Xylella fastidiosa

Pierce's disease of grapevine

General

Xylella fastidiosa is a gram-negative bacterium which is limited to xylem tissue. It has many hosts, many of which show minor or no symptoms. More and more plant species are discovered as hosts. Xylella fastidiosa is best known in grapevine, stone fruit (peach and plum) and citrus, where it causes diseases such as Pierce's disease of grapevines, phony peach disease, plum leaf scald, citrus variegated chlorosis and quick decline syndrome. Recently, it is becoming more problematic in olive.

Life cycle and appearance of Pierce's disease of grapevine

The bacteria live in xylem vessels in roots, stems and leaves and multiply. The plant may react by producing gum and tylose, a glue-like substance, in the vessels. The bacteria, gum and tyloses block the vessels, causing the plants to wilt. Transport to different plant parts depends on the plant species: in peach, bacteria are found in high numbers in the roots, whereas in plum, high numbers are found in leaves and fruits.

The bacteria are transferred from plant to plant by all kinds of xylem-feeding insects. Well known, but certainly not the only vectors are sharpshooter leafhoppers species Homalodisca coagulata and Oncometopia nigricans. The insects become infective immediately after feeding on infected plants and mature insects stay infective indefinitely. However, the bacteria are not transferred to the eggs and immature stages lose the bacteria after moulting. Long distance dispersal may occur by the accidental transport of infected vectors or dormant plants. In citrus, the bacteria can be transferred in seed used for propagation.

The bacteria are limited in many regions by winter conditions with frost periods in which the bacteria inside the dormant plants die. Also, in many regions there is no overwintering of adult vectors that may cause early infection in the new season. Therefore, the bacteria are causing the most severe problems in areas with temperate winters and overwintering adult vectors. The bacteria occur also in many wild hosts and weeds and their presence increases the risk of infection in cultivated crops. The spread from one host plant species to another is still under investigation and seems to depend on the location of the bacteria in the infected plants. For example, transfer from infected plum trees with high bacterial levels in the leaves to neighbouring peach trees with the bacteria mainly in the roots is much more successful than the other way around. In general, the low number of bacteria in peach leaves slows down the spread both within this crop as well as from this crop to other crops.

How to prevent Pierce's disease of grapevine