Xylella fastidiosa is a bacterial plant pathogen like no other and it could become one of the most important plant pathogens in history. It was voted in the top ten most important plant pathogenic bacteria by the international community in 2012 and that was before it made the 4,500 mile journey from America to Europe. It is now firmly established in Italy where it has become a major threat to the olive industry and there are numerous other well documented reports of it being elsewhere in Europe, namely France Spain Germany and the Czech republic.
The success of X fastidiosa is reflected in its host range. X fastidiosa can infect over 350 species representing 204 genera in 75 different botanical families. Many hosts are woody trees and bushes both ornamental and fruit crops. However, other hosts include fodder crops (clover, lucerne and ryegrass), vegetable crops such as brassicas, herbs (e.g. rosemary) and weeds such as dandelion, wild oat and chickweed. The fact that a very high percentage of diverse plant species can host Xylella fastidiosa without symptoms makes identification and management extremely challenging.
The bacterium was originally studied by the world’s first professional plant pathologist Newton B. Pierce who stated that he was going to work on this disease (originally called Anaheim disease after the town in California where it was first observed) and not move onto any other disease until he had cracked it. This pathogen was a particularly poor choice for Pierce’s career, unlike almost every other plant pathogenic bacterium it cannot be directly inoculated from one plant to another and was only recently (in the 1980s) cultured in the laboratory.
Over a century since Pierce’s death we still have not “cracked it”, and it is cruel irony that one of the most environmentally important, but least understood plant diseases, bears his name; “Pierce’s disease of grapevine”.
X. fastidiosa can be divided into four subspecies which have evolved in distinct geographical regions.
- X. fastidiosa subsp. fastidiosa, thought to be native to southern Central America, is associated primarily with Pierce’s disease of grapevines and almond leaf scorch.
- X. fastidiosa subsp. multiplex, thought native to temperate and subtropical North America, is associated with scorch disease in a wide range of trees, including phony peach disease and plum leaf scorch.
- X. fastidiosa subsp. pauca, thought to be native to South America, is associated with strains causing disease in citrus and coffee.
- X. fastidiosa subsp. sandyi, believed to originate from the southern region of the USA, is associated with oleander leaf scorch.
Unlike almost all other bacterial plant pathogens X. fastidiosa requires an insect vector to transmit the disease, which in itself has consequences as global warming is likely to increase range of existing vectors helping spread the disease. Thus, upon arrival in a new geographical region, it is difficult to predict which potential vectors will spread the pathogen. It is interesting to note that the invasive X fastidiosa in Europe is not all of the same subspecies. That found in France is subspecies multiplex whereas in Germany its subspecies fastidiosa and Italy subspecies pauca. The Baleric islands now hosts all three subspecies. This suggests several recent independent arrivals in Europe rather than spread between European countries. This creates further uncertainty about the range of possible insect vectors in Europe, given that each X. fastidiosa subspecies shows considerable host specificity.
Image from the EPPO Gallery: http://photos.eppo.org/index.php