Insect vectors are required for plant-to-plant transmission of X. fastidiosa. Although most of the work on how X. fastidiosa is transmitted has been done using Pierce’s disease of grapevines as a model system (grapevines as a host plant, Graphocephala atropunctata as a vector, and grapevine-infecting isolates from California), research has also been performed with other species and the characteristics of transmission appear to be conserved. So what are the basics of X. fastidiosa transmission?
- no transtadial transmission
- meaning that insects loose the bacterium at each molt – Purcell and Finlay 1979, Almeida and Purcell 2003
- this is also evidence that X. fastidiosa does not circulate within vectors
- no transovarial transmission
- females that are infected do not transfer the bacterium to their offspring – Freitag 1951
- no vector species – X. fastidiosa genotype specificity
- this is an important trait, it means that any insect species belonging to the Cicadellinae, Cicadidae, and three families of Cercopoidea are potential vectors of any X. fastidiosa genotype – reviewed by Almeida et al. 2005
- no latent period
- insects can acquire and inoculate X. fastidiosa within hours – Severin 1949, 1950, Purcell and Finlay 1979
- this is also evidence that X. fastidiosa does not circulate within vectors
- transmission efficiency increases with plant access period
- the proportion of insects transmitting X. fastidiosa to plants increases with increments in plant acquisition and inoculation access period, for up to 48-96 hours – Purcell and Finlay 1979, Cornara et al. 2016
- X. fastidiosa is persistent in vectors
- in insects that do not molt (i.e. adults), or until they molt (in the case of nymphs), X. fastidiosa is persistent, meaning that insects are able to transmit the pathogen likely until they die. Studies have shown that transmission occurs months after acquisition – Severin 1949, 1950, Hill and Purcell 1995, Almeida and Purcell 2003
- X. fastidiosa multiplies in vectors forming a colony on the cuticle of the foregut
- in addition to being persistent, cells multiply and form a microcolony on the surface of the foregut of insect vectors – Purcell et al. 1979, Hill and Purcell 1995, Almeida and Purcell 2006, Killiny and Almeida 2009
- bacterial populations in plants impact acquisition efficiency
- the size of live bacteria populations in infected source plants has been shown to be correlated with acquisition efficiency – Hill and Purcell 1997
- vector behavior impacts transmission efficiency
- insects may preferentially feed on plant tissue that harbors lower X. fastidiosa populations; that would lead to lower probability of pathogen acquisition – Daugherty et al. 2010
- insects may also discriminate against infected and symptomatic plants, leading to avoidance of these plants and ultimately less transmission – Daugherty et al. 2010, Zeilinger and Daugherty 2014
It is very important to remember that although X. fastidiosa transmission characteristics appear to be conserved, only a subset of important taxa have been studied. In addition, it is clear that different plant-pathogen-vector genotype combinations impact overall transmission rate. In other words, change the vector species and transmission rates will likely change, the same is true if using different plant varieties. Therefore, transmission rate/efficiency is a context dependent trait and should be carefully studied for each disease system.