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: http://www.omafra.gov.on.ca/english/crops/pub838/p838order.htm.

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.

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.