What is the reason why an artificially grown tree is liable to diseases more than a natural trees

Explanation:

At the first sign of the disease in the spring or early summer, spray the affected tree or vine with Captan Fungicide. Repeat the spray evey 10 days during the growing season. To prevent the spread of Downy Mildew, spray susceptible plants located in close proximity to diseased trees and plants.

your answer is "the 1950's"