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Brown Spots on Basil: Reasons & Treatments

Brown Spots on Basil: Reasons & Treatments

Basil is a favorite herb for growing both indoors and out, and it’s generally quite easy to care for. Like every plant, it’s vulnerable to a number of conditions that can cause problems, including unsightly brown spots.

Some of those conditions can be treated, but there are a few without good remedies.

Knowing the difference can save not only your basil plant, but nearby plants too!

 

Why does my basil have brown spots?

The primary reasons for brown spots on basil are diseases caused by fungi or bacteria, some of which can be treated, and some of which can only be contained. Other causes of brown spots include lack of nutrients, and damage from pests such as aphids and slugs.

 

Fungal Diseases

One of the primary causes of brown spots and other damage to basil plants is fungal disease. These are some of the most common culprits.

 

Fusarium Wilt

Caused by the soil-borne fungus Fusarium oxysporum.

Symptoms: Brown streaks on the stems beginning when the plant is eight to ten inches tall, followed by brown spots on leaves, then leaf death, and eventually the complete death of the plant.

Treatments: Unfortunately, there aren’t any! This is a very difficult disease to treat, and it can linger in the soil for years. Remove any potentially infected plants at once. You can help avoid introducing fusarium wilt by buying only plants and seeds from reputable growers whose products are certified free of diseases.

 

Downy Mildew

Caused by the oomycete (a fungus-like parasite) Peronospora belbaharii.

Symptoms: Leaves will begin to yellow, brown, and curl, much as they would if nutrient deficient, but the giveaway is the presence of grey or purplish fuzz, like mold, on the bottoms of the leaves.

Treatments: There are no good treatments for basil downy mildew, but you can help avoid it by maintaining good airflow around your plants, watering at soil level, growing downy mildew resistant varieties, and buying from reputable growers whose plants are disease-free.

 

Bacterial Diseases

Though less common than fungal diseases, bacteria can also infect and damage your basil plants.

 

Bacterial Leaf Spot or Basil Shoot Blight

Caused by the bacterium Pseudomonas cichorii.

Symptoms: Brown or black spots on the leaves, as well as brown or black streaks on stems.

Treatments: Again, there are no good treatments once your plant is infected, but you can help prevent infection by watering at soil level and improving drainage and air circulation.

 

Pests

Like most plants, basil is susceptible to plenty of creepy-crawly pests that like to munch on the leaves, causing browning, holes, and other damage.

 

Aphids

Aphids are tiny soft-bodied insects that suck sap and nutrients from your plants’ leaves and stems.

Symptoms of Aphid Damage: A sticky black substance on leaves, curled and spotted leaves, stunted growth.

Treatments: Many insecticides will easily treat aphid infestations, including organic options like insecticidal soaps and neem oil. Make sure your treatment is safe for edible plants, and follow directions carefully.

 

Slugs

Soft, large-leaved plants like basil are especially vulnerable to these slimy pests.

Symptoms of Slug Damage: Ragged holes and bleached or brown spots, coupled with tell-tale slime trails.

Treatments: Use diatomaceous earth or crushed eggshells around the base of plants as a barrier slugs can’t cross. Encourage toads in your garden—they’re natural slug predators—by adding toad houses. Improve air circulation to remove the moist, dark areas where slugs thrive.

 

Other Environmental Factors

Sometimes, brown spots on basil aren’t due to disease or pests at all. Other factors in your plant’s environment, from the soil to the sun, can cause damage to the leaves and stems.

 

Sunburn

Watering from above causes droplets to pool on the leaves, which can then act like little magnifying glasses, focusing sunlight until it burns your plant’s leaves.

Symptoms of Sunburn: Look for small bleached or brown spots on the tops of the leaves. Make sure to rule out other causes of damage, such as disease or insect damage, since the treatment for those is different.

Treatments: Water your plants in the early morning or early evening to avoid the brightest parts of the day and only at soil level so water doesn’t pool on the leaves. This will also help prevent bacterial and fungal diseases.

 

Frost

Basil is native to sub-tropical areas and very sensitive to cold temperatures.
Symptoms of Frost Damage: Leaves and stems sagging, looking mushy or brown. Can be patchy, or include the whole plant.

Treatments: If your entire plant was affected by frost, unfortunately, there’s no saving it. It’s best to remove the entire plant so it doesn’t attract pests and disease as it decays. If only part of it was affected, however, you may still be okay. Cut away all the damaged leaves and stems and make sure to protect the remaining plant from any cold temperatures by covering it when frosts are expected.

 

Nutrient Deficiency

Though nutrient deficiency isn’t the most common cause of sickly-looking basil plants, if you’ve ruled out other options, it may still be to blame.

Symptoms of Nutrient Deficiency: The most common deficiencies in basil are magnesium and potassium, both of which can cause leaves to turn yellow or brown, typically in the spaces between leaf veins while the veins themselves remain green. Older leaves are more susceptible as the plant begins to redistribute scarce nutrients to new growth.

Treatments: A dose of fertilizer will usually do the trick for potassium deficiency. Check the label for the NPK ratio (nitrogen to phosphorus to potassium) and opt for something with plenty of potassium, but not too much nitrogen. If you suspect magnesium deficiency is the problem, water with a solution of Epsom salts—also known as magnesium sulfate, Epsom salts are a great source of nutrients for your plant.

 

Frequently asked questions on basil plants (with brown spots)

 

Which basil varieties are most resistant to disease?

Varieties that are resistant to downy mildew include Eleonora, Rutgers Obsession, Rutgers Devotion, Rutgers Passion, Prospera Italian Large Leaf, and Prospera DMR. Varieties resistant to fusarium include Elidia, Aroma 2, Nufar, Prospera Italian Large Leaf, and Prospera DMR.

 

Are basil leaves with brown spots still safe to eat?

It depends on the cause of the damage. If your plant is only sunburned or nibbled on by pests, the leaves should be fine to eat once washed, but leaves damaged by disease should not be eaten. Make sure that any treatments you use to control pests or diseases are safe to use on edible plants, and always wash the leaves before using them!

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