I have been aware that grafting can sometimes be used as a means to test for virus presence, but am trying it myself for the first time today. The idea is that if you’re not sure if a variety contains a virus, you graft it to another compatible variety which is more prone to showing symptoms if it becomes infected with viruses.
Why I’m doing it:
The dwarf cultivar of Chaenomeles japonica called ‘Chojubai’ is known for occasionally producing leaves with whitish margins. Some suspect this may be due to a virus. However, I’ve not encountered any claims as to what virus that may be, or why we haven’t seen it transfer to other Chaenomeles cultivars. I want to gather evidence to determine if a virus is the most likely culprit or if it may actually just be a genetic component of this cultivar.
Why I chose this graft method:
In theory, a range of graft styles should work. However, I want to set this experiment up for a low liklihood of failure. Therefore, I am using an approach graft of sorts to connect a long stretch of branch between the ‘Chojubai’ and the test subject to each other (both being sliced to expose the cambium so they can heal in connection to each other. This eliminates the risk of potential graft failure adding in an unwanted variable. Both varieties will get a large amount of exposure to each other while still remaining on their own roots.
Some unknowns that will affect the outcome:
Since we don’t know what virus it is (if in fact it even is a virus), I can’t know if the test cultivar I’m grafting to is sensitive enough to the virus to show symptoms. In fact, we don’t know if ANY cultivar is sensitive enough to show symptoms other than ‘Chojubai’. Therefore, if no virus symptoms show up in the test cultivar it will only indicate that a virus is less likely, but will not 100% rule out that it is a virus. However, if the test cultivar DOES begin to show the same occasional leaves with whitish margins, then that will be evidence that it IS caused by a virus!
Note: The cultivar I’m using to test the ‘Chojubai’ for virus is a selection of Chaenomeles speciosa called ‘Victory.’
Update:
The long stretch of approach graft between the two clones has been healing well, with enough expansion that the grafting tape is splitting away. There is now plenty of opportunity for virus sharing (if present).
Here’s a really good example of the type of variegation which can sometimes occur on ‘Chojubai’. No one seems to know for sure if it is caused by a virus or if it’s just the genetic programming of this clone. Hence the motivation for this test.
So far no signs of variegation have shown up on the normal green leaf cultivar. Of course it’s only been a couple of months. At this point, the possibilities are either that it’s not a virus, or it is a virus, but one which travels slowly and will take more time to show symptoms in the new host. I’ll be very interested to see what the situation is later in this season, and then next year.
Very cool test. If no symptoms ever show up in the test cultivar, you could try mechanical inoculation. Several viruses have been brought into Chaenomeles through the process.
I speculate that the reason the virus (if it is a virus) has not spread to other Chaenomeles is because there is no insect vector and it is only spread through grafting, mechanical means, or possibly seed.
I did manage to grow a seedling from ‘Chojubai’ and it has had perfectly green leaves only. I don’t think I understand what “mechanical” inoculation would be. If grafting doesn’t count as mechanical, I don’t know what does.
Grafting is definitely mechanical. My post wasn’t very clear.
The process I was attempting to reference is where the leaves of a plant suspected of having a virus are ground. A paste made from the leaves is then applied to the wounded (through abrasive powders, razor blades, chemical foliar sprays, etc.) leaves of an indicator plant. Isolates are sometimes even injected directly into the petioles or stem.
I’m sure there are probably more accurate names for the different methods like “Leaf Abrasion Inoculation,” but I’ve seen them often just labeled “Mechanical Inoculation.”
I imagine grafting provides higher transmission rates than other methods in most circumstances. The leaf inoculation methods may allow you to:
Test multiple cultivars at the same time.
Test as early as the cotyledon stage.
Test on non-graft compatible indicator plants (less important for Chaenomeles).
All of which could potentially yield different results.
With all of that said, I’m not even sure how much of it you’d even be able to do given Chojubai‘s slow growth and small leaves.
Just out of curiosity, is your seedling a dwarf?
Thanks for all your posts. After years of reading, posts like this are the reason I finally signed up.
Thanks, I understand now. That “mechanical” method does sound like it may be a useful tool in a number of circumstances.
Regarding the seedling from ‘Chojubai’:
I had assumed ‘Chojubai’ would not be able to set fruit with self-pollination so I had hand pollinated it with pollen from ‘Victory’. That said, I usually have very poor results from hand pollinating Chaenomeles and find it’s better to just put varieties I want to cross side by side and let the bees do it, so in addition to hand pollination I placed the two varieties side by side while blooming. Fruit set ended up being very minimal with three fruits held till ripe. Once cut open (if I recall correctly), two of the three had no seeds and the third had only a very small number of seeds. From those few seeds I was able to grow out a single seedling. Wanting to push growth as fast as possible I have fertilized it VERY heavily. The overall leaf shape so far looks just like ‘Chojubai,’ but a little bigger. However, it still looks smaller than any other *Chaenomeles I grow. I expect without heavy fertilizing the foliage would be smaller, but not quite as small as the original ‘Chojubai.’ Actually, even though I had intended to make a cross, I’ve been suspicious that it may actually have been the result of self pollination, but I will need to observe it more years before I solidify that opinion.
