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What to Look For
1. Suspicious Tree Symptoms That Should Be Examined Closely
Citrus trees normally grow well and vigorously, with a healthy complement of dark green foliage. Almost any deviation (yellowing of the leaves) from the normal should be checked more closely to try to ascertain why such deviation is occurring. There are a number of causes for chlorosis (yellowing) in citrus trees, and many of these will be presented in following sections. However, the first step in problem diagnosis is to examine any and all leaf chlorosis patterns more closely.
Usually, the first symptom of greening is manifest as the yellowing of one or more twigs or branches, i.e., yellow shoots. As the disease progresses, twigs begin to exhibit a greater degree of yellowing, sometimes accompanied by erect growth that may exhibit "rabbit eared" leaves. In time, twigs begin to die and the tree begins to decline rapidly towards its ultimate death. In some cases, the advancement of the symptoms results in an excessive drop of leaves and fruit.
One other key symptom of citrus greening is misshapen or lopsided fruit, so the presence of such fruit should be examined closely for the presence of other symptoms.
2. Leaf Chlorosis Caused by Insects or Other Diseases
There are numerous leaf chlorosis patterns that can be observed in citrus. Fortunately, most are caused by something other than the citrus greening bacterium. Because some of the symptoms of citrus greening are quite similar to symptoms caused by insect feeding or disease problems other than greening, this section presents some of the alternate causes of the chlorosis patterns to help decide whether or not they are cause for concern.
3. Symptoms of Citrus Greening Disease on Fruit
There are several important symptoms of citrus greening on the fruit, both externally and internally. External fruit symptoms include lopsided or misshapen fruit, non-uniform fruit color (the blossom end remains green as the rest of the fruit turns yellow), small and non-uniform fruit size.
Internally, the seeds are usually aborted (brown to black), the central axis of the fruit (in vertical cross section) is curved in response to the fruit’s misshapenness, and there is a yellowish-brown discoloration in the albedo (white) tissue beneath the button, often extending down into the central axis of the fruit. On red fleshed varieties, the discoloration may appear to be more reddish-brown.
In addition, non-visible symptoms include an acrid taste.
4. Leaf Chlorosis Patterns Caused by Nutrient Deficiency or Other Cause
There are numerous leaf chlorosis patterns that can be observed in citrus. Fortunately, most are caused by something other than the citrus greening bacterium. Because some of the symptoms of citrus greening are quite similar to symptoms caused by nutrient deficiency or other problems, this section presents more of the alternate causes of the chlorosis patterns to help decide whether or not they are cause for concern.
In the case of zinc deficiency, the typical leaf symptoms of citrus greening almost always occur together with zinc deficiency, so zinc patterns should be examined especially carefully.
5. Leaf Chlorosis Patterns That Are Symptoms of Citrus Greening Disease
The most telling of the leaf chlorosis patterns of citrus greening is the presence of blotchy mottling on the leaves. As a general rule, the pattern of the mottling is asymmetric across the leaf midrib, but some other chlorosis patterns are also asymmetric. Too, the intensity of the mottling may vary with the relative length of time that the tree has been infected. (In some citrus, there may be more “blotch” than “mottle”, so various examples of citrus greening on different types of citrus will be shown below under "Blotchy Mottle Symptoms on Various Citrus")
Other symptoms include green islands (spots) within almost totally yellowed leaves. Very often, the midrib and major lateral veins of the leaf turn yellow. Also, these same veins can develop a corky exterior on the upper side of the leaf.
Finally, zinc deficiency patterns almost always occur in combination with blotchy mottle and the green islands symptoms.
6. Blotchy Mottle Symptoms on Various Citrus
As indicated earlier, probably the most universal symptom of citrus greening is blotchy mottle of the leaves. Blotchy mottle is very similar across most citrus, though there may be subtle differences in the appearance of the mottling between different species. For example, some citrus types may have more “blotch” than “mottle”. In this section, additional blotchy mottle symptoms are shown for several different types of citrus, including mandarins, lemons and limes, as those are among the most popular citrus trees in home landscapes across Texas.
7. Other Symptoms That Should Be Tested
Because the citrus greening bacterium can be present in a host plant for several years before some of the more obvious disease symptoms become visible, it is important to test any symptom that appears to be an early manifestation of the blotchy mottle or of fruit deformity. Only through early detection is there any chance to eradicate this disease before it becomes widely established in Texas.
In this section are images that are believed to be among the first manifestations of the symptoms of citrus greening. This is not to say that the symptoms depicted here are, in fact, citrus greening. It does mean, however, that because these symptoms could be citrus greening, they should be tested.
