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5 Causes of Brown Spots on Gardenia Leaves (With Treatment)

Brown spots on gardenias are one of the hardest things for a gardener to deal with out of all the problems they can have.

There are several reasons their leaves have these ugly spots, which I’ll talk about today.

The bacterial and fungal leaf spot diseases Rhizoctonia and Cercospora are common culprits in developing brown spots on gardenia leaves.

Cut off diseased branches, spray with a fungicide, and make sure your plant has adequate water and ventilation.

Low soil acidity, pests, iron chlorosis, and too much fertilizer can also cause brown leaf spots.

What Portions of The Plant Are Infected? 

  • Leaves: Older leaves and those closest to the ground are particularly prone to brown spots. Typically, brown spots on leaves are caused by a leaf spot disease caused by fungi or bacteria.
  • Flowers & Buds: Brown spots can spread quickly; eventually, they may cover all of your gardenia’s blossoms and buds in discoloration. Sometimes these spots join together to form larger brown areas that spread to the stems. Bugs like bud mites that cause brown spots on gardenias to like to eat the flowers.
  • Stems: If the brown spots result from a soil-based problem, like fungus gnats, root rot, or an incorrect soil pH, the branches may be affected as the rotting spreads upward from the roots. The base of the stem is where softening, decay, and browning of outer tissue first appear.
  • Roots: Brown spots may be caused by root rot, poor drainage, overwatering, excess fertilizer, poor water quality, or other issues that affect roots. If you look closer, you might notice some mushy, brown, or black, slimy roots that have a sour or putrid odor.

Why Are There Spots on My Gardenia Leaves?

[1] Low Soil Acidity Levels

Gardenias are acid-loving bloomers, meaning they’ll struggle and show signs of distress on alkaline soil but thrive in the slightly acidic range of 5.0 to 6.0.

Soil pH above 7.0 prevents gardenias from efficiently absorbing vital nutrients like iron. This will cause severe iron deficiency in your plant, manifesting as iron chlorosis. (Source)

Iron deficiency can be easily identified by the telltale symptoms of brown spots on the plant’s foliage.

Remember that chlorophyll, the green pigment responsible for the evergreen appearance of gardenia leaves, is produced only when the iron is present.

It’s also essential in photosynthesis, which helps to facilitate.

Yellowing or bluish-green foliage is another sign of iron deficiency caused by low soil acidity.

This condition is called chlorosis, defined by the loss of green pigmentation.

Young plants are not uncommon to completely lose their green coloration, leaving only the veins and the tissue along the edges.

In some cases, older leaves may only turn yellow at the edges. As a result, the growth of the entire gardenia plant will be stunted.

Solution

You must first test the pH of your soil. A good soil pH test kit is cheap and a must-have in your gardening toolbox (Check the latest price on Amazon here).

Before you plant or pot your gardenia, check the soil pH to ensure it falls between 5.0 and 6.0.

Then, mix some aluminum sulfate, peat moss, organic compost, or conifer bark chips into a fresh potting mix before you plant or pot your gardenia.

However, there are several ways to adjust the soil’s acidity if iron deficiency is the problem:

  • Mulch your gardenia with a layer of organic compost or mulch two to three inches thick. This will ensure that the roots are not disturbed.
  • Feed your gardenia a fertilizer explicitly designed for acid-loving houseplants (Check the latest price on Amazon here). Use only once, at the start of the spring and summer seasons. It is best to follow the label instructions for strength and feeding amount.
  • You can also fertilize your plant with fish emulsion or blood meal.
  • As a last resort, use chelated iron to acidify your gardenia soil. Only apply once a year. Follow the manufacturer’s instructions as you would with any fertilizer.

[2] Rhizoctonia Leaf Infection

Rhizoctonia Leaf Infection is a fungal leaf spot disease that causes unsightly brown lesions on the foliage of your gardenia.

The pathogen responsible for the fungal infection is usually Rhizoctonia solani, but other Rhizoctonia fungi can also cause it.

Prolonged leaf wetness caused by water splashing, overhead irrigation, and overwatering favors the fungal disease.

In addition, excess humidity and poor air circulation around your gardenia (due to overcrowding) may aggravate the situation.

Warm temperatures of 70-90°F (12-32 °C), especially when combined with continuous leaf wetness, promote the spread of the fungal infection.

Symptoms of Rhizoctonia Leaf Infection

Spots of dark tan to reddish brown appear on the leaves as an early indicator. They can reach a diameter of a quarter of an inch and are roughly circular with zones.

If your gardenia is poorly aerated and overwatered, the disease will likely strike the oldest and lowest leaves first and then spread upward.

These brown spots may grow and join together to form larger ones. Whole leaves may become discolored and die off if the disease progresses.

Root rot can also be caused by the Rhizoctonia fungus, which is worth noting. Brown leaf spots are just one of many visible symptoms of fungal rot disease.

Symptoms of Rhizoctonia root rot in gardenias include widespread leaf yellowing, wilting, and browning.

In addition, Overwatering, improper drainage, and waterlogged soil frequently amplify the fungal disease. As a result, the affected roots will be dark brown or black in color and soft, smelly, and slimy.

Control and Management of Rhizoctonia Leaf Disease

To prevent the spread of Rhizoctonia leaf spot disease, any infected houseplants must be placed in quarantine.

If your gardenia has any infected leaves or other parts, please remove and dispose of them immediately. After each use, disinfect the knife with bleach or rubbing alcohol.

When you plant or move your gardenias, use a sterile potting mix.

Fungicide sprays and biological microorganisms like the bacillus bacterium can help eliminate or prevent Rhizoctonia fungus. Good cultural and preventative practices, such as:

  • Getting cuttings from disease-free, healthy gardenias for propagation.
  • Avoiding wetting leaves when watering (so, overhead watering is a big no)
  • Avoiding overwatering.
  • Improving drainage – Change to a well-draining potting soil and well-drained pot
  • Never reusing old soil.
  • Ensuring ample air circulation around your gardenias.
  • Watering your gardenias early in the morning.

[3] Bacterial Leaf Spot

Bacterium Xanthomonas campestris causes bacterial leaf spots in most gardenia varieties and cultivars, including G. jasminoides ‘Aimee’ and G. jasminoides ‘Fortuniana.’

Warmer temperatures in the 72-82°F (22-28°C) range and allowing foliage to stay wet for extended periods favor bacterial infection of the leaves.

Symptoms

 Bacterial leaf spot is a highly contagious disease that eventually affects gardenia leaves’ vitality, appearance, and vigor.

Small brown water-soaked spots on leaves (beginning with old foliage) and blossoms are the primary symptoms.

The leafage that is affected wilts turns yellow, eventually falling off the plant.

The brown spots grow larger before turning black as the bacterial infection progresses.

It may spread to the stems in severe cases, causing cankers and softening the branch. A yellow halo surrounds brown spots on the leaves.

Symptoms of bacterial leaf infection may also appear on the leaf margins.

As a result, the leaves turn yellowish-brown along the edges, where the tissue dries and breaks off, and the affected foliage appears papery.

Control and Management of Bacterial Leaf Spot of Gardenia

Remove and dispose of infected foliage as soon as possible to prevent bacterial pathogens from spreading to neighboring leaves.

There are no known chemical treatments for gardenia bacterial leaf spots. However, using a copper spray early in the disease’s lifecycle can be beneficial.

Reduce leaf wetness by:

  • Avoiding overhead watering.
  • Not splashing water onto the foliage.
  • Not handling leaves with wet hands.
  • Avoiding overwatering.
  • Improving aeration around your gardenia.
  • Watering your plant early in the morning.

For the best outcome, report your gardenia in a fresh batch of sterilized potting mix and a new sterilized pot.

[4] Cercospora Leaf Spot

Cercospora leaf spot is a fungus that attacks various ornamental plants, including azaleas and gardenias. Cercospora spp. Fungus is frequently responsible for the disease in the latter.

The fungus reproduces and spreads more quickly on wet leaves and in warm temperatures ranging from 68 to 95 degrees Fahrenheit (20 to 35 degrees Celsius).

As a result, disease symptoms are more likely to appear in the early fall and late summer.

Overwatering, overcrowding, and poor drainage are all factors.

Symptoms Cercospora leaf spot

Cercospora fungus is distinguished by the appearance of dark brown spots with a yellow halo on the surfaces of the leaves.

The disease also causes yellowing and dropping of leaves. As the disease progresses, the leaf spots may turn black.

Control and Management of Gardenia Cercospora Leaf Spot

As with any fungal disease, removing infected leaves and other plant parts is critical as soon as possible.

First, of course, you must immediately isolate your infected gardenias.

Antifungal sprays every week are one of the chemical controls for the Cercospora leaf spots. 

I prefer copper-based fungicides (Amazon link), but thiophanate-methyl, myclobutanil, and chlorothalonil-containing products can also prevent and eliminate the disease.

Combine good preventive and cultural practices with chemical controls for the best chances of eradication. These are some examples:

  • Avoid wetting leaves or allowing foliage to stay wet for long periods. Irrigating your gardenias early in the morning will allow the leaves to dry.
  • Avoid using overhead irrigation and, instead, use watering from below or self-watering pots.
  • Gardenias should not be overwatered.
  • After handling infected leaves, sanitize your hands and prune the shears with rubbing alcohol or a bleach solution. This is necessary before maintaining healthy leaves and other plants.
  • Increasing air circulation around and between the leaves of your gardenia. Consider spacing your houseplants and pruning your gardenias regularly.

[5] Phytophthora Root Rot

Soil-borne fungi, such as Rhizoctonia, Pythium, or Phytophthora, are frequently responsible for root rot in gardenia plants.

However, Phytophthora root rot is the most common and often causes extensive above-ground symptoms, such as brown spots on foliage.

This rot-causing disease poses a serious threat to the health and vitality of your gardenia if it is allowed to go untreated for an extended period.

For infection, the fungus that causes rot disease prefers overly deep roots, poor soil drainage, and wet or soggy soil conditions.

Symptoms of Phytophthora

Yellowing leaves (beginning with older and lower foliage), brown spots on foliage, stunted growth, and stem dieback are common symptoms. Affected leaves typically turn yellow, brown, wilt, and fall off.

You will likely find many decayed or decaying roots if you dig up a gardenia plant with Rhizoctonia root rot.

They usually have a black or dark brown appearance and a slimy or mushy texture.

In addition, the roots may have a distinct sour or pungent odor to your nose.

The symptoms of Rhizoctonia root rot worsen until your gardenia collapses and succumbs to the disease.

Control and Management of Rhizoctonia Root Rot in Gardenias

Generally, buy, plant, or pot disease-resistant gardenia cultivars and species. For propagation, take cuttings from healthy, disease-free plants.

To prevent the spread of the disease, remove and destroy infested foliage as soon as possible.

When treating Rhizoctonia root rot, repotting is your best bet. Use a sterile potting mix that drains well and a new, well-drained pot.

How to Prevent Brown Spots on Gardenia

  • Avoid overwatering by keeping the soil moist to the touch.
  • Use drip irrigation or a self-watering system instead of overhead irrigation.
  • Remove and dispose of fallen leaves and dead plant matter.
  • Avoid splattering water on the foliage.
  • Water your gardenia first thing in the morning.
  • Make sure your gardenia has plenty of air circulation.
  • Surround your plant with mulch or compost.
  • Maintain soil pH between 5.0 and 6.0.