With the rapid development of the greenhouse industry in Iowa in the last decade, lettuce has become the most important vegetable crop grown under glass in this state. In some seasons a disease known as downy mildew is one of the most serious troubles met with in forcing this crop. Competition with garden lettuce from the Pacific Coast and the Gulf States, together with the high cost of production in recent years, makes the control of downy mildew an urgent question with many Iowa growers.

The downy mildew of lettuce, long known as a destructive garden and greenhouse disease, is caused by a parasitic fungus. Brenda lactucae, which is widely distributed in both Europe and America. It is somewhat similar to the fungus which causes rose mildew, and develops typical white, downy growths which are readily seen. These are found especially on the lower surface of the leaf. They consist of grayish masses of fungous growth, which produce myriads of spores, conidophores, by which the fungus is propagated.