Symptoms

Despite all the care given to the crop, over-incubated seed potatoes (phase III) have a slow, staggered, heterogeneous emergence. The developing aerial parts remain stunted and the root system undeveloped, rendering the plants vulnerable to drought. The slow growth of the shoots makes them extremely susceptible to attacks by soil pathogens (especially black scurf). As the tubers are formed early, the period of vegetation is shortened (which can be beneficial in the case of early potatoes). In extreme cases, notably when the temperatures immediately following planting are low, or when the tubers are desprouted, the plant tuberises in the soil with no preliminary formation of aerial stems.

Conversely, under-incubated seed potatoes (in phase I) result in late emergence and a limited number of stems (numerous single-stem plants) but subsequently have normal vigour. This phenomenon is observed mainly with early export and prompt planting in autumn.