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.
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|
|‘Aroma’ (susceptible cultivar)||5||9||10|
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’.
‘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.