Foliage Diseases

• The alternaria brown spot (Alternaria alternata) affects tangerines, tangelos and grapefruit trees. It does not appear on orange trees. The tree's young leaves shows signs of brownish-black lesions that sport a golden halo. Fruit can show lesions, and premature fruit drop may occur. Leaves suffering from the fungus greasy spot (Mycosphaerella citri) exhibit yellow spotting and brown blistering. As the fungus becomes ubiquitous on the tree, defoliation occurs. Both alternaria brown spot and fungus greasy spot respond well to applications of strobilurin containing fungicides or copper based fungicides.
  
  

Sooty Mold

• Aphids, mealybugs, scales and whiteflies feed on the citrus tree's leaves by sucking the tree's nutrients. The insects secrete a honeydew like substance. The substance attracts a wide variety of ants onto the tree's foliage. As the honeydew becomes plentiful, it begins to develop a black sooty mold that covers the tree's vegetation and branches. A tree that suffers from widespread black sooty mold has reduced healthy fruit production. Its leaves will be unable to successfully photosynthesize sunlight. The ripening of the fruit becomes delayed and exhibits poor coloration. Control black sooty mold on the tree's leaves by controlling the insects. Use horticultural oils to loosen the mold from the foliage.
  
  

Insects

• The brown citrus aphid feeds on the foliage of the tree by sucking its nutrients. The insect's heavy feeding causes the leaves to twist and distort. Consider purchasing ladybugs to help control the aphid population naturally. The citrus leaf miner causes damage on the foliage of young trees. The insect feeds on the leaf's epidermal layer, which gives it a silvery appearance. Control is often difficult. Commercial pesticides and bioinsecticides are available, and they can help. Asian citrus psyllid insects cause the tree's foliage to curl and become distorted. Use horticultural oils for control.
  
  

Nutritional Deficiencies

• Abnormalities in the citrus tree's vegetation can represent a nutritional deficiency. Lack of adequate nitrogen causes the tree's leaves to turn yellow, and the leaves may appear smaller. A magnesium deficiency causes the tree's leaves to have yellow blotches. Leaves that appear white but still show green veins indicate a lack of iron. A zinc deficiency s the leaves to appear green striped with a yellow or whitish background. The leaves may also appear thin and smaller than normal. Yellow leaves with striking green veins indicate a lack of manganese. Apply nutrient foliar sprays to treat manganese, zinc and magnesium deficiencies. Use soil additives for nitrogen and iron deficiencies.