Elisabeth Hodgdon, Cornell University

Cameron Cedeno & Carolee Bull, Penn State

Leafy greens are a high value crop for many vegetable producers. Brassica baby leaf salad greens in particular are a great fit for many operations in the Northeast given their cold tolerance, quick growth, and varietal diversity. Recently, more brassica leafy greens growers in the Northeast have reported issues with bacterial disease in their crops, particularly in arugula. Bacterial blight, caused by the pathogen Pseudomonas cannabina pv. alisalensis, was first detected in arugula in New York State in 2021. Here, we summarize current recommendations for managing this disease and new results from current research projects on arugula bacterial blight.

Symptoms of bacterial blight in arugula appear as black and tan spots on leaves that are surrounded by yellow halos. The lesions spread quickly under moist conditions, leading to blighted leaves. Because of the high value of baby leaf greens, there is little tolerance for spotting on arugula leaves in most retail and wholesale markets. Thus, growers need effective solutions to manage bacterial blight in their arugula.

Because bacterial blight of arugula and other baby leaf brassicas is new to our region and elsewhere in the US, little is currently known about specific management strategies. Best management practices for plant disease prevention, particularly when multiple strategies are used in tandem, seem to be effective for many farms who experience this disease, including:

• Use certified disease-free or treated seed when available: Hot water bath treatment for brassica seeds is recommended to prevent bacterial disease, however, arugula seed cannot be hot water treated due to “gumminess” of the resulting wet seed. Steam and UV treatments are currently being tested for treatment of arugula seed. The bacterial blight pathogen is believed to be seedborne in some cases; research is underway examining this pathway of transmission.
• Increase air flow and reduce humidity in growing environments: Very dense plantings of arugula under row cover and in high tunnels often experience higher incidence of bacterial blight. Activities that improve air flow, such as removing row cover as often as possible and ventilating tunnels, can reduce disease. Some producers have been able to grow arugula crops outdoors with no bacterial blight, while their indoor plantings experience this disease, although this is not always the case.
• Use drip irrigation versus overhead: Practices that reduce leaf wetness help prevent spread and progression of bacterial blight and other disease.
• Chemical control: Very little is known about pesticide efficacy on bacterial blight in arugula. Copper and hydrogen peroxide products may aid in preventing or slowing progression of disease. A test spray should be performed first to assess phytotoxicity under your growing conditions. Be aware that for any pesticide used in high tunnels in NYS, the label must explicitly list “greenhouse” (“hoophouse,” “indoor,” or comparable terms) use in order to legally be applied to a crop within a high tunnel. If the label is silent on greenhouse use, you may not use the product in your tunnel.
• Crop rotation: Rotation of tunnel beds or fields away from brassicas is important in disrupting the disease cycle of this pathogen, although there are known non-brassica hosts (e.g. oats).

As part of a Northeast SARE Research and Education grant, we (the NorthEast Arugula Team, or NEAT) conducted a series of arugula variety susceptibility trials, growing over 20 different varieties of arugula that were inoculated with the bacterial blight pathogen. Unfortunately, no salad arugula (Eruca sativa, e.g. Astro, Standard, Esmee, etc. common varieties) cultivars showed resistance to bacterial blight. Wild arugula varieties (Diplotaxis tenuifolia; e.g. Sylvetta, Nemesis, and Bellezia), however, exhibited significantly lower disease severity in both field and greenhouse experiments. If wild arugula is a good fit for a farm’s salad greens markets, it may be worth growing versus salad arugula if bacterial blight is a problem.

Do you suspect you have bacterial blight in your arugula or other brassica greens? If so, please reach out to the NEAT team by contacting Elisabeth Hodgdon at eh528@cornell.edu or (518) 650-5323 to coordinate submission of samples to Penn State for diagnosis. Funding for this project was provided by Northeast SARE Research & Education grant LNE23-463. For more information about the NEAT team and projects: https://plantpath.psu.edu/research/labs/bull/research/neat

 

Bacterial blight symptoms in arugula. Photo credits: Elisabeth Hodgdon (left) and Margaret McGrath (right).