Temporal Dynamics of ‘Ca. Phytoplasma mali’ Load in the Insect Vector Cacopsylla melanoneura

Abstract

The transmission of phytoplasmas is the result of an intricate interplay involving pathogens, insect vectors and host plants. Knowledge of the vector’s competence during its lifespan allows us to define more sustainable well-timed control strategies targeted towards the most worrisome life stages. We investigated the temporal dynamics of ‘Candidatus Phytoplasma mali’ load in Cacopsylla melanoneura in the different developmental stages in Northwest Italy. The phytoplasma load in the vector was evaluated in overwintering adults, nymphs and newly emerged adults after different acquisition access periods. Moreover, we followed the multiplication of the phytoplasma during the aestivation and the overwintering period on conifers. Our results confirmed the ability of remigrants to retain the phytoplasma until the end of winter. We also highlighted the high acquisition efficiency and vector competence, based on phytoplasma load, of nymphs and newly emerged adults. Therefore, particular attention should be paid to the management of overwintered C. melanoneura as soon as they return to the orchards, but also to newly emerged adults, particularly in orchards with a high infection rate and when the migration to conifers is delayed.

Multiple factors driving the acquisition efficiency of apple proliferation phytoplasma in Cacopsylla melanoneura

“…For instance, newly emerged adults from the populations ET and M carried relatively high phytoplasma concentrations and therefore might have been able to transmit phytoplasma to healthy plants. Previous studies showed that the concentration of phytoplasma in newly emerged psyllids reared on infected trees is higher compared to the concentration measured in overwintered individuals (Oppedisano et al 2019;Candian et al 2020). When comparing the transmission e ciency of phytoplasma in C. melanoneura to the other AP vector C. picta from NE, the concentration of the latter is around 10-times higher (Oppedisano et al 2019), thus suggesting a different type of interaction between 'Ca.…”

“…P. mali' subtypes (Cainelli et al 2004;Martini et al 2008;Casati et al 2010), making the pathosystem C. melanoneura -AP phytoplasma a good study system for the investigation of factors involved in the acquisition and transmission of similar pathogens. Previous studies focused on the natural infection rates and were based on arti cial acquisition studies using a high number of randomly caught wild specimens feeding on plants infected with a single phytoplasma subtype (Mattedi et al 2008;Oppedisano et al 2019;Candian et al 2020). For the rst time, we compared the phytoplasma acquisition e ciency of a high number of newly emerged adults of C. melanoneura (full siblings) from populations with high (Northwest Italy, NW) and low (Northeast Italy, NE) vector e ciency.…”

“…Although we generally found low phytoplasma concentrations, the fact that 'Ca. P. mali' is able to multiply in C. melanoneura during the overwintering period (Candian et al 2020) indicates that these individuals might become competent vectors during reproductive diapause prior remigrating back to apple trees. Future studies are therefore needed to investigate whether these individuals can transmit the AP phytoplasma to a healthy plant.…”

“…Wherever C. picta is present, it is considered the main vector; whereas C. melanoneura plays an important role only in the regions where C. picta is absent (Mattedi et al 2008). Italy constitutes an interesting area: C. picta is exclusively present in the NE, where it is considered the main vector (Oppedisano et al 2019), whereas C. melanoneura is present both in NE and NW but it is considered a main vector only in the NW, such as Piemonte and Valle d'Aosta (Tedeschi and Alma 2004;Tedeschi et al 2012;Fischnaller et al 2017;Candian et al 2020). This has been demonstrated in transmission studies where C. melanoneura collected in NE Italy were less e cient both in acquisition and transmission of 'Ca.…”

Cacopsylla pruni (Hemiptera, Psyllidae) in an apricot orchard is more attracted to white sticky traps dependent on host phenology

“… Cacopsylla pruni and C. melanoneura are very close in terms of their life cycle and vectoring capabilities, such as long effective latency (the period needed from the acquisition of the pathogen to become infective) ( Thébaud et al 2009 , Candian et al 2020 ). The long latency has a consequence that most phytoplasma transmission is performed by overwintered adults.…”