Phoma basal rot is a disease of romaine lettuce which first appeared on the coast of California in 2000. Caused by the fungus Phoma exigua, the disease causes brown and black cavities to develop on the root crown, as well as the upper region of the taproot. These cavities are hard and do not produce visible fungal growths. The disease weakens and ultimately cracks the crown, resulting in the visible above-ground symptoms—stunted and uneven growth. Crisphead lettuce is also affected by Phoma basal rot, but to a lesser extent than romaine lettuce.
Unlike many fungal diseases which cause soft, waterlogged spots, Phoma basal rot is noteworthy for the hard depressions it produces in the upper root system of infected plants.
Because the most notable symptoms caused by Phoma exigua are belowground, growers typically first realize that their plants are affected by the disease when growth slows, despite their best efforts. Leaves yellow and wilt, with fungal growth often stronger on one side. Eventually, infected plants experience complete collapse. When these plants are pulled from the soil, the cause of death becomes evident.
The dry, rotted root lesions caused by Phoma basal rot sink deep into the surface. The lesions are dark brown or black in appearance—sometimes so dark that affected tissues look burned. These hard, sunken lesions can penetrate quite deeply into root and taproot tissues, causing cracks which are colonized by secondary pathogens. In cases where infection of the roots is extensive, the aboveground portion of a plant can easily be broken off from the roots with modest force.
Lesions are typically concentrated on one side of the roots, which accounts for the unevenness of aboveground growth. Taking a cross-section of infected root tissue shows how the infection penetrates from one side of the root, all the way to the center of the crown or taproot. Because infection is confined to one side of the root, many infected plants survive to maturity. However, surviving plants exhibit very lopsided development, which typically renders them unsellable.
Phoma exigua, a member of the genus of soil-dwelling fungi Phoma, received little attention until the last twenty years.
Through the 1980s, Phoma exigua received more attention for the gangrene it causes on potatoes. But in the early 1990s, the organism was isolated in greenhouse lettuce grown in the United Kingdom. Then in 2000, it first appeared in California’s Salinas Valley.
In regions of California where the disease is known to be present, the spores of P. exigua should be considered ubiquitous in outdoor soils.
Unfortunately, there have been few studies of the disease, the hardiness of the organism, and the mechanisms by which P. exigua infects and reproduces within hosts are poorly understood.
Fortunately for growers impacted by Phoma basil rot, P. exigua has been shown to be responsive to fungicides currently on the market.
Testing has shown that two fungicides in particular are effective in reducing the incidence of Phoma basal rot: azoxystrobin (Amistar) and boscalid (Endura). The use of a combination of cyprodinil and fludioxonil has also been shown to be effective.
As with many fungal pathogens, growers can work to prevent initial infections by ensuring that proper drainage is implemented, so that plants are not exposed to standing water. The use of drip hoses and other ground-based irrigation methods are also advisable, as this avoids the wetting of leaves which provides a more hospitable environment for infection.
(Thumbnail image source: Jeff Vanuga, USDA Natural Resources Conservation Service. Licensed via public domain.)