[Virus] Citrus tristeza virus (CTV) (family Closteroviridae) (genus Closterovirus)
Tristeza, quick decline (QD)
Tristeza is vectored by several species of aphid the most efficient being the brown citrus aphid Toxoptera citricida (Kirklady). Where T. citricida does occur, Aphis gossypii (Glover), the cotton/melon aphid, is the primary vector. The aphid can acquire the virus after feeding on infected plants for 5-60 minutes; but loses the ability to transmit the virus after 24 hours. CTV is also graft-transmitted, but not transmitted through seed. The virus exists in many forms known as strains that vary in the type and severity of symptoms manifested in host plants. Some strains are mild and produce no noticeable symptoms however; other strains are severe causing decline and death of the tree or deep pits in the trunk and stem. CTV is phloem-limited and the largest of the known plant viruses.
There are three distinct syndromes of CTV infection: quick decline, stem pitting, and seedling yellows. The most notorious is quick decline (QD) and is associated with the name Tristeza. It is a three-component malady consisting of a sweet orange variety used as a scion grafted onto sour orange rootstock and infected with a quick decline strain of CTV. In this case, the virus affects the cambium layer right below the bud union and prevents the normal development of cambium cells (that mature into xylem and phloem cells). As a result, the flow of photosynthetic products from the upper portions of the tree down to the roots and water and nutrients adsorbed by the roots and transported up the tree are blocked. The decline may not occur for several years, or it may occur rapidly after a heat and/or water stress event, leaving fruit shriveled on the tree and leaves brown and dehydrated. When the decline is slow, often a bulge occurs above the bud union when a window is cut in the bark (inner flap).
Severe strains of CTV cause symptoms such as stem pitting (SP) and seedling yellows (SY), regardless of the rootstock. Grapefruit varieties are most susceptible to stem pitting strains, but these strains can be severe for many other citrus varieties. When the bark is peeled away, pits in the wood can be observed ranging from short and narrow to elongated and deep; gum is sometimes associated with the pits. Trunks may be so severely impacted that they have a ropey appearance. Severely affected trees are chlorotic, stunted, and generally have a low yield of poor quality fruit. Citrus macrophylla or alemow is most susceptible to SP strains.
Seedling yellows (SY) is typically not seen in field situations with the exception of topworking SY-infected trees with grapefruit or lemon budwood. It affects primarily young seedlings, and nursery workers rogue out affected plants. The SY reaction is frequently used to describe field collected isolates of the virus in bio-characterization experiments performed under controlled greenhouse conditions. A severe strain often causes the SY symptom in sour orange, grapefruit and lemon host indicator plants.
Leaf - chlorotic leaf flecking, vein clearing, leaf cupping, corking of leaf veins, and stem pitting.
Fruit - reduced fruit size.
Whole tree- the symptoms are similar to root injury. These symptoms include thinning of foliage, twig dieback, retardation of growth and possibly tree collapse.
Trunk - inside of the bark a honeycomb or stem pitting appearance can be detected with the unaided eye. In the trunks and limbs of larger trees, there sometimes is a bumpy or ropy appearance caused by the pitting.
The host range of this pathogen is very complex as it varies with rootstock and scion combinations, CTV strain, and environmental conditions. In general, the tristeza virus infects almost all species, cultivars and hybrids of Citrus in addition to other genera in the Rutaceae such as Aegle, Microcitrus, and Passiflora. Some pummelo genotypes and Poncirus trifoliata are resistant to most isolates. Most trifoliate orange clones and their hybrids are tolerant to most isolates and are widely used as rootstocks. Sour orange and alemow are highly susceptible. Mandarins are typically tolerant.
CTV occurs in every citrus producing area throughout the world; however not all have severe isolates. The virus primarily spreads by the sharing of infected budwood. Most countries counteract with activity by the implementation of budwood certification programs which include the testing of budwood for graft-transmissible pathogens, budwood-clean-up through thermal therapy and shoot-tip grafting, and distribution through certified nurseries.