Avoid Sour Orange Rootstock To Reduce Risk of CTV Decline

By Amit Levy

Although HLB is the primary concern in Florida citrus groves, citrus tristeza virus (CTV) can also cause a severe disease leading to tree death. This severe disease, caused by the T36 isolate of CTV, is called decline. It occurs when a citrus scion is grafted onto sour orange rootstock.

CTV decline begins with phloem necrosis below the bud union that reduces the flow of sugars to the roots. This results in loss of feeder roots that cannot take up the 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.

T36 is already naturally present in Florida. In addition, new therapies for HLB based on the T36 isolate that has been programmed to produce an antimicrobial peptide, have received EPA approval for use in Florida counties. Therefore, it is important to understand CTV and its risks, especially when dealing with sour orange rootstock and/or applying these therapies.

CTV can be spread between trees by the brown citrus aphid. While the aphid levels are currently low, all aphids are capable of periodic outbreaks when conditions are right, and brown citrus aphid is no exception. Thus, it is not possible to know if or when conditions will be ideal for spread of CTV.

Even though the specific CTV version that is used for the therapy delivery is exhibiting low transmission levels, trees may already naturally contain other mild CTV isolates that may complement these transmission defects. There is currently an increase in the use of sour orange rootstock for new plantings in Florida, and with the high number of expected new trees treated with T36 virus, there is a risk of spreading T36 into these sour orange-based trees.

Always be cautious not to use any CTV T36-based product on a tree grafted on a sour orange rootstock. To reduce virus transmission risk, aphid control can help. However, the simplest and most effective approach is to avoid sour orange rootstock. This will efficiently reduce the risk of CTV decline, either from naturally infected or treated plants.

Amit Levy is an associate professor at the University of Florida Institute of Food and Agricultural Sciences Citrus Research and Education Center in Lake Alfred.