PI-009: Seed Potato Tuber Inspection
Appendix 4: Dry Rot

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Causal agents:

Dry rot is the term used to describe dry decomposition, usually starting in cuts, bruising or other injuries. Dry rot is most often caused by Fusarium spp. (7,8).Other conditions or pathogens may result in dry rot. White knot or Starch spot (9) is a bruising which appears as a hard white lump in the tuber flesh a few millimeters below the skin; this is similar to Black spot except the coloration is lacking. Both White knot and Black spot (10) start out as a bruising injury but may develop into a dry rot in severe cases. Additional pathogens resulting in dry rot include, Late blight - Phytophthora infestans (11), Early blight - Alternaria solani (12), Blackleg - Pectobacterium carotovorum subsp. carotovorum, as well as severe Skin Spot - Polyscytalum pustulans, among others. As well, mechanical and physiological damage which has dried, such as pressure bruising, or dried frost and freezing injuries often result in dry rot. In storage below 4°C, dry rot fungi in small lesions may remain dormant, however upon planting may be quickly attacked by soft rot bacteria which rot the tuber very quickly. Certain pathogens may be causative agents of both dry rot and soft rot.

Scoring:

As in the case of soft rot or wet breakdown, it is not always necessary to identify the specific pathogen affecting the tuber. Dry rot is scored under the dry rot tolerance when it affects the flesh of the potato and penetrates more than 6 mm in depth or is less than 6 mm in depth but covers more than 10% of the surface area. It may be slightly moist but not wet, and thus is not considered a "wet" rot. Refer to Seeds Regulations Part II for tolerances.

7 | Fusarium stem end rot
potato, Fusarium stem end rot
Details for Image 7

Fusarium spp. The potato is a light-yellow colour in a horizontal position, with the state of dry decomposition beginning on its left side. The decomposing area takes up 30% of the viewed side of the potato, it is a purple-brown colour which demonstrates bruising, and has a few wrinkles on the bruised area.

8 | Fusarium dry rot
potato, Fusarium dry rot
Details for Image 8

This image of a potato is another demonstrating a form of dry rot caused by Fusarium spp. The potato is cut in half so that the inside of the potato is visible. The decomposition begins on the left side of the potato, takes up 75% of the cut surface area, and is a dark-brown colour. The decomposition occurs in one large spot and has a few surrounding smaller spots. Several hollow areas have formed due to decomposition.

9 | White knot (Starch spot)
potato, White knot
Details for Image 9

This is an image of a potato demonstrating dry rot in the form of a white knot or starch spot. This form of decomposition involves bruising which appears as a hard white lump in the tuber flesh a few millimeters below the skin. In this image there is one main spot shown on the potato; a small portion of the skin has been peeled off, but is still attached, demonstrating that the white spot occurs below the potato skin. The spot in this picture is shaped like an upside-down teardrop, with a tiny dark spot showing another area beginning to decompose.

10 | Black spot
potato, Black spot
Details for Image 10

This is an image of a potato demonstrating dry rot in the form of a black spot. Black spots are similar to white spots in that they are a hard lump in the tuber flesh a few millimeters below the skin, but in this case they are a dark black/brown colour. In this image only a portion of the skin the size of a nickel has been peeled off to demonstrate this form of decomposition below the skin.

11 | Late blight-tan to brown granular dry rot
potato, Late blight-tan to brown granular dry rot
Details for Image 11

This is an image of a potato demonstrating dry rot in the form of a Late Blight – tan to brown granular dry rot. The potato in this image has a light brown skin and has been cut in half. On the half that is exposed, 75% of its cut surface area appears as a dark brown colour, demonstrating its decomposition, leaving the other 25% being its natural cream colour.

12 | Early blight-shallow infection, granular dry rot dry corky and brown
potato, Early blight-shallow infection, granular dry rot dry corky and brown
Details for Image 12

This is an image of a potato demonstrating dry rot in the form of an Early Blight-shallow infection, dry corky and brown. It is a red-skinned potato with the rot taking up approximately 10% of its surface area. The area with the decomposition appears dry, crusted, and somewhat bumpy. It is a medium and dark-brown colour, and with a crack forming in the middle.

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