Phytoplasma mali
(apple proliferation)

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Identity

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Preferred Scientific Name

• Phytoplasma mali Seemüller & Schneider, 2004

Preferred Common Name

• apple proliferation

Other Scientific Names

• apple proliferation phytoplasma Seemüller et al., 1994
• Candidatus Phytoplasma mali Seemüller & Schneider, 2004

International Common Names

• Spanish: proliferaciones del manzano
• French: maladie du proliferation du pommiers

Local Common Names

• Croatia: proliferacaija
• Czechoslovakia (former): proliferace janoble
• Denmark: heksekost
• Germany: Triebsucht des Apfels, Besenwuchs, Proliferation
• Greece: skupa milias
• Hungary: almaseprusodes
• Italy: scopazzi del melo
• Netherlands: proliferatie

English acronym

• AP
• AT

EPPO code

• PHYPMA (Phytoplasma mali)

Taxonomic Tree

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• Domain: Bacteria
•     Phylum: Firmicutes
•         Class: Mollicutes
•             Order: Acholeplasmatales
•                 Family: Acholeplasmataceae
•                     Genus: Phytoplasma
•                         Species: Phytoplasma mali

Notes on Taxonomy and Nomenclature

Top of page Apple proliferation is a disease consistently associated with the presence of phytoplasmas in the sieve tubes of diseased apple trees (Seemüller, 1976).

Phytoplasmas, formerly known as mycoplasma like organisms (=MLOs), are unusual, self-replicating bacteria, possessing very small genomes, lacking cell wall components and displaying genetic economy that requires a strict dependence on the host for nutrients and refuge. They are transmitted from diseased to healthy plants by phloem-feeding vectors such as leafhoppers or planthoppers. They are evolutionary descendants from the low G+C-containing, Gram-positive bacteria and through chromosome reduction, represent the smallest self-replicating life forms. These prokaryotes belong to a monophyletic clade of organisms related to Mollicutes (ICSB, 1993, 1997). Their closest relatives are the genus Acholeplasma and Anaeroplasma (Davis et al., 1997; Weisburg et al., 1989). These relationships have been clearly stated by sequence analysis of conserved genes (Lim and Sears, 1992; Gundersen et al., 1994).

Although the culturable Mollicutes are differentiated and classified on the basis of phenotypic and genotypic characteristics (ICSB, 1995), the phytoplasmas are characterized by serology and, mainly, by dot and Southern blot analysis, RFLP analysis of PCR-amplified conserved DNA and sequence analysis of conserved DNA (ICSB, 1993, 1997).

Twenty major groups have been distinguished (Seemüller et al., 1998b). The apple proliferation group phytoplasmas have been reported only in Europe, with the exception of the peach yellow leaf roll agent, which occurs also in USA. The peach yellow leaf roll and pear decline phytoplasmas are closely related (Kirkpatrick et al., 1994; Kison et al., 1994). Apple proliferation phytoplasma is one of the three phytoplasmas belonging to the apple proliferation group (=clade).

Further information on the molecular classification of phytoplasmas belonging to the apple proliferation clade may be found in Seemüller et al. (1998b).

In 2004 it was proposed to accommodate phytoplasmas within the novel genus 'Candidatus (Ca.) Phytoplasma', and apple proliferation as the species Candidatus Phytoplasma mali (Anon., 2004; Seemüller and Schneider, 2004).

Description

Top of page The causal agent of AP is found in the sieve tubes of the current season's phloem. The phytoplasma is highly pleomorphic, approximately 200-800 nm in diameter, bounded with a trilaminar cytoplasmic membrane but lacking a rigid cell wall (Seemüller, 1990).

Distribution

Top of page Records from Austria, Belgium, Bulgaria, Greece, Norway, Romania, Switzerland, former USSR, former Yugoslavia (Németh, 1986), India and South Africa (Seemüller, 1990) are based on symptoms and may require further confirmation.

See also CABI/EPPO (1998, No. 249).

Distribution Table

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The distribution in this summary table is based on all the information available. When several references are cited, they may give conflicting information on the status. Further details may be available for individual references in the Distribution Table Details section which can be selected by going to Generate Report.

Hosts/Species Affected

Top of page Apples are the main host, and most cultivars are susceptible (EPPO/CABI, 1997).

Apple cultivars known to be affected by AP phytoplasma include: susceptible, Belle de Booskop, Gravestein, Golden Delicious and Winter Banana (Anon., 1997); highly susceptible, Florina, Prima and Priscilla (Loi et al., 1995); of medium susceptibility, Idared, McIntosh, Starking and Starkrimson (Németh, 1986); and tolerant, Roja de Benejama (Anon., 1997), Antonokova, Cortland, Spartan, Yellow transparent, Wealthy (Németh, 1986). In northern Italy serious epidemics have been reported to occur on the cultivars Golden Delicious, Florina, Canadian Renette and Granny Smith, grafted on different rootstocks (Osler et al., 2001).

The cultivars Prima, Florina and Priscilla, which are known to be resistant to scab (Venturia inaequalis), were derived from cultivars susceptible to apple proliferation such as: Golden Delicious, Starking Delicious, McIntosh, Jonathan, Rome Beauty and Malus floribunda 821 (Kartte and Seemüller, 1988). These authors stated that 'care should be taken that the high susceptibility of some Malus spp. is not combined in the breeding products'.

The phytoplasma can also be artificially inoculated to Malus baccata, M. coronaria, M. domestica, M. floribunda, M. fusca, M. gloriosa, M. ionensis, M. platicarpa, M. purpurea, M. robusta (Németh, 1986).

Magnolia species and cultivars have been found to be hosts for phytoplasmas of both the apple proliferation and aster yellows phytoplasma groups, although the specific phytoplasma within the AP group/clade was not identified. Infected plants have distorted and stunted growth (Kaminska, 2003).

Host Plants and Other Plants Affected

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Growth Stages

Top of page Flowering stage, Fruiting stage, Vegetative growing stage

Symptoms

Top of page The leaves of infected plants roll downward and become brittle, they are finely and irregularly serrated and are smaller than normal; they also turn red in autumn in contrast to the yellow coloration of healthy plants. The summer leaves are often chlorotic. Early defoliation may occur. A rosette of terminal leaves sometimes develops late in the season in place of normal dormant buds. Stipules are abnormally enlarged while petioles are rather short; this is an important symptom in nursery surveys.

Shoots develop prematurely from axillary buds and give rise of secondary shoots forming witches' broom. The angle between the secondary shoots and the main shoot is abnormally narrow. Leaf rosette may appear on the shoot ends or the shoot tips may die back; this is also an important symptom in nursery surveys.

In some cases, flowers show numerous petals and the peduncles are abnormally long. They fail to set and may stay on the tree for a long period. Fruits are reduced in size, incomplete coloration and poor flavour. Symptoms are unevenly distributed on the whole plant, often seemingly healthy branches are found with normal fruits. Affected trees are less vigorous, but rarely die. Sometimes after a shock phase, trees can produce normally, especially if adequately fertilized. The fibrous root system of infected trees forms compact felt-like masses of short roots so that larger ones are unable to develop. Root weight is reduced by 20-40%. The trunk circumference and crown diameter are reduced compared with healthy trees (Kunze, 1979).

List of Symptoms/Signs

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Biology and Ecology

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In the field, the disease seems to spread naturally by root fusion and by insect vectors. In terms of the vectors, AP was experimentally transmitted from apple to apple and to Catharanthus roseus by the leafhopper vector Philaenus spumarius; from apple to C. roseus by Aphrophora alni and Lepyronia coleoptrata; and from infected celery plants to apple seedlings by Arhianus interstitialis (Hegab and El-Zohairy, 1986). These experiments showed that nymphs can acquire the pathogen and transmit it in the adult stage. Adults were found to maintain the ability to transmit the phytoplasma up to the end of their life. The incubation period in apple lasts 1-2 years (Krczal et al., 1988; Krczal and Bliefernich, 1992).

More recently, the leafhopper Fiebierella florii was considered as a putative vector of apple proliferation phytoplasma. AP-DNA was revealed in the total DNA extracted from F. florii trapped in orchards with proliferation-diseased trees (Vega et al., 1993; Bliefernich and Krczal, 1995). Also, it has recently been demonstrated that the psyllids Cacopsylla costalis, C. mali, and C. melanoneura are vectors (Grando et al., 1998; Jarausch et al., 2003; Tedeschi et al., 2003; Tedeschi and Alma, 2004) and, according to Tedeschi and Alma (2004), psyllids seems to be the most important vectors for AP.

Schmid (1975) reported that, in terms of spread, 73% of trees in an apple orchard became infected over a 12-year period; an average increase of 18% per year was noted.

As indicated above, the distribution of the phytoplasma within the infected tree is restricted to the functional phloem elements, with colonization of above-ground parts following a seasonal pattern. The phytoplasma disappears in the above ground parts during winter, when the complete inactivation of sieve-tubes in aerial parts of pome fruit trees occurs. During this period the phytoplasma still survives in the roots and in spring begins to re-colonize the stem and shoots after the development of a new phloem circle. The pathogen can then be acquired by sap-sucking insects, such as those listed above, within which it can multiply, circulate to the salivary glands, and be expelled during feeding probes to infect other plants. Once a vector is infected, it retains the ability to transmit the phytoplasma for the duration of its life span, and overwintering adult psyllid vectors have been shown to still carry the phytoplasma the next spring (Tedeschi et al., 2003; Tedeschi and Alma, 2004).

Temperature seems to have a significant impact on disease expression and therefore impact. An AP survey conducted in Germany (Seemüller et al., 1998) showed that the phytoplasma was widely distributed outside of the area in southwest Germany where the disease is of greatest economic importance. In the southwest region it seems that the warmer temperatures enhance the growth of the phytoplasma, leading to high populations in the tree tops and more obvious symptoms. However, in the warmer regions of southern Europe, such as in the Emilia-Romagna region of Italy, temperatures may be too high for good symptom development, whereas in the more northerly areas of Europe the temperatures may be too cool for good symptom development. This suggestion corresponds at least in part with results of an earlier study by Ducroquet et al. (1986) who found that symptoms of AP developed at temperatures of 21-24°C, but not between 29 and 32°C.

Means of Movement and Dispersal

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Vector transmission

The psyllids Cacopsylla costalis, C. mali and C. melanoneura are vectors (Grando et al., 1998; Jarausch et al., 2003; Tedeschi et al., 2003; Tedeschi and Alma, 2004;) and, according to Tedeschi and Alma (2004), psyllids seem to be the most important vectors for AP. The leafhopper Fiebierella floriii is considered as a putative vector of apple proliferation phytoplasma. 

Accidental introduction

The disease is graft-transmissible, so long-distance spread could occur with the human movement of infected propagation materials, such as scionwood or rootstock materials. The vegetatively propagated rootstocks are especially hazardous as they are generally symptomless.

There is no seed or pollen transmission.

Vectors and Intermediate Hosts

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Impact

Top of page AP is considered one of the most important phytoplasma diseases of apple (EPPO/CABI, 1997), particularly in the northern areas of southern Europe, where temperatures are the most conducive to symptom expression. Outside of this region, where cooler or warmer growing conditions occur, the disease appears to be of less importance (Seemüller et al., 1998a).

AP is reported to affect almost all apple varieties, causing reductions in (i) fruit size by up to 50%, (ii) fruit weight by 63-74%, (iii) fruit quality, through the reduction of sugar and acid content, and (iv) tree vigour. It also increases susceptibility of infected trees to other plant pathogens, such as powdery mildew (Podosphaera leucotricha; Maszkiewicz et al., 1980), or the silver leaf fungus (Chondrostereum purpureum; Németh, 1986). Most significant losses (up to 80%) are incurred during the acute phase of the disease (ie. shock phase), although a considerable percentage of fruit remains undersized even after this period. In some cases, AP can also lead to premature death of infected trees (Németh, 1986; Seemüller, 1990; Smith et al., 1988).

Diagnosis

Top of page Biological Assay

Positive identification requires transmission to a woody indicator species (see Detection and Inspection Methods). Malus dawsoniana, a very sensitive indicator, when grafted in June on the scion, develops symptoms the following autumn. Using the double budding technique, the reaction appears after budbreak. (Anon., 1997). The use of DAPI reagent (1,6 diamidino 2-phenylindole) can help to detect the fluorescence of phytoplasmas in the sieve tubes of phloem tissue under the bark of infected apple trees

Serological Assay

Apple proliferation can be detected in infected apple trees using monoclonal antibodies (MAbs) to AP phytoplasmas obtained from AP phytoplasma-infected Catharanthus roseus as source of antigen. These MAbs react specifically in ELISA and immunofluorescence (IF) (Loi et al., 1998). Their patent is pendent. ADGEN Ltd. have also produced an AP phytoplasma specific antisera, which works with apple extracts if there is sufficient antigen present. Usually the concentration of phytoplasma is a limiting factor.

Molecular Assay

DNA extraction

Midrib and main vein tissue (1 g) should be used from leaves collected in August-September, avoiding any large necrotic areas. Use the following enriched phytoplasmas procedure (Ahrens and Seemüller (1992) to obtain DNA for PCR.

- Incubate the tissue in 6 ml of grinding buffer on ice for 10 min.
- Grind the tissue, then add 8 ml of fresh buffer and grind once more.
- Centrifuge at 1100 g and 4°C for 10 min.
- Decant the supernatant then centrifuge at 14,600 g and 4°C for 25 min.
- Resuspend the pellet in 1.5 ml of warm (60°C) extraction buffer.
- Incubate at 60°C for 30 min.
- Add to the lysate an equal volume of chloroform/isoamylalcohol (24:1, v/v).
- Precipitate the aqueous layer with two-third volume of -20°C isopropanol.
- Centrifuge at 15,000 g and 4°C.
- Wash the pellet with 70% ethanol.
- Dry under vacuum.
- Add 50 µg/ml RNAse at 37°C for 30 min.

Extract with chloroform/isoamylalcohol (24:1, v/v), precipitate with ethanol, wash and dry pellet as described.

Grinding buffer

potassium phosphate 125 mM
ascorbic acid 30 mM
sucrose 10%
bovine serum albumin (BSA fraction V) 0.15%
polyvinylpyrrolidone (PVP 15) 2%
pH 7.6.

Extraction buffer

Hexadecyltrimethyl-ammonium bromide (CTAB) 2%
NaCl 1.4 M
2-mercaptoethanol 0.2%
Ethylendiaminetetraacetic (EDTA) 20 mM
Tris- hydroxymethylaminomethane hydrochloride (Tris-HCl) 100 mM
pH 8.0

Polymerase Chain Reaction (PCR)

Several primers intended for diagnostic procedures for the diagnosis of phytoplasma diseases have been identified, among them are those specific to the apple proliferation group (see Lee et al., 1993, 1995; Firrao et al., 1994; Jarausch et al., 1994; Lorenz et al., 1995; Smart et al., 1996).


Molecular diagnosis of apple proliferation phytoplasma by direct PCR

Primer pair: f01/r01

Sequence:
5' - CGG AAA CTT TTA GTT TCA GT - 3'
5' - AAG TGC CCA ACT AAA TGA T - 3'

Amplified product size: ca 1100 bp

Phytoplasma: all phytoplasma isolates of AP group (AP, PD, ESFY, PYLR)

Reference: Lorenz et al. (1995)

Primer pair: fPD/r01

Sequence:
5' - GAC CCG TAA GGT ATG CTG -3'
5' - AAG TGC CCA ACT AAA TGA T - 3'

Amplified product size: ca 1000 bp

Phytoplasma: all phytoplasma isolates of pome fruit subgroup of AP group (AP, PD)

Reference: Lorenz et al. (1995)

Primer pair: fAT/rAS

Sequence:
5' - CAT CAT TTA GTT GGG CAC TT - 3'
5' - GGC CCC GGA CCA TTA TTT ATT - 3'

Amplified product size: ca 500 bp

Phytoplasma: all phytoplasma isolates of pome fruit subgroup of AP group (AP, PD)

Reference: Smart et al. (1996)

Primer pair: AP5-AP4

Sequence:
5' - TCT TTT AAT CTT CAA CCA TGG C - 3'
5' - CCA ATG TGT GAA ATC TGT AG - 3'

Amplified product size: ca 483 bp

Phytoplasma: specific for apple proloferation phytoplasma

Reference: Jarausch et al. (1994)

Primer pair: AP3-AP5

Sequence:
5' - CTA AAA CTC ACG CTT CAG CTA CTC - 3'
5' - TGA GAT TTG CTA AAA CTC ACG CTT - 3'

Amplified product size: ca 670 bp

Phytoplasma: specific for apple proliferation phytoplasma

Reference: Firra et al. (1994)


The pair fO1/rO1, AP3-AP5 and fPD/rO1 prime in the 16S rDNA sequence and are, respectively, the first AP group specific, the second AP specific, and the third pome fruit subgroup specific. The pair fAT/rAS primes in the 16S/23S rDNA spacer region and is specific for pome fruit subgroup phytoplasmas.

The pair AP5-AP4 was designed from a randomly cloned nucleotide sequence (Gene Bank accession number L22217) of the German isolate AT (1812 bp) of the previously cloned Hind III fragment IH196 (3,7 Kb) sequenced from both ends. This pair is specific for AP phytoplasma.

Molecular diagnosis of apple proliferation phytoplasma by nested PCR

Primer pair: R16F2/R2

Sequence:
5' - ACG ACT GCT AAG GAC TGG - 3'
5' - TGA GGG GCG GTG TGT ACA AAC CCC G - 3'

Amplified product size: ca 1200 bp

Phytoplasma: all phytoplasmas

Reference: Lee et al. (1993)

Primer pair: R16(X)F1/R1

Sequence:
5' - GAC CCG CAA GTA TGC TGA GAG ATG - 3'
5' - CAA TCC GAA CTG AGA CTG T - 3'

Amplified product size: ca 1100 bp

Phytoplasma: all phytoplasmas of AP group (AP, PD, ESFY)

Reference: Lee et al. (1995)


PCR products initially amplified by using the universal primer pair R16F2/R2 are diluted (1/40) with sterile deionized water and used as template DNA for a subsequent PCR with the pair R16(X)F1/R1 This pair primes on the 16S rDNA sequence. (Lee et al., 1995). All these primer pairs work at different reaction mixtures and conditions.

The following are reaction mixtures and conditions for PCR with different primer pairs designed for diagnosis of AP group and AP phytoplasma:

Primer pairs: fO1/rO1; fPD/rO1

Reaction mixture: final volume: 40 µl
template, 100-200 ng
Primers: 0.5 µM
Buffer: 1x; MgCl2 1.5 mM
dNTPs: 100 µM
Enzyme: 0.20-1 Unit/reaction.
Reaction conditions: 35 cycles
95°C for 30 s denaturation
55°C for 75 s annealing
72°C for 90 s extension

Primer pair: fAT/rAS

Reaction mixture: final volume: 30 µl
template: 50 ng
Primers: 0.5 µM
buffer: 1x
dNTPs: 150 µM
Enzyme: 1 Unit/reaction
Reaction conditions: 30 cycles
94°C for 60 s denaturation
55°C for 60 s annealing
72°C for 120 s extension

Primer pair: AP5/AP4

Reaction mixture: final volume: 40 µl
template: 10-100 ng
Primers: 0.5 µM
Buffer: 1x
dNTPs: 125 µM
Enzyme: 0.5 Unit/reaction
Reaction conditions:
95°C for 60 s predenaturation step
40 cycles
95°C for 10 s denaturation
58°C for 15 s annealing
72°C for 45 s extension
72°C for 240 s elongation

Primer pairs: R16F2/R2; R16(X)F2/R2

Reaction mixture: final volume: 50 µl
template: 20 ng
Primers: 0.4-1 µM
Buffer: 1x
dNTPs: 200 µM
Enzyme: 1 Unit/reaction
Reaction conditions:
94°C for 120 s predenaturation step
35 cycles
94°C for 60 s denaturation
50°C for 120 s annealing
70 °C for 180 s extension
72°C for 600s elongation

Primer pairs: AP3-AP5:

Reaction mixture: final volume: 50 µl
template: 2 µl
Primers: 300 ng
Buffer: 1x
dNTPs: 100 mM
Enzyme: 1 Unit/reaction

Reaction conditions: 25 cycles
94°C for 30 s denaturation
63°C for 30 s annealing
72°C for 30 s extension
72°C for 600 s elongation

Restriction Fragment Length Polymorphism analysis (RFLP)

The use of PCR amplification of the 16S rDNA to differentiate phytoplasmas belonging to the same group is difficult due to the high similarity of their sequences so that it is necessary to digest the amplification products by restriction endonucleases.

Molecular characterization of AP phytoplasma with RFLP analysis of amplified products obtained with different primers

Primer pair: f01/r01; fPD/r01
Endonuclease: Ssp I, Sfe I
Phytoplasma: AP
Reference: Lorenz et al. (1995)

Primer pair: R16F2/R2+R16(X)F2/R2
Endonuclease: Rsa I
Phytoplasma: AP
Reference: Lee et al. (1995)

Primer pair: AP5-AP4
Endonuclease: Ssp I, Spe I, Hinf I
Reference: Jarausch et al. (1994)


Dot-Blot Hybridization

Differential diagnosis of apple proliferation and European stone fruit yellow phytoplasmas can be made by oligonucleotide hybridization in the presence of tetramethylammonium chloride (Malisano et al., 1996). This diagnostic approach may be more practical than PCR plus RFLP in differentiating the two phytoplasmas that cause apple proliferation and plum leptonecrosis, which are economically significant diseases in the main European fruit tree-growing areas. It is also suitable for automatic evaluation of a large number of samples.

DNA Amplification

The primer pair P1-P4 should be used:

P1 5' - CAG CAG GYC CGC GTA ATA CAT A - 3'
P4 5' - RMC CCG AGA AGC TAT TCA CCG - 3'

Y = C or T; R = A or G; m = A or C

This pair primes on 16S rDNA sequence and amplify a segment of ca 865 bp (Firrao et al., 1993).

- Add 2 x SSC to 5-45 µl of PCR mixture (1 x SSC: 150 mM NaCl and 15 mM Na citrate, pH 7.0).
- vacuum-filter on nylon membranes with a 96-well dot-blot manifold apparatus
- place dried membrane sequentially on 3 mm paper sheets saturated with: 1.5 M NaCl and 0.5 NaOH for 5 min; 0.5 M Tris pH 7.4; 0.5 M Tris pH 7.4 and 1.5 M NaCl for 5 min.
- dry and bake for 30 min at 120°C
- pre-hybridize in 5 x SSC; -0.1% laurosylsarcosine; 0.02% SDS; 1% blocking reagent; 42°C for 2 h
- hybridize in solution as above containing 20 pmol oligonucleotide probe at 42°C for 6 h
- washings :2 x 30 min in 5x SSC at 4°C; 2 x 20 min at 57°C in tetramethylammonium chloride wash solution (wash solution: 3 M TMACl, 50 mM Tris-HCl; 2 mM EDTA; 0.1% sodium dodecyl sulfate (SDS), pH 8)
- digoxigenin labelling and detection can be performed according to manufacturer's instructions.

Probe sequences:

LN 11 5' -GTG CGT AGG CGG TTA AA- 3'
AP 11 5' -GTG TGT AGG CGG TTA AA- 3'

(Malisano et al., 1996)

Requirements for positive molecular diagnosis:

- Water instead of template DNA must be considered and added to reaction mixture for use as negative controls in PCR experiments.

- In case of negative result, the use of universal rRNA primers is recommended to check the presence of phytoplasmas other than apple proliferation phytoplasma. Suggested universal primers: fU5/RU3 (Lorenz et al., 1995); F2n/R2 (Lee et al., 1995); and P1/Tint (Smart et al., 1996).

- Then use PCR, with other group- or species-specific primers, and RFLP analysis for differentiation (Lorenz et al., 1995; Smart et al., 1996).

- Use DNA of reference strains, belonging to the same and to other groups, in all molecular diagnostic methods.

Detection and Inspection

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Apple proliferation is routinely detected by visual observation (see Symptoms) and by field testing on Malus domestica Golden Delicious (side interstock grafting) using five repetitions for 2 years (OEPP/EPPO, 1991/1992).

For laboratory diagnosis at least five samples per plant should be randomly collected in order to avoid false negative results due to the low titre and erratic distribution of these pathogens in the phloem of the plant. Samples from leaves, petioles, shoots and canes should be collected at the end of August. The use of fresh material is recommended in diagnostic assays. However, if this is not possible, the test material should be stored at -20°C. (See Diagnosis.)

Similarities to Other Species/Conditions

Top of page The criteria used to differentiate apple proliferation from the other phytoplasmas and the related Mollicutes are: full-length of 16S rDNA; 16S/23S spacer region sequence; RFLP of PCR-amplified ribosomal DNA products; and RFLP patterns of non-ribosomal DNA (Southern analysis).

Prevention and Control

Top of page As a preventive measure, the use of healthy propagation material is a prerequisite. Intensive control of weeds and insect pests in the orchards is also essential. Removal of root suckers is a control measure to reduce the diffusion of leafhoppers living on weeds. Phytoplasmas are sensitive to tetracycline, but this antibiotic is bacteriostatic rather than bactericidal. After antibiotic treatment, symptoms can noticeably diminish for 2 years (Casanova et al., 1980), but often this is only observed as a temporary effect (Schmid, 1983). The use of tetracycline can unintentionally select for resistant bacterial populations. At the present time, resistant rootstock is the most efficient way to control the disease (Seemüller, 1990).

References

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Anon, 1997. Apple proliferation phytoplasma. In: Quarantine Pests for Europe. II ed,. Smith, McNamara, Scott, Holderness, Burger eds., CABI and EPPO pub, 959-962

Anon, 2004. 'Candidatus Phytoplasma', a taxon for the wall-less, non-helical prokaryotes that colonize plant phloem and insects. International Journal of Systematic and Evolutionary Microbiology, 54(4):1243-1255

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Davis RE, Dally EL, Gundersen DE, Lee IngMing, Habili N, 1997. "Candidatus phytoplasma australiense", a new phytoplasma taxon associated with Australian grapevine yellows. International Journal of Systematic Bacteriology, 47(2):262-269; 45 ref

Ducroquet J-P, Dosba F, Lansac M, Mazy K, 1986. Effet de la température sur l’expression des symptômes de la prolifération du pommier. Agronomie, 6(10):897-903

EPPO, 2011. EPPO Reporting Service. EPPO Reporting Service. Paris, France: EPPO. http://archives.eppo.org/EPPOReporting/Reporting_Archives.htm

EPPO, 2014. PQR database. Paris, France: European and Mediterranean Plant Protection Organization. http://www.eppo.int/DATABASES/pqr/pqr.htm

EPPO, 2019. EPPO Global Database. Paris, France: EPPO.https://gd.eppo.int

Firrao G, Gobbi E, Locci R, 1993. Use of polymerase chain reaction to produce oligonucleotide probes for mycoplasmalike organisms. Phytopathology, 83(6):602-607

Firrao G, Gobbi E, Locci R, 1994. Rapid diagnosis of apple proliferation mycoplasma-like organism using a polymerase chain reaction procedure. Plant Pathology, 43(4):669-674

Grando MS, Forti D, Vindimian ME, 1998. DNA sequences of the apple proliferation phytoplasma found in psyllid collected from diseased apple trees. Journal of Plant Pathology, 80(3):257

Gundersen DE, Lee IM, Rehner SA, Davis RE, Kingsbury DT, 1994. Phylogeny of mycoplasmalike organisms (Phytoplasmas): a basis for their classification. Journal of Bacteriology, 176(17):5244-5254

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