Autors: Jesus G. Töfoli; Ricardo J. Domingues; Josiane T. Ferrari, LDFH, CPDSV, Instituto Biológico

E-mail: jesus.tofoli@sp.gov.br

Main Host: Lettuce (Lactuca sativa)

Causal agent: Sclerotinia sclerotiorum, Sclerotinia minor

Etiology: The species Sclerotinia sclerotiorum (Lib.) de Bary and Sclerotinia minor Jagger belong to Kingdom Fungi, Phylum Ascomycota, Class Leotiomycetes, Order Helotiales, Family Sclerotiniaceae. The two species differ mainly in terms of sclerotia size, life cycle and host spectrum. S. sclerotiorum produces large (20 - 10 mm in diameter), smooth, rounded sclerotia whereas in S. minor these are smaller (0.5 - 2 mm in diameter), rough and angular. The two species also differ in their life cycle. S. minor sclerotia rarely produce apothecia in nature. When these germinate, they form a whitish cottony growth on the soil surface that, in direct contact with senescent tissues of the host, initiates the infectious process. In addition to germinating directly, S. sclerotiorum sclerotia also have the ability to produce apothecia under specific conditions. Apothecia are fruiting bodies that produce ascospores that are ejected and then dispersed by wind or water. These in contact with the plant germinate and initiate the infection. Ascospores can survive for up to two weeks before starting an infection. As for the infection method, S. sclerotiorum produces ascospores that can reach long distances by the action of wind or water, while S. minor only affects plants that are close to the germinated sclerotia. The sclerotia are resistance structures of these fungi that allow them to survive in the soil under adverse conditions for periods of 8 to 10 years. Sclerotia survival is closely related to soil type; pH; previous crop; its location in the soil profile; humidity; temperature and presence of microorganisms that cause their degradation. In general, S. sclerotiorum is considered to have a greater destructive potential when compared to S. minor. By having carpogenic germination, S. sclerotiorum can reach a greater inoculum potential, since each sclerotia can generate from one to more than 20 apothecia and these release more than 2,000,000 ascospores, in a short period of time. It is noteworthy, however, that the production of sclerotia by S. minor is 10 to 100 times greater than that of S. Sclerotiorum. The disease is favored by humid periods and temperatures ranging from 10 to 20°C, being more severe after the crop is closed. All types of lettuce are susceptible to both species.

Symptoms: Caused by the fungi Sclerotinia sclerotiorum and Sclerotinia minor, the disease affects the base of the plants, causing the stem and leaves close to the ground to rot. Affected plants show symptoms of progressive wilting, followed by yellowing, generalized collapse and death. The lesions initially appear moist, light or dark brown in color, and are covered by a dense white mycelium and black sclerotia. Affected tissues rapidly and severely degrade, assuming the appearance of soft rot. Despite being considered a typical soil disease, its occurrence has also been reported in hydroponic cultivation. The control of lettuce drop is considered difficult due to the large number of hosts that the disease presents and its ability to survive in the soil for long periods in the form of sclerotia. The adoption of joint strategies, related to crop practices, use of fungicides and use of biological control agents, are fundamental for the sustainability of the crop.

Economic importance: Lettuce drop is a disease, typical of winter and spring, capable of causing high losses to the producer if not managed correctly. Is one of the most economically damaging diseases of lettuce.

Distribuition: Worldwide.

Management: The main control measures are:

- When possible, avoid planting in areas with a recent history of the disease. - Plant in light, well-drained soils. - Avoid planting in lowlands, areas subject to moisture accumulation. - Deep plowing in order to bury sclerotia and eliminate possible compacted areas that favor the accumulation of moisture in the surface layers of the soil.

- Adopt raised beds to avoid accumulation of moisture in the soil.

- Planting healthy seedlings.

- Adopt appropriate spacing for the type of lettuce grown, in order to allow air circulation between the plants and reduce moisture levels in the leaves. Density of plants should be avoided in favorable seasons due to it creating a microclimate favorable to the disease.

- Balanced fertilization. Excess nitrogen causes tender tissues and dense foliage, favoring the development of lettuce drop.

- Adopt the planting of more upright cultivars such as those from the group Romana. These allow a smaller accumulation of humidity between the plants, in comparison to the “cabbage-like” cultivars.

- Perform crop rotation with non-susceptible crops such as corn, oats, sweet corn, sorghum, wheat or pasture for periods of 3 to 4 years.

- Eliminate susceptible volunteer plants and host weeds such as: wild poinsettia (Euphorbia heterophylla); large-fruited amaranth (Amaranthus deflexus), Japanese morning glory (Ipomoea nil); winged false buttonweed (Borreria alata); gallant soldier (Galinsoga parviflora); arrowleaf sida (Sida rhombifolia); black-jack (Bidens pilosa) and maria-mole (Senecio brasiliensis).

- Reduce irrigation in periods that are critical and favorable to the disease. Plan the watering so that the plants can be drier at the end of the day. - Avoid contact of the plant with the soil through the use of cover (mulch).

- Eliminate and destroy crop residues in order to reduce inoculum sources.

- Cleaning and disinfestation of implements.

- Carry out cultivation operations and crop treatments first in disease-free areas and then in areas with a history of the disease.

- Solarization with transparent polyethylene for at least 60 days is recommended for the control of S. sclerotiorum and S. minor and other soil pathogens. This practice should be employed during the summer.

- Cleaning and sanitizing hydroponic systems with chlorine-based solution.

- The biological control of the disease can be performed with formulations of Trichoderma sp. The microorganism must be applied pre-planting and must be used in combination with other control strategies such as solarization in order to achieve good levels of control.

- Fungicides. Applications of registered fungicides (AGROFIT- http://agrofit.agricultura.gov.br/agrofit_cons/principal_agrofit_cons) must be started seven days after transplanting, repeating every 10 days (maximum of 3 applications/cycle). In critical situations, the seedlings can be watered with a fungicide before being taken to the field. In direct seeding systems, spraying should be carried out from thinning to the rosette stage. The use of fungicides must be carried out within integrated control programs and must follow all the manufacturer´s recommendations regarding dose, volume, intervals, number of applications, use of personal protective equipment (PPE), safety interval, product storage and packaging disposal. To avoid cases of resistance to specific fungicides, it is recommended that they be used alternately or formulated with non-specific products (contact); that the repetitive use of fungicides with the same mechanism of action is avoided and that curative applications are not applied in situations of high disease pressure. The technology for applying fungicides is important for successful control of the disease, given that the disease starts on the lower leaves. Factors such as: type of nozzles, application volume, pressure, boom height and tractor speed must always be considered in order to provide the best possible distribution of the product in the crop.

References:

KOIKE, S.T.; GLADDERS, P.; PAULUS, A.O. Vegetable Diseases: a colour handbook. St. Paul: APS. 2007. 448 p.

KRAUSE SAKATE, R. et al. (Ed.) Manual de fitopatologia: doenças das plantas cultivadas. 5. ed. São Paulo: Agronômica Ceres 2: 2016, p.33-40.

LOPES, C.A.; QUEZADO-DUVAL, A.M.; REIS, A. Doenças da alface. Brasília: Embrapa Hortaliças. 2010. 68p.

TOFOLI, J.G.; DOMINGUES, R.J. Manejo e métodos de controle de doenças fúngicas da alface. PROSAF. Instituto Biológico. http://www.biologico.sp.gov.br/uploads/files/pdf/prosaf/apostilas/doencas_alface.pdf TÖFOLI, J.G.; DOMINGUES, R.J. Doenças causadas por fungos. Aspectos Fitossanitários da Cultura da Alface. Boletim Técnico - Instituto Biológico, no 29, p. 28-26, 2017.

Keywords: Lettuce, Sclerotinia sclerotiorum, Sclerotinia minor.

Suggested Citation:

TÖFOLI, J. G.; DOMINGUES, R.J.; FERRARI, J. T. Lettuce (Lactuca sativa L.): Lettuce drop: Sclerotinia sclerotiorum (Lib.) de Bary and Sclerotinia minor Jagger. In: Instituto Biológico. Guia de Sanidade Vegetal. Available at: http://www.sica.bio.br/guiabiologico/busca_culturas_resultado_ok.php?Id=217&Vlt=3. Accessed on: month day, year