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Integrated Pest Management Program

Department of Plant Science and Landscape Architecture, Department of Extension

Fact Sheets > Greenhouse > Herbs,Fact Sheets > Greenhouse > Diseases

Basil Downy Mildew in the Greenhouse



Downy mildew on basil was first reported in the US in 2007 in Florida.  Since, then it has been found both in the greenhouse and the field in the Northeast, including Connecticut.  Downy mildew on basil is seed borne, so infections can begin in the greenhouse.  It is also readily spread by air blown spores.


Symptoms are often confused with a nutritional problem because you first see mottling and yellowing of the foliage.  But, if you turn over the leaf, you will see the purplish brown to grayish fungal sporulation on the underside of the leaves.  The underside of the leaves will have a “dirty” appearance.

Conditions Favoring Downy Mildew on Basil

Downy mildew on basil is especially severe when foliage stays wet for extended periods (6 to 12 hours) after inoculation (arrival of a pathogen on a host plant ).   Leaf wetness of 24 hours is needed after the symptoms begin to see the characteristic fungal sporulation. 

Causal Organism and Host Range

Basil downy mildew is caused by the fungus like organism Peronospora belbahrii. Unfortunately, the popular Genovese type sweet basils are very susceptible, whereas the citrus and spice basil varieties tend to be less susceptible.


Scout crops regularly and promptly remove and destroy infected plants.

 Figure 1: Yellowing between the veins

Figure 2: Sporulation on leaf underside


  •  Reduce leaf wetness periods by plant spacing, improving air circulation, and watering when plants will dry quickly.
  • In the greenhouse, use a combination of heating and venting to reduce humidity and condensation, especially when warm days are followed by cool nights.
  • If you see these symptoms, immediately destroy the infected plants.   Preventative sprays are not recommended for greenhouse production because if it is present in the greenhouse, it most likely came in on infected seed.  Once plants become infected, the disease is inside the plant tissues and fungicides will not be effective in stopping the downy mildew within the infected basil plants. 
  • If you detect the disease, after you discard and destroy the infected plants, you can protect the adjacent plants with the following fungicides:  potassium salts of phosphorous acid (Fosphite) (Group 33), and cyazofamid (Ranman), (Group 21) which is now registered for greenhouse use.  Organic Products include:  potassium bicarbonate (Milstop), hydrogen dioxide (Oxidate 2.0), neem oil (Triact 70), and Steptomyces lydicus  (Actinovate).

When you are selling basil plants to retail customers, encourage home gardeners to make their pesto early!   Basil downy mildew does not overwinter in Connecticut, but the windblown spores move in from the South and infections often begin in late July to early August in gardens and fields. 


Allen, J. A.  2012. Downy Mildew on Basil.  UCONN Factsheet

Garibaldi, A., D. Beretti and M.L. Gullino. 2007. Effect of leaf wetness duration and temperature on infection of downy mildew (Peronospora sp.) of basil. Journal of Plant Diseases and Protection. 114(1) 6-8.

Garibaldi, A., G. Minuto, D. Beretti and M.L. Gullino. 2004. Seed transmission of Peronospora sp. of Basil.  Journal of Plant Diseases and Protection. 111(5): 465-469.

Gill, S., K. Rane, A. Ristvey and S. Klick. 2013.  Downy Mildew on Basil.  University of Maryland Greenhouse TPM/IPM Bi-Weekly Report. March 27, 2013.

Homa, K, R. Pyne, B.Barney, A. Wyrenandt, and J. Simon.  2013. Controlling Basil Downy Mildew. Rutgers Plant & Pest Advisory.

McGrath, M.  2013.  Expect and Prepare for Downy Mildew in Basil. Cornell Vegetable MD online

Wyenandt, C. A., J. E. Simon, M. T. McGrath and D.L. Ward. 2010.  Susceptiblity of Basil Cultivars and Breeding Lines to Downy Mildew (Peronospora belbahrii).  HortScience. 45(9): 1416-1419.


By: Leanne Pundt, Extension Educator, University of Connecticut, and Joan Allen, Asst. Cooperative Extension Educator University of Connecticut, 2013

Photos by Leanne Pundt, used with permission


The information in this document is for educational purposes only.  The recommendations contained are based on the best available knowledge at the time of publication.  Any reference to commercial products, trade or brand names is for information only, and no endorsement or approval is intended. The Cooperative Extension System does not guarantee or warrant the standard of any product referenced or imply approval of the product to the exclusion of others which also may be available.  The University of Connecticut, Cooperative Extension System, College of Agriculture and Natural Resources is an equal opportunity program provider and employer.