Basil downy mildew

The disease is caused by the fungal pathogen Peronospora belbahrii and may have a serious impact on commercial basil production.

Basil downy mildew is caused by a fungal pathogen. It initially appears as tiny greyish specks on the underside of leaves and progresses to cover larger areas of the lower leaf surface. The fungus may appear raised from the leaf surface or “fluffy” in appearance. As the disease progresses areas of the affected leaves may turn yellow, brown and die. While affected leaf material may not be viable for commercial sale, the disease itself does not have an impact on human health.

Symptoms of basil downy mildew include the presence of grey/black spores or fruiting bodies on the underside of leaves; yellowing of upper leaf surface which can appear blotchy. Severe symptoms can lead to leaf death.

Basil downy mildew has become an important disease in sweet basil production worldwide in the past decade.  Global production is at significant risk because of the lack of genetic resistance to this disease and the ability of the pathogen to be distributed on seed. With favourable weather conditions, basil downy mildew can spread rapidly and result in complete crop loss.

The pathogen can be carried on seed, transplant material or fresh leaves.  Spores can also be dispersed long distances via wind. The disease may spread within an infected crop by wind, water splash and through management activities which may spread the spores from an infected plant to others.

The pathogen that causes basil downy mildew, Peronospora belbahrii, has not been reported as infecting other plants grown for herb production in Australia.

All seedlings and transplants should be monitored closely for symptoms of the disease. If suspect plants are identified, they should be removed and destroyed immediately in an effort to minimise the threat to other plants. Burning, bagging or burial are good options for plant destruction, as these methods ensure that fungal spores cannot re-enter the crop from the removed plants.

Controlling the disease has proven to be challenging overseas. Fungicide efficacy studies have shown limited success.

There are currently 5 minor use permits available for the control of downy mildew in basil:

• PER84875 Mandipropamid / basil (field and protected situation) / expires 31 August 2019
• PER13790 Mancozeb / herbs and spices (field and protected situation) / expires 30 June 2018
• PER13702 Copper / herbs (field situation) / expires 30 June 2020
• PER85543 Oxathiapiprolin / basil (field and protected crops) / expires 31 December 2019
• PER85584 Azoxystrobin / basil (field and protected crops) / expires 31 December 2020.

It is recommended that good air movement is provided between plants to allow leaves to dry quickly after any moisture (rain, irrigation, fog or dew).

There are no known varieties of sweet basil which are resistant to the disease.  Observations in the United States are that red leaf basil varieties and lemon flavoured varieties may display lower disease levels.

International plant breeding efforts are focussing on introducing downy mildew resistant genes into commercial sweet basils.

Due to the disease’s ability to spread long distances on air currents, formal quarantine arrangements are not likely to be effective in containing the disease and will not be applied to infested properties.

Basil growers can play their part in minimizing the spread of downy mildew by:

• sourcing disease free planting material
• promptly destroying infected plants
• not moving infected plants
• not sharing or moving contaminated equipment unless it has been thoroughly cleaned and is free of plant material
• creating an environment less conducive to disease development (don’t overhead irrigate; increase plant spacing for better airflow).

All growers are encouraged to implement and maintain good farm biosecurity practices.