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Black Spot Fungus: Getting Rid Of Black Leaf Spot

Black Spot Fungus: Getting Rid Of Black Leaf Spot


By: Jackie Rhoades

You’re strolling through your garden enjoying the lush growth the spring rains have produced. You stop to admire one particular specimen and you notice black spots on plant leaves. Closer inspection shows black spots on leaves throughout a whole section of your garden. This can’t be! You don’t have any roses. Unfortunately, you don’t need them. Your garden has been infected with black spot fungus.

What is Black Spot Fungus?

Don’t let the name fool you. Diplocarpon rosae, or black spot fungus, isn’t just a disease of roses. It can attack any plant with fleshy leaves and stems if the conditions are right. You’ve already taken the first step in treating black leaf spot. You’ve been inspecting your garden on a regular basis and you’ve caught it early.

Black spot fungus begins to develop in the spring when temperatures reach into the sixties and the garden has been continuously wet for six to nine hours. By the time temperatures reach into the seventies, the disease is running rampant and won’t slow down until the daytime temperatures rise above 85 F. (29 C.). It starts with tiny black spots on leaves, no bigger than a pinhead. As the fungus develops, those black spots on leaves are ringed with yellow. Soon the entire leaf turns yellow and falls.

Treating Black Leaf Spot Fungus

Getting rid of black leaf spot must be a two-pronged attack. Because its spores travel on the wind and plash from leaf to leaf during watering, treating black leaf spot should be first on your agenda.

There are several good fungicides on the market, several of which claim to be organic. They come in handy bottle sprayers, but if your garden is large, you might want to buy it as a concentrate to mix in your tank sprayer.

Neem oil is another alternative for treating black leaf spot. It’s an oil pressed from an evergreen tree. It’s all natural and has shown some remarkable results as an effective garden fungicide.

For those of you who prefer Grandma’s solutions to garden problems, try this: Mix one heaping tablespoon of bicarbonate of soda (baking soda) into a gallon of water for your sprayer. Add a dash of horticultural oil or horticultural soap and Voila! You have a method of treating black leaf spot that works by changing the pH on the leaf surface to one the fungus can’t survive. The oil or soap makes the solution stick and the cost is around four cents a gallon.

The next step in getting rid of black leaf spot is prevention and maintenance. The first, we already talked about. Inspect your garden regularly in the spring. Black spots on plant tissues will spread quickly. Start preventative spraying before the temperatures hit sixty. Read the label directions for the method you choose and follow it closely. For Grandma’s recipe, a light weekly dose should be sufficient. Continue spraying until temperatures are hot enough to get rid of black spot fungus without it.

Avoid watering your plants on cloudy days. Bright sun and good air circulation are essential for getting rid of black leaf spot.

During an outbreak, all affected debris should be disposed of. It may not be ideal as far as looks go, but affected plants should be cut back, and in the fall every bit of garden debris should be thrown away or burned. The spores can overwinter on plant material, but can’t survive in bare soil.

The good news is that black spot fungus rarely kills the host plant. Getting rid of black leaf spot takes a lot of diligence, but in the end, the rewards are worth it.

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The issue with black spot is that the defoliation that occurs when the fungus is allowed to get out of control weakens the plant. And not just for the current growing season, but for the next season as well. The main function of plant foliage is to absorb energy from the sun and perform photosynthesis. If your plant loses all or most of its leaves to black spot, it will be unable to collect and store energy for the winter, as well as for the burst of growth that occurs in spring. You'll see weaker growth and reduced bloom in the season following severe black spot damage.

Black spot spores overwinter on infected foliage and canes, including infected foliage that has fallen and been left on the ground.   In spring, spores are splashed up onto newly emerging foliage during rains or irrigation. Once the weather begins to stay consistently warm and humid, the spores germinate and infect the plant within one day. Visible symptoms (black spot and some yellowing) will be evident within five days, and it will produce and spread new spores within ten days. The new spores will infect other parts of the plant, or be carried on the wind to any other nearby rose bushes.


What Does Black Spot Do?

Black spot will look like somewhat circular black spots on leaves. It usually occurs on the upper sides of leaves, but can also develop on the undersides. The outer margins of the black circles are ragged or feathery and they are usually surrounded by a ring of yellow.

Spots typically begin on the lower leaves and move upward. They can appear as early as when the leaves first unfurl. These spots can enlarge and eventually merge. Affected leaves often fall off the plants, and if left unchecked, the entire plant can defoliate.

The fungus can also infect young canes, causing dark purple or black blisters on the canes, and even the flowers may show some red spotting. Infected plants will set fewer flower buds and without leaves, the plants become stressed and susceptible to more problems.


Black spot

Black spot disease is the scourge of rose growers. Not only does it make the plant look unsightly, but it also causes the leaves to drop prematurely, resulting in weakening the plant if it occurs repeatedly.

Description

Black spot is a common fungal problem on roses but can be found on other plants too. The spots can be of various colours – grey, brown or black. The spots are in fact dead leaf tissue caused by the fungus which spreads the disease. Spots can sometimes join together to form larger areas of dead tissue. Roses can also develop smaller black spots on their stems.

Symptoms

There are a few other symptoms than the spots themselves. However, diseased plants frequently have other problems causing weaknesses, which enable the fungal disease to develop. It is worthwhile carefully checking any plant with leaf spot.

Treatment And Control

General Tips

Immediately remove and destroy all infected leaves and plant parts. Pick up any fallen foliage and destroy it, whenever it happens but especially in autumn. To reduce the chances of re-infection for the following season, prune back plants hard. Where roses were infected with black spot disease in the previous season, then start spraying with a suitable fungicide as new leaves open and repeat at fortnightly intervals.

Chemical Control

Spray with a suitable fungicide from April onwards. Continue spraying at recommended intervals throughout the summer.


How to Treat a Black Spot on Hydrangeas

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Hydrangeas, including bigleaf (Hydrangea macrophylla), smooth (Hydrangea arborescens) and panicle (Hydrangea paniculata), grow in U.S. Department of Agriculture plant hardiness zones 8 through 10, and are all subject to black spotting on their leaves in the right conditions. Black spotting can originate from a number of fungal diseases, including black spot, powdery mildew and cercospora. Treatment for black spots involves taking a cultural, organic and chemical approach to the managing the fungi that cause them.

Trim the oldest and largest stems to the base each spring and after flowering using bypass pruners. Trimming the stems increases aeration and reduces the dampness and humidity that cultivates the fungal diseases that cause black spots on leaves.

Remove all dropped, diseased leaves from the ground and discard. Although not part of the plant, fungal spores can become airborne and reinfect the hydrangeas and other plants in their vicinity.

Apply 2 inches of fresh compost to the soil to smother fungal spores and to aid in disease prevention. Although you can use bark mulch or plant and grass trimmings, compost wards off the spread of fungus better.

Remove all diseased, dry and dying or dead branches from the hydrangeas with bypass pruners as needed, particularly in spring.

Fill a spray bottle with 1 pint of tepid water and 1/2 tablespoon neem oil and mix well. Spray the stems and tops and bottoms of the leaves of the hydrangeas and discard the solution, as it breaks down within hours after mixing. Neem oil treats black spot and a host of other fungal diseases, such as cercospora. Spray it on the hydrangeas every seven days.

Mix 1 gallon of water with 6 teaspoons of fungicide containing 48 percent copper salts of fatty and rosin acids as the active ingredient. Spray the leaves and stems of the hydrangeas every seven to 10 days depending on severity of the spotting. Copper-based fungicides treat leaf spots and powdery mildew, which causes dark spots to appear on the leaves along with grayish white powder.


Treat A Black Spot On Hydrangeas

Trim the oldest and largest stems to the base each spring and after flowering using bypass pruners. Remove all dropped, diseased leaves from the ground and discard. Apply 2 inches of fresh compost to the soil to smother fungal spores and to aid in disease prevention. Fill a spray bottle with 1 pint of tepid water and 1/2 tablespoon neem oil and mix well. Spray the stems and tops and bottoms of the leaves of the hydrangeas and discard the solution, as it breaks down within hours after mixing. Spray it on the hydrangeas every seven days. Copper-based fungicides treat leaf spots and powdery mildew, which causes dark spots to appear on the leaves along with grayish white powder.

  • Lay down 2 inches of fresh compost around the base of the plant at the beginning of each season.
  • Spray the stems and tops and bottoms of the leaves of the hydrangeas and discard the solution, as it breaks down within hours after mixing.

Based in the American Southwest, Bridget Kelly has been writing about gardening and real estate since 2005. Her articles have appeared at Trulia.com, SFGate.com, GardenGuides.com, RE/MAX.com, MarketLeader.com, RealEstate.com, USAToday.com and in "Chicago Agent" magazine, to name a few. She holds a Bachelor of Arts in English with a concentration in creative writing.

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How to Treat a Quaking Aspen with Leaf Spots

The poor quaking aspen has so many disease problems it's a wonder there are any around to grow in cultivation. With thin bark, the slightest encounter with a lawn mower or weed-eater or even a speck of a hole from an insect can lead to life-threatening fungal diseases. One of the most serious foliar diseases of the quaking aspen is black leaf spot, caused by a Marssonina, a fungus. Symptoms include dark spots on the foliage in spring that eventually grow together to form large dead areas.


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