Navel orange trees, which first appeared in 1820 in Brazil, grow from cuttings, since the inverted twin orange that grows into the main fruit produces no seeds. Grown in Florida, California, Washington, Arizona and in other countries, navel orange trees can develop diseases such as foot rot, root rot, Fukumoto bark rot, navel orange split and citrus stubborn disease, all of which affect their ability to thrive.
Foot rot, a soil-borne disease, affects citrus fruit by attacking the trunk of the tree, either by moving up the trunk or down into the roots, according to James W. Ferguson, author of "Your Florida Dooryard Citrus Guide - Common Pests, Diseases, and Disorders of Dooryard Citrus." Disease symptoms include oozing of gum from the tree, as well as uneven patches. Eventually, cracking, dried bark becomes visible. Also, brown staining shows up on the wood underneath the bark. The disease also causes leaves to die or fail to grow, since the foot rot interferes with moving nutrients from the roots to the tree canopy.
Phytophthora Root Rot
When a navel orange trees suffers from phytophthora root rot, the leaves become light green or yellow and may fall off. Tree vigour declines, too, since the disease causes the root's outer layer to separate from the centre layer and turn soft, according to the article "Citrus Phytophthora Root Rot" by the University of California Integrated Pest Management Online. Though the tree can regenerate, severe damage to the roots prevents the tree from taking in water and nutrients, offsetting its ability to revive. The disease has similar symptoms as other tree diseases and pests, requiring a lab test to accurately identify it.
Fukumoto Bark Rot
Fukumoto bark rot causes dying, gumming branches, and cankers to appear on navel orange trees. The disease affects trees "one to two years old" as well as the Fukumoto variety more than others, according to Craig Kallsen, University of California farm adviser, in the article "Fruits and Nuts." Researchers suspect a copper deficiency causes the disease, since adding lime and copper to the soil shows some symptom relief. Gardeners will notice Fukumoto bark rot during hot temperatures in the spring or summer. The gumming may appear frothy and have a beer-brewing scent.
Navel Orange Split
Experts do not know the exact cause of navel orange split, but some suggest that the disease may occur on stressed trees, according to the article, "Navel Orange Split," by Pamela Geisel, University of California Cooperative Extension farm adviser, and Carolyn Unruh, University of California Cooperative Extension staff writer. Fluctuations in soil, temperature, fertiliser level and humidity as well as a combination of these may cause the orange to split at the navel before the fruit ripens. Drought-stricken trees taking water from the fruit or waterlogged trees triggering the fruit to swell too quickly all cause the oranges to split.
Citrus Stubborn Disease
Having first affected navel trees in California, the citrus stubborn disease causes stunted growth in young trees. Agriculture experts noticed the citrus stubborn disease, caused by the spiroplasma cirri virus, in 1915. Other symptoms include dense foliage with leaves "cupped and unnaturally thick" as if nutrient deficient, according to the article "Stubborn Disease of Citrus" by Lawrence G. Brown of the Florida Department of Agriculture and Consumer Services. Some symptoms may not appear obvious on mature trees, but citrus stubborn disease causes the production of smaller, fewer and misshapen oranges. Though only affecting orange groves in the warmer regions of California and Arizona as well as North Africa, the eastern Mediterranean Basin and the Middle East, Florida growers and researchers consider the disease a threat.
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- University of Florida IFAS Extension: "Your Florida Dooryard Citrus Guide - Common Pests, Diseases, and Disorders of Dooryard Citrus"; James W. Ferguson
- University of California Integrated Pest Management Online; "Citrus Phytophthora Root Rot"
- Craig Kallsen, University of California Cooperative Extension farm adviser; "Citrus/Subtropical Horticulture, Pistachios"; June 2001
- Pamela Geisel, University of California Cooperative Extension; "Navel Orange Split"
- Lawrence G. Brown of the Florida Department of Agriculture and Consumer Services; "Stubborn Disease of Citrus"