Strategies to manage Phoma basal rot, a serious disease of romaine lettuce in portions of coastal California, have been learned, although how the pathogen moves about remains a mystery.
After first appearing in the spring of 2000, the disease was considered established in 2004, particularly in Monterey and Santa Cruz counties. Quite virulent on romaine, it is less a threat to iceberg varieties.
It was also detected in romaine in the Santa Maria area, but it has thus far not shown up in fields in other coastal areas, in the desert, or in the San Joaquin Valley.
Its distinctive symptoms include yellowing and wilting of lower leaves, followed by stunted growth on the infected side of plants and eventual death of plants. Black cavities appear on the crown and taproot of infected plants, which break off easily at ground level.
Steve Koike, Monterey County farm advisor, has led research on the disease, or Pbr, since it appeared. He reported results of his latest studies recently before the California Lettuce Research Board at Seaside.
“We have found some fungicides that are very effective on Pbr. This year Endura was used quite commonly on romaine in infested fields,” Koike said. Second applications were observed in his trials, but he said a single treatment would be sufficient.
However, he said very little of the disease was found during the 2005 season and he hesitated to say the fungicide was a direct cause. Quadris and Switch have also been very effective in preventing the disease from developing.
Avoid infested fields
Beyond fungicides, he added, another strategy is avoiding planting romaine in fields known to be infested with Pbr. “That's a good practice if you can do it. Or you might want to stay away from lettuce, period, in those fields because it is a soilborne problem in certain places.”
He said his multi-year research indicates that most iceberg and leaf-type lettuce varieties have resistance to Pbr. “These have been grown in fields where if you had romaine you would see 30 to 50 percent disease.”
Still to be resolved is where Pbr came from and how it moves from place to place. Koike said many infested fields of romaine had been previously fumigated and planted to strawberries. After the strawberries were taken out, the following romaine had tremendous amounts of the disease.
That prompted another investigation funded by the board into whether strawberries, either transplants or field planted, conventional or organic, could be the source.
No connection found
After two seasons of examining strawberry roots, Koike said no Phoma was detected on them. “It would be premature to say there is no connection with strawberries, but we can say we have not been able to document it.”
Continuing the research, Koike had soil microplots fumigated to remove any possible Phoma contamination. In November 2004, these were planted to strawberries to be grown through 2005. Romaine will be planted in 2006 and the plots assayed for Pbr.
In a related field study, he will treat plots of infested fields with various fumigants to see if they incite the disease in 2006 and romaine will be planted to them in 2007.
In other work, Koike has begun surveys on the threat of tomato spotted wilt virus (TSWV), discovered in parts of fields in the Salinas Valley during the past two seasons. It is not commonly seen in coastal counties, although outbreaks have occurred in the Huron area. TSWV has a long list of hosts among crops, hosts among crops, ornamental plants, and weeds and is vectored by thrips.
Responding to yet another new disease threat to lettuce, Koike is collaborating with Tom Gordon, plant pathologist at the University of California, Davis, in tracing reports that two or three fields in the Salinas Valley showed symptoms of Fusarium wilt on head lettuce this year.
The causal pathogen, Fusarium oxysporum, had previously only been confirmed in the Pajaro Valley. Koike and Gordon are testing samples from fields to learn if they are, in fact, the disease or only secondary decay organisms. Fusarium wilt has been identified in inland lettuce fields but had not earlier been detected along the state's coastline.
In his report, Gordon noted that the main defense against the wilt is use of resistant varieties. “Although most head lettuce cultivars are susceptible to Fusarium wilt, our research has confirmed that the ‘Salinas’ variety is relatively resistant to the disease.”
Aware that research previously published in Japan showed that higher temperatures favor development of the disease, Gordon is following up on observations that it is most damaging during warmer growing periods.
He is working with geneticists at UC, Davis on resistance to Fusarium and said studies thus far suggest the resistance in “Salinas” is controlled by more than one gene.
He said the severity of the wilt is governed not only by the susceptibility of the cultivar but by the abundance of the pathogen in the soil. “The only practical means by which populations can be reduced is by crop rotation.”
His continuing studies are focused on learning how long the disease can persist in the soil. He has measured viability in artificially infested soil to about 14 months, but those levels are only slight compared to the initial amount.
Carolee Bull, USDA plant pathologist at Salinas, is principal investigator in a board-funded project aimed at selecting resistant materials to assist plant breeders in developing lettuce plants to fend off bacterial leaf spot (BLS) and corky root (CR).
BLS causes blackened lesions on leaves and occurs sporadically in coastal and inland counties. Bull told the board she received several calls from growers about it during the fall of 2004 and the spring of 2005.
In greenhouse and field trials subjecting plants to heavy BLS pressure, she found resistance in several cultivars, mainly Waldmann's and Little Gem, along with several progeny from a cross between Batavia Reine des Glaces and Holborn Standard. These are in further testing.
CR is economically important for the California lettuce industry, and resistant crisphead cultivars have reduced it.
“However,” Bull said, “resistance is not available for all types. Identification of additional resistance in romaine types and the development of alternative disease management strategies will help the industry prevent an increase in the impact of this disease.”
A part of the project is determining whether mycobacteria are a biocontrol of Rhizomonas suberfaciens, the pathogen that causes CR.
Another dimension of her research is to develop field plots where the disease is distributed uniformly to indicate disease pressure more accurately when testing new plant materials.