2019 ASHS Annual Conference

With the endemic presence of huanglongbing (HLB) in Florida interest in the use of rootstocks as a strategy to mitigate the devastating effects of the disease has increased considerably. The importance of rootstock has been recognized since the advent of Phytophthora root rot in the mid-1800s and the occurrence of citrus tristeza virus in the 1940s, which decimated citrus industries around the world. In addition to rendering a tree tolerant or resistant to diseases, rootstock also influences tree vigor, fruit quality, yield, and adaptability to abiotic stresses.

Contrary to most commercial scion cultivars, which are susceptible to HLB, several rootstocks show high levels of tolerance when grown on their own. In a grafted tree, rootstock tolerance usually does not transfer to the scion to a level that reduces HLB incidence and prevents disease development. However, commercial field trials conducted under high HLB pressure demonstrated that productivity of a citrus tree is improved when favoring some rootstock cultivars over others. This has resulted in the high demand for specific cultivars such as ‘US-942’, a hybrid of mandarin (Citrus reticulata) and trifoliate orange (Poncirus trifoliata), and other newly developed rootstocks for which no seed source trees are yet available.

Traditionally, citrus rootstocks are propagated by seed, which due to the phenomenon of nucellar embryony results in genetically identical plants. In the absence of sufficient numbers of seeds, propagation must occur by alternative methods, namely cuttings or tissue culture. Although tissue culture is widely used for propagation of other tree crops, in Florida there is concern about the increased costs associated with this technology and the potential inferior growth of trees.

In this part of the workshop, results from field trials assessing the performance of a wide range of commercially available and experimental rootstocks, propagated traditionally and by other technologies, are presented. We will discuss the advantage of using rootstock as a management strategy to combat HLB, particularly as it relates to cultivar-specific differences in vigor and productivity, and potential pitfalls arising from alternative propagation methods. In addition, we will discuss the term ‘rootstock tolerance’ and the possible mechanisms associated with this trait.