Lettuce drop (white mold or Sclerotinia drop) is caused by two species of Sclerotinia, S. sclerotiorum and S. minor. These pathogens cause a brown, soft decay that eventually destroys the plant crown, resulting in wilting of the entire plant. Note the abundant white mycelia and small dark sclerotia (resting bodies). Corky root of lettuce is caused by the aerobic, gram-negative bacterium Rhizomonas suberifaciens. Severely infected roots become very brittle, resulting in interveinal chlorosis, necrosis on the lower leaves and smaller heads. Bottom rot, caused by soil-borne fungus Rhizoctonia solani, is most commonly associated with plants at or near maturity. Disease is favored by warm, wet conditions during which lesions can expand rapidly, causing midrib and leaf blade rot. Bacterial leaf spot caused by Xanthomonas campestris pv. vitians. Initial symptoms are small, angular, water-soaked lesions on mature leaves. As disease progresses and lesions enlarge, necrotic areas become dry and eventually collapse.

Sporulating downy mildew lesions on lettuce leaf. This disease is caused by Bremia lactucae. Young lesions have a characteristic light green or slight chlorotic appearance. These lesions turn necrotic so that affected parts eventually die. Older leaves are usually attacked first. Root-knot nematodes (Meloidogyne spp.) are obligate endoparasites that complete most of their life cycle within their host roots. Note the swelling of the roots. Stunting and uneven growth patterns result in crop loss.