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higher susceptibility to blackspot bruising. Cultural practices, such as fertilization and irrigation, or environmental conditions that physiologically age tubers will result in tubers with a higher potential to be blackspot-damaged during harvest and handling. Environmental conditions like hot weather during the growing season have a tendency to physiologically age tubers. Not all potato varieties are equally susceptible to blackspot bruise. For example, Ranger Russet is very susceptible to blackspot bruise while Russet Burbank is rated as susceptible. Varietal susceptibility to blackspot bruise is a combination of its physical and chemical properties. Physical properties include tuber and cell size. Chemical differences in blackspot bruise susceptibility among varieties are largely due to tyrosine. Varieties with higher tyrosine levels are more susceptible to blackspot bruise. Tubers with high specific gravity, under the right conditions, have a higher potential to blackspot-bruise than those with lower specific gravity. However, specific gravity variation within a variety has a greater effect on blackspot bruise than the specific gravity among varieties. That is, not all varieties with high specific gravity are more susceptible to blackspot bruise than those with lower specific gravity. TESTING BLACKSPOT POTENTIAL A grower-field survey was conducted by University of Idaho researchers in Idaho in the mid-1990s to determine the effects of vine maturity, specific gravity and soil moisture on blackspot bruise potential. Tubers harvested from vines that were rated in late August as more mature (less green) tended to blackspot bruise more than tubers harvested from plants that were less mature (more green). For Russet Burbank, more blackspot bruising occurred on tubers harvested from fields that had 70 percent or more dead vines in August, and for Ranger Russet more blackspot bruising resulted from fields with 20 percent or more dead vines. Higher specific gravity tubers in the survey were generally more susceptible to blackspot bruising. Russet Burbank tubers with specific gravity above 1.080 and Ranger Russet tubers with specific gravity above 1.085 were more susceptible to blackspot bruising. A third factor in the survey they measured was available soil moisture. Fields with lower available soil moisture (less than 50 percent) at harvest tended to have more blackspot bruising. When the data from one year were analyzed, they found that vine maturity accounted for 21 percent of the variability in blackspot bruising, and specific gravity plus vine maturity accounted for 35 percent. Altogether, vine maturity, specific gravity and field moisture accounted for just over half (53 percent) of the measured blackspot bruise. This data shows that maintaining green vines for Russet Burbank and Ranger Russet and maintaining soil moisture above 60 percent will help reduce the potential for blackspot bruise at harvest. However, tubers can still be damaged if not properly handled. Producers cannot control factors such as genetic disposition to bruise, but they need to manage factors that are under their control, such as vine-kill date, soil moisture, drop height and temperature. Although producers cannot directly control temperature, which influences tuber bruising, they can harvest at the correct tuber pulp temperature. Regardless of bruise type, colder tuber pulp temperatures increase damage. Both shatter bruise and blackspot bruise are more severe at colder temperatures (Table 1). Ideally, tubers should be harvested when pulp temperatures are 50 to 60 degrees F. If tubers are harvested warmer than 60 degrees, it could be challenging to remove field heat in storage, resulting in higher rot potential. A critical harvest management tool for minimizing bruising is to monitor tuber pulp temperature of every truckload. Also note in Table 1 that at six- and 12-inch drop heights at 42 or 56 degrees F, blackspot bruise develops more readily than shatter bruise; as drop- height increases to 24 inches, shatter bruise increases proportionally more than blackspot bruise. Another critical harvest management tool for minimizing bruising is to minimize drop heights on all equipment on which tubers are handled. PG TABLE 1. Tuber temperature and force effects on blackspot and shatter bruise of Russet Burbank potatoes. BLACKSPOT BRUISE HEIGHT2 (IN) 6 12 24 42° (3%) 53.5 66.5 41.0 SHATTER BRUISE 42° (3%) 0.8 5.0 51.3 1 Adapted from Smittle, D.A. et al. 1974. Amer Potato J: 51: 152-164. 2 Dropped a 3.5-oz plug one time on the bud and stem ends of tubers. 3 Data is percentage of bruised areas developing detectable damage. Putting Rows to Bed Saving resources, increasing yields with bed format BED TESTS Go to our website at www.potatogrower.com, go to “Extras” and click on the link for “Harvest Management chapter from Potato Production Systems (2003).” There you’ll find more discussion about managing tuber bruising. www.potatogrower.com 25 BLACKSPOT BRUISE 42° (3%) 12.3 49.3 70.3 SHATTER BRUISE 42° (3%) 0 0.8 10.8