Basil Downy Mildew Confirmed in Ontario – July 28, 2020

Basil downy mildew has been confirmed in a home garden in Norfolk County. The symptoms first appeared in the past days, which means infection probably occurred over a week ago. It is likely that the disease is present in most of southern Ontario. This is about 1 to 2 weeks earlier than average.

Conventional growers have several fungicide options for controlling basil downy mildew and these should be rotated to reduce the chances of resistance developing. All basil crops need to be protected now. Once widespread infection occurs, it is often too difficult to control the disease and prevent further spread. There are several “resistant” cultivars now available on the market, but many are just tolerant of the disease and can still be infected. It is important to still protect these cultivars through regular fungicide sprays. Fungicide options are listed in the Herbs section of OMAFRA Publication 838 Vegetable Crop Protection Guide:

There are no pest control product options for organic basil growers. Once the disease begins it will defoliate the crop within two weeks unless the cultivar grown has some tolerance or resistance. If the disease has just begun, there may still be time to harvest the remainder of the crop, especially if it is a tolerant cultivar. However, keep in mind that the disease can continue to develop postharvest, since basil cannot be chilled low enough to stop disease progression. If no disease symptoms are present in the field, the development of the disease may be slowed by ensuring good airflow around plants through pruning and weed management. Avoid overhead irrigation, which would increase humidity within the canopy.

Basil downy mildew is caused by an oomycete (water mould) called Peronospora belbahrii that first appeared in Canada around 2009. It usually first appears in the lower canopy as chlorosis (yellowing) of sections of the upper leaf surface (Figure 1). Once symptoms appear, sporangia are produced on the lower leaf surface and these appear early in the morning as a purple/grey dust in the sections of the leaf that have discoloured. The sporangia are released with the first breezes in the morning and blow many kilometres on the wind to infect additional fields. There are no other diseases that cause chlorosis of sections of the basil leaf. The presence of sporangia on the lower leaf surface serve as an additional confirmation of the disease, especially when viewed under a microscope.

Three basil leaves laid out showing yellowing of sections of each leaf
Figure 1. Chlorosis (yellowing) of sections of sweet basil leaves due to basil downy mildew.

Unless the basil is protected through regular fungicide applications, the disease can defoliate a plant within 2 weeks. Infected leaves usually fall off the plant once the entire leaf area is symptomatic. Since there is usually an approximately 1-week period between infection and symptom development, leaves that appear healthy at harvest can continue to show symptoms after harvest and then continue to spread. Harvest may only be possible if there is a very low incidence of the disease in the field and/or the basil is harvested for immediate processing.

About Sean Westerveld

Ginseng and Medicinal Herbs Specialist, OMAFRA
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2 Responses to Basil Downy Mildew Confirmed in Ontario – July 28, 2020

  1. james says:

    Something is not quite right here. We have never used a fungicide again after we started using hydrogen peroxide. Now for about ten years already and I still see these articles in which growers are warned that there are no organic ways of dealing with downy mildew.

    • Sean Westerveld says:

      Hi James. Technically hydrogen peroxide is a fungicide and as such must be registered by the Pest Management Regulatory Agency on each crop and pest. As a result, use of hydrogen peroxide against basil downy mildew would be illegal in Canada. There are no organic products, including hydrogen peroxide, currently registered for use against basil downy mildew in Canada. There are hydrogen peroxide products registered for use against downy mildews of other crops, but they only provide suppression or partial suppression of the disease. This is because downy mildews live inside the leaf, so cannot be killed by hydrogen peroxide once the leaf is infected. Sporulation and spore germination are all that can be affected by a hydrogen peroxide application, but frequent reapplication would be required for this to have any benefit. This is in contrast to powdery mildews that live mostly on the leaf surface and could be directly contacted by any spray. Since downy mildew attacks the harvested portion of the basil plant, suppression or partial suppression do not provide sufficient control. This is supported by several research studies that compared control of many organic and conventional products against the disease.

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