The Effects of Rootstock on ‘Ray Ruby' Grapefruit Fruit Quality

The Indian River district of Florida is the world’s major production region for fresh grapefruit and Indian River grapefruit are valued for their high quality.  In the past, the rootstock of choice for Indian River grapefruit was Sour Orange (Citrus aurantium L.).  However, sour orange is susceptible to citrus tristeza virus (CTV) and when the brown citrus aphid, vector of CTV, arrived in Florida sour orange fell out of favor and ‘Swingle’ citrumelo ((‘Duncan’ grapefruit (C. paradisi Macf.) x Trifoliate orange (Poncirus trifoliata)) became the rootstock of choice.  Unfortunately, ‘Swingle’ has proven to be a poor rootstock for the acid flatwoods soils typical of the Indian River district, and alternatives are needed.  We report results here on the effects of seven rootstocks on fruit quality of ‘Ray Ruby’, an important red grapefruit in the Indian River district.  Four recent releases from the USDA rootstock breeding program, US-852,  US-897, US-942 and US-812 (all C. reticulata x P. trifoliata hybrids), X639 (C. reticulata x P. trifoliata), along with industry standards Sour Orange and ‘Swingle’ were included in the trial.   The trial was planted in 2005-6, in Indian River Co., FL.  Fruit quality data were collected in 2012 (eight harvests), 2013 (six harvests) and 2014 (single harvest).  In each year, rootstock effects on fruit size, total solids, and solids acid ratio were significant.  Sour Orange and ‘Swingle’ produced the largest fruit, whereas US-897 (a semi-dwarfing rootstock) produced the smallest fruit.  Peel thickness (measured only in the 2011-2012 season) was greatest in Sour Orange early in the season, but not towards the end of the season.  Misshapen (“sheep nosed”) fruit occurred more frequently on Sour Orange than on other rootstocks, although sheep-nosing was not a major problem with any rootstock.  Analysis of variance for fruit quality data collected in January of each of the three years confirmed that Sour Orange and Swingle produced the largest fruit and US-897 the smallest fruit.  Total solids were highest in US-897 and lowest in X639 and US-852.  Total solids/acid ratio was significantly lower on X639 than all other rootstocks.    Our results indicate that ‘Ray Ruby’ grapefruit produced on the new USDA trifoliate hybrid rootstocks for the most part produced fruit with quality equivalent to Sour Orange.  Although tree performance and yield are the most important characteristics when selecting rootstocks for a particular area, our data indicate that fruit quality can also be strongly affected by rootstock.