Bacterial leaf spot of lettuce has been affecting Salinas Valley crops for many years. The disease was first noted in California in 1964 and became an economic concern in the 1990s. Bacterial leaf spot now occurs in the Salinas Valley, to some degree, every season.
However, in 2009 the disease was widespread and caused significant damage to lettuce.
• Symptoms: Early symptoms of bacterial leaf spot are small (eighth of an inch to quarter of an inch), water-soaked spots that usually occur only on the older, outer leaves of the plant. Lesions are typically angular in shape because the pathogen does not penetrate or cross the veins in the leaf. Lesions quickly turn black—this is the diagnostic feature of this disease. If disease is severe, numerous lesions may coalesce, resulting in the collapse of the leaf. Older lesions dry up and become papery in texture, but retain the black color. Lesions rarely occur on newly developing leaves. If disease is severe, secondary decay organisms (bacteria, Botrytis cinerea) can colonize the leaves and result in a messy soft rot of the plant. In many cases in 2009 this secondary soft rot decay was more damaging to lettuce yields than the primary bacterial leaf spot disease itself. Bacterial leaf spot can occur on iceberg, romaine, leaf, and butterhead lettuce types.
• Pathogen: Bacterial leaf spot is caused by Xanthomonas campestris pv. vitians. The pathogen can be isolated on standard microbiological media and produces yellow, mucoid, slow growing colonies typical of most xanthomonads. This bacterium is a pathogen mostly limited to lettuce, though under greenhouse conditions several weeds in the same plant family can develop bacterial leaf spot disease when inoculated. To our knowledge, naturally infected weeds showing leaf spot symptoms in the field have not been documented.
Some researchers indicate that X. campestris pv. vitians from lettuce can infect very different plants such as pepper and tomato when such are artifi cially inoculated; however, naturally infected pepper and tomato have never been found. Bacterial leaf spot disease of lettuce should not be confused with other Xanthomonas diseases. For example, bacterial spot disease of tomato and pepper is caused by a distinct pathovar (Xanthomonas campestris. pv. vesicatoria); this pathogen will not infect lettuce.
Disease cycle. The pathogen is highly dependent on wet, cool conditions for infection and disease development.
Splashing water from overhead irrigation and rain disperses the pathogen in the field and enables the pathogen to infect significant numbers of plants. The pathogen can be seedborne, though the extent and frequency of seedborne inoculum is not currently known. If lettuce transplants are grown from infested seed, the pathogen may become established on plants during the greenhouse phase of growth.
The bacterium can survive for up to five months in the soil. Therefore, infected lettuce crops, once disked into the soil, can supply bacterial inoculum that can infect a subsequent lettuce planting. The bacterium has also been found surviving epiphytically on weed plants, though the significance of this factor is not known.
• Control: Clearly the elimination or reduction of the use of overhead sprinkler irrigation will significantly curtail this disease in all situations, except when rains occur. Some resistant lettuce lines have been identified, though resistance is not widely available in currently used cultivars. Residual bacterial inoculum, left in the soil following an infected lettuce crop, will potentially cause problems for the next lettuce planting unless that planting is delayed for five months or longer. Therefore, crop rotation schemes will need to be evaluated if bacterial leaf spot is a chronic problem in fields heavily planted to lettuce. Effective foliar sprays have not been identified for this disease.
The exact role of seedborne inoculum is not currently known. We have no information on how frequently commercial seed lots might be infested and to what degree. Thresholds (the levels at which seedborne inoculation becomes economically important) for bacterial leaf spot have not been established. Therefore, additional research might be useful in assessing frequency of contaminated seed lots, sensitivity and accuracy of currently used seed tests, and in establishing seed testing standards and thresholds.