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My Basil Plant Leaves are Getting Black Spots

Updated February 21, 2017

Basil is a staple for any home garden, as it is easy to grow and is a tasty herbal addition to pasta and other dishes. Fast-growing and sun-loving, the basil plant does well in pots or planted directly in the garden soil. Growers should beware of a few bacterial and fungal diseases that may appear as black spots on basil leaves. Many such infections are relatively common, and basil growers should monitor their plants to prevent their spread,

Leaf Spot

Also known as Colletotrichum sp., this disease is characterised by dark leaf spots with tissue that dies and falls through the holes. It can damage or kill leaves and stems, sometimes killing off the plant entirely. To combat this disease, sow seeds in sterilised containers and soil. Improve air flow by increasing space between plants, and control moisture levels by limiting irrigation and decreasing humidity levels.

Bacterial Leaf Spot

Another form of leaf spot, Psudomonas cichorii, causes water-soaked dark leaf spots in irregular or angular shapes. Basil grown in wet or humid environments are especially prone to this disease. It is spread by splashing water or by transferring bacteria from one leaf to another with your hands, and is also spread through infected seeds. To combat this disease, implement moisture-control mechanisms as mentioned above, and use disease-free seeds and seedlings.

Downy Mildew

Basil Downy Mildew, or Peronospora belbahrii, is a new and destructive disease first identified in the U.S. in 2007. Infected leaves turn yellow, then brown and eventually die off, called necrosis. The disease is also identifiable by the purplish or black mildew spot appearing on the undersides of leaves, called sporulation. As leaves die off, they are often splodged with black. Downy Mildew is thought to be a seed-borne pathogen that can also spread through spores on leaves, seeds and other items. The disease's extent has spread rapidly in recent years, as it is highly contagious and transferred easily by wind.

Causes and Prevention

The above diseases can be monitored by controlling moisture levels, improving air flow between plants and using only disease-free plants and seeds. If leaf spot or mildew is identified, diseased plants should be harvested and immediately separated from healthy plants to reduce the chance of further infection. Pull off and eliminate diseased foliage the moment it is sighted. Regular fungicidal application can help control fungal infections. Sterilise equipment and growing containers and use sterilised soil to help prevent diseases from arising.

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About the Author

Carly Fiske has been writing professionally since 2009. She writes for websites including greenanswers.com, openoffer.com and thirdage.com. Fiske holds a Bachelor of Arts in cultural anthropology from the University of Redlands.