Favorite Citrus Trees and Fungal Infection

The orange tree in our Mediterranean-styled courtyard produced delicious fruit. Each winter, direct sunlight ripened its large sweet Navel Oranges. My uncle loved the oranges from this tree so much we called it "Uncle Charlie’s tree" and made sure that he received dozens of tree-ripened oranges each year. Little did I know that a fungal infection, encouraged by excessive moisture, lurked near the base of this tree.

My uncle, Carlos Smith, said these oranges reminded him of the ones he’d treasured growing up in Lemon Grove, California, where his family worked in the citrus orchards and at one of the largest fruit packing plants in San Diego County during the 1930s. As a young man, he drove flatbed trucks to deliver citrus fruit, mostly from the groves to the packing plant. 

The Washington Navel Orange, Citrus sinensis (L.) Osbeck, was introduced to North America from the Brazilian state of Bahia in the 1800’s so it was well established in California by the time Uncle Charlie grew to love them.  Horticulturists at the University of California, Riverside, who maintain a citrus variety collection, report the Washington Navel was so named after being held in USDA quarantine in our nation’s Capitol.  It first flourished in Riverside, California, adjacent to San Diego County.

Uncle Charlie especially loved to eat tree-ripened oranges.  Citrus tend to stay on the tree even after they are initially ripe so the freshest and sweetest oranges must have been available to him.  I can imagine my uncle reaching out and picking select oranges as he drove through the endless rows of fruit-laden trees. He also, undoubtedly, passed the occasional lifeless citrus tree.

One afternoon, I became alarmed to see our orange tree in rapid decline, I inspected it and found a canker, or fungal infection, with the diameter of about 4 inches at the base of the trunk.  Sorely, I was reminded that moist citrus bark invites fungal growth. In this case, moist and rotting leaves had accumulated and promoted the growth of fungi. Such diseases generally flourish under these conditions and begin to disrupt the trunk’s ability to function.  The dieback had all the symptoms of stress because the tree’s internal transport system failed as the fungus thrived within the plant tissue that conducted water.

The University of California at Davis website on Integrated Pest Management lists several known fungal diseases of citrus related to the amount, frequency and method of irrigation: Brown Rot, Armillaria Root Rot, Dry Root Rot, Phytophthora Gummosis, and Phytophthora Root Rot (www.ipm.ucdavis.edu).  Any application of water that allows the trunk, root, and limbs to get wet and stay moist creates an environment favorable for these fungal diseases. Early citrus growers and farm workers encountered debilitating fungal growth on trunks, and roots because early irrigation systems were less precise and many Southern California soils contain fungi that can infect citrus trees.

To avoid fungal diseases in citrus, keep the trunk free from standing water and do not allow water to keep the trunk moist. Adequate air circulation helps evaporate excessive moisture remaining on the bark after irrigation or rainfall so prune the lowest branches, called 'the skirt', in order that all leaves and shoots are at least 12 inches from the ground. Plant your citrus tree in well drained soil where moisture at the base of the trunk will drain and evaporate between watering.  Keep compost, mulch, and decaying leaves from accumulating and remaining in contact with the trunk. 

Minimizing excessive moisture may help your favorite citrus tree remain free of fungal diseases and produce memorable fruit for those close to you.