Iron spot

Iron spot, commonly referred to as internal brown spot or rust spot, is a non-parasitic physiological disorder that primarily affects potato tubers. Unlike infectious diseases caused by pathogens, this condition is driven by internal metabolic imbalances within the plant, triggered by specific soil and environmental stress factors during the tuber bulking stage.

The primary symptom of iron spot is observed internally when a tuber is cut. The flesh displays scattered or clustered necrotic spots that vary in color from rusty brown to dark red. These spots consist of localized dead tissue cells where iron compounds have accumulated and oxidized. In many cases, the external skin of the potato remains perfectly smooth and appears completely normal.

Development of this disorder is heavily influenced by soil conditions. Low calcium availability is the most significant factor, often exacerbated by high soil acidity. Environmental stressors such as extreme heat and fluctuating soil moisture levels during the peak growing season interfere with the plant's ability to transport nutrients to the tubers, leading to the rapid localized death of cell tissue.

The economic impact of iron spot is significant for potato growers and processors. Tubers affected by this disorder are downgraded for retail, as they lack the aesthetic quality required for fresh markets. Furthermore, severe internal discoloration makes these potatoes unsuitable for processing industries, such as chip or fry manufacturing, and increases the susceptibility of the crop to post-harvest storage rots.

Prevention focuses on optimizing soil health and nutrient management. To mitigate the risk of iron spot, farmers should:

• Test soil pH and apply lime to correct excessive acidity.
• Ensure a consistent and adequate irrigation schedule to prevent drought stress.
• Balance nutrient applications, ensuring sufficient calcium levels to support cell wall development.
• Use balanced fertilization programs based on annual soil analysis to avoid nutrient deficiencies or toxicities.