Cause Rhizoctonia solani, a fungus. The disease is widespread, occurring most often on muck soil. It commonly attacks 'Big Boston' types of head lettuce.
Symptoms The disease appears as heads approach maturity. Necrotic spots appear on stems and midribs of lower leaves and may enlarge over the entire stem. A brown exudate appears on the surface of the spots, and infection continues to produce sunken brown spots. The fungus may advance through the leaf blade until the entire head is destroyed. Brown sclerotia develop near leaf axils, and the fungus may produce a noticeable network of growth around the head. The head becomes slimy and dries to a blackened mummy. Bacterial soft rot often follows bottom rot invasion.
- Use well-drained soil or raised beds.
- Rotate with nonsusceptible crops such as sweet corn or onions if practical.
- Nevado 4F (Group 2) at 1.5 to 2 pints/A on 10-day intervals. Do not apply within 14 days of harvest. 24-hr reentry.
- Quadris Flowable (Group 11) at 0.4 to 0.8 oz/1,000 row feet applied in a 7-inch or less band either in-furrow at planting or as a directed spray at cultivation. 4-hr reentry.
- Rovral 4 Flowable (Group 2) at 1.5 to 2 pints/A in at least 40 gal water by ground. Apply at the three-leaf stage of growth and again 10 days later. A third application can be made 10 days later if disease pressure persists. Do not apply within 14 days of harvest. Do not cultivate after application. 24-hr reentry.
- Double Nickel LC at 0.5 to 4.5 pints/A for soil application on 14- to 28-day intervals. Can be applied the day of harvest. 4-hr reentry. O