A Career Perspective on Citrus Rootstocks, Their Development, and Commercialization

Professor Hume was right: rootstocks are critical and much is still to be learned. Rootstocks and scions are the foundation of many tree fruit industries of the world. Together, those components establish profitability, but it can be argued that the rootstock is the critical component; otherwise, scions would be grown on their own roots everywhere. There is no precedent for the failure of a citrus industry because of an inadequate scion variety, but serious problems have occurred because of a less than satisfactory rootstock. Therefore, after 35 years in the field conducting rootstock and scion evaluation research and working with our University of Florida citrus plant improvement team, here is an historical perspective on citrus rootstocks along with some observations and reflections on the human or social side of research and grower cooperation. It is well established from decades of experience that citrus rootstocks bring many advantages and profitability to commercial enterprise. In some instances, citrus rootstocks are the sole determining element that allows citrus to be grown in particular circumstances. Yet, despite their popularity and necessity, every rootstock has one or more undesirable traits that preclude its universal use. Those limitations have sometimes been discovered during research and development; at other times, they have not been revealed until after significant commercial use. Until the mid-1800s, citrus was grown primarily as seedlings. Commercial operations were limited in scope and size as compared with today and certainly were not operated with the same level of technical knowledge or experience. It is generally known that two grand disease events were apparently responsible for creating the initial interest in citrus rootstocks and changing the direction of citriculture; first, the damaging effects of Phytophthora foot rot when ownrooted sweet orange trees were common. Many of those trees were actually seedlings with their own inherent disadvantages. The second disease problem was citrus tristeza virus, which devastated trees of sweet orange and other varieties on the highly susceptible sour orange, the most popular rootstock in many parts of the world before the spread of the virus. As these problems spread and were encountered by citrus growers worldwide, budding, i.e., using a rootstock, became the accepted practice. Grafting or budding is actually a centuries old horticultural technique but one not widely practiced in citriculture until the late 1800s or the early 1900s. It was not until the 1940s, however, that the cause of tree decline on sour orange came to be associated with a graft-transmissible entity later shown to be a virus. The ‘‘modern’’ era of citrus rootstock use and subsequent history could be described as beginning in the early part of the 20th century in response to the diseases mentioned (Webber, 1967). Field trials with usually a small number of rootstocks were initiated and eventually followed with ones of larger scale (Batchelor and Rounds, 1948; Gardner et al., 1967; Moreira and Salibe, 1969). Those early trials also revealed the importance of other unknown virus and viroid diseases that affected the rootstocks being evaluated. However, they were conducted under considerable constraints, in retrospect, given the genetic and breeding tools available in 2009. Formal breeding programs have emerged and use new technologies such as somatic hybridization and molecular markers that are altering and expanding the range of possibilities for genetic manipulation leading to improved rootstocks (Kahn, 2007). As a result, there is a new era of citrus rootstock research and development underway that is linked to different ways of thinking about rootstocks and their role in future citrus orchards. It is conceivable that genetic advances might eventually eliminate the need for rootstocks as rootstock traits are incorporated into the scion variety.