After 5 nights between 16 and 22 degrees from Dec. 24-28, everything suffered from cold damage at the Citrus Variety and Rootstock Trials at JL Lomax Elementary School. This freeze event has by far been the biggest challenge that the northern Florida and southern Georgia citrus industry has faced in the last 10 years. I did not freeze protect the Owari satsumas that were planted in 2014 and 2015. The Sugar Belles planted in 2018 were not protected. I did freeze protect the Tango trial that was planted in 2020. While it is too early to know what type of damage there is to the wood, there are some differences in foliage shed on the different varieties. The satsumas have retained the most live foliage and the Sugar Belles have retained the least live foliage. The Tangos lost a lot of foliage as well. Being that these trees are all on different rootstocks, I rated each tree on January 9 to see how much live foliage was retained. By this time, it appeared the foliage drop was finished. This was a subjective rating so I had 2 other people rate the percentage of live foliage remaining on the trees. I took the average of the three ratings. I wanted to see if there was a difference between foliage retention on the different rootstocks for Owari, Sugar Belle, and Tango. It does appear there will be some damaged wood on the Sugar Belles (which looked the worst of the three) and the Tangos (which have a lot of young growth). Fruit load may also be a significant factor in cold-hardiness. Unharvested heavily-fruited Owari trees adjacent to the harvested Owari trees had significantly more damage (Fig. 1).
The results in the tables show the three highest yielding trees, US-942, Cleopatra, and US-852 retained less foliage and the lowest yielding tree, Rubidoux, had the highest foliage retained along with US-812. One thing to note is that the Owari trees on Rubidoux were planted two years afterwards and have never caught up in growth, but we know Rubidoux is a good rootstock for satsumas. US-812 has a mid-range yield and the highest retained foliage (Table 1).
|% Foliage Retained
This is not the first time the US-812 rootstock has appeared to look the best after freezing temperatures. In April of 2015 the trees had been planted for 8 months and survived several non-consecutive nights in the low 20’s. Much of the foliage had dropped from many of the trees. Without knowing the rootstock, I had 3 people choose the 10 trees with the most remaining foliage. Each person had 5 of the 6 trees on US-812 in their top 10. The rest of their top 10 trees were other random rootstocks. Could this mean that the US-812 rootstock improves Owari’s cold hardiness? With the Sugar Belle’s, Rubidoux appeared to have retained the most foliage (Table 2). Again, the highest yielding tree had the lowest percentage of foliage. In both the Owari’s and Sugar Belle’s, Rubidoux rootstocks retained the most foliage.
|Sugar Belle Rootstock
|% Foliage Retained
With the Tango’s, I only harvested half of the crop that was nicely colored with a plan to harvest the others after the freeze but before I could harvest them they dropped off so I could not get the total yield (Table 3).
|% Foliage Retained
|Super Sour #2
Overall it appears that satsumas have proven their cold-tolerance compared to other varieties, but it does appear the Sugar Belle and Tangos trees will survive. Having other tree variety options is important to the Georgia Industry. Cold tolerance may be greatly diminished on trees with a large fruit loads, and harvesting fruit as early as possible may improve cold-hardiness. I think we are fortunate not to have experienced this cold event at the rootstock trial last year as the Owari trees had a tremendous crop load. It also appears that Rubidoux rootstock providing cold hardiness does have some merit in satsumas and Sugar Belles. US-812 rootstock may also provide similar cold-hardiness to Rubidoux.