Factors contributing to the blackspot bruise potential of Idaho potato fields

Blackspot bruise is a major problem in the fresh market and frozen french f ry industry. The blackspot bruise potential of Russet Burbank and Ranger Ru sset in Idaho potato fields was determined by surveying commercial fields d uring 1993 and 1994. Management factors were monitored to determine what pr actices were contributing to blackspot susceptibility in addition to mechan ical damage. The survey included 17 Russet Burbank and 3 Ranger Russet fiel ds in 1993, and 28 Russet Burbank and 8 Ranger Russet fields in 1994. The 1 993 season was unusually cool and wet whereas 1994 was warmer than normal, resulting in a wide range of environmental conditions for the 2 year study. Blackspot bruise potential was determined at different stages of tuber phy siological maturity by collecting samples several weeks prior to normal har vest, immediately before harvest, and after storing sub-samples for several months. The blackspot bruise potential was measured by both impact and abr asive peel tests. Field maturity was the factor most consistently related to blackspot potent ial both years. In 1994 a multiple regression of 3 independent variables - field maturity index, specific gravity, and percent available soil water at tuber sampling, compared with the blackspot potential as the dependent var iable gave a correlation coefficient of r = 0.73 (p = 0.001). Due to the co ol, wet growing season in 1993, there was not enough variability in specifi c gravity and available soil water among the fields sampled for these facto rs to correlate with the blackspot potential. The available fertility data, although not complete for all fields, indicated no direct relationship bet ween N, P, or K fertilization and blackspot potential. Preharvest samples i n late August had lower blackspot potential than harvest samples in mid Sep tember, and storage samples in February had the highest susceptibility. The re was a consistent increase in blackspot severity when tubers were equilib rated at 4 C compared with 10 C prior to bruising.