Location: Crop Improvement and Protection ResearchTitle: Frequency of Verticillium species in commercial spinach fields and transmission of V. dahliae from spinach to subsequent lettuce crops Author
Submitted to: Phytopathology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 7/29/2014
Publication Date: 1/1/2015
Citation: Short, D.P., Gurung, S., Koike, S.T., Klosterman, S.J., Subbarao, K.V. 2015. Frequency of Verticillium species in commercial spinach fields and transmission of V. dahliae from spinach to subsequent lettuce crops. Phytopathology. 105:80-90. Interpretive Summary: Verticillium dahliae is a plant pathogenic fungus with a broad host range, and can infect spinach, lettuce, strawberry and many other crops. The pathogen can also be seedborne, as is the case for spinach. Unlike the lettuce seeds, which can also be infected with V. dahliae, spinach seeds are more commonly infected with the fungus. Therefore, the introduction of V. dahliae on spinach seeds, which are often planted at a high seeding rate, represents a potential threat to the production of crops that are grown in rotation with spinach and susceptible to this fungus. This study was undertaken to determine the distribution of V. dahliae and related Verticillium species in spinach seedlings in commercial fields, and to determine whether V. dahliae can be transmitted from infected spinach seed to lettuce planted in the same infested soil. The results of the work indicate that Verticillium isaacii is the species of Verticillium most commonly isolated from spinach seedlings in commercial fields followed by V. dahliae and that V. dahliae can be transmitted from infected spinach seeds to subsequent crop of lettuce planted in the same infested soil. The study emphasizes the needs to curtail the introduction of the amounts of V. dahliae-infected seeds into soils in which other susceptible crops will be grown. It also emphasizes the need to determine the involvement of other Verticillium species present on spinach seeds in causing Verticillium wilt.
Technical Abstract: Verticillium wilt, caused by Verticillium dahlia,e is a devastating disease of lettuce in California. The disease on lettuce is currently restricted to a small geographic area in the central coastal California, even though cropping patterns in other coastal lettuce production regions in the state are similar. Infested spinach seed has been implicated in the introduction of V. dahliae into lettuce fields yet direct evidence linking this inoculum to wilt epidemics in lettuce is lacking. In this study, we surveyed 100 commercial spinach fields (69 in 2011 and 31 in 2012) in four coastal California counties and evaluated the frequency of Verticillium species recovered from spinach seedlings and assessed the area under spinach production in each county. Of the total number of isolates recovered, 160 from 2011 and 198 from 2012 isolates from 2011 and 2012, respectively, were randomly selected and identified to species using the Verticillium species-specific PCR assay. Regardless of the county, Verticillium isaacii was the most frequently isolated species from spinach followed by V. dahliae and, less frequently, V. klebahnii. The frequency of recovery of Verticillium species was unrelated to the occurrence of Verticillium wilt on lettuce in the four counties but was related to the area under spinach production in individual counties. The transmission of V. dahliae from -infested spinach seeds to lettuce was investigated in microplots. Verticillium wilt developed on lettuce following two or three spinach plantings in independent experiments in 2011-12. In a greenhouse study, we demonstrated the transmission of a GFP-tagged mutant strain of V. dahliae from spinach to lettuce roots following two cycles of incorporation of the infected spinach residue into the soil. This study presents conclusive evidence for V. dahliae introduced by spinach seed causing Verticillium wilt in lettuce.