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What is Tipburn?
Tipburn is a breakdown of the leaf margins which is of particular concern on the internal heart leaves which are not obvious at harvest. External tipburn can also occur on the outer wrapper leaves but these can be trimmed at harvest.
Figure 1. External tipburn of Cos lettuce
What Tipburn does?
Tipburn is a critical defect which limits the appearance and shelf life of lettuce (fresh market lettuce and minimally processed salad mixes). Internal tipburn is a real problem for summer lettuce growers because its incidence is variable, some plantings are affected more than others and it may not be apparent 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 which dramatically reduce shelf life.
What causes Tipburn ?
Tipburn in lettuce has been generally recognised as a calcium deficiency disorder.
Calcium strengthens plant cell walls and Tipburn is more accurately related to the inability of plants to supply enough calcium to developing leaves during periods of rapid growth. Calcium moves from the roots to the leaves of the plant along with water drawn by the transpiration process. Rapidly transpiring outer leaves draw most of the water and accumulate most of the calcium. Enclosed lettuce heart leaves, which are growing rapidly, have a much lower transpiration rate and draw less water and consequently less calcium. With less calcium available, the rapidly growing heart leaves form weaker cell walls which may collapse and die as the leaves expand close to harvest. This is seen as internal tipburn. These breakdown sites allow entry of bacteria which results in further breakdown and unmarketable product.
While Tipburn is generally considered a calcium deficiency problem, symptoms can occur despite plentiful supplies of calcium in most vegetable growing soils. The problem is moving sufficient calcium to the rapidly growing inner leaves. External tipburn can occur for similar reasons but can also be caused by windburn, sand blasting or other physical damage to the delicate growing leaf tips.
Tipburn is a feature of rapidly growing summer lettuce (but it can also occur in spring and autumn) and reflects the inability of plants to move sufficient water and nutrients to the rapidly growing leaf tissues enclosed in the heart of the lettuce plant.
The answer is that tipburn is induced by a number of factors including growth rate, which are a function of climate, water and nutrient availability, supply of calcium and any stress imposed on the plant which results in uneven growth rate.
Sap Calcium Levels
The sap calcium trend for both summer and winter crops is similar, as indicated in the graphs below. Field work has shown that close to harvest, concentrations of calcium in the sap of “winter” lettuce fell to levels comparable with summer lettuce. Tipburn does not usually develop in winter plantings because of the slower and more consistent winter growing conditions. Leaf sap-calcium is not the sole factor influencing the development of tipburn in lettuce and growing conditions and growth rate of the crop is critical.
Figure 2. Sap calcium levels in winter lettuce crops
Figure 3. Sap calcium levels in summer lettuce crops
What can be done to reduce the incidence of tipburn?
Relationship between Nutrients and Tipburn
Calcium (Ca)
Concentrations of leaf-sap calcium in summer iceberg lettuce generally fall below the desirable range within 3 weeks of transplanting even though there may be 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.
Supplementary calcium applications using foliar sprays did not effectively increase calcium levels or reduce tipburn and are not 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. However leaf-sap calcium concentrations did not 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 induced premature tipburn. Foliar supplements containing nitrogen did not increase growth rates or alter sap nitrate levels within the plants.
General nutrition
There is some potential to control growth rate and reduce potential tipburn by reducing excessive application of nutrients such as nitrogen and by managing irrigation more effectively.
Trials evaluating reduced fertiliser input following a previous crop have shown no reduction in growth rate indicating under the conditions evaluated high soil reserves of key nutrients.
Potassium (K) may 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.
Growers may be able to exploit residual nutrient reserves by reducing fertiliser applications to the first lettuce plantings following another crop. This highlights the importance of carrying out soil tests and basing fertiliser application on soil requirements and previous cropping history.
Crop growth rates
High summer growth rates are clearly one of the major factors leading to tipburn in lettuce because high growth rates place a greater demand on the individual plants ability to provide adequate water and nutrients to rapidly growing leaf tissues. Even brief calcium deficiencies may be sufficient to weaken plant cell walls enough to increase the risk of tipburn. 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 may cause surges in growth rate and result in tipburn.
Growth rate is critical to lettuce quality and it is 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. There is potential to control growth rate by reducing excessive application of nutrients such as nitrogen and providing consistent, even soil moisture levels.
Cultivar selection is very important
Cultivars can have the most significant effect of any individual factor on the incidence of tipburn. In field trials cultivars differed significantly in their susceptibility to tipburn throughout the season.
The iceberg cultivars Raider, Casino, Toronto, Ponderosa and Kingsway showed good tolerance of tipburn at different sites and times of the season while Silverado was consistently tolerant at three sites across southern Victoria. Of the cos cultivars evaluated Verdi, Donatus, and Cosmic showed the best tolerance to tipburn while Nero also showed some tolerance. It is important when choosing cultivars for times of the year when tipburn may be a problem to consider using some cultivars with tipburn resistance. Check with your seed suppliers for appropriate cultivars.
Cultivars also differ in their susceptibility to tipburn with the incidence increasing as they approach maturity. The incidence of tipburn in the cultivar Casino did not 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 may be due to a number of factors. 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 although growth patterns were the same.
Root development and root to shoot ratio are likely to play an important role in nutrient uptake and the development of tipburn in field grown lettuce. Tipburn sensitive cultivars have been shown to have less root development than more resistant varieties.
Irrigation Practices
Good irrigation practices are critical to maintaining a good even crop growth rate and facilitating 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.
Iceberg and Cos lettuce have different water requirements and are best grown in separate irrigation blocks over the summer season.
It is essential to maintain an even plant growth rate and variable supply of moisture will result in uneven growth rates and variable uptake of nutrients and lead to increased incidence of disorders such as tipburn. To maintain an even moisture level in the soil, the soil moisture levels should be monitored and irrigation requirements scheduled according to need. Sandy soils will require more frequent irrigations with clay soils the least in frequency.
To minimise the impact of poor water quality, and maximise the uptake and translocation of nutrients around the plant, particularly calcium, irrigation should preferably be applied at evening or night. Watering at night improves the supply of calcium to developing heart-leaves and lowers the incidence of tipburn.
Where water containing high salt or sodium levels is used then this will minimise the impact on the crop. Irrigating at night (early morning) will improve the supply of nutrients to the heart leaves and reduce leaf burn from poor quality water.
However it is also important to take into account the impact on disease of irrigating at night where prolonged leaf wetness can lead to increased incidence of diseases such as Downy Mildew and Anthracnose. On warm or hot nights this will not be a problem as the leaves will not remain wet for an extended period.
Taking these factors into account the best time to irrigate is from around 4.00 am to around 8.00 am. If earlier there will be a longer period of leaf wetness and slightly higher disease problems but better uptake of nutrients, and if irrigated later, uptake and movement of nutrients around the plant will not be as effective. This will be a management decision based on time of the year, cultivar used and climatic conditions.
Avoid over-and under-watering resulting in variation in soil moisture levels.
Incidence of Tipburn can be reduced
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, the only way to inspect your crop is to physically open the lettuce.
It is important not to 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 to 50%.
Better management practices are more critical with crops planted mid-summer.
Maintain a consistent crop growth rate by maintaining an even supply of nutrients and water throughout the growth of the crop, particularly after hearting.
Tipburn is a more consistent problem in January harvests rather than April harvests at all growing regions.
Monitoring sequential plantings revealed that tipburn developed earlier and was more extensive in mid summer plantings.
Make sure the crop is provided with adequate water overnight or early morning to maximise water and nutrient uptake.
Cos should not be intercropped with iceberg lettuce over the critical summer period. Cos lettuce requires specific management to reduce incidence of tipburn.
Consider using tipburn tolerant cultivars in summer plantings.
There is no strong evidence of any value in using calcium foliar sprays.
The previous version of this note was published in September 2003.
The advice provided in this publication is intended as a source of information only. Always read the label before using any of the products mentioned. The State of Victoria and its employees do not guarantee that the publication is without flaw of any kind or is wholly appropriate for your particular purposes and therefore disclaims all liability for any error, loss or other consequence which may arise from you relying on any information in this publication.
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