Description
BIOLOGY AND DAMAGE
The phytoplasma Ca. P. mali is a virus-like micro-organism without a cell wall that can only survive within the host plant or vector insects. In the host plant, the phytoplasma settles in the phloem vessels where it multiplies until it colonises the entire plant, including the roots. Spread within the plant can take several years and is influenced by factors that are still little known. During vegetative rest, the phytoplasma is only found in the roots as the phloem loses its functionality, and at the next vegetative restart it returns to colonise the aerial part.
The damaging effect on the plant is due to the alteration of phloem flow with repercussions on normal vegetative growth and fruit quality. A reduction in production of up to 60% is estimated, as well as high costs for plant replacement and pest control.
The recovery of infected plants, i.e. the eradication of the pathogen, has never been documented. The remission of symptoms, a phenomenon known as recovery in English, results in a transient disappearance of symptoms while the presence of the phytoplasma persists in the roots. In the event of recovery, the asymptomatic apple plant remains infected and this can lead to the transmission of infection from an asymptomatic infected plant to a healthy plant via vector insects.
SYMPTOMS OF THE DISEASE
The manifestation of bark beetle symptoms may be more or less obvious depending on many factors that are not yet well known. It is possible that the plant does not show any symptoms despite being infected with the phytoplasma; in this case we speak of latent infection. These plants are potentially dangerous as they are a source of inoculum for vector insects and healthy plants, but cannot be recognised even by careful visual inspection. It is therefore important to be able to identify at least the plants showing symptoms.
The symptoms of the disease may affect the whole plant or, much more commonly, only a part of it; however, the whole plant must be considered to be diseased.
Symptoms may occur as early as the vegetative restart or during the growing season. The autumn period, before the beginning of leaf fall, is the time when by far the greatest number of symptomatic plants are visible (which is why field inspections have always taken place during this period). Another time when symptoms are visible fairly easily is in spring, in the phenological phase between well-spread outspread bunches/pink buttons and the beginning of flowering; up to 50% of symptomatic plants can be detected at this time.
Symptoms can be distinguished into specific and non-specific symptoms.
Specific symptoms: the presence of a single symptom is sufficient to state with certainty that the plant is infected. Specific symptoms are:
- "witches' broom", i.e. the year's shoots branch out because the lateral buds open early, forming a clustering of the vegetation that resembles an upturned broom (Fig. 1). This symptom is typically autumnal and is clearly visible and easy to recognise.
- Enlarged stipules with serrated margins: these are clearly distinguishable from normal stipules (i.e. the leaf stalks present at the base of the petiole of each leaf); they are especially visible in the flower clusters and at the base of shoots as early as spring (Fig. 2) and until leaf fall (Fig. 3); in autumn they may also appear in the apical part of new shoots (Fig. 4). This symptom is more difficult to detect; it must be carefully observed inside the plant.
Non-specific symptoms: they do not allow an AP infection to be identified with certainty unless at least two are present at the same time. These are non-specific symptoms:
- small apples with a long stalk (generally tasteless and poorly coloured);
- early reddening of leaves at the end of the growing season.
Other symptoms that can help identify AP-infected plants as early as spring, in addition to enlarged stipules with serrated margins, are the following
- early vegetative regrowth
- reddened vegetation;
- elongated clusters;
- crowded vegetation;
- late re-blooming.
More photographs on the symptoms of apple bark beetle are available on the Fondazione E. Mach dedicated to phytoemergencies: https://fitoemergenze.fmach.it/scopazzi-del-melo
SPREAD AND PHYTOSANITARY STATUS
'Candidatus Phytoplasma mali' was first reported in Italy in 1950, in Veneto and Trentino, and is currently widespread in all the main European fruit-growing areas. The disease has been endemic in Trentino since 2006 when the first large wave of infected plants was recorded. A second wave was recorded in 2013 in the Valsugana area, while in the last few years a third wave is being witnessed with serious infection rates throughout the Trentino territory.
One of the main causes of the increase in the incidence of the disease is orchard management. In fact, abandoned orchards or those in which the infected plants have not been grubbed up on time or in which no targeted treatments against vectors have been carried out are the most affected by the disease.
Phytoplasma was classified and treated as a quarantine pest until the entry into force of the new phytosanitary regime in 2019, when it was downgraded to a non-quarantine regulated pathogen. Despite its phytosanitary status, due to the economic impact that this disease can have on the provincial fruit-growing sector, it is necessary to maintain a high level of attention and take phytosanitary measures to contain its spread, which is why the recent Resolution No 1442 of 26 September 2025 was issued.
TRANSMISSION ROUTES
The spread of phytoplasma from one plant to another can occur via: vector insects, root anastomosis, infected nursery material, grafting. Phytoplasma is neither transmitted through scissors or hacksaws used for pruning nor through wounds.
Insect vectors
The two psyllids Cacopsylla melanoneura (Förster) and Cacopsylla picta (sin. costalis) have been indicated as vector insects of 'Apple Proliferation' in Trentino - South Tyrol, although it cannot be excluded that other species may be involved. Both species are univoltine, i.e. they complete one generation per year and both overwinter at the adult stage on refuge plants, mainly conifers. At the end of winter, the adults migrate again from the forests to host plants, where they reproduce and where the juvenile stages develop. During their trophic activity, with their stinging-sucking mouthparts they are able to transmit phytoplasma from infected to healthy plants in a persistent-propagating manner, retaining their potential infective capacity for life.
In our environments, the migration of C. melanoneura from wintering sites to orchards is generally recorded between late January and mid-March, while C. picta migrates from late March to April. The presence of the overwintering generation of C. picta on apple trees often occurs in conjunction with the flowering phase, especially in fruit-growing areas at higher altitudes or in years when flowering is late or prolonged. In their transmission capacity C. picta has been shown to play a more important role as a vector than C. melanoneura . The results of transmission research carried out on psyllids show that both C. picta and C. melanoneura play an important role in the spread of the disease, especially under conditions of high populations and in the presence of numerous sources of inoculum, i.e. many infected plants.
Root anastomosis
The formation of natural root bridges appears to be very common in orchards. Epidemiological studies have highlighted the role of root bridges in the spread of apple bark beetle disease, especially in medium to older plants. Root bridges occur not only between viable plants, but also between newly planted young apple trees and the still viable remnants of roots remaining in the soil after the previous orchard was uprooted. Such roots can remain viable for up to five to six years after the uprooting of the infected plant and could still be tested for 'Ca. P. mali'.
Nursery material and grafting
The nursery material used to establish new plants can also play an active role in the propagation of the phytoplasma. For this reason, it is necessary to start with propagation material that is phytosanitary sound and genetically compliant with the varietal characteristics, and thus conforms to the EU certification system.