American Phytopathological Society has selected the article Effective Downy Mildew Management in Basil Using Resistant Varieties, Environment Modifications, and Fungicides, co-authored by Meg McGrath as an Editor’s Pick.
“This article succinctly summarizes progress to date in breeding and cultural and chemical control. Patel et al. suggest an integrated approach and highlight how multiple control measures combined can be much more effective than relying on a single control method.
Congratulations Meg!
Meg explains the results in lay terms:
“The great progress that has been made in managing downy mildew affecting basil is described in this review article. It has been just 20 years since this disease started appearing throughout the world. The pathogen is able to spread well because it can be in seed and it produces an abundance of spores dispersed by wind.
Multiple management tactics have been developed, evaluated through research, and are now being implemented by growers. An integrated management approach with multiple practices is recommended to achieve the very high level of disease control needed for a leafy herb.
Determining that spice types of basil are less susceptible than the more commonly grown sweet (Genovese) basil was the start of finding resistance genes to breed into basil. It takes years of conventional breeding to develop a variety with disease resistance plus good horticultural characteristics. Several resistant basil varieties are now commercially available with newest shown to have good to high levels of resistance. But there is concern resistant varieties may not be durable based on recent observations of downy mildew becoming more severe than expected on them. Other downy mildew pathogens have demonstrated ability to evolve to overcome genetic resistance.
Several chemically-different fungicides suitable for conventionally-produced basil were shown through replicated experiments to have good efficacy and cause no damage to basil (phytotoxicity), information that the manufacturers needed to register these fungicides for commercial use. Alternating among fungicides applied on a preventive (starting before symptoms seen) weekly schedule is recommended to achieve effective control and minimize opportunity for the pathogen to develop resistance to these fungicides. No fungicides approved for organic production were effective when tested on susceptible varieties.
Additional practices for greenhouse-produced basil which are based on knowledge about the pathogen’s biology include fanning leaves to keep them dry so pathogen spores cannot germinate, turning red (best) or white lights on during night because the pathogen needs at least six hours of darkness to produce spores, and dehumidifying air. Brief exposure of basil to ultraviolet light at night is a promising new practice warranting additional research.
What excites Meg about this research?
Results from translational (applied) research like this are immediately useful for growers. It is rewarding and satisfying to be able to help growers be more successful, especially when it pertains to managing an important disease. Basil is the most grown herb crop. Downy mildew is its most common and devastating disease. It has rendered crops completely unmarketable.
Some management practices for basil downy mildew are exciting because they are cutting edge and very innovative: using lights at night in greenhouses to prevent the pathogen from being able to produce spores and developing resistant varieties with modern molecular tools (silencing the susceptibility gene in basil).
For plant pathologists it is exciting to work with such an interesting pathogen. For almost 70 years there were no reports of Peronospora belbahrii following first report of it causing downy mildew of basil in Uganda in 1932. Since 2001 there have been many reports throughout the world. Cause of this huge change in the pathogen’s occurrence remains unknown.
The unpredictable nature of some pathogens makes them exciting to work with for plant pathologists. Other downy mildew pathogens have demonstrated potential to evolve rapidly to overcome resistant plant varieties and to become resistant to fungicides. We suspect this will happen with Peronospora belbahrii. Not knowing if and when this might occur, and which varieties and fungicides will be affected creates scientific excitement.