Internal black spot

• Symptoms

Internal black spot (internal bruising, blackspot, blue spot) appears just under the undamaged skin of the tuber in the form of roughly circular blue-grey spots (ash spots) (photos 1 and 2).

This colouration is due to the formation of pigments (melanine) by enzymatic oxidation of the main phenolic component of the potato, i.e. tyrosine. They accumulate in the damaged tissues (lesions in the cell membranes).

Thus the symptom usually appears 1 to 3 days after the cell damage, hence the impossibility of detecting it during handling operations. Placing the tubers at high and humid temperature accelerates the biochemical reactions and the symptoms will be revealed on peeling the tubers around twelve hours after the impact.

• Risk factors

While tuber susceptibility is dependant on the cultivar, it can increase, sometimes significantly, according to dry matter content and especially with a high tyrosine level. A dry matter content above 21% often means a significative level of tuber susceptibility. Potassium deficiency in the plant, low handling temperatures, excess CO₂ and lack of turgor pressure also increase the susceptibility of the tuber to this type of damage.

• Control measures

To reduce tuber susceptibility and the risk of injuries during harvesting, special attention should also be paid to specific points concerning agronomical practices: sufficiently deep and crumbly soil preparation, planting depth consistent with the limit of tillage, sufficient and well balanced potassic fertilisation, well planned irrigation, choice of date for haulm destruction taking into account the maturity of the tubers and their dry matter content.