Valencia trees on high-vigor (left) and low-vigor (right) inducing rootstocks in a Central Ridge field trial. Note the higher yield efficiency of the smaller trees on the right and the leaning of these trees due to Hurricane Irma.
By Ute Albrecht
Rootstock selection is critical for longevity and productivity of a grove. The decision should be based on compatibility with the scion, desired tree size and planting density, soil type, pest and disease pressure, and other factors. Planting the right rootstock will provide higher economic returns without any additional cost.
What is an HLB-tolerant rootstock? Tolerance to a disease is generally defined as the ability to be productive in the presence of the disease-causing organism. In contrast, resistance is the ability to evade the disease-causing organism due to specific mechanisms. Many rootstock cultivars, especially those with a trifoliate orange parent, are highly tolerant to HLB when grown as seedling trees. However, after grafting with an HLB-susceptible scion (such as sweet orange), rootstock tolerance is diminished. Thus far no rootstock has been identified that completely prevents HLB-induced damage of the grafted scion. Despite the inability of currently available rootstocks to eliminate HLB, field trials have clearly demonstrated that some rootstocks are superior to others and induce healthier and more productive trees.
Among the rootstocks that appear to perform well in Central Ridge and flatwood-type locations in Florida are several citrandarins (hybrids of mandarin and trifoliate orange), such as US-942, US-812, C-54 and X-639.
X-639 is one of the most vigorous rootstocks and produces large and healthy canopies. Perhaps vigor is one of the factors contributing to grafted tree tolerance. A downside of high vigor is that trees can be less productive during the early years because much energy is invested into vegetative growth. Increasing production during the early years can be achieved through high-density planting in combination with less vigorous rootstocks. Smaller-size-inducing rootstocks usually have a much higher yield efficiency, meaning that they produce more fruit per unit canopy. These fruits are typically also higher in soluble solids. However, trees on some of the less vigorous rootstocks may be more vulnerable to wind-induced toppling than vigorous rootstocks. C-22 is one of these rootstocks.
With so many factors to consider, rootstock selection is not easy, and diversification is advisable. Remember the destruction of citrus industries worldwide because of citrus tristeza virus-induced decline of trees on sour orange. Fortunately, citrus breeders are very productive, and many cultivars are available or in the pipeline to help the industry.
See https://edis.ifas.ufl.edu/publication/HS1260 for guidance on rootstock selection. Read more on rootstock-scion graft compatibility here.
Ute Albrecht is an assistant professor at the University of Florida Institute of Food and Agricultural Sciences Southwest Florida Research and Education Center in Immokalee.
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