Tristeza disease complex—Citrus tristeza virus
Citrus tristeza virus (CTV) is one of the most important plant pathogens affecting older citrus trees. Trees infected with the virus are widespread throughout Southern California. The disease is more limited in the Central Valley and desert valleys because of environmental conditions and actions to remove infected trees.
Identification
Symptoms of tristeza disease complex include rapid decline and death of trees, stem pitting beneath bark as described below, and yellowing of foliage. The yellowed leaves may have colorless (relatively clear) veins.
Quick decline (rapid tree death) commonly occurs when sweet orange varieties are grown on sour orange rootstock and then become infected with certain strains of CTV. Quick decline occurs because phloem just below the bud union dies, preventing movement of starch from the scion downward to roots. During periods of drought stress or heat stress the infected tree rapidly dies, often within a few weeks. Prior to rapid death scattered limbs may prematurely drop leaves. The pulp of fruit on the tree may be atypically dry.
Prior to tree death, quick decline is sometimes detectable by peeling bark off the bud union. Look for thickening in bark just below the graft union. Wood around the union can develop a honeycomblike appearance, a band of brownish discoloration, or both. Note that when citrus trees are stressed and weak, the fungus Fusarium solani can cause a dry root rot and tree death that resembles CTV quick decline.
Certain strains of CTV can cause stem pitting, in which the vascular tissue in trunks and branches does not produce normal cells, leaving elongated sunken areas or pits. Stem pitting is considered a severe symptom in citrus because it affects the health of tree branches and limbs and reduces fruit yield. Most citrus types are susceptible to stem pitting disease, even if they are grafted onto a CTV-resistant rootstock such as trifoliate orange and its hybrids. Stem pitting can be recognized by peeling bark off branches or the trunk and looking for pits in wood and protrusions or ridges on the underside of bark. Varieties prone to stem pitting include grapefruit, tangelo, and tangor. Many mandarin varieties are resistant.
Rootstock scion combinations that are susceptible to CTV and infected by the virus may show pale green to yellowish foliage, reduced growth flushes, and premature leaf drop. Infected young trees may bloom early and begin producing fruit 1 to 2 years before uninfected trees. The fruit may remain small due to poor root growth. Yellowing of leaves and shoots occur in the summer when the tree's water needs cannot be met by the declining root system. Yellowing symptoms caused by CTV are difficult to diagnose and may resemble symptoms caused by certain environmental disorders or other diseases.
Visual diagnosis of tristeza is difficult and commonly inconclusive. Plant diagnostic laboratory tests using enzyme-linked immunosorbent assay (ELISA) or molecular methods such as polymerase chain reaction (PCR, which targets specific genes) are used for a rapid diagnosis of CTV and may indicate the severity of the strain(s) detected.
Life cycle
Citrus tristeza virus is spread through budding and grafting during propagation. Aphids feeding on infected citrus can vector the virus when they move to and feed on other citrus trees. Aphids that vector CTV include the black citrus aphid (Toxoptera aurantii), cotton aphid, or melon aphid (Aphis gossypii), and spirea aphid (A. spiraecola). The cotton aphid is the primary vector of the CTV strains found in California. The rate of spread of the disease depends on the aphid species, the particular virus strain(s), and the susceptibility of the tree such as the cultivars of its scion and rootstock combination. Peak periods of transmission occur in spring and fall when the trees are flushing leaves, virus is abundant in infected trees, and the aphids are flying and feeding.
Damage
Tristeza occurs primarily in old trees where sweet orange scion is grafted to a sour orange rootstock. In the past, sour orange was a commonly used rootstock because it provides resistance to various fungal diseases and is productive in a wide range of soil conditions. Sour orange rootstock is no longer widely used because of the potential for quick decline caused by CTV.
Damage symptoms of Citrus tristeza virus are those described above under Identification. However, trees may be infected with more than one strain of CTV and one strain can mask the expression of symptoms of another strain. Many tristeza symptoms resemble damage caused by other citrus diseases and disorders.
Solutions
Tristeza is manageable only by preventive measures. Obtain new plants from a reliable commercial supplier that provides trees certified as disease free. Avoid planting trees with a sweet orange scion grafted to a sour orange rootstock.
Observe quarantine restrictions to avoid spreading tristeza. No citrus plants or plant parts should be moved from infested areas of Southern California to other parts of the state.
In Southern California where tristeza is widespread, you may want to remove an infected tree only when it becomes unproductive. If a tree provided with a good growing environment and appropriate cultural care is not performing as desired, remove and replace it.
Adapted from Integrated Pest Management for Citrus and Pest Management Guidelines: Citrus, University of California Statewide Integrated Pest Management Program (UC IPM). |
