Post-harvest diseases may be spread in the field before harvest by the use of infected seed or other planting material. Many diseases can survive by using weed plants or other crops as alternate or alternative hosts. They are also spread by means of infected soil carried on farm implements, vehicles, boots, etc. Although relatively few post-harvest losses of fresh produce are caused by attacks of insects or other animals, localized attacks by these pests may be serious. Insect damage is usually caused by insect larvae burrowing through produce, e.g. fruit fly, sweet potato weevil, potato tuber moth.
Infestation usually occurs before harvest.
Post-harvest spread is a problem where produce is held in store or is exposed to lengthy periods of transport. Rats, mice and other animal pests again are sometimes a problem when produce is stored on the farm. There are no generally accepted methods for evaluating post-harvest losses of fresh produce. Whatever evaluation method may be used, the result can refer only to the described situation. In the appraisal of an existing marketing operation, the accurate evaluation of losses occurring is a problem.