Rootstock Influences Cold-Hardiness in Sugar Belles 

Josh McGillfreeze, Mandarins, Rootstocks

Sugar Belles on four different rootstocks in Georgia showed different levels of damage from the December 2022 freeze, reported Jake Price, Lowndes County Extension coordinator for University of Georgia Extension.

Sugar Belles on June 27 prior to the December 2022 freeze (left) and on March 15 after the freeze (right).

The Sugar Belles at a Valdosta research plot planted in 2018 have put on the first flush of foliage after being mostly defoliated. The trees are snuggly located between windbreaks to the west and north and are protected by four rows of trees to the east. All trees have rebounded after having no freeze protection, but Price said there are differences in wood damage and bark splitting. 

The Sugar Belle row consists of 20 trees on four rootstocks with five repetitions. The rootstocks are US-897, US-942, Rubidoux and US-852. Trees on US-852 had the largest diameter limbs removed by pruning after the freeze; Rubidoux had the smallest limbs removed.

Price said trees on US-852 lost the most canopy while trees on Rubidoux lost the least canopy.

“I also recorded the percentage of trees with bark splits due to the freeze on the trunks and major limbs,” Price stated. “Sixty percent of trees on US-852 had bark split on the trunk, and 100% had bark split on the major limbs. Rubidoux had no bark splits on the trunk or limbs.”

Bark splits on the trunk and major limb of a Sugar Belle on US-852 rootstock

From these results, it would appear that Rubidoux increases the cold-hardiness of Sugar Belle trees more than the other rootstocks, Price concluded. He stated that US-942 would be second best, US-897 next, and that US-852 had the most damage.

The rootstock US-852 is a hybrid between a large-flowered Poncirus trifoliata and a Changsha. “That combination would make one think this rootstock would increase cold-hardiness and be a good rootstock for Georgia but that was not the case in this situation,” Price stated.

Price noted that the Sugar Belle row is a small sample size, but the information may be helpful to growers wanting to plant Sugar Belles and trying to decide on a rootstock.

Table 1 shows the data discussed. “There is a chance more limbs could die and alter this data. But as of April 10, this is what we know,” Price stated. 

Table 1. Post-freeze rootstock performance on Sugar Belle

Source: Georgia Citrus Association