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24, chemin de Borde Rouge -Auzeville - CS52627 31326 Castanet Tolosan cedex - France

Last update: May 2021

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Potato tuber blemishes: understanding & diagnosis

Other discolorations

Discolorations due to physiological disorders

Browning after skinning
Visual diagnosis

The skin of the tuber is partially or completely peeled off, exposing the underlying tuber flesh to the air

As a consequence, due to enzymatic reaction, rust-brown areas are formed on the tuber surface


Mechanical damage on immature tubers with unset skin

skinning 180


skinning 3.180
Potato greening-NK-650
Visual diagnosis

Partial or complete greening or pink discoloration of the tuber skin


Excessive exposure to light:either in the soil (open ridge) or following unadequate storage conditions

Green or pink discoloration depends on the variety

Discolorations due to biotic factors (under construction, to be completed)


PMTV (Potato Mop Top virus)


TRV (Tobacco Rattle Virus)

Visual diagnosis

Brown irregular lines and blotches

External symptoms linked to brown arcs and necrotic areas internally

Spraing due to PMTV or TRV are generally not typical enough to allow a precise diagnosis

Confirmatory diagnosis

Confirmatory diagnosis relies on serogical or DNA diagnostic tests

PVYNTN  (Potato Tuber Necrotic Ringspot Disease = PTNRD)



PVY ntn
Visual diagnosis

Brown blisters in the form of arcs, rings or lines

The necotic tissues become later depressed and may even crack

The symptoms remain limited to the periderm

Confirmatory diagnosis

Confirmatory diagnosis relies on serogical or DNA diagnostic tests


Violet root rot (Rizoctoniacrocorum)

Rhizoctone violet
violet root rot

Visual diagnosis

Tubers are often covered with reddish-purple mycelial network. Under the mycelial mats, tubers may have dark gray spots covered with purplish black sclerotia




Pink rot (Phytophthora erythroseptica)


Pink rot (Phytophthora erythroseptica)

Visual diagnosis

Dark brown or black irregular on the tuber surface

Symptoms may show specifically around lenticels

External symptoms linked to lesions internally

Infected tubers may leak liquid from lenticels and feel spongy

Similar symptoms may be caused by Pythium spp. and Phytophthora infestans

Confirmatory diagnosis

Isolation onto agar from the leading edge of the internal lesion

Microscopoc observation of the spores

In the case of pink rot, the internal tissue turns pink within 30 min after exposure to air

Leak rot (Pythium spp.)


Late blight (Phytophthora infestans)

Late blight


Discolations due to unknown causes

Golding bloch-like symptoms
Golding-bloch like symptoms2


Golding bloch-like symptoms-1
Visual diagnosis

Brown discoloration, golding bloch-like symptom; also occuring on red skin varieties


So far unidentified causes;  putative physiological disorders and/or uncomplete pigmentation