 Lettuce also suffers from several viral diseases , including big vein , which causes yellow , distorted leaves , and mosaic virus , which is spread by aphids and causes stunted plant growth and deformed leaves . Aster yellows are a disease @-@ causing bacteria carried by leafhoppers , which causes deformed leaves . Fungal diseases include powdery mildew and downy mildew , which cause leaves to mold and die and bottom rot , lettuce drop and gray mold , which cause entire plants to rot and collapse . Crowding lettuce tends to attract pests and diseases . Weeds can also be an issue , as cultivated lettuce is generally not competitive with them , especially when directly seeded into the ground . Transplanted lettuce ( started in flats and later moved to growing beds ) is generally more competitive initially , but can still be crowded later in the season , causing misshapen lettuce and lower yields . Weeds also act as homes for insects and disease and can make harvesting more difficult . Herbicides are often used to control weeds in commercial production . However , this has led to the development of herbicide @-@ resistant weeds and prompted environmental and health concerns . 
