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.
Primary infection of leaves results in leaf scorch. Part of a green leaf suddenly dies and turns brown with adjacent tissue turning yellow or red. This desiccation spreads and the whole leaf may shrivel and drop off. Infected stems show irregular maturation and patches of brown and green tissue. In the following seasons, these infected plants show a slower development and produce chlorotic shoots that are stunted. When the infection becomes chronic, the leaves become distorted with interveinal chlorosis and the shoots get shorter internodes. Infected vines eventually die. This happens faster in younger vines than in older ones and faster in susceptible cultivars (within 2-3 years) than in more tolerant cultivars, which may survive more than five years.
Stone fruit (peaches and plums):
In peach, the canopy of infected plants is compact and umbrella-like due to the shorter internodes. The foliage is denser and the leaves are a darker green than usual. Also, young infected trees show dwarfing. Leaves and flowers appear early and remain on the plant longer, but fewer and smaller fruits are produced.
In plums, the first symptoms are irregular chlorosis or browning of leaves along the margin or the tip from early to midsummer. The necrotic area spreads over the leaf from the margin and is marked by a chlorotic band. In the beginning, only a few leaves per branch are showing symptoms, but later the growth stops and the plants die, usually within a few years.
Symptoms are variable interveinal chlorosis of the leaves, resembling zinc deficiency. Infection becomes systemic more rapidly in young trees than in older trees. In the latter, it may stay limited to certain branches. When the leaves mature, slightly raised gummy lesions occur on the underside of the leaves. These lesions start out in a light-brown colour, turning to dark-brown or necrosis. Fruits of infected trees are much smaller and their sugar content is higher. The early blossoming and fruit set in peach does not occur in citrus, there is no difference between infected and healthy trees. Normal fruit abortion does not happen so total production remains similar, with more smaller fruits in infected trees. The infected trees grow slower than normal and look stunted. Twigs and branches die and the foliage becomes thinner, but the trees don’t die of the disease.
Scorch symptoms and desiccation of twigs and small branches occur on infected leaves. This usually starts in the upper part of the canopy on a few branches and then extends to the rest. Eventually, the whole root system is killed and the trees die.