There may be another way of obtaining a tetraploid banana without Colchocine or Oryzalin.
"Crossing of a triploid banana with a diploid variety generates diploid, triploid, tetraploid, aneuploid and hyperploid progeny"
I believe this may be because sometimes an unreduced gamete escapes, and in the case of triploids, an unreduced gamete (still with all 3 sets of chromosomes) is much more likely to escape meoisis intact and be viable than a triploid that underwent chromosomal division.
So an unreduced 3n gamete from pollen + a normal haploid gamete from the female parent = a tetraploid offspring 4n
https://www.sciencedirect.com/science/article/pii/S0254629905000463#bbib22I know the same thing can occur in citrus but it's usually only somewhere around 1 out of 25 offspring.
Another article seems to suggest with bananas it may be as high as half:
"On the assumption that Cavendish cultivars have low fertility, the Banana and Plantain Breeding Program at the Honduran Foundation for Agricultural Research (FHIA), starting in 2002, pollinated 20,000 bunches, approximately 2 million fingers, of the Cavendish cultivars ‘Grand Naine’ and ‘Williams’ with pollen from 10 Cavendish cultivars for the development of Cavendish tetraploids. As a result, 200 seeds with 40 viable embryos were obtained, from which 20 tetraploid hybrids were developed. These results confirmed the assumption that Cavendish cultivars have low fertility, which allows their use in conventional breeding methods to create new progenies. The selected tetraploid progenies were crossed with improved FHIA diploids for the development of second generation triploid hybrids."
http://www.fhia.org.hn/dowloads/banano_y_platano_pdfs/Improvement_of_cavendish_banana.pdfmuch of this is because seeds that would have had problematic chromsomal abnormalities don't form in the first place, i.e. half the seeds that do manage to grow from a normal seedless banana, out of the very few number of seeds, will be tetraploids. Then these tetraploids can be crossed again with a normal (diploid) seeded banana to give a seedless banana offspring.