Ustilago segetum var. The cause of plant damage Climatic factors like excessive temperatures like heat or frost, heavy or prolonged rain, storm. The host must be susceptible to the disease. Some plants are resistent to different agents. Organ specifity There must be an organ specifity of the agent.
Some agents can infest only specific organs like flowers or roots to which they are specialized.
Pleospora betae can attack the whole plant. Phase specifity Some agents have a specifity to flowers. They have wait till the blossoms open to attack the host, other have a phase specifity to leaves. Survival of the agent Some agents have developed survival strategies to overcome the time between one host to another. Virus cannot survive for long time outside a host cell.