Symptoms and Signs
Tipburn is characterized by browning of the margins of young, maturing leaves in head and leaf lettuces. The brown area may be limited to a few small spots at or near the leaf margin, or the entire edge of the leaf may be affected. Many pinpoint necrotic areas along the margin may give that tissue a speckled appearance. Brown veins may occur near the brown lesions.
Comments on the Disorder
Tipburn is a result of calcium deficiencies in growing tissue, which can occur even if calcium levels in the soil are adequate. Calcium is important for cell wall strength and membrane integrity. Calcium mostly moves in the transpirational flow in the plant, therefore, the highest concentration of calcium is in outer leaves where higher level of transpiration occurs.
Tipburn is rarely the result of low soil calcium, but more commonly is due to water stress and low evapotranspiration (ET) that causes transient deficiency of calcium in rapidly expanding leaf tissue.
- Head formation reduces transpiration in the inner leaves of iceberg lettuce. Inner leaves of head lettuce or leaf lettuces transpire less than outer leaves and are more likely to develop tipburn.
- Rapid growth creates conditions for the development of tipburn.
- Low evapotranspiration (ET) due to weather conditions like fog reduces transpiration in leaf lettuces.
- Soil water stress can also reduce the transport of calcium to the plant.
Cultivars vary in their susceptibility to tipburn. In areas and seasons in which tipburn is likely to occur, use resistant cultivars. Foliar sprays of calcium have reduced tipburn in studies in which the nutrient is applied directly to the tissue; however, this is ineffective in head lettuce because the nutrient will not reach susceptible tissue deep inside the head in time to prevent the development of tipburn. Calcium fertilizer applications to the soil are generally ineffective because soils in California's lettuce producing districts generally already have sufficient calcium to supply the plant.