Grain can be traded in the futures market before harvesting. If grain production is predicted to be insufficient, grain futures prices will rise; if grain harvests are predicted, grain futures prices will fall. This morning, meteorologists predicted that from tomorrow there will be much-needed rainfall in the grain-producing areas. 
A pathogen that has destroyed some cereal crops will spread more widely before the end of the growing season, agricultural experts announced today
 since sufficient moisture is very important to the survival of current grains, the price of grain futures today will fall sharply.