Measurement of black spot susceptibility of potatoes

The black spot reaction was initiated by dropping plugs of varying weights from different heights onto the stem end of potatoes with known temperatures. The degree of discoloration was assessed in a scale from 0 to 5, the results were added and the sum was divided by the number of tubers. Relationships between black spot incidence, temperature and impact were very close in samples containing 100 or more tubers. This was not always the case, however, if samples were of a more manageable size of 20 to 25 tubers. Results indicated the desirability of testing under a standard set of conditions—such as 100 g falling 30 cm onto the stem end of tubers with a temperature of 10 C. The results of visual assessment of the black spots were closely related to the results of a more objective, but also more laborious, method in which the amount of discolored tissue was weighed.