Spots on basil leaves

Basil Plant Turning Yellow: How To Treat Yellow Leaves On Basil Plants

Versatile and easy to grow, basil is an attractive culinary herb valued for its aromatic leaves, which are used either dry or fresh. Although basil is usually grown as an annual, it is suitable for growing year round in USDA plant hardiness zones 10 and above. Although the herb is relatively trouble-free, it is susceptible to certain pests and diseases that can cause yellowish leaves on basil plants.

What Causes Basil Leaves to Turn Yellow?

There are a number of reasons for a basil plant turning yellow, and determining the reason isn’t always easy.

Improper watering – Root rot, a result of too much water, is one of the most common reasons for yellow leaves on basil plants. Water basil only when the top 1 to 2 inches of soil is dry, and remember that slightly dry soil is healthier than soggy soil. As a general rule, one deep watering every seven to 10 days is adequate. If you grow basil in a container, be sure the pot has at least one drainage hole.

Fungal disease – Although several fungal diseases can cause yellow leaves on basil plants, downy mildew is one of the most common. Downy mildew is a fast-spreading fungus recognized by yellowish basil leaves and a fuzzy, gray or brown growth. If you catch the problem early, you may be able to stop the spread by clipping affected growth. However, badly affected plants should be removed and disposed of carefully.

Growing conditions – Chilly temperatures are another reason for yellowish basil leaves. Basil prefers daytime temps above 70 F. (21 C.). Nighttime temperatures should be above 50 F. (10 C.) Lack of sun is yet another common cause of yellowish basil leaves. Basil prefers bright sunlight for six to eight hours per day. Basil grown indoors will likely need artificial light during the winter, ideally for 10 to 12 hours per day.

Aphids – Aphids are tiny pests that suck the juice from tender foliage, thus causing yellow leaves on basil plants. Look for aphids on the undersides of leaves and on the joints of stems and leaves. Aphids are easy to control with insecticidal soap, but be careful not to apply the soap when the sun is directly on the leaves or on hot days, as the soap can scorch the plant.

Root knot nematodes – These small, soil-dwelling pests can cause yellowish basil leaves and small galls on the roots. The best recourse is to harvest the plant and use the healthy leaves. Next time, plant resistant varieties in soil not affected by nematodes.

Lack of nutrients – Basil is a hardy plant that does well in poor soil, but it still requires nutrients in order to thrive. Fertilize basil regularly to prevent yellowish basil leaves, using an all-purpose balanced fertilizer.

Got black spots on your basil leaves and looking for a solution? Basil is one of the most popular culinary herbs in the world. It is featured in cuisine from Italy, Northeast Asia, and its native land of India. Many cooks and chefs call basil the “king of herbs.”

Although there are literally hundreds of varieties of basil around the world, those most commonly grown in the U.S. include various types of sweet basil, cinnamon basil, African blue, holy basil, lemon basil, and lettuce-leaf basil.

Most of these are derivatives of the sweet basil family and all are basically the same plant in terms of how they’re grown in the garden. A common problem in most strains of basil is black spots on the leaves, usually caused by temperature or fungus.

Causes of Basil Black Spots

Basil has natural enemies as well as some problems that can be caused by the environment in which it’s grown. Many people mistake black spots on the leaves as a fungal infection when, in reality, it’s due to early frost nipping the leaves. Basil leaves are very fragile, so even a short frost can cause the leaf edges and the more vulnerable top leaves to die.

If the spots are from a fungal infection, there are several culprits to choose from, all commonly referred to as blight. Because they are all caused by fungi, the diagnosis, symptoms and treatments are generally the same as well.

The spots may appear at the center, on the edge, or anywhere else on the leaf or stem of the basil plant. In most cases, a look underneath the plant will show a fungal growth below the spot. These growths are encouraged when plants are weak from too much or not enough macronutrients (especially nitrogen) in the soil or when damp conditions persist for days, encouraging fungi.

Treatment and Prevention of Basil Black Spots

For spots that could be caused by sudden drops in temperature, protecting the plants against frost is the best line of defense. Cover them with bell jars, tents, etc. Hoop tents are the most popular way to cover and preserve basil from short-term cold weather.

Trimming the plants down to harvest as many of the leaves as possible before the cold sets in, preserving them by drying or other methods, is another good option.

Proper nutrition in the soil is also important. Over- and under-application of nitrogen is the most common culprit for blight-like symptoms. Using good, well-made, organic compost rather than commercial mixtures is a sure way to prevent this type of imbalance in the soil.

For fungal infections, keeping plants well aerated by trimming regularly and giving them plenty of sun is a must. Watering in the morning and at the base of the plants rather than evening also keeps moisture levels down in cooler temperatures. Many fungicides are available to kill the fungus if it does appear. Natural options include fungal soap, a baking soda and water mixture, etc. Trimming away and removing affected leaves and then treating the rest of the plant is the best bet.

Most basil plants can get through black spots on their leaves if the underlying problem is treated quickly. Basil is fast-growing, so removing blighted leaves doesn’t mean your plants will not produce plenty of harvest.

Want to learn more about black spots on basil leaves?

See these resources:
Fresh Basil Production Guidelines For Hawaii and other warm climates from University of Hawaii Cooperative Extension Service.
Sweet Basil Plant Disease Management Guide from University of Florida Extension

Plant Problem Q&A: What Are These Brown Spots?

Q: My basil gets brown spots on some of the leaves. Not too cold or windy. I thought it was being burned by too much sun, but you say 6 hours of full sun per day are needed. What could be causing the brown spots? Some of the spots are on the edges and some are right in the center of the young leaves.

A: There are several possibilities here, and most depend on the size, shape and coloring of the spots. In general you will want to prune the affected leaves and improve air circulation with further pruning, adjusted position, changing your watering habits or staking.

Basil can suffer from a wide variety of afflictions during the growing season.
Photo by Laura Taylor/Courtesy Flickr

Brown and black spots on the leaves and streaks on the stem: These are likely symptoms of bacterial leaf spot, which occurs when soil infected with Pseudomonas cichorii is splashed onto the leaves during watering. There is no direct cure for bacterial leaf spot, but damage can be minimized by modifying watering practices to eliminate splashing and increasing air circulation around the plant by increasing spacing or adjusting placement. You will also want to remove the affected leaves.

Watering from above can also result in brown discoloration of the leaves. If water sits on the leaves it can act as a lens for sunlight, resulting in the leaves being burnt. This is most likely to happen if you water in the afternoon, when the sun is at its hottest. Watering in the morning or evening and at soil level may correct the problem. Additionally, if your area has low levels of humidity, the basil may be getting too much heat and sun for the amount of water it receives. Additional watering or adjusted placement may help.

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Brown or black spots on older or lower leaves may be a symptom of potassium deficiency. This deficiency is most common in chalky or sandy soils. Other symptoms include a “scorched” look to leaf edges and yellow coloration between veins. A potassium fertilizer may help, but will not immediately correct the problem. One homemade potassium fertilizer in a comfrey tea, and others include seaweed and composted banana peels. Another possibility is that the potassium levels in the soil are fine but the plant is not getting enough water to properly absorb the nutrients.

Fungal or bacterial disease can also cause discoloration and spotting (black, brown or gray, sometimes ringed with yellow). Fungal conditions are often accompanied by gray, purple or white fuzz, but a particular diagnosis is needed before you can choose the right treatment. Take a sample to a local nursery or send detailed photos to your local extension office to determine the type of disease, then use an appropriate organic fungicide or antibiotic soap or spray.

Read More: The Encyclopedia of Herbs by Tucker and DeBaggio
The All-New Illustrated Guide to Gardening by Bradley and Cole
Basil Problem – Growing Small Farms
Why is My Basil Dying? – In the Herb Garden

If you know of another problem brown leaf spots could indicate, let us know what it is and what to do about it in the comments!

The Goal

One of last week’s topic in the UF’s GreenHouse 101 was discovery and identification of plant diseases. The teachers asked us to explore diseases in the plants we are growing. I am growing basil. I love basil. I put it liberally on my salads, pizza, red sauces…. To meet my cravings, I try to grow as much basil as I can outside starting in early summer months.

Sadly, one of my basil plants is sick.

The goal of this post is to discuss a disease I am seeing on one of the basil plants. I’ll start with observations and finish with a diagnosis.

Observation

Here is a picture of the sick basil plant:

Here is the front of the leaf:

the area around the dark spots has a wax-like “look and feel.”

…and the back:

when I first saw this picture of the back, I thought pests might be involved. However, on closer inspection with a 10x magnifier, the areas that looked like there might be a webbing or small pest turned out to be dirt and/or dust.

Diagnosis

The black spots are most likely due to either a bacteria or fungi. Within the section on Plant Health (week 4) of the Greenhouse 101 course, the teacher (Brian J. Pearson) noted fungi on plants is typically dry whereas bacteria is typically waxy. In this case, the black spots and area around had a waxy feel. This leads me to believe the black spots are do to a bacteria infecting the plant. While there are many posts on the Internet that discuss black spots, I found this post to help me. This diagnosis makes sense not just because observations appear similar to images provided for black spots on plants that are infected with a fungi or bacteria, but also because it most often occurs in late Spring/early Summer when the plant leaves stay wet longer than they should. We live in the Pacific Northwest of the US where it rains quite a bit during this period.

The class material on diseases pointed out those based on bacteria or fungi will find it’s way to other plants since wind, rain, insects… will transmit the bacteria/fungi to other plants. At this point, this was the only affected plant.

Fix

Not every leaf had the black spots. I removed those that did from the basil plant, leaving the healthy looking leaves alone. For now I’ll keep an eye out for other plants that start showing black spots. If the disease continues, I’ll consider a treatment such as a fungicide. If that doesn’t work, I’ll assume the soil is contaminated with the disease and will remove the soil where the plants were growing that had leaves with black spots. I also sterilized the scissors I used to clip off the infected leaves.

Ah yes. Not a very exciting or technical post. For me – fascinating and exciting to evolve my understanding of plant disease debugging as part of my seemingly endless effort to grow healthy vegetables and herbs.

Thanks for reading this far. Please find many things to smile about.

Diseases And Problems With Growing Basil

Basil is one of the most popular herbs to grow, but that doesn’t mean there are no basil plant problems. There are a few basil diseases that can cause basil leaves to turn brown or yellow, have spots or even wilt and fall off. Keep reading to learn more about the diseases that can cause problems with growing basil.

Common Basil Diseases

Fusarium wilt is among the most common basil diseases. This basil wilt disease most commonly affects sweet basil varieties, but other basil varieties are still somewhat vulnerable.

Symptoms of fusarium wilt include:

  • stunted growth
  • wilted and yellowing leaves
  • brown spots or streaks on the stem
  • severely twisted stems
  • leaf drop

Fusarium wilt is caused by a fungus that can be carried by either the soil that affected basil plants have been growing in or by seeds from infected basil plants.

There is no remedy for fusarium wilt. Destroy infected plants and don’t plant basil or other mint plants in that area for two to three years. Even if a basil or mint plant cannot be hurt by fusarium wilt, they can carry the disease and infect other plants.

Bacterial Leaf Spot or Basil Shoot Blight

This basil disease is caused by a bacteria called Pseudomonas cichorii. Symptoms of bacterial leaf spot are black or brown spots that appear on the leaves and streaking on the stems of the plant.

Bacterial leaf spot occurs when infected soil is splashed onto the leaves of the basil plant.

While there is no fix for bacterial leaf spot, you can minimize the damage by making sure that your basil plants have plenty of air circulation and that they are watered in a way so that the bacteria is not splashed onto the leaves.

Downy mildew is a relatively new basil disease that has only started to affect basil in the past few years. The symptoms of downy mildew include yellow leaves that have fuzzy, grey growth on the undersides of the leaves.

Downy mildew is aggravated by overly wet conditions, so if it appears on your basil plants, make sure that you reduce overhead watering and that the basil plants have good drainage and good air circulation.

Other Basil Plant Problems

The basil diseases listed above are specific to basil plants, but there are a few other problems with growing basil that can happen. They include:

  • Root rot
  • Nitrogen deficiency
  • Slugs
  • Thrips
  • Aphids

What’s causing these small black spots on my basil plant?

Leaf Miner damage and controlLooks like a leaf-miner of some sort…the light colored wiggly lines is what was eaten and the black is fracas…poop. Gotta see the insect itself, really to suggest how to treat. Is this plant outdoors or indoors? Please send pictures of the undersides of the leaves. Some leaf miners actually live IN the leaf. If you could turn the leaf over, look for slight bumps. Use a razor blade and carefully cut the bump to shave leaf and see if you can find insect. It’ll probably look like a tiny worm…investigate while leaves are warmed from the sun and also while night. If you can’t see anymore damage happening it might be because the insect has matured and flown away leaving eggs inside the leaf. Just use leaves that are new and undamaged for now.

In this article I’ve attached there is a paragraph on how to find the insect…if it is still possible. Putting a few leaves in a ziploc so that when larvae are matured they can’t fly away. Those tiny flies WERE probably the adult leaf miners! Are these basil plants planted in garden soil or sterilized potting soil?? And the article talks about using NEEM. They said it was NATURAL. Hey ‘Natural’ can be VERY TOXIC. I’ve used NEEM and it works very well on ornamentals but I wouldn’t use it on my basil!! Heck, take nicotine! Very natural, but it has got one of the most toxic LD50 (lethal dose for half the population so lower the number, higher the lethality)!! When did you first see this damage? Using garden soil in pots is a possible reason your plant got this insect. Big no-no! Always use sterilized potting soil (infused with bacteria and mycorrhizae) to plant anything in pots! Basil is an annual and does have a very short useable life. Next batch scrub those pots and use potting soil. No rocks or gravel in the bottom…actually inhibits drainage!! I am just trying to cover bases by making lots of assumptions!! Let us know what you find and send a few more pictures!!

White Spots on Basil Leaves

Basil is either an annual or short-lived perennial flowering plant native to many parts of Europe and Asia. It produces many leaf colors, from dark to light green and from pale lavender to almost black-purple. White spots on the leaf may be benign, or may signal a cultural problem.

White Insects

Colonies of tiny insects or even individual insects on basil leaves give them a spotty white appearance. Thrips are tiny insects the size and shape of a comma, and they’re usually found individually on the leaves, often folding the basil leaves over themselves for protection from predators. Mites leave white webbing on the undersurface of the leaves. Aphid juveniles are often white, and may gather in colonies. Whiteflies congregate around the leaves, and fly up in a cloud when the plant is disturbed. Mealy bugs are white, cottony insects that congregate on the leaves and stems. All of these insects suck the juices from basil leaves, and most can be controlled by drenching the plant in insecticidal soap.

White Fungus

Powdery mildew is a common problem with basil planted in areas with low light, low circulation and cool evening temperatures. It starts with patches or circles of white to gray powdery substance on the leaves, and can spread so that the leaves look as though they’re completely covered with white dust. Oil sprays and copper fungicides work well for heavy powdery mildew infestation.

Natural Variegation

Some cultivars of basil, such as “Pesto Perpetuo” basil (Ocimum x citriodorum) have natural variegation, or spots of white on the otherwise green leaves. In these cases, the white should be on just about all of the leaves, and ought to follow a pattern. If the white areas all start to discolor or disfiguration of the leaf accompanies them, it is likely not variegation, but a disease or insect problem.

Considerations

Prevent most problems with basil, including both insect and fungal problems, with good cultural practices. Basil prefers light soil high in organic matter with good drainage, and plenty of ventilation and sunshine to discourage fungi and sheltered spots for insects. Adding fertilizers causes bursts of succulent or leafy growth that attracts both insects and fungi to come and feed on the new, relatively unprotected leaves.

Plant Problems: Basil Diseases

Disclosure: Some links in this post are affiliate links, so, at no cost to you, I may earn a commission if you click through one and make a purchase. This in no way influences my opinions or recommendations.

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.

Source: Flickr User Scot Nelson

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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Basil

Basil (Ocimum basilicum) is one of the most popular and easy to grow garden herbs. There are many varieties and related species that make up the group of annual basils, but all are warm-season, sun-loving herbs. Basil originally came from India, but found its way into popular Italian tomato and Thai recipes. This relative of mint is often used in tomato sauces, pesto sauce, and as a flavoring agent for oils, vinegars and teas.

‘Genovese’ basil (Ocimum basilicum)
Joey Williamson, ©2009 HGIC, Clemson University

Culture

To grow any cultivar of basil, choose a sunny site in the garden. Although basil prefers full sun, it grows reasonably well in part-sun (six hours of sun per day). Have the soil tested, and till or mix into the soil the recommended amount of dolomitic limestone to bring the soil pH to between 6.0 and 6.5. This is also the best pH range for most vegetables. It is best to apply the lime during the fall to allow adequate time for the lime to reduce the soil acidity and to provide the necessary calcium and magnesium for the plants.

Mix organic matter, such as compost, manure or composted pine bark, into the garden soil before planting, as basil grows best in a rich, well-drained soil. The addition of organic matter will improve the ability of sandy soils to hold water, and aid in water drainage within clayey soils. It also greatly improves the ability of the soil to hold onto fertilizers.

Seed are typically sown indoors four to six weeks ahead of the outdoor planting time. Plant basil seeds ¼-inch deep. Germination should take 5 to 10 days at 70 ºF. Once the seedlings have 2 sets of true leaves, individual plants should be potted into small containers for faster growth. For more information, see HGIC 1259, Starting Seeds Indoors.

Basil should not be transplanted into the garden until the daytime temperature is consistently in the 70s, and the nighttime temperatures are above 50 ºF. If a late frost does threaten, the plants can be covered with pine straw, inverted pots or buckets until the temperature rises the next day.

Basil plants should not be over-fertilized because the flavor in the foliage will be reduced during rapid plant growth. A soil test is always recommended before planting a garden to be sure the correct amount of nutrients is added. In the absence of a soil test, evenly spread 3 pounds (3 pints) of 5-10-10 per 100 square feet of garden before planting and mix the fertilizer into the soil. With a spacing of 2 feet between plants and 3 feet between rows, a 100 square foot garden space has room for 16 plants.

Approximately two months after planting, the basil may be fertilized again if plant growth is reduced and foliage color has become paler green. Evenly spread ¼ to ½ pound of calcium nitrate over the 100 square foot mulched area and water in the fertilizer. Calcium nitrate is the typical side dressing fertilizer used with vegetable crops and is available at most farm supply stores.

Although basil plants need well-drained soil, they still need a relatively constant supply of soil moisture. Mulch the plants with pine needles or ground up leaves to a depth of 3 inches. This will help maintain a more constant soil moisture level, keep the foliage clean from rain-splashed soil and reduce the occurrence of weeds. It is best to water by soaker hoses, drip irrigation or by hand at the base of each plant. Do not wet the foliage, as it will be more prone to disease.

The terminal growth of basil plants may be pinched out periodically to encourage branching. This will create thicker plants with more tender foliage for harvest. By mid-summer, many cultivars of basil will begin producing flower spikes. For the most flavorful foliage, flower buds should be pinched out as soon as they appear. However, some cultivars have more colorful flowers of purple or pink, and these may be left if the plants are grown more as an ornamental. Some flower spikes may be allowed to develop if seed is to be collected.

If basil seed is to be saved from the current crop and remain true to type, the different cultivars must be separated by 150 feet to prevent cross-pollination. The seeds are formed in seed capsules, which can be easily harvested and separated by hand. Seed can be stored in vials, old film canisters or baggies. If well-sealed and stored in a cool, dry room, the seed will remain viable for several years. A couple of basil diseases can be seed-borne, that is, be present in the seed to affect the next year’s crop. If the current crop had Fusarium wilt or bacterial leaf spot, the seed should not be saved.

Before the first fall frost kills the basil, plants can be cut back for harvest, and the root systems dug up and potted in containers for continued growth indoors. Put plants in a sunny window. Water the plants regularly, but do not over-water, and do not let the plants stand with water in their saucers.

Harvesting & Storage

Basil leaves may be harvested individually or the ends of branches can be pinched off with one or two sets of leaves. Leave enough foliage on each plant after harvest so that plant growth is not significantly reduced. If larger quantities of basil are anticipated for the season, choose cultivars with larger leaves for a less time-consuming harvest. Harvested basil leaves do not store long, even under refrigeration.

Alternatively, the foliage may be air-dried or dried in a food dehydrator at less than 125 ºF until leaves are dry. Also, small bundles of cut stems with foliage may be hung upside down in a warm, well-ventilated room to dry. Crumble the completely-dried leaves and store in small, air-tight containers for up to 12 months. See HGIC 3086, Drying Herbs, Seeds & Nuts for more information.

Cultivars

‘Genovese’ is a vigorous, large leaf basil cultivar with a sweet, spicy taste that is most commonly used for making pesto. Plants typically will grow to 3 to 4 feet tall and should be spaced at 2 feet apart.

‘Pesto Perpetuo’ is a variegated-leaf basil with small leaves edged in creamy white. This is one of the few basil cultivars, which does not produce flower spikes. ‘Pesto Perpetuo’ is a very attractive plant and would make a beautiful addition to any annual and perennial flower planting. Plants will grow to approximately 16 inches tall.

‘Pesto Perpetuo’ basil (Ocimum x citriodorum)
Joey Williamson, HGIC, Clemson University

‘Purple Ruffles’ basil has dark purple leaves and a mature plant height of 18 inches. It was an All-American Selections winner in 1987, and offers an interesting ruffled foliage texture for the garden. With its purple color, Purple Ruffles basil has additional uses as an attractive garnish and in herb vinegar. The large, purple foliage also makes a nice contrast with green-leaf cultivars in the garden.

‘Serata’ basil has unique green ruffled leaves and grows 22 to 24 inches tall. The leaves are large enough for easy harvest, and the flavor is very good for pesto. ‘Serata’ basil does not make many flower spikes. ‘Green Ruffles’ basil is very similar to ‘Serata’, has serrated and quilted leaves, and also grows to 24 inches tall.

‘Serata’ basil (Ocimum basilicum)
Joey Williamson, HGIC, Clemson University

‘Siam Queen’ basil is a 1997 All-America Selections winner with licorice fragrance and flavor. The leaves are large enough for easy harvest. ‘Siam Queen’ basil is often used in salads, sauces and in Thai cooking. Mature plants will reach 24 inches tall with purple flower spikes.

‘Siam Queen’ basil (Ocimum basilicum)
Karen Russ, HGIC, Clemson University

‘Dark Opal Purple’ basil was a 1962 All-America Selections winner and is a purple-leaf cultivar that reaches 24 inches in height. Its purple leaves also make an attractive garnish, can be used in salads or to make tangy, purple vinegar. The cultivar ‘Rubin’ was released in 1993 and is very similar to ‘Dark Opal’, but has more consistent purple coloration.

‘Sweet Dani’ basil has aromatic lemon-scented leaves and is excellent for drying. This 1998 All-American Selections winner is a slender plant with narrow leaves and grows to 26 inches tall.

‘Newton’ is a more recent introduction with intermediate resistance to Fusarium wilt. It is an Italian type basil with 4-inch long leaves.

‘Prospera®’ is a new release with strong resistance to downy mildew, as well as Fusarium wilt resistance. The leaves are 3-inches long, glossy, and dark green.

Many other interesting basil cultivars are available. Some have fragrances of lemon, lime, cinnamon, licorice or anise. There are dwarf cultivars, such as ‘Spicy Globe’, that make nice ornamentals for containers or edging flower beds. A few are not used in cooking, but have unique, sweet fragrances, such as ‘Holy Basil’ and ‘African Blue’ basil. Basil seeds are readily available from many mail-order companies, with several selling organically-grown seed.

Diseases

Fusarium Wilt: Fusarium wilt of basil is caused by a soil-borne fungus (Fusarium oxysporum f. sp. basilicum). The fungus attacks the water-conducting tissue (xylem) within the stem. Infected basil plants will grow normally until they are six to twelve inches tall, then become stunted and exhibit browning of terminal growth. Once water uptake is totally blocked, the basil plants will suddenly wilt. Promptly remove and dispose of any wilt-infected plants.

Fusarium wilt of basil (Fusarium oxysporum f. sp. basilicum)
Debbie Roos, NCSU Agricultural Extension Agent, Chatham County, NC

The Fusarium wilt pathogen can survive for many years in the soil; therefore plant resistant basil varieties. Three Genovese-type cultivars of basil have been selected for resistance to Fusarium wilt. The first cultivar released was ‘Nufar’, which grows to 24 inches tall and has medium-sized leaves with mild flavor. Later, ‘Aroma 1’ and ‘Aroma 2’ were released and both have very good fragrance. ‘Newfar’, ‘Newton’, ‘Elidia’, and ‘Prospera’ are newer releases with stronger resistance to Fusarium wilt. These cultivars are readily available from mail-order seed companies.

Gray Mold: Basil is an herbaceous plant that is highly susceptible to gray mold (caused by Botrytis cinerea). Infections usually begin on the surface of stem cuts that are made during harvest. The gray mold pathogen then moves down the stem, and kills the leaves and secondary leaf buds. Infected leaves will fall off. If the infection reaches the main stem, the entire basil plant will die.

Gray mold outbreaks often occur when individual leaves and stem ends with foliage are harvested during rainy days or when watered by over-head irrigation before the wounds can heal. The wounded stems are very susceptible to infection soon after harvest. However, by 24 hours after harvest, the stems are much less susceptible to infection. At 48 hours after harvest, the cut stems have completely healed and are no longer susceptible to gray mold. To reduce the chance of gray mold or other foliar disease, do not wet the foliage with irrigation water, and do not harvest in wet weather. Pick up and dispose of any diseased plant tissue on the ground around the plants, and cut out and dispose of any infected plant parts to reduce disease spread.

Bacterial Wilt: Bacterial wilt (caused by Ralstonia solanacearum) has been commonly diagnosed on tomatoes grown in South Carolina. Basil is also susceptible to this soil-borne disease; therefore do not plant basil in garden sites where tomatoes have died from bacterial wilt. The bacteria invade the root system and multiply rapidly inside the water-conducting tissue of the lower stem, filling it with slime. The plants rapidly wilt, while the leaves stay green. This bacterium will remain in the soil for several years after disease has occurred. Since there are no resistant varieties of basil, rotate the location within the garden where the basil is grown. Infected plants and the soil immediately around each plant should be removed and disposed of promptly.

Leaf Spots: Three fungal leaf spots (caused by Cercospora spp., Alternaria spp., and Colletotrichum spp.) occur on basil in South Carolina. To reduce the incidence of foliar disease, always water at the base of the plants and never wet the leaves with sprinklers. Pull off and dispose of diseased foliage at first sign of disease. For minor fungal foliar disease occurrence, basil plants may be sprayed weekly with a fungicide containing potassium bicarbonate, such as Bonide Remedy, Monterey Bi-Carb Old Fashioned Fungicide, or GreenCure Organic Fungicide.

Basil with Cercospora leaf spot (Cercospora sp.)
Margaret Williamson, Plant Disease Diagnostician, Plant Problem Clinic, Clemson University

Downy Mildew: Mid-summer conditions were conducive for a severe outbreak of downy mildew (caused by Peronospora belbahrii) in South Carolina during 2014. Basil downy mildew is spread by wind-dispersed spores and by infected seed, and can quickly devastate a home garden planting of basil.

Downy mildew on basil foliage with initial symptoms of pale yellowing and small necrotic (dead) spots. Joey Williamson, ©2014 HGIC, Clemson Extension

Initially, the symptoms of downy mildew on basil will be a slight yellowing of the foliage, with a spread that is limited by the major leaf veins. At first, the yellowing of the foliage looks similar to a nutritional deficiency. Soon, brown spots begin to develop within the yellow leaf tissue, which continue to cover the leaf surface. Sporulation occurs on the lower leaf surfaces that correspond to the yellowed areas above. The spores are purplish-gray and are easily wind-disseminated. Soon the diseased foliage will drop from the plant.

Downy mildew rapidly spreads during mild, humid summers. The key to cultural control is to reduce leaf wetness and promote good air circulation around the plants. Water basil with drip irrigation or soaker hoses to reduce plant wetness. Plant basil in a very sunny site and space the plants far enough apart to promote rapid leaf drying from dew and after rainfall. If disease occurs, prompt removal and disposal of plants showing symptoms along with any fallen leaves may slow down the spread of this disease. The cultivars ‘Prospera’, ‘Rutgers’ Devotion’, and ‘Rutgers Obsession’ are new releases with strong resistance to downy mildew. Rotate where basil is planted in the garden each year. Downy mildew can be controlled by sprays every 7 to 10 days during warm, wet weather with products containing the biocontrol bacterium Bacillus subtilis QST-713 (such as, Serenade Garden Disease Control Concentrate or Ready To Use, from Planet Natural) or potassium salts of phosphorous acid (such as, Monterey Garden PHOS or Organocide Plant Doctor). Do not apply to basil plants if they are under heat or drought stress.

Root Rots: Two root rots (caused by Pythium spp. and Rhizoctonia spp.) were diagnosed on basil in South Carolina. Both Pythium and Rhizoctonia root rots will cause a yellowing and unthriftiness of basil plants. Branch dieback will occur starting at the tips. With Pythium root rot, a sloughing off of the outer layer of the roots will be noticed. With Rhizoctonia root rot, brown lesions on roots are first observed, and roots will eventually turn completely brown.

To reduce the occurrence of root rot, provide excellent soil drainage by the addition of compost or composted pine bark to improve the drainage of heavy clay soils. However, basil must be watered often enough so the plants do not wilt. A layer of mulch in the garden will reduce the need to water as often. Diseased plants and soil immediately around each plant should be removed and disposed of promptly. To reduce the chance of root rot, rotate the position of basil in the garden each year.

Insect Pests

The most common pests of basil are Japanese beetles, slugs and aphids. Japanese beetles are usually present for about a month in the summer. They skeletonize the foliage (i.e., eat the leaf blades, but do not consume the larger veins of the leaves). The result is a lace-like appearance to the foliage. Japanese beetles can be hand-picked from the plants and crushed or dropped into soapy water.

Aphids, spider mites, whiteflies and other soft-bodied pests can be controlled with an insecticidal soap, such as:

  • Bonide Insecticidal Soap Concentrate
  • Natural Guard Insecticidal Soap Concentrate
  • Safer Insect Killing Soap Concentrate
  • Espoma Earth-tone Insecticidal Soap Concentrate
  • Garden Safe Insecticidal Soap Insect Killer Concentrate

Always spray plants in the early evening to reduce the chance of damage to the foliage from the insecticidal soap. Many of these pests will be located on the lower leaf surface and must be directly contacted with the spray, so pay special attention to these areas when spraying.

Alternatively, basil may be sprayed with azadiractin, which is a natural product extracted from Neem trees. Azadiractin products (such as Align, Azatin, Neemex, Ornazin, or Gordon’s Azatrol) will control numerous insect pests on basil, including aphids, beetles, thrips, spider mites and whiteflies.

Slugs climb up on basil plants and eat ragged holes in the foliage. Mulch, although quite beneficial to plants, provides a hiding place for slugs during the daytime. Protect basil plants by sprinkling diatomaceous earth over the mulch around the plants. Diatomaceous earth is very sharp and scratches the skin of the soft-bodied slugs, resulting in their dehydration and death. It must be reapplied after a rain or watering.

Newer, safer slug baits are now available. These products contain iron phosphate, such as in:

  • Monterey Sluggo – Kills Slugs & Snails
  • Garden Safe Slug and Snail Bait
  • Gardens Alive Escar-Go (Slug & Snail Control)
  • Garden Safe Slug & Snail Bait
  • Bonide Slug Magic Pellets – Makes Slugs Disappear
  • Whitney Farms Slug & Snail Killer
  • Bonide Bug & Slug Killer (also contains spinosad)
  • Gardens Alive Garden Pest Bait (also contains spinosad)
  • Brandt Sluggo Plus T & O (also contains spinosad)
  • Monterey Sluggo Plus (also contains spinosad)

Iron phosphate will stop feeding by slugs quickly and is much less harmful to pets, birds, and non-target insects than the older product, metaldehyde. Spinosad is natural insecticide that will also kill cutworms, ants, and earwigs.

Basil-Walnut Pesto Recipe

For a delicious use of fresh basil, please consider this basil-walnut pesto recipe.

Pesto ingredients:

2 peeled, medium-sized garlic cloves

½ cup walnut pieces

1 cup Parmesan cheese

4 cups packed basil leaves

⅓ cup extra-virgin olive oil

In a food processor, briefly mince two garlic cloves. Add ½ cup walnuts and process until thoroughly ground. Add 1 cup Parmesan cheese and briefly mix with garlic and walnuts. Add 2 cups basil leaves and half of the ⅓ cup olive oil. Pulse the food processor until leaves are ground. Add the remaining 2 cups of basil leaves and remaining olive oil. Again, pulse leaves until ground. Cook ½ pound pasta and drain well. Return the drained pasta to the cooking pot. Add half of pesto, and mix thoroughly with the pasta. Pesto tends to adhere best to either angel hair or rotini pasta. Serve hot.

Scrape the remaining half of pesto from the food processor into a quart-size freezer bag, label and date, and immediately freeze. Pesto freezes well without loss of flavor or color. To use frozen pesto, thaw in refrigerator or defrost in microwave. Don’t allow pesto to overheat in microwave, as it should not cook. Once pesto is warm, spoon onto hot pasta, mix thoroughly and serve.

The leftover pesto should be dated and immediately refrigerated. It is best to use pesto within 3 days. Pesto, or other herbs in oil, should not be canned because of the risk of botulism (please see HGIC 3051, Most Frequently Asked Canning Questions).

Basil is one of the easiest herbs to grow at home and is far less susceptible to pests and disease than some other herbs. That said, problems can always arise and it’s best to diagnose them right away in order to treat them before they get out of hand. Here’s a list of some common problems when growing basil.

DISEASES

Damping-Off
Damping-off is a name given to a condition that attacks the seeds or the roots of young plants during wet, cool conditions. Seedlings may look healthy and then suddenly topple over and die. This is caused by a number of different fungal agents, but if your basil is planted in soil with poor drainage and fertility, it is all the more susceptible. Do not over water. Basil can’t stand sitting in damp conditions. Sterilizing containers before planting basil is also a must, since diseases can hang around for a while even after the previous plant is long gone.

Root Rot
Usually caused by poor drainage and over watering, root rot is another type of fungal disease that will stunt your plants and eventually kill them. They may lose color and begin to die. Roots may appear dark and slimy. Lay off watering. If your soil is poor and your basil does not recover, you may need to replant.

Leaf Spot Disease
Leaf spot is caused by a bacterial infection that causes spots on the leaves and streaks on the stems. Remove any infected leaves as soon as you see them so this disease does not spread. Overhead watering and damp foliage can contribute to leaf spot. Water at the base of the plant and try to water in the morning so the foliage has adequate time to dry throughout the day.

Fusarium Wilt
Sweet basil is most susceptible to fusarium wilt, which causes stunted plants, wilted yellow leaves, twisted stems, brown spots and streaks on the stems, and the leaves to drop. This fungus grows in the soil and can be transmitted through infected leaves. Destroy infected plants and avoid planting in that area for 3 years.

Downy Mildew
Downy mildew is any number of microbes that can attack basil plants. Look for yellow leaves with fuzzy, grey growth on the underside. Good air circulation and keeping the foliage dry is the best way to avoid downy mildew. Remove infected leaves and stems to prevent it from spreading.

PESTS

Root Knot Nematodes
Nematodes are worm-like creatures that live in the soil and cause your basil to wilt and turn yellow. The roots may be swollen and disfigured when nematodes are present. Till your soil deeply to disturb any possible nematode presence but crop rotation is the best way to avoid them. Otherwise, try growing basil in pots and replanting every year.

Aphids
Aphids suck plant juice and leave behind a sticky substance called honeydew. They can be found on the undersides of the leaves. They are usually semi-green or yellow (but can be red, black or other colors too), with almost transparent bodies. Blast them off with water. If let be they can do serious damage to your basil plant.

Flea Beetles
Flea beetles chew tiny holes in the basil leaves. They can usually be blasted off with water. Spray them with insecticidal soap if that doesn’t do the trick.

Whiteflies
When you shake your basil plant and a ‘cloud’ of insects rise up and then settle back down, you have whiteflies. Much like aphids, they suck the life out of plants if left unhindered. Spray them with water or insecticidal soap.

Snails and Slugs
Chewed leaves accompanied by silver streaks is likely the work of snails or slugs. Pick them off if you see them. Use beer traps or copper fencing if they are a real problem.

OTHER PROBLEMS


Nitrogen Deficiency
Wilting, yellow leaves and poor growth may be a symptom of nitrogen and other nutrient deficiency. Feed your plants with a liquid organic fertilizer that is high in nitrogen to reverse the problem. Disease can also cause these symptoms. So if fertilizing doesn’t correct it, look to nematodes or another problem listed above.

PREVENTATIVE CARE
Always plant basil in fertile soil that drains really well.

Water wisely. Poor drainage combined with too much water is one of the fastest ways to see fungus and other diseases settle in your basil.

Harvest regularly. Even if you aren’t using all that much basil, picking leaves often will encourage new growth and keep air circulation good.

Feed or replant container basil to ensure they have all the nutrients they need.

Do not plant too early. Basil does well indoors, so avoid putting it outside before it’s time.

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