Basil Bounces Back With Downy Mildew Resistant Cultivars

Basil downy mildew arrived in the U.S. (Florida) in 2007 and has been devastating Italian sweet basil crops in Maryland and other states ever since.

The disease (technically a water mold) can only survive on live basil plants so it does not overwinter outdoors in Maryland. The infection moves from southern states northward each summer.  Infected leaves turn yellow between major veins and this symptom eventually spreads across the leaf.  The characteristic sign of the pathogen appears as a fuzzy grayish-purple coating (sporangia) on the lower leaf surfaces.  Infected leaves eventually turn brown and plants collapse during warm, humid weather.


Spore structures
Spore structures (sporangia) of basil downy mildew on leaf undersides.


Resistance to the Rescue!

Thankfully, breeders have been busy developing resistant cultivars for this popular crop. Over the past two years Rutgers University has released four cultivars, ‘Obsession,’ ‘Devotion,’ ‘Passion,’ and ‘Thunderstruck’. An Israeli breeder developed ‘Prospera’ and Proven Winners came out with ‘Amazel,’ which is seed sterile and propagated from cuttings- so only available as plants.


Trials in Maryland

A limited field evaluation of four basil cultivars- three with reported resistance and one susceptible cultivar- was conducted during the 2019 growing season at three Maryland sites: Westminster, Finksburg, and Ellicott City (Central MD Research & Education Center).

Plants were established in the fourth week of May. Field observations of downy mildew symptoms were noted weekly. Plants were rated on a score of 0 – 10 with zero being free of disease. Disease ratings below were combined from the three sites.

BASIL August 10, 2019 Sept 3, 2019 Oct 7, 2019
‘Amazel’ 0 0 0
‘Prospera’ 0 0 0
‘Devotion’ 0 1 2
‘Obsession’ 0 1 2
‘Aroma’ (susceptible cultivar) 5 9 10
Basil cultivars
Basil cultivars left to right ‘Amazel’, ‘Aroma,’ and ‘Prospera’ (8/15/19)


Basil cultivar ‘Obsession” with mild downy mildew symptoms (9/5/2019)


Disease symptoms were first noted on August 10 on ‘Aroma,’ and progressed rapidly.  Observations of the resistant cultivars continued through October 8.  By September 3rd, downy mildew had caused very faint yellowing on the lower leaves of ‘Devotion’ and ‘Obsession’. In general, all the resistant basil cultivars performed quite well compared to the susceptible cultivar ‘Aroma’.  Mild disease symptoms on the Rutgers cultivars ‘Devotion’ and ‘Obsession’ did not progress; plant damage was very minor. Prospera’ and ‘Amazel’ never developed disease symptoms during the trial.


HGIC also received positive reports from UME Master Gardeners and other gardeners about the productivity and resistance to downy mildew of ‘Prospera,’ ‘Obsession,’ and ‘Devotion’.


Seed Availability

‘Devotion,’ ‘Obsession,’ and ‘Prospera’ seeds are sold by Johnny’s Seeds and High Mowing Seeds. Harris Seeds is carrying ‘Prospera’ seeds.


Other resistant cultivars: ‘Pesto Party’ has shown limited suppression of basil downy mildew (seeds available from Burpee Seeds). Be on the look-out in 2021 for the other new cultivars from the Rutgers breeding program, ‘Thunderstruck’ and ‘Passion,’ that show very good resistance to downy mildew.




By Jon Traunfeld and Dave Clement, Ph.D., Extension Specialists, UME

Tulsi Basil and Anise Hyssop: Easy, Useful, and Adaptable

My brother mailed me some anise hyssop seeds 20+ years ago and a UM student gave me two tulsi basil plants several years ago at “Maryland Day” on the College Park campus. I am deeply indebted to them both for introducing me to these mint family members that quickly became mainstays in my garden. I love both plants for being easy to grow, healthful, and attractive to many species of beneficial insects. They grow abundantly in my yard with little human assistance. They seem pretty dependable under the extreme weather conditions of climate change, although varieties within each species will certainly differ. They tolerate hot, dry weather, as well as periods of high rainfall, as long as soils don’t stay wet. Big bonus: deer don’t seem very interested in these plants!

Every part of anise hyssop (Agastache foeniculum) smells and tastes of anise. It is native to the U.S. upper Midwest and Great Plains and is also commonly known as blue giant hyssop and lavender giant hyssop. It grows well across Maryland as an annual or tender perennial (overwinters most years at my Howard Co. home). Although it self-sows readily it has not been terribly aggressive; I haven’t seen it spread beyond its main planting sites.

Robust anise hyssop plants in mid-summer holding their own in a weedy bed. Photo credit: Jon Traunfeld
Robust anise hyssop plants in mid-summer holding their own in a weedy bed. Photo credit: Jon Traunfeld
Bumble bee on anise hyssop flower. Spikes produce many flowers that are frequently visited by insects. Photo credit: Jon Traunfeld
Bumble bee on anise hyssop flower. Spikes produce many flowers that are frequently visited by insects. Photo credit: Jon Traunfeld

Tulsi basil (Ocimum sanctum or Ocimum tenuiflorum) is also known as holy basil. It is revered in India as a sacred, medicinal plant. Tulsi grows rapidly and blooms continually from June through first frost. Individual plants can easily cover 10 sq. ft. but can be pruned to fit smaller spaces. It’s one of the very few types of basil not infected by basil downy mildew, the scourge of basil lovers. It is not a culinary substitute for Italian basil but is widely used in South Asian cuisines. Continue reading

Basil downy mildew is back in Maryland gardens

Hurry up and enjoy your basil plants while they are healthy. That insidious disease, downy mildew, is infecting basil across the mid-Atlantic region. I first noticed it in a Howard Co. community garden last week and then in my home garden on July 15th.

Caption: Early, subtle symptoms
Early, subtle symptoms

The initial symptom is splotchy light yellowing of leaf topsides. Those areas eventually turn brown and die. Leaf undersides develop a fuzzy grayish fungal mat with spore structures that appear as tiny dark specks.

Sporulation on leaf undersides

The disease spreads rapidly. You can cut back infected plants to encourage new growth but the young leaves quickly become infected. Planting multiple times in different locations can be a useful strategy. Once plants are infected it’s best to pull them out, harvest usable leaves, and trash the rest of the plant.

The good news is that a Rutgers University team developed four new resistant cultivars available to commercial growers this season. Hopefully, there will be plenty of seed harvested this year so that we can all enjoy the benefits of these new releases.


By Jon Traunfeld, Extension Specialist