South the presence of the AT1 strain increased: there was a gradient distribution of the AT2 and the AT1 strains. The AP strain is completely absent.
In 2004 the situation was slightly modified. While in the high infection zone the distribution is stationary, in the low infection zone the percentage of AT1 strain is proportionally reduced to half its value.
A high correlation between the strain and the geographic area is evident and it seems that as time goes by the AT2 strain is spreading more also in the South of Trentino.
The increase in the collection does not show a difference in the distribution of the strain. The strain distribution in the micro-area roughly reflects the situation present in the areas where they are located. The novelty is the presence of one sample of AP strain in Bonifica - Rumo microarea in Revò - Alta Val di Non area and in Focus Non - Sporminore micro-area in Denno - Cunevo - Campodenno area.
In 2005 the region of Nanno represents the actual situation in Val di Non. AT2 strain is widespread and there are a few samples infected with the AP strain. The AT1 strain is now absent. This might be because of its gradual decrease each year.
The model region of Piovi also shows the distribution of the strains in the areas of the low infection zone. The AP is completely absent and a low percentage of the AT1 strain is present. Also in this case this might be because of its gradual decrease each year. This region is to the North of Trentino, where there is a lower concentration of the AT1 strain in the low infection zone. It should be interesting to notice another model region to the South of Trentino where, probably in the future, there will not be any AP strain and the AT1 strain will become more frequent.
It would be interesting to monitor the diffusion of the AT1 and the AP strains in the respective regions to understand if their distribution could be an instrument to verify the progress of the disease.
It appears that the distribution of the strain is not in relation to the age of plant and neither in relation to the rootstock.
The psyllid C. picta overwintering was found more infected than C. melanoneura. One presumes that the C. picta is already infected when it comes back to the apple trees after winter, while C. melanoneura acquires the phytoplasma later. The low presence of phytoplasma in insects of the new generation could be connected with the time required for the incubation of the phytoplasma and the gathering of the insects.
During the last few years the percentage of infected insects has greatly decreased in all the species.
The number of infected insects is low and the population of phytoplasma can be analyzed taking into consideration the two infection zones only. However, in the same infection zone the analysis of the variability of the population of the phytoplasma in the psyllids confirms the situation present in the plant samples, both in 2002 and 2004, and also the correlation between the strain and the geographic area.
The distribution of the strain in the psyllid species and in those zones is the same. The above is obvious because C. picta is mainly present in the North-West of Trentino and C.
melanoneura lives mainly in the low infection zone.
There should be an increase in the collection of insects in the future to complete the missing data in certain areas. In the last few years the number of psyllids is greatly reduced. Hence it will be difficult to have more information on regarding this.
According to data collected, the concentration of phytoplasma in the samples when they are infected by AT1 strain, is higher than when they are infected by the AT2 or AP strain.
However, the number of AT1 infected samples is very low. The number of this data ought to increase to verify whether the concentration in the samples is related to the kind of strain.
The most interesting data given by the quantitative analysis of the model regions is that concerning the rootstocks and the age of plants.
The samples with M9 rootstock presented a higher concentration. Statistically, the value in these samples were significant different from the Franco and M11 rootstock.
It seems that in Nanno the concentration of the pathogen is indirectly proportional to the age of the plant. The plants aged 9 years show a high concentration of the pathogen. It seems that the situation is inverted in Piovi: the concentration increases with the age. But in Nanno the interval of the plants analysed was 61 years (from 9 to 70 years) while in Piovi the interval was 18 years only (from 0 to 18 years). In future the older plants coming from Piovi must be analysed to verify whether in these two model regions the trend is different or whether a major concentration is present in the relatively younger plants.
The quantitative data drawn of the psyllids does not furnish the necessary information.
It would be very important to deepen the quantitative data linked to the strain. If the concentration is connected with the intensity of disease symptomatology it should possible to choose the best material for the orchards (the best rootstock). Again, if there should be a relation between strain, concentration and symptomatology it should somehow associate the presence of the symptom with the strain.
The study on the population dynamics of Ca. Phytoplasma mali has highlighted the situation
the disease. This data could have an important role if, in the future, the in vitro analysis will be able to evaluate the virulence of the different strains.
It would be interesting to relate this data with the recent information furnished by the genetic variability analysis on some regions of the phytoplasma genome that can be associated with the mechanism of the infection.