Delayed Graft Incompatibility In Maddenii Rhododendrons
Several years ago I received cuttings of Rhododendron nuttallii. I grafted them onto a plant of 'Fragrantissimum' since it seemed to be a fairly closely related plant. Nine out of the ten grafts took. Buds set the following summer, and in less than two years the grafts bloomed famously. The area of the graft union gradually swelled until the area immediately above the graft union was at least twice the diameter of the scion above. At first the stock plant put up several vigorous branches which I cut. Then it broke no more buds. Over the three year period that it was top worked, the 'Fragrantissimum' did not grow in diameter. The pencil thin branches of the 'Fragrantissimum' stock, the grotesquely swelled graft unions, and the thick branches of the R. nuttallii scions made a very odd sight. Just before the second flowering, almost three years from grafting, the grafts all started to wilt. The flowers were very small and wilted, and all the grafts soon died. I removed all the grafts, and soon buds appeared on the leafless stock plant. Growth picked up, and now the 'Fragrantissimum' is flourishing.
In May, 1982, I also top worked two plants of 'Cilpinense', one with scions of R. nuttallii x taggianum and the other with R. nuttallii x lindleyi. These were grafted the same way as the 'Fragrantissimum'. The branches of the stock plants rapidly increased in diameter until now they blend almost imperceptibly into the scions. The plants are now over four years old and are growing and flowering vigorously.
Generally, the more closely two plants are related, the more likely will be the success of their graft, but this generalization is fraught with exceptions. Incompatibility can be immediate or delayed. Just because a scion grows well soon after the graft has taken doesn't mean that it won't fail later. Signs of possible incompatibility include overgrowths at the graft union, a great difference in growth stock and scion, loss of vigor, a clean break at the graft union, and of course, death of the scion. I have described several of these symptoms in the case of delayed incompatibility of R. nuttallii on 'Fragrantissimum'. Hartmann and Kester (1983) gives a very good account of graft incompatibility as well as other topics of interest to rhodoholics.
I was surprised to find incompatibility between two fairly closely related plants, one a hybrid between sections Edgeworthia and Maddenia, and the other in the section Maddenia. Other grafts between section Maddenia and a hybrid between sections Maddenia and Moupinensia seemed to be compatible. I feel that it would be helpful for a hobbyist to try a number of grafts and publish the results in the Journal. A lot of people graft elepidotes, so compatibility in that group is fairly well known. Much less is known of graft compatibility in the lepidotes, and the results of a number of grafts could give us food for thought.
Hartmann, H. T. and D. E. Kester (1983) Plant Propagation, Principles and Practices, 4th ed., Prentice-Hall, Englewood Cliffs, N.J., 727 pp.
Pat Halligan, a member of the Seattle Chapter, is an active rhododendron hybridizer and propagator.