PI-009: Seed Potato Tuber Inspection
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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.
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.
6b: Rhizoctonia - 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).
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||Percentage Coverage|
|Trace||0% - 1%|
|Light||>1% - 5%|
|Moderate||>5% - 10%|
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