Phoma basal rot is established in various California coastal lettuce fields, both romaine and iceberg, but researchers report control of it with timely application of any of three, unregistered fungicides.
The disease, identified in its early stage on romaine by yellowing and wilting of lower leaves, showed up first in 2000 near Watsonville and was for a time known as crown rot. It has since spread into Monterey County as well as the Santa Maria area.
Recently in Seaside, Steve Koike, Monterey County farm advisor, told the California Lettuce Research Board his continuing trials with other specialists show that Quadris, Endura, and Switch each have significantly reduced the disease.
However, he added, none of the three was available to growers at the time of his remarks. Quadris has California registration but not for this disease, while California registrations for Endura and Switch were pending.
Koike said critical timing appears to be the key to efficacy of the products, which act as preventatives and must be applied early. “This is not surprising, since it is also true for Sclerotinia and other problems. These fungicides are not designed for eradication.”
In one trial, application was delayed a week by the field's irrigation schedule, and none of the materials gave adequate control. “It didn't matter which product, at high or low rates or one or two sprays,” he said.
In the past two years of trials the researchers made applications for Phoma basal rot the same as those for Sclerotinia, with banded sprays at or soon after thinning. Where multiple applications were made, they were at one-week intervals. The equivalent of 100 gallons per acre of spray solution was applied.
Currently registered fungicides for lettuce, such as Rovral, Maneb, and Botran, did not provide control, Koike said.
The team has been returning to the same fields with their investigations, and Koike said major questions now are whether the rot goes to other crops and whether related pathogens will also attack lettuce cultivars.
“We know from going back to the same fields that the fungus survives over the short term of two to four years. Now we need to learn what will happen to its populations if lettuce is not planted in infected fields.”
The pathogen is thought to be Phoma exigua, one of a group of soilborne species, some of which are weak and go to plant stems. Laboratory tests show that subspecies of P. exigua exist and samples taken from infected romaine consistently infect both romaine and iceberg cultivars, while those from iceberg consistently cause disease on iceberg and romaine.
Field investigations have pointed to links with strawberries. Koike said most of the fields infected with Phoma basal rot were lettuce following strawberries. This coincides with other studies showing Sclerotinia and Verticillium present in lettuce fields after strawberries where fumigation preceded the strawberry planting.
He said they do not know if fumigation somehow triggers the pathogen, or if strawberries are a source of the fungus. They are screening strawberry transplants from Northern California to see if the fungus can be recovered from roots.
Phoma basal rot
Signs of advanced Phoma basal rot are irregular growth on the stunted, infected portion and normal growth on the opposite side. Eventually the entire plant becomes stunted, wilts, and collapses.
Infected plants show black cavities on the crown and upper taproot, allowing plants to be easily broken off at ground level. The black cavities are firm and show no signs of fungal growth or spores, which distinguishes this disease from other crown rot pathogens.
Koike said the disease has not been detected on commercial leaf lettuce varieties.
The disease was first reported on lettuce in the United Kingdom in 1990. In greenhouse lettuce in Europe, it produces circular, dark gray to black leaf spots of up to one inch in diameter, although those symptoms have not been detected in California.
In another report to the board, Tom Gordon, plant pathologist at the University of California, Davis, disclosed his project on management of Fusarium wilt of lettuce.
The disease, evident in the San Joaquin Valley near Huron for the past decade and in desert production areas in Arizona more recently, was confirmed in the Watsonville area in 2002.
“Where fields are affected by Fusarium wilt,” Gordon said, “growers are likely to rotate out of lettuce to other crops that are not susceptible to the disease. This allows population levels of the pathogen to decline before the affected field is again cropped to lettuce.”
His approach is to test crops such as broccoli, cauliflower, celery or spinach that might be grown in the Watsonville and Salinas areas as well as watermelon, cantaloupe, tomatoes, and cotton that might fight the pathogen in the SJV.
Gordon said although genetic resistance could eventually be brought into commercial varieties, reduction of inoculum would slow development of new virulent forms of the disease that would challenge resistant cultivars.
A field trial was established near Watsonville, but the intensity of the disease was not sufficient to measure the extent of colonization by the disease.
As an alternative, an experimental field was set up at UC, Davis, where lettuce was planted in a Fusarium wilt-infested field. The crop will be incorporated and left to decompose.
Later, “candidate rotation crops,” Gordon said, “will be sown into the infested soil and their roots evaluated for extent of colonization by the lettuce wilt. Similar tests also will be conducted under greenhouse conditions.”
When the disease was first encountered in coastal fields, it occurred only in limited areas of one field, but in those areas 35 percent to 50 percent of the crop was unmarketable.
Fusarium, which is more severe in higher temperatures, causes vascular tissue to become red or brown, and in older plants, leaves turn yellow, wilt, and become necrotic. Other symptoms are a hollow cavity in the taproot, and failure to form heads.
Its symptoms may also resemble those of ammonium toxicity or Verticillium wilt, although the Verticillium infection creates a darker vascular discoloration.