PI-009: Seed Potato Tuber Inspection
Appendix 6

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6a: Rhizoctonia

Causal Agent:

Rhizoctonia solani (Black scurf; Rhizoctonia) is a common seed and soil borne fungus which survives as black resting structures called sclerotia. In the soil, the fungus can also survive as fungal threads, called mycelia on organic debris. Infections start when sclerotia in the soil or on seed tubers germinate, producing mycelia which colonize stems and roots near the seed piece. The pathogen enters tubers in storage by infecting lenticels, eyes, and skin. The fungus does not penetrate deeply into the tuber tissue. Pathogen growth on the exterior of the tuber can spread to other tubers in storage. The disease is favoured in tubers that are harvested late and stored under cool conditions. The fungus forms the characteristic black sclerotia (black scurf) on tuber surfaces before and after harvest. Planting seed potatoes infested with sclerotia increases the amount of Rhizoctonia inoculum in the soil. Rhizoctonia solani can survive in the soil for at least eight years.

Tuber Symptoms:

Rhizoctonia is identified by the presence of hard, black or dark brown sclerotia which adhere very tightly to the surface of the tubers, but do not penetrate the skin. Frequently referred to as "the dirt that will not wash off", sclerotia will vary in size from small specks to large masses up to 2.5 cm across. Infected seed may exhibit brown lesions on sprouts.

27 | Sclerotia range from 0.1 to 2.5 cm
27 | Sclerotia range from 0.1 to 2.5 cm. Description follows.
Description for 27

This image shows sclerotia, a symptom of Rhizoctonia, ranging from 0.1 to 2.5 cm on a yellow potato. You can see the sclerotia a black or dark brown, hard substance covering three quarters of the visible potato as well as in between its crevices.

28 | "The dirt that will not wash off"
28 | "The dirt that will not wash off". Description follows.
Description for 28

This image shows a close up of "the dirt that will not wash off" i.e. sclerotia. The black/ dark brownish substance appears rough and hard on the yellow potato’s surface. The sclerotium has white specks on its surface. It appears to be firmly attached to the potato.

29 | Fungal threads (mycelia) surrounding lesion
29 | Fungal threads (mycelia) surrounding lesion. Description follows.
Description for 29

This is an image of fungal threads (mycelia) surrounding a lesion on a yellow potato. The lesion is magnified in the image so that it appears to be 1 inch wide, descending deep into the potato. In the area around the lesion there are white dry spots which seem elevated in comparison to the rest of the visible potato.

30 | Sclerotia can be picked off of the tuber surface
30 |  Sclerotia can be picked off of the tuber surface. Description follows.
Description for 30

This image portrays sclerotia that can be picked off the tuber's surface. The image shows a close up of the sclerotia which appears like a brown, flakey scab that could be removed from the surface of the dry, yellow potato.

6b: Rhizoctonia - Scoring

Scoring:

Rhizoctonia is rated in the same manner as scab and is scored either singly (if only one disease is present) or in combination with scab when both are present on the tubers being inspected. Refer to current tolerances as per Seeds Regulations Part II. In order to approximate the percentage coverage of the tuber, the surface can be divided into three dimensional quadrants where each half of the tuber represents 50% of the surface area (31). Both sides of the tuber are examined (32) and the disease level is estimated by the size of quadrant the sclerotia would occupy if pooled to a single area.  Examples of trace to severe coverage are given below (33).

31 | Proportions of coverage
31 | Proportions of coverage. Description follows.
Description for 31

This image demonstrates the surface area percentages a defect can cover. The first tuber has a “50%” super imposed over it, in white, showing that the entire visible side of the tuber represents 50% of the total surface area.

The second tuber is divided with a black marker to show that ½ of the visible area of the potato represents "25%", 1/3 denotes "12.5%", 1/8 represents "6.25%", 1/16 of the potato signifies "3.13%", and 1/32 represents "1.57"”. Each marked area of the tuber has the corresponding percentage super imposed on it.

The third tuber is marked similarly to the second tuber but does not have the percentages marked on it.

Disease Level and Percentage Coverage
Disease Level Percentage Coverage
Trace 0% - 1%
Light >1% - 5%
Moderate >5% - 10%
Severe >10%
32 | Tubers seen from both sides
32 | Tubers seen from both sides. Description follows.
Description for 32

This image is divided into 2 parts: The first part has two pictures of the same tuber, one picture for each face of the potato. The potato in the first part is a yellow skinned potato with no sclerotia on the first side and the second side has very few sclerotia covering a small surface area.

The second part of the picture shows two images of the same potato but this potato has very few sclerotia on the first side and on the second side it has more sclerotia, covering approximately 50% of its face.

33 | Spectrum of Rhizoctonia, trace to severe
33 | Spectrum of Rhizoctonia, trace to severe. Description follows.
Description for 33

This image shows the gradient of sclerotia coverage from trace to severe. There are six tubers shown one image for each side in order they are: trace with 1% or less coverage of Rhizoctonia, light with >1% - 5% coverage of Rhizoctonia, top-light with 5% of Rhizoctonia, moderate with >5% - 10% coverage of Rhizoctonia, top -moderate with 10% coverage of Rhizoctonia, and severe with more than 10% coverage of Rhizoctonia.

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