Downy Mildew of Basil - Organic Management Strategies

Downy mildew is a new disease of field and greenhouse basil. Caused by the fungus Peronospora belbahrii (a different pathogen than the one causing downy mildew in other vegetable crops), basil downy mildew first appeared in Switzerland in 2001 and has since spread to numerous countries in Europe, Africa, North and South America. It was first reported in Ontario field basil during the 2010 growing season, with most reports occurring towards the end of the growing season. Although basil downy mildew is not toxic to humans, it renders basil plants unsightly and unmarketable.

This disease can be spread by wind and by marketing infected leaves. It can also be spread on contaminated seed, which may explain reports of the disease from greenhouses that have not previously had basil. Consequently, it is important to closely monitor transplants for symptoms of the disease prior to planting in the field, where large numbers of spores can be spread great distances on wind. Basil downy mildew has caused significant crop losses since it first appeared in Ontario. Its first occurrence in Ontario basil fields has always been late July or early August, although it can occur much earlier in greenhouse basil.

Organic management of any downy mildew can be very difficult. Downy mildews are very aggressive and can spread rapidly. They are caused by oomycetes, fungus-like organisms that are often called water moulds. Because they are not true fungi, they are not easily controlled by the same cultural or other practices used to manage other fungal diseases, such as powdery mildew.

There are no pest control products (organic or conventional) registered in Canada for the management of basil downy mildew. Homemade pest control products, such as compost teas, are not recommended, in part due to food safety risks associated with applying these products to a fresh market crop, and also because these products have not proven effective for control of basil downy mildew.

Over the past two years, trials have been initiated to identify methods to prevent, reduce or delay this disease. The first trials, conducted by OMAFRA staff in 2010, included a limited survey of different basil varieties for relative susceptibility to the disease. Starting in 2011, a larger research project was initiated at the University of Guelph to compare a wide range of basil varieties and to evaluate organic pest control products.

The studies compared many types of basil including, Genovese, bush, purple, thai, and spice. The only type of basil that did not develop symptoms was spice basil. While spice basil has some uses, it is not an acceptable alternative to the common basils. Almost all other varieties of basil had nearly 100% of leaf area damaged by early September. The exception was the sweet basil variety 'Medinette', which had 40% of leaf area affected by the same date. While this level of disease is still not acceptable for market, the disease development was slower on this variety, which may extend production by a couple of weeks.

Three organic products were also tested in 2011 for control of the diseases, including one based on an extract of giant knotweed (Reynoutria sachalinensis), one based on sesame oil, and one based on the bacteria Bacillus subtilis. None of these products are currently registered in Canada for control of downy mildew on basil. The products did not significantly reduce disease compared to an untreated control. However, there was a slight increase in total yield of basil with the Bacillus subtilis product. Research is continuing this winter to evaluate a wider range of organic control options.

Several cultural management strategies appear to help reduce or delay the development of this disease in Ontario basil fields. Basil planted in an open site and exposed to the prevailing winds tends to have delayed onset of the disease. Wider plant spacing will also encourage good airflow around the plants, which will also reduce disease, as long as weeds are controlled between the plants. Growers should avoid overhead irrigation, especially after the middle of July. Note that these techniques do not appear to completely prevent the occurrence of the disease, however they can sometimes delay onset of symptoms long enough to allow for an additional harvest. Ensure you buy your seeds from a reputable supplier. Seeds should not be collected from fields in which downy mildew has occurred. The disease is less severe when growing conditions are hot and dry.

Growers should continually scout their fields for symptoms of downy mildew. As soon as the first symptoms appear, growers should consider harvesting because the crop will likely be unmarketable within a few weeks. However, once leaves are infected with downy mildew, it takes at least a week for symptoms to develop. As a result, seemingly healthy leaves at harvest may develop symptoms post-harvest. This is not an issue for dried basil, as long as drying occurs as soon as possible after harvest.

By combining cultivar choice, purchase of high quality seeds or transplants, site selection, and spacing organic growers can delay the onset of basil downy mildew. Research is continuing to test a wider range of basil varieties and organic pest control products. Funding for this project is being provided by the Production Systems and Emergency Management themes of the OMAFRA/University of Guelph Partnership.

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