Field Test for Tristeza
Conventional tests for plant viruses are carried out in a laboratory with expensive equipment. What growers really need are tests which are cheap, simple and give quick results. Some new molecular tests have now been developed for citrus viruses.
The Test Kit
The test kit for citrus tristeza contains a plastic vial (called an "eppendorf"), a bottle of liquid buffer and some test strips. Extension staff or growers can use these simple test kits after only a few minutes' training. The test kits give a result in a few minutes. And they are cheap! This field test for tristreza virus costs only 80 cents per tree!
Taking the Sample
As for most tests of this kind, it is very important to take the right kind of sample. You should test one tree at a time. Only a small amount of leafy material is needed, the equivalent of a few leaves.
It is best to test more than one leaf, by taking three or four small pieces of leaf from different parts of the tree. This is because the virus is unevenly distributed throughout the plant. Typically, a tree with tristeza virus has large amounts of virus particles in some branches, while others are almost free of virus. You can't tell whether virus is present by looking at the leaves. Sampling leaves from different parts of the tree increases your chances of detecting any virus.
Also, it takes some time for virus particles to colonize new shoots. Don't take young shoots for your test sample, take mature or immature leaves.
Use fresh leaves, and test them straight after they are picked.
The Procedure
Put the pieces of fresh leaf into a glass bottle or test tube, and grind them into mush. You can use a clean wooden stick to do this, or a bamboo chopstick.
Then add about a teaspoonful of the buffer, and mix it thoroughly with the crushed leaves. Pour the solution into the plastic vial. Dip a test strip into the mixture of buffer and leaves, and wait for two or three minutes.
If the tree is infected with virus, two bands appear on the test strip. If there is only one band, then the leaf sample is from a healthy tree.
Index of Images
Figure 1 Grinding the Leaves to Test Them for Citrus Tristeza Virus
Figure 2 The Results of the Test. the Test Strip on the Right, with a Double Band, Is from an Infected Tree. the Strip on the Left, with One Band, Is from a Healthy Tree.
