2014 was the worst year for basil downy mildew in the Northeast since the disease was introduced to the US in 2007. This was, partially due to the distribution of infected transplants nationally and widespread planting of infected material leading to earlier than usual disease and high levels of inoculum in the environment. Here are some recommendations for reducing the impacts of this disease in 2015.
Disease Spread. Basil downy mildew is caused by the oomycete, Peronospora belbahrii, (not the same as Impatiens Down Mildew). It is an obligate parasite, meaning that it cannot survive outside of a living host. It does not produce overwintering oospores, but survives from year to year on living plants where basil production occurs year round, such as in Florida. From these sites the pathogen spreads via wind-dispersed sporangia that can travel great distances due to their dark pigmentation, which protects them from UV radiation. There is also evidence that the disease can be spread by contaminated seed, though we do not yet understand how this occurs and how important contaminated seed is as a source of primary inoculum.
Symptoms. Early symptoms can be mistaken for a nutritional deficiency. Photos of the signs and symptoms are available at: http://vegetablemdonline.ppath.cornell.edu/NewsArticles/BasilDowny.html
Management Recommendations. Purchase seed or plugs from reliable sources. We know that the pathogen may be seed borne, but the mechanisms involved are not well known and testing is difficult. Since Peronospora belbahrii requires a living host, it cannot be grown in a lab culture, making it more difficult to test for presence of the pathogen on or in seed. Seed testing of any pathogen is only a proxy, since you can’t test all of your seed. Any sample you submit for testing may not be wholly representative of the seed lot, leading to false negatives. Therefore, our recommendation is to buy seed from a trusted source. Talk with your seed supplier about how the seed was produced, if it has been tested and also if the variety exhibits any resistance to the pathogen.
Do not buy plugs/transplants that were grown in the south where the disease starts earlier. If you purchase plugs or transplants, inspect them carefully upon arrival.
During greenhouse production: Monitor plants at least once a week. Once plants become infected the disease is inside the plant and fungicides will not be effective. Once detected, remove infected plants or seedling trays and treat adjacent plants.
Reduce humidity and leaf wetness in the greenhouse by heating and venting greenhouses–especially when warm days are followed by cool nights. Use HAF fans to improve air flow in greenhouses. Irrigate plants in the morning so foliage dries or use sub/drip-irrigation rather than overhead. Provide space between plants to allow for good air circulation.
Advice for garden retailers. Inform customers about this disease, in particular the fact that it starts from spores that can be wind dispersed long distances, therefore it is difficult to avoid on plants grown outdoors. Dr. Meg McGrath, Cornell University suggests encouraging home gardeners to grow some plants in containers that can be brought inside when humidity outside is high (overnight and on rainy days). The pathogen needs at least 85% humidity to be able to infect.
Basil planted outdoors in the ground should be planted and harvest early. The pathogen tends to arrive in MA around mid-July. Last year the disease occurred earlier (June 22) because of the early, widespread distribution of infected plants from nurseries further south. Keep track of where the disease is being found via pest alerts in UMass Extension Veg Notes and via the basil downy mildew monitoring program (Google Docs spreadsheet) that Meg McGrath at Cornell University has put together and maintains, accessible here.
Basil downy mildew is not soil-borne so it will not stay in the soil. Advise home gardeners to plant in well-drained sites with good air circulation; orient rows parallel to the prevailing winds; control weeds; increase plant spacing; and harvest/prune so as to improve airflow through/around plants if practical.
Relative susceptibility of basil cultivars. All sweet basil (Ocimum basilicum) cultivars such as ‘Genovese,’ ‘Italian large leaf,’ ‘Poppy Joe’ and ‘Nufar’ are very susceptible to downy mildew. The least susceptible basils included the lemon and spice types such as O. x citriodorum and O. americanum , cultivars , ‘Lemon Std’, ‘Lemon’, ‘Lime’, ‘Spice’, ‘Blue Spice’ and ‘Blue Spice Fil’. There are no cultivars with resistance to basil downy mildew. Efforts to breed new basil varieties with resistance to downy mildew are ongoing at Rutgers University. ‘Eleonora’ (available from Johnny’s) is a cross of Thai basil and sweet basil with “intermediate” resistance. Thai basil has a higher disease tolerance, while sweet basil has the more desirable flavor, though some of that spicier Thai basil flavor does carryover in this cross. ‘Eleonora’ also has flatter leaves and a more open habit which contributes to disease tolerance. Resistant varieties are one important piece of the puzzle, but will need to be used as part of an integrated management plan incorporating cultural practices and chemical controls to effectively manage this disease.
Chemical control. Pay close attention to labels. Basil is a minor crop and is not always found on pesticide labels. Research trials have shown that the phosphite fungicides (eg. K-Phite, Prophyt, Fungi-phite) are among the most effective chemical controls. Other effective materials include mandipropamid (eg. Revus) and cyazofamid (eg. Ranman).
OMRI-approved products labeled for basil downy mildew include potassium bicarbonate (MilStop), hydrogen dioxide (OxiDate 2.0), Streptomyces lydicus (Actinovate), Reynoutria sachalinensis (Regalia) and neem oil (Trilogy). There is not strong efficacy data for these materials and they should be used as part of an integrated plan. Be sure to make applications preventatively (scout under leaves, follow pest alerts) and keep up regular sprays.
Note: It is the grower’s responsibility to read and follow label instructions.
Adapted from the article "Tips for Managing Basil Downy Mildew in 2015" published in UMass Extension Vegetable Notes, Jan 15, 2015, page 3 by Susan B. Scheufele, Robert L. Wick and M. Bess Dicklow, UMass Extension
Also read an interview with Rob Wick, UMass about BDM here:
http://awaytogarden.com/basil-pressure-fight-devastating-downy-mildew/
Link added Aug. 2015: Shining a red light to slow down downy mildew on basil
Tina Smith, UMass Extension and Leanne Pundt, UConn Extension
