Gordon Johnson, Extension Vegetable & Fruit Specialist; email@example.com
We are currently seeing lettuce affected by bottom rots and leaf drop. There are several disease organisms in our region that can cause these diseases.
Bottom rot is caused by the soil-borne fungus Rhizoctonia solani, and is found wherever lettuce is grown. Infection occurs when sclerotia (dormant resting structures) germinate and produce mycelium that penetrates healthy or wounded tissue. The pathogen infects lettuce over a wide range of temperatures, but is favored by warm (77 to 81ºF), moist conditions. Rhizoctonia solani is a very common soil inhabitant, and some strains of the fungus can attack other hosts including potato, onion, dry bean, wheat, corn, and several weeds. The pathogen survives between lettuce crops as sclerotia or mycelium in soil and crop debris, pathogenically on alternate hosts, but can also be introduced into a field by wind- or water-disseminated spores (basidiospores).
Bottom rot symptoms typically develop first on lower leaves in contact with the soil, and appear as small, rust-colored brown spots, primarily on the underside of leaf midribs. Symptoms generally are most pronounced at heading. Bottom rot can rot midribs and lettuce leaf blades rapidly when conditions are favorable; stems are relatively more resistant to bottom rot and are the last portion of the head to decay. Decaying heads are at first slimy and brown but become dark brown to black as they collapse and dry. A webbed network of white to brown mycelium often grows over lesions, and small gray brown sclerotia later are apparent.
Plant lettuce varieties with an upright architecture to reduce foliage contact with the soil. Practice a three-year or longer rotation to non-hosts. Destroy crop residues by plowing deeply after harvest to reduce pathogen survival. Practice effective weed control to eliminate alternate hosts between lettuce crops. Plant lettuce on high beds to promote air movement, drainage, and minimize foliage contact with the soil. Avoid irrigation near harvest.
This information is from https://wiki.bugwood.org/HPIPM:Lettuce_Bottom_Rot
Two species of Sclerotinia infect lettuce and cause the lettuce drop disease. Sclerotinia minor only infects the stems and leaves in contact with the soil. Once infection takes place, the fungus will cause a brown, soft decay that eventually destroys the plant crown tissue. Older leaves then wilt and later the entire plant will wilt and collapse, making it unharvestable. Plant collapse usually occurs when lettuce is near maturity. Profuse amounts of white mycelia and small [up to 0.125 inch (3 mm)], black, hard, resting bodies (sclerotia) form on the outside of the decayed crown.
Sclerotinia sclerotiorum can also infect lower leaves and stems, causing symptoms similar to those of S. minor. In addition, S. sclerotiorum has an aerial spore that can infect any of the upper leaves. Spores usually infect damaged or senescent tissue when the weather is cool and moist. Infection results in a watery, soft rot that is accompanied by white mycelial growth and formation of sclerotia. Sclerotinia sclerotiorum forms sclerotia that are larger (0.25–0.50 inch) than those of S. minor.
Sclerotia of both species enable the pathogens to survive in the soil for 2 to 3 years without susceptible hosts. Wet soil conditions favor disease development of both species. For S. sclerotiorum, cool and moist conditions are necessary for development of the fruiting structure (apothecium) that produces the airborne spores.
This information is from https://www2.ipm.ucanr.edu/agriculture/lettuce/Lettuce-drop/ and https://content.ces.ncsu.edu/lettuce-drop
Control for these diseases from the Mid-Atlantic Vegetable Production Recommendations are as follows:
caused by Rhizoctonia. A midsummer application of a soil fumigant will be beneficial for a fall crop. For the spring and fall crops, all fields should receive one of the following fungicide applications one week after transplanting or thinning and at 10 and/or 20 days later if conditions warrant and/or cultivation has been done: iprodione 4F, Uniform 3.66SE, or Endura 70W.
(Sclerotinia sclerotiorum). The pathogen has a wide host range including allium, brassica, and solanaceous crops. Proper and adequate crop rotations are necessary since the pathogen can survive in soils for many years. Apply one of the following as a directed spray at transplanting and/or thinning (rotate between the following fungicides if more than one application is needed): iprodione 4F, Endura 70W, Miravis Prime 3.34SC or 2.09SC, Cannonball 50WP, or Regalia (biological control).