Mechanical injury and cracking, nonpathogenic
Mechanical damage has repeatedly been singled out as a factor contributing to potato losses during the first 3 months of storage. Injuries provide an "avenue of entrance" for many fungal and bacterial pathogens in storage including Pythium leak (11), Fusarium tuber rot (9), and bacterial soft rot (5b). Mechanical damage may consist of bruises, cuts, and deep abrasions that appear harmless externally, but internal examination often reveals the presence of infected tissue. "Harvest" or "thumbnail" cracks are small curved cracks that result from mild bruising and subsequent drying of tuber surfaces. Cracks may become apparent in marketing channels because of low relative humidity. These defects are usually shallow (up to 1 / 16 in. deep), but are sufficient to allow pathogen entry. They result from glancing impacts of tubers with other tubers or with harvesting and handling equipment; mechanical shock occurs in tubers with high turgor pressure. Controls for mechanical injury include gentle handling, harvesting only physiologically mature tubers, avoiding harvest from cold and wet soils, and using chemical vine killers to speed periderm maturation. Seed should be warmed before grading, or special care should be used in grading cold tubers.