Lettuce (Lactuca sativa)-Drop (Sclerotinia Rot)


Cause Two fungi, Sclerotinia sclerotiorum and S. minor, that overwinter on lettuce refuse on the ground and as sclerotia in soil. There are two phases: the damping-off phase, which attacks seedlings; and the important field phase called "drop", which causes a watery soft rot. It is severe whenever lettuce is grown under cool, moist conditions. Sclerotia of S. minor are small (2 to 3 mm) compared to S. sclerotiorum (20 to 30 mm). S. minor is not a problem in Oregon. Many vegetable and field crops are susceptible to S. sclerotiorum, including bean, carrot, pea, lettuce, potato, green pepper, cucurbits, parsnip, radish, sunflower, herbs including mint, fruit trees including apple, nectarine, and sweet cherry, and a large number of ornamentals both woody and herbaceous. Weeds can also be host to S. sclerotiorum; dandelion (Taraxacum officinale) is a very susceptible weed host. These fungi survive as sclerotia, which may be associated with infected plant residues, contained in seed lots (external or internal to seed), or persist in the soil. Sclerotia can survive three to five years in the soil, depending on environmental conditions and cropping practices. Typically, strains of S. sclerotiorum outside of tropical zones require a conditioning period in moist soil of several weeks at 40°F or cooler temperatures, after which sclerotia can germinate in soil when the soil water potentials are ≥-100 kPa and temperatures are between 41°F and 68°F. The production of apothecia by S. sclerotiorum appears to be prevented when soil matric potentials are below -0.05 MPa. The upper temperature threshold for conditioning and germination of sclerotia is 68°F and 77°F, respectively, for S. sclerotiorum. Typically, sclerotia germinate and then produce a small, stalked, cup-shaped fruiting structure known as an apothecium, which forcibly ejects millions of ascospores into the air. Sporulating apothecia may persist five to ten days while released ascospores can survive up to two weeks, depending on environmental conditions. Since ascospores require an exogenous nutrient source before infecting a plant, the primary sites for initial infection are senescing tissues such as spent blossoms and declining leaves or plant tissues that have suffered mechanical injuries or wounding through infection by other pathogens. For infection, ascospores require free moisture or a relative humidity close to 100% with a temperature between 50°F and 86°F. Sometimes sclerotia germinate and produce undifferentiated mycelium that directly infects plant portions just below or at the soil line. After infection of the plant has occurred the mycelium spreads by hyphal growth, the fungus can then invade any healthy, vigorous part of a plant with which it comes in contact, and the pathogen may grow from plant to plant if a diseased plant is in direct physical contact with neighboring plants. Moist conditions within the plant canopy favor infection, as do rain, dew, and/or irrigation practices that keep foliage wet for long periods.

Symptoms Under moist conditions, outer leaves wilt, and there is a slimy rot of the plant. A cottony fungus growth appears on the stem near the soil level. Then the drop phase appears in midsummer or later after leaves wilt; it begins with the oldest leaves and continues until the plant collapses. Hard sclerotia develop throughout the diseased portion of the plant.

Cultural control

  • Use well-drained land and avoid crowding plants in the field.
  • Remove and destroy old plant trash from previous crops if possible. Do not dump the trash at the field's edge-inoculum can blow back into the field.
  • Corn, cereals, and grasses are not susceptible and make good rotation crops in some situations.
  • In irrigated or wet coastal areas, flooding the soil when the ground is not in production helps reduce sclerotial survival, particularly of S. minor.
  • S. minor can be controlled economically by removing infected plants from the field promptly before leaves start to drop off. However, that is not feasible for S. sclerotiorum.

Chemical control

  • Cannonball WG (Group 12) at 7 oz/A on 7- to 10-day intervals. May be applied the day of harvest. 12-hr reentry.
  • Carboxamide (Group 7) formulations are registered for use. Do not make more than two (2) sequential applications before alternating to a labeled fungicide with a different mode of action.
    • Endura at 8 to 11 oz/A. On direct-seeded lettuce, make the first application immediately after emergence or prior to disease development. On transplanted lettuce, make the first application immediately after transplanting or prior to the onset of disease. If conditions continue to favor disease development, or if soil surface is disturbed by cultivation, make a second application. Preharvest interval is 14 days. 12-hr reentry.
    • Fontelis at 16 to 24 fl oz/A on 7- to 14-day intervals. Preharvest interval is 3 days. 12-hr reentry.
  • Iprodione formulations (Group 2) are labeled.
    • Nevado 4F at 1.5 pints/A on 14-day intervals. Do not apply within 14 days of harvest. 24-hr reentry.
    • Rovral 4 Flowable 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.
  • OSO 5% SC (Group 19) at 3.75 to 13 fl oz/A on 7- to 14-day intervals. Can be applied the day of harvest. 4-hr reentry. O
  • Regalia (Group P5) at 0.5 to 4 quarts/A plus another fungicide on 5- to 10-day intervals. Does not benefit from the addition of an adjuvant. Preharvest interval is 0 days. 4-hr reentry. O
  • Premixes of fungicides are available.
    • Miravis Prime (Group 7 + 12) at 13.4 fl oz/A at planting. After at least a 7-day interval, a second application can be made if either the soil is disturbed by cultivation or thinning, or if conditions continue to favor disease. May be applied day of harvest. 12-hr reentry.
    • Switch 62.5WG (Group 12 + 9) at 11 to 14 oz/A starting at thinning and again 2 weeks later. No more than two (2) sequential applications 12-hr reentry.

Biological control Efficacy unknown in Oregon.

  • Double Nickel LC at 0.5 to 4.5 pints/A at planting and again at cultivation, can repeat on 10- to 14-day intervals. Can be applied the day of harvest. 4-hr reentry. O
  • Serenade Opti at 14 to 20 oz/A on 10- to 14-day intervals. Applications can be made up to and the day of harvest. 4-hr reentry. O
  • Stargus at 6 to 8 fl oz per 1,000 ft row as an in-furrow treatment, 3 to 4 quarts/A as a soil drench (drip or chemigation) on 10- to 21-day intervals, or 2 to 4 quarts/A as a foliar spray on 7- to 10-day intervals. Preharvest interval is 0 days. 4-hr reentry. O