Naval oranges belong to the group of sweet oranges. They were brought to the New World by Columbus in 1493. The navel in the navel orange is actually a second fruit that is embedded in the end of the orange that was attached to the tree. They are susceptible to disease and to attack from insects as well.
The Asian Citrus Leafminer
The Asian citrus leafminer attacks new, young leaves, causing them to become stunted and distorted. They can also leave a trail on the fruit. The tree has four growth periods a year and leafminers attack during each one. The leafminer will not kill the tree, and spraying for them is not often recommended because it will also kill their natural enemies--which are actually the best type of control there is.
Citrus canker is considered to be one of the most serious diseases. It is a highly contagious bacterial infection that can appear on the leaves as small, round, whitish blisters that turn to tan and then brown with a yellow halo. As it progresses, it develops a watery oily lesion around the spot.
Lesions on the twigs are similar, only larger--and on the fruit they are similar in the early stages, but eventually more pronounced and noticeable. Those that start on the fruit usually do not spread, but other bacteria can develop, causing the fruit to drop. It causes the leaves to drop off, damage to the fruit, and a decline in the general health of the whole tree.
Melrose is a fungal disease that attacks the twigs, leaves and fruit. It occurs in times of high humidity and it usually has no effect on mature tissue. It will appear on the leaves as small, dark, rough depressions with a yellow edge. As the disease progresses, the spots become raised, the leaves turn yellow and drop off.
The early signs on the twigs are the same as on the leaves, but the will become more raised. It will appear on the fruit as small, light brown, sunken spots. As it progresses, the spots will become dark and raised. If the spots are close together, the surface will be rough and the condition is called sandpaper melanose.
In times of heavy dew or light rain, the spores can be washed over the fruit surface, leaving a tear-streaked pattern. Then again, large sections of the fruit can crack open, causing mudcake melanose. A particularly popular time for an infection is in years following a freeze. A freeze will cause the death of twigs and these become breeding grounds for the infection.