The injuries affecting tubers fall into four categories according to the level of damage to the skin and the underlying cell layers:
In all cases, mechanical damage observed on tubers has been caused by shocks they have received during harvesting or handling operations (bulk or box filling and packing etc.). The force, angle and direction of the impact, the degree of suberisation of the skin and the physicochemical characteristics of the tuber (dry matter and tyrosine content, cell turgor pressure etc.) result in the appearance of one of the symptoms described in this chapter.
Symptoms on tubers
The symptoms of pressure bruising usually appear on tubers after several months in storage. Although they may be visible on the tubers sitting on the top of the stack of store boxes, they are usually more numerous on those at the bottom and near ventilation ducts (hence their name “ventilation flats”).
On the surface of the tubers, depressions or flat areas are present, sometimes marked and extensive, and sometimes accompanied by skin wrinkling and a dull appearance (photos 1 to 4). They may be associated with a lesion, which is usually quite superficial and fairly small. In the most serious cases and after a long stay in storage, a symptom of internal blackening may appear in the underlying tissues.
These symptoms should not be confused with natural flattening observed in the field when growing conditions are bad (poor soil structure, poor soil preparation, presence of clods and stones or deep planting).
Control
The localised collapse of the tuber’s surface tissues is due to two main causes which may act separately or in combination.
This damage is generally due to localised dehydration caused by ventilation that is either too strong or too dry. It is exacerbated when the ventilation is directed at tubers that are still immature, suffering mechanical damage or insufficiently healed.
Stack pressure can also produce or exacerbate symptoms when the top of the stack is too high (above 3.5 m), or when the shape and grade of the tubers reduce the contact surfaces between them and indirectly increase contact pressure.
In severe cases, there is damage to the cell membranes of the underlying internal tissues accompanied by local asphyxia causing the appearance of internal blackening after the removal of the tubers from storage
Placing only mature and undamaged tubers in storage will reduce the risk of subsequent dehydration; this means paying very special attention to haulm destruction, to retention time in the soil and to harvesting operations. With bulk storage, it is advisable to limit the height of the stacks to between 3 and 4 metres.
As soon as the tubers go into storage, the ventilation rate must be adjusted according to the actual quantity of tubers stored in the building. It is important to ensure a wound healing period of 10 to 15 days following store or box filling and to avoid a too rapid cooling rate. The drying nature of the ambient or refrigerated air can be minimised by reducing the temperature difference between the air and the tubers and by maintaining high moisture levels. The use of air humidifiers can help in this respect.