By Amit Levy
Citrus tristeza virus (CTV) infects the citrus phloem. CTV infects almost all citrus varieties, but it causes different disease symptoms depending on the virus isolate, the citrus variety and scion-rootstock combination. Florida has three dominant strains of CTV:
- T36, a decline-causing strain
- T30, which normally does not cause severe disease
- VT, a stem-pitting causing strain
CTV is transmitted by four aphid species. The brown citrus aphid (Toxoptera citricida) is the most efficient vector.
The most severe disease caused by CTV is decline. Decline disease only occurs on sour orange rootstocks. The disease is initiated by phloem necrosis below the bud union that reduces the flow of sugars to the roots, resulting in loss of feeder roots that cannot take up water and nutrients. Affected trees show defoliation and mineral deficiency symptoms and can die very quickly when rapid water uptake is required. This aggressive type of decline happens mostly in mature trees, whereas newly planted trees will usually be stunted and poorly developed. These symptoms can easily be confused with HLB symptoms.
A second important disease caused by CTV is stem pitting. This disease does not kill trees but results in highly reduced growth and yield. In stem pitting, CTV causes changes to vascular tissues. The phloem is penetrating the xylem tissue, causing pits in the wood of the stem and branches. Mild cases show no effect on plant vigor, but more severe cases cause stunting, small and misshapen fruit and chlorotic leaves. Lime, grapefruit, sweet orange, rough lemon and alemow (Citrus macrophylla) are highly susceptible to stem pitting.
Even though HLB is the main concern in Florida today, it is important to realize that CTV did not go anywhere. In surveys throughout Florida, CTV is found in many groves, including the T36, T30 and VT isolates, as well as unknown isolates that have not been identified yet. Many trees are being found with VT isolates that have mild stem pitting symptoms (Figure 1). Fortunately, no severe symptoms have been found so far.
More importantly, we find decline CTV isolates in trees on sour orange rootstocks, and this situation can quickly lead to tree decline when the conditions are right. The most simple and effective thing to do is to avoid sour orange rootstocks as much as possible. In addition, it is important to keep an eye on stem pitting symptoms. If you see them, please contact a University of Florida Institute of Food and Agricultural Sciences (UF/IFAS) citrus Extension agent.
As always, keep using clean, healthy trees from registered nurseries. Thanks to the Citrus Nursery Stock Certification Program and the transition to non-sour orange rootstocks, CTV has been successfully controlled in Florida. Let’s keep it that way.
Amit Levy is an assistant professor at the UF/IFAS Citrus Research and Education Center in Lake Alfred.
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