Loss estimates are necessary to properly evaluate damage caused by plant diseases. Development of this type of information is needed to determine what course of control action is needed and how much money can be spent on programs to reduce losses. Damage caused by disease organisms is often attributed to factors such as weather conditions instead of the disease itself. This natural tendency occurs because disease producing organisms are microscopic in size and not visible to the unaided eye.
It is rare indeed to be able to calculate the actual loss due to diseases and for this reason estimates are necessary. For example, a field of grain may be uniformly infected with an organism such as rust. This means that an unaffected area is not available for yield comparison. It is possible for one to use chemical control to protect an area but even then control may not be complete or the fungicides may alter plant growth.
Since we do rely on estimates, they should be considered as such and not referred to as fact. This does not brand estimates as inaccurate, but places them in proper perspective. Even factual evidence becomes an estimate when it is extended to refer to disease occurrence in another field or in another year. Carefully considered estimates are valuable and can be used to plan research, educational or control decision making activities.
After estimates are carefully established, errors in calculated losses are often made. This happens when an individual figures his estimated loss percentage against what was actually made when it should be considered against the potential yield without any disease. The following problem will depict the correct and incorrect method of figuring losses by using an estimated percentage.
PROBLEM: A farmer made 2,500 pounds of peanuts per acre. He did not control leaf spot and it is estimated that the disease produces a 15 percent loss. How many peanuts did he lose?
Several steps can be taken to improve the accuracy of estimates. Using the most effective chemical control possible permits one to determine how much loss can be recovered. Replicated treated and untreated plots improve accuracy. Even though this type of control is not usually 100 percent, one gets information that has a high level of reliability.
Using disease resistant varieties in comparison with susceptible ones is often beset with inaccuracy because of differences in yield potential. Observations may be helpful, but one should resist the temptation of over-extending confidence when other varieties are involved.
There is no substitute for careful field observation. Mathematical computations may enhance the presentation of findings, but it still is no more accurate than the initial input. Carefully made estimates are valuable and necessary for decision making.