The seedborne disease can be classified into either monocyclic (simple-interest) diseases or polycyclic (compound-interest) disease based on their epiphytology. Environmental conditions greatly influence the epiphytology of seedborne pathogens. The disease cycle of Gloeotinia temulenta, blind seed disease, and Claviceps purpurea, ergot, which invade only the inflorescence also are regarded as simple-interest diseases by P. D. Hewett, but conidia produced by both fungi on infected plants may result in secondary spread. These diseases should be considered as compound-interest diseases. Epiphytotics of compound-interest diseases are influenced by the environment. Under favorable conditions even a low initial seedborne inoculum can result in an epiphytotic. Many seedborne pathogens belong to this group. The threshold level for pathogenicity of any seedborne inoculum is not constant since it is affected by inoculum level, inoculum location, seed- and soilborne microflora, soil temperature, moisture, pH, environmental factors, insect vectors, and wind velocity. Epiphytotics vary with different pathogens.