Adisease can be a problem one year but not the next in the same crop because a. there is no way to predict where disease will occur. b. variables, like weather, impact the presence of disease. c. figuring out how to combat a disease usually takes about a year. d. most pesticides require a lot of time to become active, sometimes months.

Variables, like weather, impact the presence of disease.

Explanation:

Findings from climate change disease projection  work have shown that it is not easy to understand that if a disease hits a crop one year, will it again in next year too. This is because climate changes like weather, precipitation and winds have huge impact on the onset of disease and crop productivity.

Recently, scientists have developed some tools that can help in assessing the landscape of a region and help on the prediction of diseases. But more investigations are required to increase the disease resistance in crops and improve their productivity when there are climatic changes hitting the crops.

Source:

Modelling impacts of climate change on arable crop diseases: progress, challenges and applications

Fay Newbery1, Aiming Qi2, 3Bruce DLFitt

Hope it help :)

the denominator has a very specific role.