Exploring Mechanisms of delayed incompatibility for grafts

[FlyingFoxFruits]

An interesting subject to me...I wonder what exactly causes delayed incompatibility...I imagine, in certain cases, it's caused by chemicals (hormones? enzymes? Or something comparable to antibodies found in animals?)

I have a sugar Apple that was grafted onto custard Apple, which lasted a good 2yr, growing wonderfully, before suddenly dying...I assume it was delayed incompatibility....and not a pathogen that afflicted the plant...because the rootstock (Annona reticulata) is now growing happily...I wonder if the rootstock has a mechanism to reject the scion, in order to free itself from the oppression of the scion (A. squamosa), so it can reproduce.

[DimplesLee]

I had a few plants I purchased from various nurseries that I lost years after I've bought them - usually they break off at the graft joint when they get top heavy 
I think my experience would fall under:



F. Symptoms of incompatibility


1. Low % take 


2. Clean break at the graft union.


As described above, the 12 year old, extremely delayed, incompatible graft union between a red maple cultivar and a seedling of the same species exhibited a smooth surface at the point of separation between stock and scion, except the approx. 1 inch diameter central cylinder, which splintered. Apparently, stock and scion were compatible for the first few years after the graft was made, so that xylem (wood) bridged the gap only during that period. Later, after the delayed incompatibility set in, no further bridging xylem formed, so that smooth stock and scion surfaces were in contact but not anatomically joined, resulting in a "clean break" at the graft union
This splintered, not smooth, break just below the graft union between an apple fruiting variety grafted onto an M9 rootstock, snapped off due to an excessively heavy fruit load. This was a result not of incompatibility but rather the short wood fiber cells typical of this dwarfing stock, which tend to make it "brittle".


https://courses.cit.cornell.edu/hort494/mg/specific.grafting/compatibility.html

[HIfarm]

I have always heard that plants have no comparable immune system to animals so I doubt that that is the cause (that is why plant viruses are so serious & so easily spread, for example).  It might even be different growth patterns in cells of different species (you can see this in wood of various trees -- fine grain, coarse grain, straight grain, twisting / interlocking grain, etc).  Perhaps the two species are dissimilar enough that you do not get a homogeneous growth at the graft union and this causes a weak spot that eventually rears its ugly head when the tree gets enough top growth (weight & wind resistance from the leaves) to snap the weak spot when there is enough wind, ice, whatever.

I recall years ago I had a mountain ash graft that grew into a pretty large tree & fruited regularly but snapped off at the graft many years later.

I see DL snuck one in while I was writing ...

John

[FlyingFoxFruits]

thanks for your contributions to the discussion.

my situation is perplexing, because the graft union was beautiful, and clean.  The scion just suddenly died one day, and the rootstock took over, and is now growing vigorously.