The causes of these diseases can completely destroy the lettuce crop, prompting the use of various control measures.
Young plant decay is caused by the following species: Rhizoctonia solani, Pythium spp., Fusarium oxysporum, and Sclerotinia spp.
Preventive measures against Rhizoctonia solani include treating seeds, practicing crop rotation, sterilizing the soil in protected areas before planting, and removing infected plants if an outbreak occurs.
Downy Mildew (Bremia lactucae)
This pathogen attacks lettuce, endive, and radicchio, with plants being susceptible at all developmental stages.
Primary sources of infection include:
- Oospores in infected plant debris
- Infected neighboring crops
- Wild species of the genus Lactuca
- Contaminated seeds (of less importance as seeds are usually treated with fungicides)
The optimal temperature for the germination of fungi (specifically conidia) is 10°C, although the range extends from 2°C to 28°C. Once conidia penetrate the plant, the incubation period lasts 4 – 7 days at a temperature of 20 – 22°C.
A preventive measure is to remove infected plant debris, which should not be plowed under or left in protected areas as they serve as a source of infection. Despite preventive measures, lettuce must be treated with chemical agents; in Croatia, the registered active substance for this purpose is fosetyl.
White Rot (Sclerotinia minor and Sclerotinia sclerotiorum)
This disease affects lettuce in protected areas and, if crop rotation is not observed, can cause damage in open field cultivation. Infection occurs in the root collar area, where soft rot appears, disrupting capillary connections, causing the head to lose turgor and wilt. White mycelium appears at the site of rot. Lettuce is susceptible to this disease at all developmental stages but is most commonly affected before technological maturity. These pathogens prefer light, well-aerated soils where excess water does not accumulate.
Crop rotation and the removal of infected plant debris are the most important preventive measures against white rot. Chemical soil sterilization provides partial protection, while proper soil sterilization with steam yields the best results. Lettuce should be treated with fungicides immediately after sowing and before emergence, followed by a second treatment when the plant develops 4 – 5 leaves, with the last treatment applied at the beginning of head formation. To avoid phytotoxicity, lower doses are used for the first two treatments.
Other diseases present in lettuce cultivation include wilting caused by Pythium species, concentric leaf spot (Alternaria cichorii), brown spot (Marssonina panattoniana), gray mold (Botrytis cinerea), powdery mildew (Erysiphe cichoracearum f.sp. cichorii), lettuce rust, and leaf spot.
