Whatever basil varieties you grow – holy, lemon, Thai, or Genovese – basil plants can sometimes be stricken with insect and disease problems. One of the most common is white spots on leaves. There can be different reasons for this symptom, so first take a good close look at your plant to make a diagnosis.
The first step in curing a problem is to correctly identify it.
Begin by considering your growing conditions. Some plant problems are not caused by disease or insects, but by improper cultural practices. And improper cultural practices can encourage diseases or invite insects.
So ask yourself: Did you plant your basil in well-draining soil rich in organic matter? Are you growing in full sun and watering and fertilizing properly? Did you use proper field spacing so that there is good air circulation around the plants? If you answered yes to these questions, your plants should be able to sail through the growing season with very few problems.
White spots on basil leaves can have two causes – a fungal disease or an insect infestation. Insects that feed on leaves include aphids, thrips, leafhoppers, spider mites, whiteflies, and mealybugs. Fungal diseases begin as small splotches that eventually cover the entire leaf surface.
Here’s how to tell what’s affecting your basil plant. In general, the markings caused by insect damage will be irregular and random, and individual spots do not grow larger. Fungal diseases begin as more uniform spots that grow into large patches that eventually cover the entire leaf.
Diseases begin as uniformly shaped spots on the leaves – powdery mildew is white. In contrast, downy mildew is yellow, and bacterial leaf spot and fusarium wilt are black or brown. Over time, the spots will multiply until the entire leaf is covered.
Powdery mildew is the most common disease that causes white spots on the plant leaves. It forms when days are warm and nights are cool. It begins as small white spots on the upper side of leaf surfaces and grows to cover the entire leaf in a powdery looking coating. It grows rapidly on tender new growth, so avoid over-fertilization. Except in extreme cases, powdery mildew does not cause permanent damage to plants; it’s just unsightly. But plants with powdery mildew should not be eaten.
Downy mildew begins as small yellow patches on the leaves. Spread by wind, downy mildew develops as a fuzzy gray growth on the underside of the leaves and then causes the leaves to fall off. Downy mildew grows quickly and can kill a plant in a short amount of time.
Bacterial Leaf Spot
Bacterial leaf spot (pseudomonas cichorii) develops when spores in the soil are splashed up onto the leaves. Symptoms include leaves and stems that become spotted with black or brown lesions.
Fusarium oxysporum is a wilt disease identified by brown spots, twisted stems, stunted growth, and wilted and yellowed leaves that drop
A nutrient deficiency will cause discoloration of leaves. A phosphorus deficiency manifests as small purple or brown spots; a lack of nitrogen or iron causes leaves to yellow. As deficient conditions continue, the spots become necrotic (dead) and over time will turn brittle and fade to white. A soil test can help you determine if you need to add nutrients.
Insects and Pests
Most insect feeding damage shows up as small white dots on the top surface of the leaves. These spots give a mottled or speckled appearance to the leaf. Some insect pests can multiply by the hundreds in a very short amount of time, so a good scouting program is essential to stop infestations before they overwhelm your basil plant. A magnifying glass can help you detect the presence of pests.
Aphids are soft-bodied and usually live in a colony up and down the stems and leaves. Some aphid species are white, but they can be other colors as well. They suck juices out of the basil leaves, leaving behind small spots in an irregular pattern.
Thrips (sometimes called flower thrips) are tiny and slender and tend to hide in new buds and on the undersides of leaves. They will sometimes close leaves up over themselves. The damage they cause appears as irregular silvery-white spots and trails, which turn brown as plant tissue dies.
Leafhoppers are another insect that suck plant juices from the leaf surface, leaving small white dots. They have a wedge-shaped body and are quick moving and often hard to see. The damage they cause looks like channels or trails through the leaf surfaces.
Spider mites are very tiny creatures that build fine white webbings in leaf axils and stem crotches. They suck juices out of plant leaves, leaving a mottled appearance on the leaves that look like tiny dots
Whiteflies feed on the underside of the leaves. Their light-colored bodies look like white spots. If you suspect whiteflies, give your basil plant a shake. The insects will rise up in a cloud.
Mealy bugs are small and white and are often found living in crotches and leaf axils under a cottony cover. A tropical insect, mealy bugs do not survive in areas that have cold winters, but they can be a problem on plants grown indoors or on outdoor plants in warmer climates.
Treating The Problem
Once you have identified the cause of white spots, there are some treatments you can use. Keep in mind that basil is a sensitive herb and treatments and products can be just as damaging as the pest you’re trying to eradicate!
Whether you make your own or purchase a ready-made product, apply any treatments on a day that’s not windy. A cloudy day is best (or very early in the morning or late in the evening) because the heat of the sun in combination with products can sometimes cause the leaves to burn.
Treatments for Diseases
The best way to control mildews and other fungal diseases on basil is by using good growing practices. Fungicides work best as a preventative and should be applied before fungal growth occurs. Most products need to be applied weekly – the label will guide you.
There are no cures for bacterial diseases on basil. Basil plants stricken with bacterial leaf spot should be removed and destroyed.
Potassium bicarbonate is a commercially available product sold under different brand names that is used as a spray to prevent and treat mildews and other fungal diseases. Do not try to make your own – you can kill your plants and harm the environment. Buy pre-made products such as Dr. Earth’s Organic Fungicide spray.
A homemade recipe for fungal diseases calls for 1 tablespoon of baking soda, a tablespoon of mild dish soap, and a tablespoon of vegetable oil mixed in a gallon of water. This makes an effective anti-fungal spray that can also help eliminate insect pests as well. Use weekly, and be sure touse this mixture on well-watered plants and not on sunny days as it may cause the foliage to burn.
Treatments for Pests
Insecticides cannot tell the difference between pests and beneficial insects. If you decide to spray for pests, do so when bees are not active – early in the morning or late in the evening. And an insecticide has the potential to kill the predatory insects – lacewings, lady beetles, parasitic wasps, etc. – that would do the job for you.
Whiteflies and leafhoppers are flying insects and cannot be sprayed – they just fly away. A systemic insecticide, applied to the soil and taken up by the plant, will kill insects as they feed. Be sure that any insecticide you use is labeled safe for food crops.
And sometimes, the best remedy is a strong spray of water from a garden hose. That can blast the pests right off.
Neem oil is a plant-based horticultural oil (from neem trees, native to India) that smothers insect pests. It has a strong odor that can affect your basil’s flavor, so if you decide to use it, do so sparingly. Mix 2 tablespoons in a gallon of water and spray the leaves’ tops and undersides to the point of runoff. Neem oil shouldn’t be used when air temperatures are above 80°F and should only be used on a cloudy day or at sunset because it may cause leaves to burn. Make a fresh batch every time.
Insecticidal soaps contain fatty acids that kill insect pests by suffocation and disrupt the insect’s cell membranes. The soaps also remove the protective waxy coating many of these pests have and that causes them to dehydrate and die. Small infestations of aphids, thrips, mealybugs, and spider mites can be controlled with insecticidal soaps. The spray needs to touch the insect for it to work, so be sure to cover all sides of the leaf.
Many gardeners like to make their insecticidal soap. Recipes abound on the internet – the most common is to mix 2 tablespoons of mild dish soap and 1 tablespoon vegetable oil in a gallon of water and use in a spray bottle. This mixture can also help remove honeydew and the sooty mold that often develops along with an insect infestation. The soap you use should not contain degreasers or bleach, and soft water is recommended. Make a fresh batch every time.
Again, a homemade product will kill all insects, not just pests.
Diatomaceous earth – or DE – is a powdery substance that comes from the fossilized skeletons (silica) of small aquatic creatures called diatoms. Although DE feels like a soft powder, it is very sharp to insects (and slugs and snails). These tiny, sharp particles get into the joints of insects and cause irritation and dehydration.
Diatomaceous earth will kill all insects that come in contact with it, so use it with care. DE is used dry and should be reapplied after a rain. Moisten the foliage, then sprinkle or dust the DE on the plants. Although it’s safe for people and pests, diatomaceous earth can cause irritation, so wear gloves, eye protection, and a dust mask when applying.
Using Pesticides Safely
If you chose to use pesticides, do it safely. Misuse of pesticides can be dangerous – to you, your plants, and your backyard habitat. Even though a product may be labeled as safe or natural or organic, that doesn’t always mean that it’s not hazardous.
Always uses pesticides responsibly and know that insecticides will kill all insects, even beneficial ones.
It can also be a waste of time and money if you use the wrong product or use it at the wrong time. So before you ask: “What can I spray on my basil plant?” do your homework. And then, be sure to consider the following:
- Make sure you have identified the pest or disease correctly and decide if the amount of damage warrants the use of pesticides.
- Determine if the time is right to use a pesticide. Be sure that the pest you are treating is in a vulnerable stage and that the product will be effective.
- Be sure the treatment you intend to use is the right product for the job.
- Be sure that the weather is right – no rain in the forecast to wash the product away, no wind to cause drift, and that temperatures are in the range that the product requires to be effective.
- Follow the directions on the label exactly. Be sure the product you are using is approved for use on basil and the pest or disease you are treating.
- Always prepare pesticides in a well-ventilated area.
- Never mix different pesticides unless the label tells you it is safe to do so.
- Use the proper personal protection equipment – gloves, goggles, etc.- listed on the label, and never eat, drink, or smoke while applying pesticides. Wash your hands after application.
- Always store any pesticides you use in their original container, in a dry place out of the reach of children and pets.
- Dispose of unused pesticides through a household hazardous waste reclamation program.
Is It Safe to Eat Basil with White Spots?
If the spots were caused by insect damage, the basil is safe to eat. Wash the leaves thoroughly before use. The flavor might not be as strong as you would like, and the basil leaves might not be as attractive as you’d like, but they will be perfectly fine to eat.
White spots caused by powdery mildew and other fungal or bacterial diseases should not be eaten. In addition to its unappetizing appearance, powdery mildew interferes with the herb’s flavor and can cause allergic reactions in some people.