Citrus trees in NZ orchards show considerable variablility in performance, manifested in the tree size, shape and appearance of the bud union and stock. This variability has been linked with off-type rootstocks and infection with a number of citrus virus and virus-like diseases. A couple of these viruses were not thought to be present in New Zealand.

Citrus trees may be affected by many different virus and virus-like diseases. Symptoms of these diseases vary from relatively harmless infections, such as citrus vein enation virus, to rapid death of trees, as caused by severe strains of citrus tristeza virus. Although most citrus viruses and viroids do not kill the trees, most are debilitating and will result in some degree of tree growth reduction. They have adverse effects on external and internal quality of the fruit and therefore reduce overall production of an orchard. Until recently, the only virus and virus-like diseases thought to be present in New Zealand were citrus tristeza virus (CTV), citrus vein-enation virus (CVEV) and citrus exocortis viroid (CEV).

In 1992, coinciding with the establishment of the NZ Citrus Budwood Scheme, HortResearch initiated a survey to characterise the citrus viroid complex and citrus tristeza virus strains present in NZ. During the past two years this survey has been expanded to include indexing for other citrus viruses, namely - satsuma dwarf virus, citrus vein enation virus and the citrus psorosis virus complex. This article presents the current findings of these surveys.

Citrus Viroids
Viroids are the smallest and structurally best characterised infectious agents presently known. Unlike viruses which have a protein coat, viroids are quite simply pieces of naked RNA. The two viroids of most economic importance are citrus exocortis viroid (CEV) and cachexia viroid (CVIIb).

Although we escaped the massive devastation caused by CTV by basing our industry predominantly on Poncirus trifoliata, we do have a problem with this rootstock as it is particularly sensitive to infection by CEV. Symptoms of CEV infection in trees on trifoliata first become apparent 4 to 8 years after planting, and include mild to severe bark cracking and scaling on the rootstock portion of the tree (Figure 1). The bark cracking and scaling impedes the flow of nutrients and water between the roots and canopy of the tree resulting in reduced tree vigour, stunting and losses in fruit quality and yield.

Figure 1
CEV bark cracking on trifoliate rootstock.

Citrus viroid detection and identification is based upon symptoms produced on the biological indicator Etrog citron. Graft-transmission of viroids using buds from infected field trees to Etrog citron indicator plants produces leaf symptoms indicative of viroid infection, including various degrees of leaf curling (Figure 2). The presence of the viroid(s) is then confirmed based upon banding patterns of extracted viroid RNA analysed by sequential polyacrylamide gel electrophoresis. CEV has been extracted and positively identified from Villa Franca and Genoa lemons.

Figure 2
CEV leaf symptoms on Etrog citron. Healthy plant on the LHS

Citrus Tristeza Virus
Citrus tristeza virus (CTV) represents the biggest threat to worldwide citrus production. It is believed to have originated in Asia where it existed unrecognised for centuries, possibly because the commonly grown citrus cultivars were highly tolerant. CTV quick decline of trees on sour orange rootstocks was first identified in Australia and New Zealand in 1940. The CTV had not been isolated at this time and the death of these trees was attributed to a scion/rootstocks 'incompatibility problem'. In retrospect, this misidentification of the disease was very fortunate as the NZ citrus propagators of the day had to use alternative rootstocks such as Poncirus trifoliata, citranges and sweet orange, which did not show this 'incompatibility problem'. Consequently, although we have the quick decline strains of CTV in NZ, our industry has been based on rootstock varieties resistant or tolerant to CTV and we have not suffered the devastating tree losses to CTV experienced by those countries which used sour orange as their major rootstock variety. Unfortunately, strains of CTV which cause severe stem pitting can cause losses even on CTV tolerant rootstocks. Tree stunting and yield losses due to these strains of CTV are evident in NZ citrus orchards.

Since 1993 CTV isolates have been collected from superior clonal trees of 33 different citrus cultivars grown in the different citrus growing regions of New Zealand. Screening of CTV isolates on Madam Vinous, Eureka lemon, West Indian lime and Sour Orange indicator seedlings, demonstrated that severe stem pitting (Figure 3) and seedling yellows strains (Figure 4) of CTV were widespread throughout New Zealand. Milder forms of the disease were produced by some isolates of the virus, which either lacked both the seedling yellows and stem pitting symptom components or lacked seedling yellows symptoms but expressed a mild form of stem pitting. Fortunately, the orange stem pitting CTV strain recently found in Australia and Brazil has not been found to be present in New Zealand.

Figure 3
CTV stem pitting symptoms evident under the bark on young shoots of Madame vinous sweet orange.

Figure 4
CTV seedling yellows symptoms. Healthy plant on LHS.

Satsuma Dwarf Virus
Satsuma dwarf virus is a viral disease of citrus which effectively reduces tree vigour, resulting in the trees with a dwarfed habit. Satsuma trees infected with SDV will also display a range of leaf symptoms including narrow, boat or spoon-shaped leaves, the surface of which is often distorted or crinkled (Figure 5 a & b).

Figure 5a
SDV leaf symptoms on Miyagawa satsuma mandarin.

Figure 5b
SDV leaf curling. Healthy shoot on RHS.

The virus has only been reported to be present in citrus in Japan and Turkey and is believed to be restricted to cooler climates. A preliminary SDV survey was initiated in the spring of 1993, on satsuma orchards in the Kerikeri district. Symptoms of tree stunting and/or leaf distortion in the spring growth flushes were observed in all satsuma orchards surveyed.

Biological indexing for SDV in the presence of CTV is problematic as CTV virus symptoms mask those of SDV. It is therefore necessary to pass the suspected SDV extracts through a CTV resistant plant prior to indexing. This step of the indexing procedure will be completed in September 1995 and biological indexing results for SDV will be completed in June 1996.

Citrus Psorosis Complex
Citrus psorosis is a disease of citrus commonly known as scaly bark. It causes a loss of vigour which is accompanied by reduced fruit quality and dramatic reduction in yield. Severe infections can result in the death of scaffold branches and occasionally the entire tree.

Psorosis is now believed to be a complex of diseases with each component being caused by a different virus, each differing widely in its effect on the trees. Symptoms vary from distinctive mottles and patterns on leaves and fruit to the development of bark scaling lesions on the trunk and limbs of sweet orange, mandarin and grapefruit cultivars (Figure 6). Occasionally, in a nursery situation, shock symptoms on new shoots of susceptible cultivars may be observed (Figure 7). The shock symptoms are characterised by the sudden death of young shoots.

Figure 6
Psorosis bark scaling symptoms on Silverhill satsuma mandarin

Figure 7
Psorosis shock symptoms on nursery plant.

Purification of viruses from leaf and bark tissues of trees of satsuma and tangor cultivars has identified mixed infections of two distinct virus particles: rigid and flexuous rods. Electron microscope observations of these particles suggest that these trees were infected with the citrus ringspot virus, a component of the psorosis complex. These results coupled with observations of bark scaling and leaf flecking in field trees of satsuma mandarin, NZ grapefruit, tangor and tangelo cultivars suggests that citrus psorosis virus is reasonably widespread in NZ. The majority of citrus species are symptomless carriers of this disease complex. Therefore we have recently initiated a survey of the main commercial citrus cultivars to determine how many of the cultivars will need to undergo virus elimination protocols to halt the spread of this disease and ensure that future orchards are psorosis free.

Citrus Vein-Enation Virus
Citrus vein-enation virus is widespread throughout the cooler citrus growing regions of the world. It is symptomless in the majority of citrus cultivars and is not considered of major economic importance. CVEV is linked to the formation of wood galls on the trunks and branches of rough and Volkamer lemons. Severe infection on these rootstocks has been reported to eventually cause tree decline.

At Kerikeri Research Centre, graft-transmission by bark patches from field trees of citrus to sensitive indicator plants has confirmed the presence of CVEV. Symptoms of enations have been observed on the leaves of indicator plants bark-grafted with patches from a wide range of cultivars from all citrus growing regions in New Zealand. Vein-enations appear as small galls on the veins on the underside of the leaves (Figure 8). Rough lemon is not a commercial rootstock in New Zealand and consequently, although this pathogen is widespread it does not pose a major threat to our industry.

Figure 8.
CVEV galls on underside of leaf of Sour orange seedling. Insert showing close up of galls.

Transmission
Citrus viroids are primarily transmitted through infected budwood. They are also readily mechanically transmitted on infected cutting and pruning tools. Citrus viruses can be spread by insects, by mechanical transmission and through infected budwood. Psorosis virus can be transmitted both mechanically and in infected budwood. It can also be transmitted through seed hence it is important to ensure a psorosis-free source of seed is confirmed before propagating rootstocks for production. At present psorosis is the only known seed borne citrus virus.

Both CTV and CVEV are spread by aphids. The brown citrus aphid is widespread in New Zealand, it is therefore not surprising that both these pathogens are widespread. Each of these viruses are also readily transmitted in infected budwood.

SDV can easily be spread through infected budwood or on contaminated orchard cutting and pruning tools. Unlike the other viruses, SDV is also soil borne and consequently once a site is infected, the disease may become permanently established.

Protecting Our Industry
In order to keep our citrus orchards free from debilitating citrus pathogens we must ensure that we:
* maintain secure quarantine systems to eliminate diseases in legal budwood imports.
* maintain stringent border checks to stop illegal imports of infected budwood.
* maintain good orchard hygiene to prevent virus/viroid spread.
* promote the use of indexed budwood supplied by the NZ Citrus Budwood Scheme.
* continue research into CTV mild strain selection for protection against severe
  strains of citrus tristeza virus.
* process all commercial citrus cultivars through shoot-tip grafting in order to eliminate
  debilitating virus/viroids.

HELP PROTECT THE NZ CITRUS INDUSTRY
If you ever hear of unusual or illegal importations of citrus budwood, please call the MAF Flora & Fauna Investigation Unit. Your alertness and rapid action could prevent virus disease from devastating our citrus industry.

MAF Flora & Fauna Investigation Unit
Phone: 09-357 6472

Source:
The Orchardist, September, 1995