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The following diseases are those which may be encountered in California citrus orchards, particularly in very old plantings. Since the mid 1960's, most nurseries have been using tested and registered materials originally derived from the CCPP so the presence in the field of most of these diseases is rare. The only diseases that continue to be a problem to California citrus are those with natural insect vectors, notably tristeza and stubborn.
Cachexia
In reactive hosts, Cachexia causes the wood to have pits and the bark to have corresponding ridges that fit into the pits. Brown gummy deposits form in the pits and the bark phloem. Bark scaling may sometimes occur. The overall result on the tree is stunting that can range from mild to severe.
The disease is transmitted through infected budwood and on budding and pruning equipment. Control requires the use of cachexia free propagating budwood and decontamination of all equipment between diseased and healthy trees.
Concave Gum
Mandarin and sweet oranges are the varieties susceptible to concave gum and they will show deep concavities in the trunk and main branches. Gumming and bark scaling are usually not associated with the disease although gum is deposited in concentric rings of the wood in affected trees. Affected trees have decreased vigor and production.
The principle means of control is the use of budwood that has tested negative for the disease. Also, seed used for the propagation of rootstocks should be collected only from trees that have tested negative for the psorosis complex since there are reports of possible seed transmission of the disease.
Exocortis
Symptoms on susceptible hosts like trifoliate orange and its hybrids, Rangpur lime and some lemons include bark scaling and stunting. On citron leaf epinasity may also be seen.
Control of exocortis requires the use of budwood testing free of the disease and appropriate sanitation of all pruning and grafting tools.
Psorosis
Susceptible hosts such as sweet orange and grapefruit develop scaling and flaking of the bark on trunks and limbs. The wood becomes impregnated with gum forming irregular circular patterns in the trunk that are very characteristic of the disease. Severe psorosis can be very destructive causing tree debilitation and decline. Even less severe infections will affect tree vigor and fruit production.
Control of psorosis can be easily done by using only propagating budwood from sources testing negative for the disease. Since there have also been reports of possible seed transmission, all seed trees used for rootstock production should also test negative for psorosis.
Stubborn
Stubborn affects oranges, grapefruit and mandarins where it causes severe stunting with small dense and upright growing foliage, off blooming, small and lopsided fruit and reduced production. Lemons and limes as well as trifoliate orange and its hybrids can be experimentally infected but rarely show symptoms in the field. The organism causing the disease has a very wide host range of plants other than citrus and it is from these plants that infection to citrus occurs. Tree to tree spread of stubborn within an orchard is extremely rare.
In areas where stubborn and the vector are not endemic, control can be achieved by using only budwood testing negative for stubborn for all propagations. Where both disease and vector are present control is very difficult and efforts should be concentrated on avoiding infection in young plants, by the use of vector proof enclosures or growing in disease free and or vector free areas. If this is not possible, young plantings, up to about 5 years in age, should be examined carefully for the presence of the diseased trees and these individuals removed and replanted with a healthy tree.
A viroid disease widespread in many old-line citrus cultivars where it may remain symptom less but it is particularly pronounced in some mandarins, tangelos and rootstock varieties. Often this disease has been used synonymously with xyloporosis but there is some evidence they are not the same.
This is a graft transmissible virus –like disease that was common in old budline trees and is generally thought to be part of the psorosis disease complex.
Citrus exocortis is a viroid disease that can infect most citrus varieties where it is latent, showing symptoms only in trifoliate orange, some trifoliate orange hybrids, Rangpur lime and some citrons and lemons.
This is the first virus-like disease of citrus recognized and described by Fawcett. Many citrus pathologists believe the disease to be a complex of diseases that share common symptoms. Included in this complex are such diseases as psorosis B, which may be synonymous with citrus ringspot, concave gum, blind pocket, Impietratura and Cristacortis. The disease agent may be graft transmitted to most citrus species, however, some do not develop symptoms.
Stubborn is a disease caused by a Mycoplasma called Spiroplasma citri that is transmitted to citrus by leafhoppers. The disease is found in citrus growing in the hot inland valleys of California.
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University of California, Riverside Citrus Clonal Protection Program Diseases |
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This virus disease is one of the most economically important diseases of citrus worldwide. The virus which causes the disease, citrus tristeza virus (CTV), can infect nearly all varieties and cultivars of citrus. The virus is spread naturally from tree to tree by aphids. In California the aphid vector is the cotton aphid.
This disease is caused by an aphid transmitted virus that may infect a variety of citrus cultivars but is reactive only in rough lemon, Mexican lime and sour orange.
