Cause Physiological disorder. All varieties grown in the Pacific Northwest are susceptible to some degree, but they differ considerably. Under Oregon conditions, all California varieties are susceptible. At least five environmental conditions-all of which affect the plant's calcium uptake or distribution-reportedly favor development of the disorder. (1) High relative humidity with high temperatures, particularly nights above 65°F during head development. (2) Conditions that favor rapid succulent growth such as an abundance of moisture and plant nutrients. (3) An inadequate or fluctuating water supply as the crop matures. (4) Low soil and air temperatures during early development followed by a period of high temperatures with low humidity as the crop nears maturity. (5) An imbalance in the mineral nutrition of the crop, particularly as affecting calcium. Excessively high levels of potassium and magnesium or high applications of ammonium nitrogen have been reported to predispose lettuce to tip burn, particularly if soils are low in calcium to begin with.
Symptoms Initial symptoms are small, dark brown spots along the margins of interior or exterior head leaves. Spots later merge together, and the entire margin becomes brown and necrotic. Tip burn is more serious on the internal leaves of the head because harvest crews cannot detect and eliminate the heads as unmarketable. The internal necrotic areas are ideal sites for the development of secondary rot organisms. Usually only a few leaves of the head are affected with tip burn.
Cultural control Calcium chloride treatments had some disease-reducing effect in test plots but not in field plots.
- Use the more resistant varieties: Ithaca and Summertime are recommended for main-season production.
- It is important to maintain adequate calcium levels in soil and to manage fertilizer and irrigation programs to provide even growth throughout the plant's life. Soil samples should show adequate base saturation and adequate levels of calcium.
- Nitrogen forms may be important; nitrate forms are preferred to ammonium forms.
- Harvest lettuce at optimum maturity because tip burn tends to be more serious on overmature lettuce.