The effects of N fertiliser application rates on red drupelet disorder (reversion) in 'Ouachita' thornless blackberries grown under tunnels

Red drupelet disorder (RDD), sometimes called reversion, red cell, or reddening, is a physiological occurrence in blackberries (Rubus fruticosus) which causes individual drupelets to revert to a red colour from black, reducing marketable yield. The disorder usually develops during postharvest cool storage. The objective of this trial was to assess the impact of nitrogen (N) application on fruit and foliar N concentration, with the aim to identify any link between N availability and RDD. Three levels of N (52.85, 105.7, 211.4 kg ha^-1 total) were applied via weekly fertigation during the harvest period to 'Ouachita' blackberries grown under 150 µm high-UV transmittance polythene tunnels in northern Tasmania, Australia. The design consisted of three blocks of three 105.7 m long rows arranged in a complete randomised block design, with each row receiving a N application treatment. Fruit was sampled at five dates every 15 days between January 11 and March 11, 2016 by harvesting every fruit from four randomly selected 3 m sections of cane, from which subsamples of 20 fruit per row were taken for further analysis. Fruit were assessed for drupelet disorder by counting individual drupelets expressing RDD then weighed and homogenised to measure pH, titratable acidity, and total soluble solids (TSS) (°Brix). 'High' N treatment fruit had significantly increased average number of drupelets expressing RDD relative to the fruit of 'low' and 'medium' N treatments in the first four harvest dates. Over the season the average number of drupelets per fruit showing any level of RDD was 2.2, 1.63, and 1.41 for high, medium, and low N treatments, respectively. Early season fruit across all treatments had the highest levels of reversion averaging 3.19 drupelets per fruit in pick 1 with levels declining significantly at each subsequent harvest date (2.61, 1.41, 1.17, and 0.69 drupelets per fruit in picks 2-4 respectively). This study has shown that there is an association between high N fertigation during harvest and RDD expression, and time of the season and RDD expression.