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Citrus Tristeza Virus
Citrus tristeza virus (CTV) was, prior to the invasion of Huanglongbing, the number one pathogen of citrus here in the US. As you may have read about our facility here at USDA Beltsville (insert link to EPCC page), we have over 250 examples of CTV from 40 countries in planta in our greenhouse. If extracted and put under a scanning electron microscope, the virus has the shape of a thread, or what we call a flexuous rod. Through many experiments on this virus we have found that some strains of this pathogen are " mild" producing very very few symptoms, while other are " severe " causing decline and in some cases death of a tree in a short amount of time. Symptoms in each type of citrus, orange vs lemon vs grapefruit etc, are also different. Testing a large proportion of our collection plants by grafting buds from infected plants into 5 cultivars of healthy citrus, we determined that there are at least 13 different " bio-types" of the virus based on visual symptoms alone, depending on the isolate used and the citrus host.
- Stem pitting
- Vein clearing
- Corky vein
- Leaf cupping
- Seedling yellows
- Quick Decline
Some citrus rootstocks are more tolerant of infection than others. In the 1930's the entire citrus industry of Florida and Brazil was almost wiped out because most orange trees were grown on sour orange rootstock, chosen for its vigorous growth. Sour orange trees however are very susceptible to CTV, and when CTV moved into the state it wiped out many groves. Today oranges are grown on rootstocks which are not as sensitive.
CTV is spread by, yes, you guessed it: humans! But CTV is also spread by insects known as aphids or it can be transmitted through the use of infected budwood. Our " native " aphids can transmit the virus but are not efficient. Another species of aphid known as Toxoptera citricida , or Brown Citrus Aphid (BCA), is very efficient. A single aphid feeding less than a day on an infected plant can transmit the virus to the next plant it feeds on. The mouthpart of the aphid, the stylet, is shaped and used like a straw. The aphid plunges it's stylet through the bark and the water pressure within the plant forces the plant juices into the aphid. Right after an aphid plunges in its mouth part it " spits " for lack of a better term, to clear it's mouthpart of debris etc. This action literally blows the remains of the last juices the aphid " drank " into the new plant, thus moving the virus plant to plant.
The BCA was found in South America and other
citrus growing regions but was not in the USA until the mid 1990's when it was found in Florida. The aphids may have been " blown in " on hurricane winds or as " hitchhikers " on plant material. The aphid population went from a single find to being found in half the state within a single year. The aphids have winged forms as well as non-winged forms and each aphid is female and gives live birth to between 1-17 " babies " every day.
We use the BCA to transmit CTV on purpose to healthy citrus as part of our research. We place a single aphid on an infected tree for 24hr and then carefully move it to a healthy tree and allow it to feed 24 hrs before removing it. Through this research we found that a seemingly mild strain of the virus can be hiding or " masking " a severe strain. The source (infected) plant displays no symptoms yet tests positive. Performing our 24hr feeding protocol above, the healthy plant may become infected with a very severe strain of the virus. This is because the source tree with no symptoms may have two to six strains of CTV " cross protecting " the tree from each other. Although the viruses are present, symptoms are minimal. But the BCA transmits CTV only one strain at a time, and so the cross protection effect is lost. This is how the CTV has been found in all citrus growing areas in the US: A tree may appear to be completely healthy but can be infected with the virus!
There is some natural resistance to CTV in some citrus cultivars. The tri-foliate citrus is considered to be resistant to the disease and is used as a rootstock when CTV disease pressure is severe. Some research has been done inoculating the trees through grafting with a known, " cross protecting " mild isolate, with mixed results.
For more information about citrus pathogens please contact Dr. John Hartung
For more information concerning growing citrus please contact Cristina Paul