2.4.2.j Pierce's disease

What damage is caused by Pierce's disease in the vineyard?

Foliage symptoms are mainly visible during hot, dry summer periods or when there are severe water constraints. They are characterized by the appearance of partial dryness that gradually spreads to the edges of the leaf blades until the leaf is entirely affected. Leaves become scorched, necrotic and fall off.
Branches can also be affected, growing irregularly and remaining green in places.

Is it easily confused with other diseases?

The onset of symptoms can resemble Black Rot, with the formation of spots that give a scorched appearance. The absence of pycnidia (black dots) can be used to differentiate between the two diseases. The spots caused by Black rot do not spread, however, so it is easy to tell the difference between the diseases as the infection progresses. Poor lignification of shoots is typical of Pierce's disease, which makes it easily identifiable.

What is the vector for Pierce's disease? What is its life cycle?

Pierce's disease is caused by the Xyllela fastidiosa bacterium carried by insect vectors. The insects can be of various species: mainly leafhoppers and cercopes. With the help of their phloem-feeding type mouthparts, they feed on the xylem vessels of a plant which may contain the bacterium without showing symptoms (the host plant).

The bacteria can multiply among these insect vectors which will subsequently infect other plants. Once the plant is infected, the bacterium proliferates in the aerial organs and roots and clogs the conducting vessels. This leads to the symptoms described above.
All xylem-feeding sucking insects are likely to be vectors of Xyllela fastidiosa.

How does the Xyllela fastifiosa bacterium function?

This bacterium mainly exists in production areas where winters are mild. In vitro, its optimal growth oscillates between 25 and 32°C, and is reduced or may even stop if temperatures fall below 12°C.

What are some existing prophylactic methods?

There is no specific curative treatment for Pierce's disease, however, several measures can be implemented to reduce the risk of contamination and spread of the disease in the vineyard:

• Implement immediate uprooting of affected vines, clean the plot, removing stumps and dead wood

• Management of treatments based on natural pyrethrum-based insecticides, which are authorized in organic agriculture, used to control the leafhopper that carries Flavescence dorée. This insecticide is not specific to the FD leafhopper, and it can also impact other species. Be careful, however, to respect the number of treatments with natural pyrethrum, which is limited to a maximum of three in organic viticulture.

• Control and adaptation of grassed areas to avoid insect vector refuge areas

• Use of resistant plant material when possible. Certain American rootstocks and grape varieties are more resistant to this disease.

• Cleaning of pruning tools or any other tools that can cause wounds and contribute to contamination.