Histopathology of Seed-Borne Infections - Applied Research Center ...

Location of Fungal Hyphae in Seeds 125infestion seedsduring threshingfoliarinfection stagesecondary infectionwitting plantinfestionseedsplantinginfested seedsseedling infectionstageabovegroundinfectionsoilinfestationundergroundinfectionFIGURE 5.12 Disease cycle of seed-borne safflower rust, Puccinia calcitrapae var. centaureae(From Halfon-Meiri, A. 1983. Seed Sci. Technol. 11, 835–851. With permission.)5.5.4 DeuteromycetesDeuteromycetes or fungi imperfecti are very economically important because manyof them are the causes of serious plant diseases. It is generally assumed that thesefungi reproduce exclusively by asexual means. However, in recent years, sexual orperfect stages of several taxa have been discovered. It has been revealed that specieshaving similar conidial form may have a sexual form belonging to different genera.Species of Rhizoctonia are placed in the basidiomycetous genera Thanetophorusand Cerotobasidium, while those of Phoma are placed in the ascomycetous generaDidymella (Phoma lycopersici), Leptosphaeria (Phoma lingam), and Pleospora(Phoma betae). Furthermore, sexual or perfect stages of many species belonging tosuch form genera are still unknown. The literature also reveals that the imperfect orasexual form causes disease and is associated with seed. The Deuteromycetes aresubclassified into Hyphomycetes and Coelomycetes, and they are dealt with separately.Their location in seed is presented in Table 5.5.5.5.4.1 Hyphomycetesplanting rust-free seeds5.5.4.1.1 AlternariaPlant pathogenic species of Alternaria usually cause leaf spots and blights in variouscrop plants throughout the world. They are weak to serious pathogens, and a largenumber of them are seed-borne and seed-transmitted (Richardson, 1990). Many ofthe seed-borne Alternaria species cause deep infection in seed (Table 5.5). Themycelium is dark colored in Alternaria and moderate to heavy seed infections resultin symptomatic seeds.