Some of you may know me from the old Gardenweb site as Kiwinut. I stumbled across the Growing Fruit site a few months ago (kudos to Scott!), and have read a lot, but I’m just now getting around to posting. I used to do a lot of kiwifruit growing and breeding, but now only dabbling a bit with grapes, kiwi and passionfruit.
I wanted to share some relatively new info based on recent research regarding Actinidia collections in the US and more specifically relating to the cultivar(s) ‘Issai’. The reason I say cultivars, is that there are more than one genotype sold as ‘Issai’ in the US. ‘Issai’ is a name used in Japan for any precocious, parthenocarpic Actinidia arguta selection, as it translates to “year one” or “first year”. For many years, what we called ‘Issai’ was thought to have a possible hybrid origin from a cross of A. rufa x A. arguta and is hexaploid. We now know that there are populations of wild hexaploid A. arguta in the snowy mountain areas of north central Japan, as well as tetraploid A. arguta. The ‘Issai’ selections very likely originated in these areas.
In 2017, researchers at the Univ. of New Hampshire developed genetic markers for hardy kiwi species and analyzed the germplasm available at the USDA repositories, university collections and a number of nurseries. The markers can distinguish between A. arguta, A. kolomikta and A. polygama. They found that the majority of A. arguta cultivars in the US are duplicates of the same genotypes that have been given new names by nurseries, or were mixed up over the years. The genetic analyses could also be used to determine the ploidy of all the cultivars. The work can be read here, https://journals.plos.org/plosone/article?id=10.1371/journal.pone.0170580.
One of the more interesting results involved ‘Issai’. It was determined that there are two different genotypes sold in the US as ‘Issai’. One is referred to as the “long-fruit variant” and the other as the “small fruit” or “round fruit” variant. It turns out that the small/round fruit variant is hexaploid, but the long-fruit variant (the more common one?) is actually tetraploid. Both seem to be very closely related, although I don’t know how close that really is. The other interesting discovery is the male ‘Flowercloud’, was also found to be very closely related to the 'Issai’s and most closely related to the tetraploid variant. They also found that ‘Flowercloud’ is just a precocious tetraploid A. arguta, and not a hybrid with A. kolomikta as claimed by many nurseries.
The main reason I am posting this is to ask everyone growing ‘Issai’, which version do you have and where did you get it? Both versions are present in the repositories, but there is no clear list of which version is sold by various nurseries.
I bought an ‘Issai’ from One Green World this spring, and it turned out to be the small/round fruit hexaploid variant. Many years ago, I had a long-fruit version that I think was from Edible Landscaping.
Small/round fruit variant from OGW
Long-fruit variant, photo from Edible Landscaping.