Tipburn in lettuce

Tipburn is a breakdown of the leaf margins of lettuce. It's a critical defect that limits the appearance and shelf life of fresh market lettuce and minimally processed salad mixes.

Internal and external tipburn

Tipburn is of greater concern when it affects the internal heart leaves, which are not obvious at harvest (Figure 1). Internal tipburn is a real problem for summer lettuce growers because:

• its incidence is variable
• some plantings are affected more than others
• it might not be obvious at harvest.

Tipburn can lead to internal bacterial breakdown or slime within the head and the crop can become unmarketable. Internal tipburn is critical if the lettuce is to be used for salad mixes as there is zero tolerance for defects that dramatically reduce shelf life.

External tipburn that only affects the outer wrapper leaves can be trimmed at harvest.

Cause of tipburn

Tipburn in lettuce has been mostly recognised as a calcium deficiency disorder, but is influenced by a number of factors, including climate and growth rate.

Tipburn is a feature of rapidly growing summer lettuce (but it can also occur in spring and autumn). It reflects the plant's inability to move enough water and nutrients to the rapidly growing leaf tissues enclosed in the heart of the lettuce plant.

External tipburn can occur for similar reasons but can also be caused by physical damage to the delicate growing leaf tips, such as:

• windburn
• sand blasting.

Calcium deficiency

Calcium strengthens plant cell walls, and tipburn is mostly a result of the plant's inability to supply enough calcium to developing leaves during periods of rapid growth. Rapidly transpiring outer leaves draw most of the water and accumulate most of the calcium. With less calcium available, the rapidly growing heart leaves form weaker cell walls, which can collapse and die as the leaves expand close to harvest.

These breakdown sites allow bacteria to enter, which results in further breakdown and unmarketable product.

Growth rate

Besides leaf sap calcium, a factor that influences the development of tipburn in lettuce is the growing conditions. Growth rate of the crop is critical.

Field work has shown that, close to harvest, concentrations of calcium in the sap of 'winter' lettuce fell to levels comparable with summer lettuce. But tipburn doesn't usually develop in winter plantings because of the slower and more consistent winter growing conditions.

In summer, a lettuce crop grows at more than twice the rate in winter. Periodic stress will also have an impact because fluctuations in supply of fertiliser or water can cause surges in growth rate (and result in tipburn).

Growth rate is critical to lettuce quality and it's important to maintain a consistent crop growth rate by maintaining an even supply of nutrients and water throughout the growth of the crop, but particularly after hearting.

Reducing the risk of tipburn

Crop growth rates

There is some potential to control growth rate and reduce potential tipburn by:

• reducing excessive application of nutrients such as nitrogen
• managing irrigation more effectively (providing consistent moisture levels).

Calcium (Ca) supplements and tipburn

Concentrations of leaf-sap calcium in summer iceberg lettuce generally fall below the desirable range within 3 weeks of transplanting. This can happen even when there's plenty of available calcium in the soil.

Trials of a number of calcium supplements applied by foliar sprays have been carried out to determine if the levels of calcium in the plants can be raised to reduce incidence of tipburn. The results are as follows:

• Supplementary calcium applications using foliar sprays don't effectively increase calcium levels or reduce tipburn and aren't recommended for tipburn control.
• Calcium nitrate application as a side dressing was not beneficial to calcium uptake.
• Field trials indicated that low concentrations of calcium were evident in leaf sap and dry tissue samples throughout most of the growing period in summer lettuce. But leaf-sap calcium concentrations didn't provide a consistent indication of tipburn risk because of the range of factors involved.
• In cos lettuce, additional nitrogen fertiliser increased the crop growth rate but also brought about premature tipburn. Foliar supplements containing nitrogen didn't increase growth rates or alter sap nitrate levels within the plants.

General nutrition

It is important to carry out soil tests and base fertiliser applications on soil requirements and the previous cropping history. Growers might be able to exploit residual nutrient reserves by reducing fertiliser applications to the first lettuce plantings following another crop.

Potassium (K) might have an important role in the ability of iceberg lettuce to form a proper heart. Very low concentrations of potassium in the soil appear to increase the risk of lettuce failing to form proper hearts.

Resistant cultivars

Consider using some cultivars with tipburn resistance when choosing cultivars for times of the year when tipburn is a problem (such as mid-summer plantings). Cultivars can have the most significant effect of any individual factor on the risk of tipburn.

In field trials, cultivars differed significantly in their susceptibility to tipburn throughout the season. Cultivars also differ in their susceptibility to tipburn with the incidence increasing as they approach maturity. For example, the incidence of tipburn in the cultivar Casino didn't change during the 4 days prior to harvest, providing the grower with a greater harvest 'window' than most other cultivars.

Differences in susceptibility to tipburn might be due to a number of factors:

• Nutrients – at harvest, the iceberg cultivar, Sheeba generally showed a higher concentration of sap nitrate, calcium and potassium than was seen in other cultivars such as Target, though growth patterns were the same.
• Root development and root to shoot ratio of tipburn-sensitive cultivars have been shown to have less root development than more resistant varieties. This is likely to play an important role in nutrient uptake and the development of tipburn in field-grown lettuce.

These iceberg cultivars showed good tolerance of tipburn at different sites and times of the season:

• Casino
• Kingsway
• Ponderosa
• Raider
• Toronto
• Silverado (consistently tolerant at 3 sites across southern Victoria).

Of those evaluated, these cos cultivars showed the best tolerance to tipburn:

• Cosmic
• Donatus
• Verdi
• Nero (some tolerance).

Don't intercrop cos with iceberg lettuce over the critical summer period. Cos lettuce requires specific management to reduce the risk of tipburn.

Check with your seed suppliers for appropriate cultivars.

Irrigation practices

Good irrigation practices are critical to maintaining a good even crop growth rate and helping with the effective uptake of nutrients. Lettuce has a shallow root system and, to achieve a marketable yield, requires a constant supply of moisture during the growing season.

Variable supply of moisture will result in:

• uneven growth rates
• variable uptake of nutrients.

This will lead to increased incidence of disorders such as tipburn.

To maintain an even moisture level in the soil, you must monitor the soil moisture levels, schedule irrigation requirements depending on need:

• Sandy soils will need more frequent irrigations and clay soils the least frequent.
• Iceberg and cos lettuce have different water requirements and are best grown in separate irrigation blocks over the summer season.

Water in the evening or at night

Watering at night:

• maximises the uptake and translocation of nutrients around the plant
• improves the supply of calcium to developing heart-leaves and lowers the incidence of tipburn
• minimises the impact water with high salt or sodium levels has on the crop
• improves the supply of nutrients to the heart leaves and reduces leaf burn from poor-quality water.

But when watering at night, consider that prolonged leaf wetness can lead to diseases such as downy mildew and anthracnose.

The best time to irrigate is from around 4.00am to around 8.00am. Earlier than 4.00am and there'll be a better uptake of nutrients but a higher risk of disease. Later than 8.00am and the uptake and movement of nutrients around the plant won't be as effective.

Make this decision based on the:

• time of the year
• cultivar used
• climatic conditions – on warm or hot nights, prolonged leaf wetness won't be a problem.

Planting and harvest timing

Crop management is vital from heart formation, which is the critical point for the first onset of tipburn. After heart formation, internal tipburn becomes invisible and the only way to inspect your crop is to physically open the lettuce.

Don't delay harvesting the crop as the tipburn severity continues to increase. In well-fertilised soils, harvesting 3 days earlier can reduce tipburn by as much as 30 per cent to 50 per cent.

Tipburn tends to develop earlier and be more extensive in mid-summer plantings.

Tipburn is a more consistent problem in January harvests rather than April harvests in all growing regions.

References

Murdoch, C.; Dimsey, R.; Sippo, J.; Pierce, P.; Rennick, T. (2000) Decreasing tipburn in lettuce. Conference paper: Australian Lettuce Industry Conference, Hay, New South Wales, Australia, 6-8 June, 2000. 2000 pp.69–75.

Davis RM, Subbarao KV, Raid RN, Kurtz EA (1997) Compendium of Lettuce Diseases. APS Press 78pp.