Quality analysis of ware potato

Common defects of potato


Soft rot
The most common cause of soft rot is Pectobacterium atrosepticum bacterium which causes blackleg in the field. Also P. carotovorum may cause rotting in bad storing conditions. In warm summers also Dickeya-bacteria are able to cause blackleg and rotting of tubers. Bacteria may infect tubers from the stolon end already in the field (right) or go through the tuber surface via a mechanical damage (left)

Soft rot tuber is always ineligible.

Late blight
Asexual spores (conidia) of late blight (Phythophthora infestans) reach the tubers often already in the field, when rain washes the spores to the soil, or at harvest, when the tubers mix with the contaminated soil, but the rottening of tubers may often start not until in storage. In addition, soft rot bacteria may go into the tuber through the late blight damage. It is often difficult to detect the rotten tubers at sorting.

Tubers damaged by late blight are always ineligible.

Dry rots (gangrene and dry rot)
Spores of Phoma- and Fusarium -fungi lie as latent contamination on tuber surface and cause dry rots through mechanical damages generated at lifting or grading (damage of Phoma in the picture). 

Tubers damaged by dry rot are always ineligible.

Ring rot
Ringrot (Clavibacter michiganensis spp. sepedonicus) is a nowadays quite rare bacterial disease. It can be identified by pressing lightly a halved tuber from the perimeter, whereupon rotten tissue is extruding from the vascular ring. Ring rot is a quarantine disease, which must be reported to Evira.

Tubers damaged by ring rot are always ineligible.

Leak is caused by soilborne Pythium-species. They belong to the same group of organisms as the cause of late blight. The damage is limited to the middle of the tuber, inside the vascular ring. It is often impossible to detect the rotten tuners at grading.

Tubers damaged by leak are always ineligible.


Pink rot
The soilborne organism (Phythophthora erytroseptica) causing pink rot is a near relative to the cause of late blight. It damages tubers quite rarely, predominantly in wet soil. Tuber flesh changes to pink red after about 15 minutes after the tuber is cleaved.

Tubers with pink rot are always ineligible.

Moptop belongs to the few viruses which cause disease symptoms in tubers. Brown curves and rings in tuber flesh are a defence reaction of the tuber, when it builds a layer of suberized cells to isolate the virotic areas. No resistant varieties exist, but the magnitude of disease symptoms vary according to the variety.

Tuber with moptop symptoms is always rejected.

Holes caused by R. solani (1)
Tubers may comprise holes of varying depth, especially in bud end, caused by stem canker (R. solani). Another type of Rhizoctonia-damage is dry core, which is a result of infection of lenticels.

Tuber is rejected if the depth of hole is >4 mm.

Holes caused by R. solani (2)
So called dry core disease symptom is formed when the hypha of Rhizoctonia infects open corkpores of the tuber.

Tuber is rejected if the depth of hole is >4 mm.

Holes caused by wireworms
Wireworm, which is a larva of click beetle, may dig to the tuber. Larvas appear especially when potato is cultivated after grass and in hills near grassy ditches.  

Tuber is rejected if the depth of hole is >4 mm.

Holes caused by slugs

Tuber is rejected if the depth of hole is >4 mm

Common scab and net scab are caused by soilborne Streptomyces-bacteria. Defects resembling common scab are also caused by soilborne fungi Spongospora subterranea (powdery scab) and Rhizoctonia solani (scabby areas resembling net scab)

A tuber is scabby and must be rejected, if scab covers more than 25 % of tuber surface or extends deeper than to 2 mm.

Greening occurs, when the tuber is exposed some time to light, either in hill, storage or shops. Greening as such is harmless, but it indicates an increased content of glycoalkaloids (solanin).

Because glycoalkaloids are mildly toxic to humans and give tuber a bitter taste, green tubers should be rejected. A tuber is regarded green if pale green flush is exceeding 1/8 of the surface or cannnot be removed with normal peeling.

Mechanical damage
Tubers damage mechanically at harvest and grading.

Defect is harmful and the tuber must be rejected if the damage extends deeper than to 4 mm.

Blackspot, or blue discoloration, is an internal discoloration resulting from impact that damages cells in the tissue beneath the periderm without causing an observable break in the periderm. They appear after grading of potatoes, which have softened due to water loss in long storage and are usually found only after peeling.

Defect is harmful and the tuber must be rejected if the damage extends deeper than to 5 mm.

Growth cracks
Sensitivity for growth cracks is a variety property. In susceptible varieties growth cracks form as a result of fluctuating moisture conditions in soil during the growing season. Infection of stem canker (R. solani) may worsen the cracking.  

Defect is harmful and the tuber is rejected if the crack extends deeper than to 4 mm.

Misshapen tubers
Tubers develope malformed in unfavorable growing conditions (varying moisture, period of high temperature, hard soil). Malforming is often result of second growth phenomenon. In addition, stem canker (R. solani) infection is a considerable cause of malforming.

Badly misshapen tubers are not suitable to ware potato.

Hollow heart
Sensitivity to hollow heart is a variety property. Hollowness appears most often in big tubers and is a result of fast growing of the tuber in fluctuating moisture conditions. Also stem canker (R. solani) may accomplish tubers with hollow heart. 


Tuber with hollow heart is always rejected.

Internal brown spots
Sensitivity to internal brown spots is propably a variety property. Lesions may occur shattered everywhere in tuber flesh but most often in the tissue inside the vascular ring and in the bud end of the tuber. It is propably a result of combined effect of lack of water and/or calsium in soil during periods of fast growing.

Tuber with internal brown spots is always rejected.

Blackheart is easy  to diagnose: the center of tuber is black and may contain a cavity. It occurs as a result of a deficiency of oxygen available to the tuber for respiration. Low-oxygen conditions may develope in wet soil, if potatoes are packed in tight plastic pags or the temperature of potatoes rise over 30 degrees.

Tuber with black heart is always rejected.

Low temperature injury
Cold damage is seen from a cleaved tuber as grayish areas in tuber tissue, often around vascular ring. Damaged tuber tastes clearly sweet. Soft rot backeria may later make the tuber rotten completely.


Frostbitten tuber is always ineligible.

A tuber starts to build sprouts when it is stored in too warm conditions (>4 °C).



Tubers with sprouts longer than 3 mm are rejected. Early potatoes should however be totally sprout-free.

Silver scurf
Silver scurf is a mild shallow skin damage caused by Helminthosporium solani -fungus. It is not regarded as a bad quality defect, but may worsen the tuber appearance conciderably, especially in springtime. It also increases the evaporation of water from tubers during storing and the amount of waste at optical grading.

Silver scorf is not regarded as quality defect.

Black scurf
Black scurf consist of black sclerotia of Rhizoctonia solani, which help it overwinter.



Black scurf  is not regarded as a quality defect according to UNECE standard.