I visited Leo Manuel today and he was kind enough to offer me a bunch of Brewster Lychees and also a few of his Hak Ip and Sweetheart Lychees. His Hak Ip and Sweetheart were much less fruitful than his Brewster which was completely loaded from the top to bottom with fruit.
His Lychees were planted in different parts of his yard and his Sweetheart is much larger than his Hak Ip so it's not really comparing Lychees to Lychees. The color of the leaves were slightly different but I feel it was due to being planted in different parts of the yard, age of tree, etc... I noticed that even on the same tree, the size, shape and color of the leaves can be drastically different from each other. Even the fruit from the same tree can be drastically different. Leo's Sweetheart Lychee had a few super small fruit that were about the size of a Groff but it was fully red and on the same tree, he had one fruit that weighed 49 grams and had a unique flattened heart shape very much like some of the pictures of Fei Zhi Siu from Australia.
I harvested two Hak Ip fruit and four Sweetheart fruit. The Hak Ip had an average weight of 38.1 grams and had an average Brix of 17.5%. The Sweetheart had an average weight of 36.4 grams, I only weighed 3 of 4 Sweethearts because one fruit was bad and I only used it for a Brix reading and to observe seed size. The sweethearts had an average Brix of 19.75%.
As far as taste, they both tasted the same to me even though they had different Brix levels. They both had a slight honey taste to them and I detected some Tannin taste in both varieties. Both had a medium level of Lychee/Rose flavor. I believe both varieties were harvested a bit past their prime. They both had relatively low production this year although the Hak Ip is much smaller than the Sweetheart.
In conclusion, I don't know for sure wether or not these two varieties are the same and probably the only way to find out is through sequencing their DNA. In my opinion, they are the same or at least they are so similar that I can't tell the difference.
When looking at these pictures, the color of the fruit is quite different but the differences in color can be attributed to the difference in growing conditions, age of tree, exposure to sunlight and everything else I touched upon above.
Simon

