Home gardens often include varieties of basil for their versatility, taste and ease of growth. Basil is a hardy, drought tolerant herb that is susceptible to fungal and bacterial pathogens. Additionally, the annual suffers from exposure to cold temperatures that cause the leaves to turn black, according to the North Carolina Cooperative Extension.
Overwatering is a common cause of basil plant death. Basil is a drought tolerant plant that prefers well-drained soil. Overwatering quickly results in root rot and death. Early symptoms include wilting leaves and stem rot at the base of the soil. Fungus and mold may also appear on the soil's surface.
Fusarium oxysporum causes fusarium wilt, a disease that causes basil plants to wilt. Plants with this disease appear normal until they reach 6 to 12 inches in height, according to North Carolina Cooperative Extension Service. Besides wilted leaves, another common sign is stunted growth. Although fusarium wilt is a soil-borne disease, it is also spread with contaminated seeds. Infected soil may carry the disease for up to 12 years. Purchasing certified healthy basil seeds helps prevent the spread of this disease.
Wind-blown spores and contaminated seed are responsible for spreading the pathogen known as basil downy mildew. Downy mildew first appears as yellowing of the leaves, a common sign of nutrient deficiency. Leaf-yellowing is followed by a purple-gray, downy growth on the leaves. Fungicides are the only treatment, but are not guaranteed. Downy mildew tolerates cool temperatures but grows quickly in warm weather.
Fungal and bacterial pathogens as well as foliar nematodes, microscopic round worms, cause spots on the leaves. The leaves may become brittle and fall from the plant. Stem lesions also occur with these pathogens. Bacterial and fungal conditions are worse in hot, humid weather, according to the University of Hawaii Cooperative Extension.
Root-knot nematodes damage the roots of basil plants, depriving them of nutrients and water. Early symptoms include wilting, stunted growth and yellow leaves. The disease will result in root rot if left untreated. Containerized basil plants should be repotted with new soil. Garden basil plants can be treated by adding compost to the soil to decrease the concentration of nematodes. Laying clear plastic sheets around the plant raises the soil temperature, discouraging nematodes.
Herbivorous insects leave noticeable damage on basil leaves in the form of bite marks, slivered scars, webs and honeydew, a sticky excrement left by a number of pests. Most insects are visible with close inspection of the plant. Infested portions of the plant must be removed to prevent the insects from spreading. Most insecticides are not approved for use on herbs.