The rootstock experiments have given me a lot of useful information although the sample size is still too small to be statistically significant. The double stone grafting technique is not recommended because it induces precocity to the point that a young tree just 12 inches tall will bloom and attempt to hold fruit. This issue for me, in our relatively cold climate, may actually be a benefit in warmer climates like Florida or the Philippines where a reduction in the growth rate and increase in precocity may be beneficial.
The California Super Mango rootstock works very well and significantly increases growth and because mature scions are not used, the issue of precocity is averted. I only made a couple of CSMR trees and unfortunately gophers killed both trees that were planted at the orchard. The significantly faster growth rate of these multiple rootstock seedling trees was enough for me to conclude that this technique is great for increasing the rate of growth for mango trees in SoCal.
One or two additional rootstocks is enough to see the benefits.
The CSMR trees can be a waste of rootstocks however and I came up with the simple idea of grafting seedling scions from Polyembryonic varieties such as Sweet Tart, E4, Orange Sherbet, Pina Colada onto any vigorous seedling rootstock and so far the trees are growing much better than trees grafted with mature scions.
The negative aspects of this technique is that you can’t be sure if you’re grafting a clone or the zygotic seedling. Even if you grafted the zygotic seedling, however, approximately 50% of the genetic material will one from the maternal parent that bore the fruit and you may get a fruit that still tastes great or potentially even better tasting than the parent.
Several friends have grown mango trees grafted with both mature scions and seedling scions and all have reported that the seedling scions are significantly larger and healthier than the branches grafted with mature scions.
I am sure that many more growers in SoCal will be using this technique in the future once word gets out how effective it is.
Simon