Images below were taken during December 2020. Leaves were from a home garden on Long Island where symptoms were first seen on 8 December and continued to develop throughout the month. Symptoms were not seen in 2021, despite frequent rain and mostly mild temperatures (40s F) for December, nor in previous years at this location where arugula is grown from spring until temperatures in winter are cold enough to kill arugula. This observation documents the potential for this disease to occur in the region.
Symptoms were dark brown to black, irregularly-shaped flecks visible on both the upper and lower leaf surface. White fungal growth consisting of spores (sporangia) and structures that hold them (sporangiophores) were seen on both leaf surfaces especially on leaves incubated for at least a day in a closed plastic bag. These symptoms are like those seen on broccoli and other brassicas.
Other affected crops or wild weed hosts can be sources of the pathogen for a crop because sporangia of downy mildew pathogens are dispersed in air currents potentially long distances. Contaminated seed is another potential source of the pathogen.
Cool to moderate temperatures and long periods of high humidity or free moisture favor downy mildew development. Sporulation occurs during night over a broad temperature range of 41-77 F with 59 F being optimal.
Downy mildew is considered one of the most important diseases of baby arugula grown for spring mix in Florida where excessive moisture levels occurring within the dense canopy as it nears maturity can result in total crop loss.
A few growers in the northeast have reported seeing downy mildew in their arugula. There had been very few samples submitted to diagnosticians in the region as of December 2020 when they were contacted along with some state vegetable pathologists to obtain information about occurrence of this disease. It was confirmed in upstate New York in 2008, Massachusetts in 2012 and 2016, and Pennsylvania in 2017, but has not been seen by diagnosticians or vegetable pathologists in Virginia, New Jersey, Rhode Island, Vermont, New Hampshire, or Maine.
Downy mildew could become more important in the northeast because arugula is being grown much more than in the past and it is being grown during winter when conditions are favorable due to low sunlight (thus low UV radiation) as well as being cool and moist. There are no resistant varieties.
Please report sightings of downy mildew in arugula to Meg McGrath to contribute to knowledge about it.
Following are microscopic images taken at 100X of the pathogen’s round spores (sporangia) and tree-like structures that held these spores (sporangiophores). These are the white growth seen on the leaves in the preceding images.
