Submitted to: Plant Disease
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 3/28/2006
Publication Date: 7/14/2006
Citation: Cintas, N.A., Koike, S.T., Bunch, R., Bull, C.T. 2006. Holdover inoculum of pseudomonas syringae pv. alisalensis from broccoli raab causes disease in subsequent plantings. Plant Disease. 90:1077-1084. Interpretive Summary: The 12 million dollar crucifer industry in Monterey County California provides the US with the majority of broccoli, cauliflower, and Brussels sprouts as well as other specialty crucifers such as broccoli raab consumed annually. Bacterial blight of crucifers is an emerging disease that was first observed in California but has since been reported across the country. Because the pathogen can cause disease on a wide range of plants it is important to know if susceptible plants are at risk of becoming diseased if they are planted in fields after a diseased crop has been harvested and the residue has been incorporated into the soil. This research demonstrated that the pathogen present in the leaves of a diseased crop caused disease when a second crop is planted into the ground after the diseased crop has been tilled into the soil. This is significant for growers because a susceptible crop following a diseased crop is likely to also become diseased unless enough time has passed since the diseased crop was incorporated into the soil.
Technical Abstract: Uniform plots of broccoli raab (Brassica rapa subsp. rapa) seedlings were inoculated with a rifampicin resistant strain of Pseudomonas syringae pv. alisalensis, the causal agent of bacterial blight on crucifers, resulting in 100% disease incidence of mature plants. Diseased plants were incorporated into the soil at maturity and smaller replicated plots were re-planted at various times after incorporation. Rifampicin-resistant fluorescent pseudomonads with rep-PCR profiles identical to P. syringae pv. alisalensis wee isolated from lesions on plants grown in soil into which the first diseased crop was incorporated. Disease incidence declined in mature plants as the length of time between incorporation of the first planting and seeding of the replanted plots increased. Bacterial population levels in soil decreased over time and bacteria were no longer detectable 3 weeks after incorporation of the diseased crop. In laboratory tests, population levels of P. syringae pv. alisalensis decreased in untreated soil but not autoclaved soil. Greenhouse studies demonstrated a direct correlation between population levels of P. syringae pv. alisalensis applied to soil and disease incidence in seedlings. However, the decline in bacterial populations in field soils did not wholly account for the decline in disease incidence with subsequent plantings.