Verticillium Wilt of Lettuce

Until 1995, lettuce was thought to be immune to Verticillium wilt. Crops of lettuce had been grown in fields known to contain the pathogen for many years without any development of symptoms on lettuce. However, Verticillium wilt-like symptoms were observed on lettuce growing near Watsonville, California in 1994, and it was later determined that the symptoms were the result of infection by the fungus Verticillium dahliae. A few years later, the disease was observed in California’s Salinas Valley.1 Incidence levels of up to 80% have since been observed, and some fields have been abandoned before harvest due to the disease. A 2019 publication stated that Verticillium wilt of lettuce had not been detected in Arizona, but symptoms on lettuce in high tunnels and research field plots were observed in the state of Washington in 2010 and 2012.^2,3 The disease on lettuce in Washington was found to be caused by a different species of Verticillium. Verticillium dahliae has a wide host range of about 400 identified plant species, including many crop, ornamental, tree, and weed species, and the pathogen commonly occurs in agricultural soils in temperate growing regions around the world.^4,5 Verticillium wilt of lettuce has been found in northern Italy and on the Greek island of Crete.¹

Small, black structures called microsclerotia may develop andbecome visible along the veins of outer, affected leaves and in the roots of senescing plants.^1,4 The symptoms progress to the center of the head, and the plant eventually dies. A study found that symptoms did not form on harvested heads, and that there was no additional post-harvest decline in quality. Therefore, the disease will likely have minimal impact on quality and marketability if heads are harvested before symptoms appear.^4,6 All lettuce types are susceptible to Verticillium wilt, but the most damage usually occurs on crisphead varieties.^1,4

Verticillium dahliae is a soilborne fungus that can persist in the soil indefinitely. The pathogen can survive as microsclerotia, which can remain viable in the soil for up to fourteen years.^1,5,7 The pathogen can be spread within and between fields on anything that moves infested soil, including equipment, shoes, and wind. The pathogen has also been detected on lettuce seeds and the seeds of other crops. The rate of infection from inoculum on lettuce seed appears to be low, and it may require planting infested lettuce seed over several years to result in disease development in the field.¹ However, there is good evidence that V. dahliae can be present on the seed of some common rotational crops, including other leafy green species, and that this seedborne inoculum might increase the risk of Verticillium wilt occurring on lettuce.^1,7 Infection of lettuce occurs when the microsclerotia germinate and produce hyphae (fungal threads) that penetrate near the root tips. The fungus then grows into the xylem (waterconducting tissue) and spreads upward into the stem and leaves of the plant. Verticillium wilt is a monocyclic disease, meaning that there is no plant-to-plant spread during the growing season. Microsclerotia produced on the infected lettuce plants enter the soil as the crop debris decomposes and become available to infect future crops.4 The level of disease on a lettuce crop is proportional to the amount of Verticillium inoculum in the soil, more microsclerotia result in a greater number of infected plants. However, the threshold for infection of lettuce by V. dahliae is high compared to the thresholds for some other crops.

Infection of strawberry plants requires an inoculum density of one to two microsclerotia per gram of soil. For cauliflower, ten microsclerotia per gram of soil results in about a 50% incidence of infection and sixteen microsclerotia per gram results in near 100% incidence. The infection threshold for lettuce is around 100 to 120 microsclerotia per gram soil, and levels between 200 and 600 microsclerotia per gram soil are likely to result in total loss of the lettuce crop.^1,7 Verticillium wilt can develop on lettuce in any of the seasonal plantings, but in California symptoms are often more severe and the highest yield losses are seen on fall crops because the warmer temperatures favor the growth and development of the pathogen.^1,4

It is very difficult to eradicate the Verticillium wilt pathogen once it is established in a field, and there are no rescue treatments for controlling the disease once lettuce plants are infected. Therefore, it is important to prevent the introduction of the pathogen into non-infested fields whenever possible.^4,7

Preventing the spread of infested soil and crop debris is the primary strategy for preventing the introduction of V. dahliae into non-infested fields. Planting seeds (lettuce and rotational crops) that are not infested with the pathogen is also needed to help prevent the establishment of V. dahliae. Because of the longevity of microsclerotia in the soil, with the exception of planting broccoli, crop rotation to non-host crops is not usually effective for managing Verticillium wilt.^4,5

Some lettuce varieties have some resistance to V. dahlia race 1, but no varieties have resistance to race 2. The single dominant race 1 resistance gene is also effective against V. isaacii and V.klehahnii. About 55% of V. dahliae isolates in California lettuce fields belong to race 1, and 45% belong to race ². If the race 1 resistance gene is widely used in lettuce, it will likely result in an increase in the proportion of the population of race 2 isolates.^1,4

Soil fumigants, such as chloropicrin + dazomet, can help reduce the population of V. dahliae microsclerotia in the soil. However, currently available fumigants will not completely eradicate the pathogen in a field.¹ An alternative for reducing the number of microsclerotia in the soil is to rotate the field to broccoli. It has been shown that a planting of broccoli can help reduce the population of V. dahlia, potentially below the threshold level needed for the development of wilt on lettuce.

The lower return per acre for a crop of broccoli compared to lettuce has limited the use of this management strategy.^1,7