There have been no official reports of basil downy in field grown basil so far this year. However, basil downy mildew is usually present to some extent on greenhouse grown basil in garden centers in the spring. Often the disease does not progress in late spring or early summer due to an open canopy in the field and less favourable cool and dry conditions. However, recent rains over the past week followed but hot and humid weather over the next week will greatly increase the risk of the disease in field basil. A southerly flow with occasional thunderstorms will also increase the risk of inoculum coming in from basil growing regions in the US.

Conventional growers should apply protectant fungicides before the disease develops. There are now 5 fungicides registered for control or suppression of basil downy mildew in Ontario:

• Torrent (cyazofamid) at a rate of 0.2-0.22 L/ha – field and greenhouse
• Confine Extra (phosphonate) at a rate of 3-5 L/ha (suppression only) – field and greenhouse
• Revus (mandipropamid) at a rate of 583 mL/ha – field and greenhouse
• Orondis Ultra A (mandipropamid) at 175-350 mL/ha + Orondis Ultra B (oxathiapiprolin) at 583 mL/ha – field and greenhouse
• Reason (fenamidone) at 400 mL/ha – field only

For more information on these products  consult the product labels and the Herbs section of OMAFRA Publication 838 Vegetable Crop Protection Guide.

There are no products registered for control or suppression of basil downy mildew that are approved for organic production. Organic growers should rely on promoting good airflow around plants through wide plant spacing and frequent monitoring of the crop so harvest can occur rapidly upon the first detection of the disease. New downy mildew resistant/tolerant basil cultivars are now being introduced onto the market in the US. Some of these will be available in the coming years to reduce the impact of the disease for all basil growers.

Basil downy mildew usually first appears within the canopy on older leaves. Sections of the affected leaves turn a lighter green/yellowish colour (Figure 1). When the leaves are turned over, especially in the morning, purplish/grey fuzzy spores are often present (Figure 2). Eventually the whole leaf turns yellow and falls off of the plant. The leaves may also develop brown spots as secondary weak fungi move in and cause additional damage.

Figure 1. Symptoms of basil downy mildew begin as yellowing of sections of the leaf.

Figure 2. The underside of infected leaves will develop purplish fuzzy spores.