Additional Nitrogen Application Reduced White Drupelet Disorder in 'Sweetie Pie' Blackberry

White drupelet disorder (WDD) in blackberry (Rubus subgenus Rubus) is an abiotic condition resulting from a cultivar and environment interaction. Although high temperatures and light intensities have been implicated, little is known why this disorder manifests. Other factors, such as overall plant stress, may be contributing influences. In this study, three treatments were applied to examine whether the addition of nitrogen (N) can reduce WDD on 'Sweetie Pie' erect blackberry over three seasons. An initial 50 lb/acre (56.0 kg.ha(-1)) N was applied to all plots at budbreak. Two additional N application treatments of 100 kg.ha(-1) were applied at one time (1x) or five, 20-kg.ha(-1) applications (5x), spaced 1 week apart for 5 weeks starting at bloom. One control treatment of no additional N (0x) was also included. Berries were harvested and weighed as a total, then berries with white drupelets were separated out and weighed. The two values were divided to create a proportion and were then multiplied by 100 to determine the percentage. Nitrogen application decreased the percentage of white drupelet berries from 13.0% (control) to 10.0% (one additional application) and 9.1% (five additional applications). WDD for the 0x treatment correlated negatively to maximum high daytime temperatures during May (r = -0.58, P = 0.03) over the three seasons. Occurrence of white drupelets by treatments 0x, 1x, and 5x correlated significantly with the cumulative number of rainfall events (r = 0.49, 0.47, and 0.46, respectively). Leaf chlorophyll index and photosynthesis measurements were unaffected by treatment. Although it is likely that multiple factors are involved in the development of white drupelets, additional N may reduce the problem.