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For some years now, small, black leaf spots have frequently appeared on lamb's lettuce. The cause is the bacterium Acidovorax valerinellae. Infestation does not cause the plants to die, but the quality is considerably impaired.

Host plants

The only known host plant at present is lamb's lettuce(Valerianella locusta).

In 2001, a susceptibility test of existing lamb's lettuce cultivars was conducted, and all cultivars were equally susceptible to Acidovorax valerianellae. Only two wild varieties were not infected by the bacterium.

Distribution

Probably worldwide, everywhere where lamb's lettuce is cultivated. Frequent reports of infestation of lamb's lettuce come from France, Germany and Austria.

Propagation and transmission

Prolonged periods of leaf wetness, e.g. after prolonged rainfall or irrigation measures, promote infections and the spread of infestation. In the field, therefore, an occurrence of the disease must be expected in spring and fall under warm and humid conditions. In packaging, the bacterium can continue to develop on the finished goods for sale, and the bacteria can persist in the soil. Furthermore, transmission with the seed is possible.

Economic importance

More severely infested plants are no longer suitable for marketing. In the case of minor infestation on the outer leaves, these can still be marketed by increasing the cleaning effort. If infested areas are overlooked, this may result in major damage during transport and storage, also due to secondary bacterial infestation.

Prevention and control

• Use of healthy seed
• Avoid prolonged periods of leaf wetness if possible.
• Do not recultivate lamb's lettuce on plots where the disease has occurred until at least two years have passed. The more often lamb's lettuce follows itself (even within a year), the more the disease can spread to subsequent stands

Services

Plant Health Services