When the decrease in the price of one good causes the demand for another good to decrease, the goods are: 

Prices of Related Goods and Services
In general, if a reduction in the price of one good increases the demand for another, the two goods are called complements. If a reduction in the price of one good reduces the demand for another, the two goods are called substitutes.The substitution effect states that when the price of a good decreases, consumers will substitute away from goods that are relatively more expensive to the cheaper good.

By: honey kaundal