptoms on 61% of living seedlings. These included brownish red lesions with sunken centres or discolouration, and/or rotting of the scutellum-mesocotyl, roots, and node. Following plating of dead seeds and seedling parts on carnation leaf agar, F. graminearum was recovered from all parts of living seedlings except the leaves, with the highest incidences in parts closest to the seed remains. Transmission rates of 52% and 44% recorded for surface disinfected and non-surface disinfected sublots respectively were similar to seed-borne inoculum levels, suggesting that under favourable conditions F. graminearum is effectively transmitted from maize seeds to seedlings. Koch's postulates for pathogenicity of F. graminearum isolated from seedlings infected naturally by seed-borne inoculum were fulfilled, and it was concluded that F. graminearum is seed transmitted from maize seeds to seedlings.