Plant Problems: Basil Diseases

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Basil, a fragrant annual herb that is popular in cooking — particularly in Italian dishes. It’s easy to grow indoors and outdoors, but it’s still quite a fragile plant — especially in hot conditions, and is prone to a number of diseases. Basil diseases can prevent the plant from flourishing and, in some cases, can kill your basil plant.

Fusarium Wilt

Fusarium wilt is one of the most common diseases associated with basil. It is a soil-based disease that can stay in the soil for many years. The disease causes the plant’s water vessels to block, resulting in the plant wilting, then dying. The first symptoms of fusarium wilt are yellowing shoots as well as yellow leaves that appear when the plants are 8 to 10 inches tall. In wet conditions, the plant may show white or orange fungal growth on the stems. In the late stages of the disease, the plant may develop twisted stems, and the stem tissue becomes discolored.

How to Control Fusarium Wilt

Plants affected by Fusarium Wilt should be removed from the soil as soon as you spot the infection. Refrain from planting basil for 4 to 5 years, giving the soil time to recover. Make sure that the seeds or young basil plants that you purchase come from reliable stock and that they are disease-free. Diseased basil plants in pots should be thrown away and the pots should be thoroughly disinfected.

Bacterial Leaf Spot/Basil Shoot Blight

Caused by a bacteria known as Pseudomonas cichorii, this basil disease shows itself as brown or black spots on the leaves, as well as streaking on the stems. Pseudomonas cichorii does not only affect basil but also chrysanthemums, geraniums, and other ornamental plants. The bacteria is easily spread by water splashing the plant from infected soil. Bacterial leaf spot likes warm and wet conditions

How to Control Bacterial Leaf Spot

There is no way to cure bacterial leaf spot, but there are measures you can take to prevent it and minimize the damage. Remove infected plants as soon as possible. Gently water the basil plants so that water does not splash up on to the leaves. Use drip irrigation to keep your plants watered if possible. Lower relative humidity by improving circulation of the air. Try to avoid handling the plants when the leaves are wet, as this can spread the disease.

Downy Mildew

First reported in Florida in 2007, this relatively new disease is very destructive and contagious. It’s not always instantly recognized as downy mildew because the first sign is yellowing leaves, similar to a nutrient deficiency. Only specific sections of the leaves will be affected as downy mildew cannot pass across the major veins in the leaves. As the disease progresses, a fuzzy mold grows on the lower leaves of the plant. This can either be white, gray, or purple.

How to Control Downy Mildew

The disease is spread by airborne spores that thrive in wet weather. If the plants are indoors or in a greenhouse, then try not to get the leaves too wet, as this encourages the disease. Avoid watering the plants in the evening as the leaf wetness can persist throughout the night, leading to downy mildew. Water the plants first thing in the morning and if they are in a greenhouse, leave the door open to allow air to circulate and the leaves to dry out. The basil plants should not be placed too close together as the air needs to be able to circulate around the plants for the leaves to dry out. Remove infected leaves as soon as possible and severely affected plants should be pulled up and destroyed. Once an area has been affected, avoid replanting the same area with basil for at least a year.

Root Rot

Root rot is also known as damping off. It is a fungal basil disease that occurs in wet and cool conditions. It enters the plant via the small roots and over the following week it spreads through the entire root system. This prevents the nutrients and water from reaching the plant and eventually kills the basil. The first signs of root rot in your plant are the leaves wilting and changing color to yellow and then brown. The leaves then fall off the plant. If you look at the roots, you’ll see they turn brown and feel soft. Once the roots feel mushy, then it is probable that the plant will wither and die.

How to Treat Root Rot

The most common cause of root rot is constantly wet soil, which provides the ideal conditions for the fungus to grow in. Basil should be in containers that drain well and it certainly won’t hurt the plants if you allow them to dry out in between waterings. As soon as you notice the first signs of root rot it is imperative to treat the plants. The first thing to do is remove the plant from the soil and wash the roots. Trim any of the roots that show signs of root rot and repot the plant in fresh soil and a different pot. If you use rainwater to water your plants, make sure the water container is covered to prevent any organic debris from entering the water that may have root rot pathogens on them. Basil seeds should not be overcrowded or watered too frequently.

These are some of the most common basil diseases and are mostly caused by overwatering. Although it is tempting to water your basil plants in order for them to thrive, caution must be taken that the plants are not too wet and allowing fungus and pathogens to spread throughout the plant.

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3 thoughts on “Plant Problems: Basil Diseases”

  1. First things first, I like the layout of your website. It is very clean. Secondly, coming to the basil plant, we have some of these plants in our house. Their leaves have medicinal properties and we use them sometimes, in our tea. But thanks so much for your information packed post.

  2. So that explains why I cant even grow my basil. I have tried a number of times to grow our own herbs but then I am just not to lucky to have them grow. I water them too frequently. Ive also experienced that Fusarium Wilt.

    1. Yes, herbs don’t like being over-watered. If you’re growing them outside directly in the ground, take a 1 or 2 liter juice bottle, take the lid off, cut the bottom off the bottle, then bury it 1/3 of the way into the soil with the neck downwards, a few inches from your plant. Fill it with water, then the water will slowly leak out of the bottle into the soil. It keeps your plant moist but not too wet and helps prevent these kinds of diseases.

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