Young Tree Decline

• Young Tree Decline (YTD) kills all orange trees--including tangerines and other mandarins--and this infectious blight can cause major orange tree loss. According to Purdue University's horticulture website, "substantial recovery" from YTD is achieved with "early treatment of an affected tree with 20 gals of a 1 1/2 percent solution of sodium erythorbate or erythorbic acid applied to the soil, and 10 gallons (38 liters) applied as a foliar spray, plus soil application of 5 to 7 1/2 lbs (2.2-3.3 kg) of calcium chloride or calcium nitrate--about 6 feet (1.8 m) out from the base of the trunk." Purdue recommends following with leaf spraying of urea to encourage new growth.
  
  

Yellowing Leaves on Citrus

• Overwatering or waterlogged roots due to poor drainage is the most common cause of yellow leaves. Sometimes yellowing is only along leaf veins. Leaf drop and twig dieback can also occur. The only cure is solving the water problem, which might not reverse the root damage and save a particular tree. Salt buildup in the soil can also cause yellow leaves and "salt burn," requiring more care using fertilizers and soil leaching. A threat to U.S. citrus, first found in Florida in 2005, is a bacterial disease known as citrus greening or Huanglongbing (HB), which causes leaf yellowing and complete tree decline within two years.
  
  

Other Citrus Leaf Symptoms

• Greasy spot fungus causes oily-looking, irregular spots on leaves and severe defoliation. Control its spread by removing fallen leaves. Spider mites can give a silvery, scratched look to leaves in summer and fall, and a severe infestation can cause leaves to drop in summer or fall. Infestations of whiteflies and blackfly can lead to sooty mold, or a black, soot-like growth on leaves and fruit. Consult with Master Gardeners on fly or other insect controls--sometimes insecticides won't work but parasites will--or wash off mold with soapy water.
  
  

Citrus Leaf and Fruit Drop

• Citrus canker, a bacterial disease that causes premature leaf and fruit drop, is identified on both leaves and fruit by brown lesions with a yellow "halo." Infected trees continue to decline and ultimately produce no fruit. Another fungus, Diaporthe citri, causes stem-end rot, gummosis, melanose and dieback. Foot rot related to poor drainage or over-watering can cause all leaves to drop though fruit remains on the tree, or trees might just look sick with sparse, yellow-veined foliage and dead trunk bark near the ground.
  
  

Less Serious Symptoms

• Leaves that are wind-burned in spring will have dried or dead margins and show some cupping and curling, but this is usually not serious. Aphids also cause leaves to cup and curl, especially after a flush of new bloom and fruit growth, and will usually disappear on their own. Sunburned leaves develop brown or black spots on their undersides, usually not serious damage.