Although citrus is indigenous to southeast Asia, oranges were first planted commercially in central Arizona in the late 1800s. Today commercial production is centered in several warm and low-frost-risk areas of central and southwestern Arizona. A great number of citrus varieties are also widely planted in home gardens. Many diseases of citrus have been described world wide and have colorful and descriptive names such as: blue mold, green mold, gray mold, pink mold, pink nose, brown rot, black spot, black rot, black pit, yellow vein, yellow spot, rubbery wood, lumpy rind, curly leaf, corky bark, slow decline, spreading decline, and stubborn. Other names are rooted in the many international languages of citrus such as: Italian (impietratura and mal secco), Portuguese (tristeza), or Greek and Latin (cachexia, psorosis, exocortis, xyloporosis, cristacortis, and leprosis). Fortunately, most of the more serious, widespread diseases do not occur or are of little importance in Arizona. Diseases of importance here include several caused by fungi or fungal-like organisms: foot rot gummosis (Phytophthora root rot) caused by Phytophthora nicotianae (syn. Phytophthora parasitica) and P. citrophthora, Hendersonula branch wilt (caused by Hendersonula toruloidea), brown heartwood rot of lemons caused by species of Antrodia and Coniophora, and black rot of fruit caused by Alternaria sp. Bacterial diseases such as citrus canker that require wet environments do not occur in Arizona. Symptoms often may be confusing.
Of the many virus and virus-like diseases that have been described world wide, only tristeza, psorosis and a mycoplasma disease, stubborn, are presently known to occur in Arizona. Tristeza and stubborn disease have been extensively studied in Arizona and in other citrus areas of the world. In 1956, research in Arizona identified tristeza virus in Meyer lemon, and psorosis, a virus-like disease, in several old line citrus varieties including Marsh grapefruit, Valencia orange, and Washington navel. These two diseases were probably introduced into Arizona in infected budwood and planting material prior to 1930. Stubborn disease was first identified in Arizona in 1965
One of the most important diseases found in citrus in Arizona is caused by the citrus nematode (Tylenchulus semipenetrans). This disease has become more widespread and important because present control options are limited. Many serious and widespread diseases in citrus grown in Arizona are nonparasitic in nature. These diseases are not the result of infection by living microorganisms but are caused by factors such as: excessive sun and heat, excesses of certain elements including boron or lithium, excess soil salts, mineral deficiencies, water stress, freeze injury, hail, lightning, and genetic disorders. Two excellent publications that discuss diseases of citrus are: Compendium of Citrus Diseases and Citrus Health Management both published by the American Phytopathological Society and Integrated Pest Management for Citrus, published by the University of California.