Called "the king of herbs" by the ancient Greeks, basil (Ocimum basilicum) is an easy-to-grow annual with distinctive fragrant, flavorful foliage. Although this mint family member typically requires little maintenance, you must provide it with the right care or the deep green leaves might turn yellow. Several plant diseases can also cause yellowing leaves. Identifying the cause of yellowing basil plants can help you apply the correct treatment.

Basil background
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A patch of basil plants growing in a garden.

Water and Sun

Basil plants in the garden need about 1 1/2 inches of water every week to keep them healthy, but watering them from above or using sprinklers to water them can lead to yellowing leaves. Watering from above gets the leaf surfaces wet, and sunlight beating down on standing water can cause sunscald. Watering just around plant bases with a soaker hose helps alleviate the issue, as does watering plants in the late evening or early morning. Basil thrives in warm weather and requires at least six hours of sun each day. Older leaves naturally turn yellow, as do leaves near the bottom of the plant where they don't get enough sunlight. Both are natural occurrences, so you don't need to take any steps to correct the yellowing.

Fertilizer Needs

Basil prefers nutrient-rich soil full of organic matter, and it can turn yellow if the soil isn't fertile enough. Before planting, amend the soil with a 4- to 6-inch layer of compost and about 3 pints of 5-10-10 fertilizer for every 100 square feet of planting space. Work the fertilizer and compost into the top 6 inches of soil. Sidedress with fertilizer applications about once every 14 days, using about 1/3 cup of fertilizer for every 25 square feet of growing area. Scatter the fertilizer near your basil plants and lightly water the soil.

Basil plants suffering from a potassium deficiency often develop yellow areas between the veins on the leaves. Boost potassium in the soil with a commercial potassium fertilizer, such as muriate of potash. Following the directions on the product's label, sprinkle about 1/2 cup of potash fertilizer for every 100 square feet of growing space. Lightly water the fertilizer into the soil.

Bacterial Diseases

Basil can come under attack from bacterial leaf spot, a disease that causes water-soaked, yellowish-brown to black streaking on stems and spotting on the leaves. This disease occurs when Pseudomonas cichorii fungi infect the soil, and the soil splashes onto basil plants during rains or watering. Bacterial leaf spot is particularly problematic during humid, warm weather. No cure exists for bacterial leaf spot disease, so clip off infected leaves as soon as you spot them. When pruning, use shears or scissors that have been wiped with a cloth soaked in rubbing alcohol. This helps prevent the spread of disease. You can prevent infections by watering plants at the bases of their stems to reduce splashing.

Fungal Diseases

Several fungal diseases can cause basil plants to turn yellow, with fusarium wilt and downy mildew being two of the most common. Fusarium wilt is caused by a soil-borne fungus (Fusarium oxysporum f. sp. Basilicum) that attacks the plant tissue inside of the stem that transfers water throughout the plant. Basil plants typically grow normally until they reach between 6 and 12 inches tall. After that, the disease causes brown spotting and streaking on basil plant stems as well as yellowing, wilting and dropping leaves. The first sign of downy mildew disease is a slight yellowing around the major veins on the leaves. Brown spots form within the yellow tissue before spreading to cover entire leaves. The leaves also develop fuzzy, purple-gray growths beneath the infected yellow areas. The diseased leaves quickly fall from the basil plant.

Treating Fungal Diseases

Fungicides aren't a good choice to treat fungal diseases on basil, advises Clemson Cooperative Extension. Keeping the leaves as dry as possible and improving airflow around basil plants help control both downy mildew and fusarium wilt. Make sure each basil plant has enough space around it to allow for ample ventilation, which helps reduce leaf moisture. Space basil plants 24 inches apart and leave 36 inches between rows. Remove diseased basil plants and any fallen leaves from the area. Throw the plant debris in a covered trash can rather than letting them dry on the ground. The fungal spores of both diseases can survive in the soil for many years.

Foliar Nematodes

Basil plants can attract strawberry leaf nematodes ((Aphelenchoides fragariae), microscopic worms that feed within basil leaves and initially cause yellow, twisted and stunted leaves. The yellowed holes eventually turn brown and occasionally drop from the leaf, leaving a "shot hole" appearance. This nematode requires standing water to stay on leaf surfaces, which is another reason for careful watering. No chemical controls are available to home gardeners, so remove any affected basil plants and toss them in a covered garbage can.