Lettuce disorders and diseases
Good quality products are free from diseases and disorders. Customers are not willing to purchase lettuce with rots, damages, strong discoloration or wilting. There are dozens of different diseases and disorders, which are not always easy to recognize. A good diagnosis of the product’s health is important. By recognizing the symptoms of a disease or disorder, it can be linked to the cause. If you know the cause, you know what measures can be taken to prevent the disease or disorder in the future.
Identification of lettuce disorders and diseases usually takes place by carefully looking at the symptoms. It is important to detect the quality issues as early as possible in the supply chain to reduce further incidence and to avoid dissatisfied customers. Retailers can also benefit from knowledge about disorders they may encounter. For them, it is important to know whether the cause lies in a previous stage in the supply chain or is due to their own operations. Common causes of deterioration of lettuces are discolorations, wilting and rotting.
Frequently occurring disorders and diseases
Pink rib. Photo by WUR
Pink ribPink rib is a physiological disorder in which the white ribs of the lettuce turn pink. This pink discoloration occurs after longer storage or display. Some cultivars are more sensitive than others. Heads harvested overmature are more susceptible and storage under too high temperature will boost the outbreak of the pink pigments.
A severe incidence of browning on the leaves. Photo by Angela M. Benivegna/Shutterstock.com
Brown or russet spotBrown spot or russet spot (or spotting) may be visible on the white ribs of lettuce head. Brown spots can have various causes. It can be symptoms of a storage under too high CO2 conditions. A failure in fresh-air ventilation during storage may allow CO2 accumulation, which will induce the russet spot symptoms. Storage in a room with ethylene also induces russet spot symptoms. Overmature heads will develop russet spot more easily.
Moisture loss results in loss of fresh appearance, Photo by WUR
WiltingLettuce leaves are sensitive to wilting and therefore need protection against moisture loss. When the water loss (and thus weight loss) is too severe, lettuce cells lose their turgidity, which becomes visible as wilting symptoms. Leaves and ribs then lose their crunchiness. In addition to loss of fresh appearance, this clearly also has a negative effect on taste.
Overfilled boxes can lead to damaged leaves. Photo by WUR
Mechanical damagesDuring harvesting, packaging and transport processes, lettuce leaves can be subject to mechanical damages such as injuries and bruising. The display in the retail phase also requires careful handling. Bruises are often not immediately visible, but areas will discolour later to dark green or brownish due to enzymatic processes. The wounds and bruises are more susceptible to decay.
Yellowing is a symptom of senescence. Photo by WUR
SenescenceSenescence is associated with loss of texture (soft tissue) and flavour. Leaf edges or complete leaves lose their green colour and become yellowish or brown. They become also more susceptible to decay. Lettuce with senescence symptoms may have been stored under suboptimal conditions such as too high temperature or were simply stored for too long.
Development of rot on a damaged leaf edge. Photo by WUR
Moulds and rotsExamples of fungi on lettuce are Anthracnose (small circular water-soaked lesions ) and Botrytis (grey mould). Removing the outer leaves during harvest and storage under cold conditions will reduce the likelihood of growth of these fungi. Bacterial soft rot mostly starts on bruised leaves. Prevention of damages and storage at low temperature are measures to reduce the incidence of this rot.