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
Shatter bruises (fracture damage, external or split damage) are wounds characterised by sometimes deep and extensive breaks in the cell walls. Depending on the symptoms, they will be named as follows: cracks, open shatters or internal crushing (photos 1 and 2). Breaks are directly visible on the skin of the tuber immediately following strong impact. With starch potatoes, major crushing may cause the release of starch grains, inducing “starch pile” symptoms (photos 3 and 4).
The use of colouring agents (iron perchloride, pyrocatechol) facilitates visual detection if applied as soon as the injury occurs. The lesions, which also affect the tuber flesh and take several days to heal, provide an easy entry point for different rotting agents (Phoma spp., Fusarium spp., Pectobacterium spp., etc.).
Control
The susceptibility of the cultivar plays an important role in this type of wound. Under the same conditions of production and for the same force of impact, some cultivars burst completely while others only have a few shallow cracks. Apart from this cultivar aspect, a number of other factors can increase the tuber’s intrinsic susceptibility: high turgor pressure due to heavy rain and excessive irrigation, immaturity, premature lifting and low handling temperatures.
Globally, all the mechanical equipment used to reduce tuber damage should be implemented throughout harvesting and handling operations: height of fall less than 30 cm, belt speed under 40 m/min, fall impact dampener, high-performance sheaths and foam protection, etc. During harvesting, the settings must be very accurately adjusted: chain rotation speed consistent with the engine speed (ideally, a ratio of 0.8 to 1.2), minimum shaking, minimum aggression by devices used for the elimination of haulms and clods. Preferably, harvesting must be done when the tubers have had sufficient maturation time in the soil to have a well suberised skin. At the same time, it is necessary to avoid too dry, too humid or too cold conditions. For all handling operations, the temperature of the tubers should never be less than 10°C.