Reducing Potato Internal Spotting

Internal Rust Spot (IRS) also known as Internal Fleck, is a physiological internal defect in which small brown spots, due to cell death, appear in the tuber. Internal Brown Spot (IBS) is any spot contained inside the vascular ring in the medullary tissue.


Internal spotting is usually associated with either poor or interrupted supply of calcium to newly formed and expanding cells. Calcium is incorporated into cell walls and cannot be redistributed therefore it is critical to supply calcium to new cells as they are formed. Calcium can not enter cells once cell expansion takes place and too little calcium will lead to the collapse of cells and cell death leading to the visual appearance of spotting.

Internal disorders such as internal rust spot (IRS) or internal brown spot (IBS) can be reduced by a good calcium supply, in the form of calcium nitrate, at hilling or during tuber initiation.

Calcium and internal spotting in potatoes

This study demonstrates the relationship between the rate of applied calcium, calcium content of peel and the incidence of internal rust spot.

Calcium and internal rust pot in potatoes

Experience shows that ensuring there is a minimum of 0.15% calcium in the peel, improves potato skin finish, boosts disease tolerance and minimises IRS. Peel analysis is a good way of confirming whether a disease or skin finish problem is calcium related. Even small amounts of calcium in the tuber can make a big difference.


Alongside potassium, calcium and magnesium, boron is an important element present in the cell wall. Here it acts as a cement between pectins, providing cohesive strength for cell tissues. Therefore boron affects tuber storage quality characteristics. Boron also affects calcium absorption, so supplies are important to ensure a balanced nutrition.

This trial shows how boron influences the calcium content of tubers and so also the incidence of internal rust spot.

Other crop management practices influencing internal spotting

  • Avoiding fields, with adverse factors such poor drainage or low water holding capabilities
  • Scheduling irrigation to maximize quality characteristics
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