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Plant Answer Line Question

Keywords: Plant diseases--Control, Fungal diseases of plants, Paeonia, Plant diseases--Diagnosis

I planted some peony bulbs last year and they grew nicely until they reached about 10 inches high. One was in the ground, and the other is planted in a medium sized pot outside. The one in the ground is now dead, and the other one is not looking good. It gets dark spots on the leaves, and then the leaves die. Can you help?


Without additional details, it is difficult to say what may be wrong with your peonies. The Penn State Extension has information on different diseases that can affect peony plants. What you describe sounds somewhat like peony leaf blotch or measles, as shown in Iowa State University's Plant Pathology webpage on peony diseases. Here is an excerpt:
"Peony leaf blotch is also known as measles or stem spot. Warm, humid weather provides optimal conditions for infection by the causal fungus, Cladosporium paeoniae.
The leaf spots are glossy and purplish-brown on the upper sides of leaves. On the lower sides, spots are chestnut-brown. Infection is generally more pronounced at the margins of outer leaves. Leaves may become slightly distorted as they continue growing.
Fungal infections on young stems first appear as elongated, reddish-brown streaks. As plant growth continues, infected tissue near the crown may darken and become depressed. Stems on the upper portion of the plant may show individual, raised spots. To manage peony leaf blotch, cut the stems at ground level in the fall or early spring. Rake the area before new shoots appear. Fungicides are available to help control the disease, but must be used in combination with other management practices. Also, providing good air circulation and avoiding wetting the leaves when watering can help reduce disease severity."

There are other possibilities, including peony blight, also known as Botrytis blight. The Royal Horticultural Society discusses this problem:
"Peonies collapse at soil level and the stem bases are covered in grey mould. In a severe attack the leaves are also affected and the plant may be killed or so badly weakened it fails to sprout again next spring. Infections also occur frequently behind the flower buds just before they open.
This is a disease that affects both herbaceous and tree peonies. It is caused by a fungus (Botrytis paeoniae) related to grey mould (Botrytis cinerea), which may also attack peonies in a similar way.
Wilt is encouraged by high humidity which builds up around dense clumps of peonies. Increase the circulation of air by thinning out overcrowded shoots. Also avoid over-feeding, especially with nitrogen-rich fertilisers, which encourages lush, disease-prone growth.
Cut out all infected stems well below soil level, as soon as you notice them. Don't put infected material in the compost bin but burn it or put it in the dustbin, preferably in a sealed bag. If whole plants are badly affected lift and destroy them in their entirety along with the soil surrounding the roots. This total destruction is essential as the fungus can produce black resting bodies (sclerotia), which survive for long periods in the soil ready to re-infect new peonies.
There are no fungicides available to amateur gardeners at present."

I recommend taking plant samples to your local county extension agent for diagnosis.

Date 2018-06-14
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Plant Answer Line Question

Keywords: Plant diseases--Control, Fungicides, Powdery mildew diseases, Integrated pest management, Dahlia

What can I do about powdery mildew on my dahlias? Should I throw the bulbs away, or does it only contaminate the plant above the ground? I have heard both too much water and not enough water cause this problem. Is either true?


The main thing you will need to do is destroy all the foliage affected by the mildew. The mildew can survive the winter on infected foliage, and then spread to new foliage.

Powdery mildew thrives where plants are crowded and there isn't enough air circulation, so give your plants space, a sunny site, and try watering in the morning, and watering from beneath the plants (not over the leaves) so they are able to dry off during the course of the day. As you indicated, too little water can also be a problem.

Here are two websites with additional information:
Univ. of California IPM Online Guide
Washington State University Extension Garden Tips

I did not come across any information specifically saying that powdery mildew will affect bulbs or tubers. I spoke to an experienced dahlia and begonia grower here who said that it should be all right to store and replant your tubers, as long as you thoroughly get rid of all the diseased foliage aboveground.

Some sources (such as The Organic Gardener's Handbook of Natural Insect and Disease Control, edited by Barbara Ellis, Rodale Press, 1996) suggest that a baking soda spray (1 tsp. per 1 quart of warm water, with a bit of dish soap) is protective or preventive, but Washington State University Extension professor Linda Chalker Scott disputes the efficacy of this method. She says that other methods work better:
"Other treatments have been more successful in powdery mildew control, including horticultural oils, potassium bicarbonate, potassium phosphate, sulfur, milk, and even water sprays. Probably the most field success has been found in combining SBC [sodium bicarbonate] with horticultural oils, including mineral and vegetable oils (see the Fall 2008 MasterGardener magazine). The mixtures are so effective that they've been successful even on serious powdery mildew epidemics."

Date 2017-08-08
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Plant Answer Line Question

Keywords: Plant diseases--Control, Prunus, Fruit--Diseases and pests

Have you any advice about how to combat peachtree leaf curl using natural methods at this stage in the season? I've just read about the use of thyme or oregano oil, but no advice on amount used. I would be glad of any help!


The information I was able to find about thyme oil as a treatment for Taphrina came from an application to the U.S. Patents Office, so I cannot speak for its efficacy. I did find information from Connecticut Agricultural Experiment Station which mentions thyme oil as an organic-acceptable insecticide.

The U.S. Department of Agriculture's Agricultural Research Service offers research on the use of plant essential oils in postharvest disease control,too.My impression is that the efficacy of these plant-based oils is still being studied and evaluated.

I also found information on managing peach leaf curl (Taphrina deformans)[formerly available online] from the National Sustainable Agriculture Information Service which suggests using lime sulfur, acceptable by U.S. organic standards, though European standards may differ. Below is a more substantial excerpt:

The life-cycle diagram above in Figure 2 shows that the infection period for leaf curl is when new leaves start emerging from buds in the spring. Spraying after the buds have opened is ineffective, because infection takes place as the young leaves emerge, and the fungus develops inside the leaf.

Accordingly, sprays must be applied during the trees' dormant period after the leaves have fallen and before the first budswell in the spring. Many orchardists spray just prior to budswell during the months of February and March. Orchards with a history of severe peach leaf curl benefit from a double application: in the autumn at leaf fall and again in late winter or early spring just before budswell.

Fortunately for the organic grower, lime sulfur is one of the most effective fungicides for control of peach leaf curl and is allowed in certified organic production . Bordeaux and copper fungicides are also approved for certified organic programs and are effective as well, but not as effective as lime-sulfur.

Pscheidt and Wittig (6), performed trials comparing Kocide, lime-sulfur, several synthetic fungicides, and Maxi-Crop seaweed for leaf curl control. Lime-sulfur and one of the synthetics (ziram) were best, roughly twice as effective as Kocide. Seaweed sprays, despite positive anecdotal reports, were completely ineffective.

Severe leaf curl infection can cause the tree to shed many of its leaves and to replace them with a second flush of growth. At this time the tree will benefit from a light feeding with a quickly-available soluble fertilizer such as compost tea or fish emulsion to help it recover.

There are various levels of resistance to leaf curl among varieties; however, because of the relative ease of controlling the disease, breeding for resistance has not been a priority. Redhaven, Candor, Clayton, and Frost are some of the cultivars with resistance to leaf curl, though none is immune. In contrast, Redskin and cultivars derived from it are susceptible.


The City of Seattle's Integrated Pest Management Solutions pages for landscaping professionals also suggests methods of prevention and control. Damage may be reduced by sheltering the tree from winter and early spring wet. If only a few leaves are affected, they may be removed by hand. Peach leaf curl does not usually kill the tree, though fruit yield will be reduced. This resource also mentions using copper fungicides and lime sulfur when the tree is dormant.

Date 2018-04-11
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Plant Answer Line Question

Keywords: Rosa, Plant diseases--Control, Fungal diseases of plants

What can I do about black spot on my roses? I heard that burying banana peels in the soil might help.


According to The Organic Gardener's Handbook of Natural Insect and Disease Control (edited by Barbara Ellis; Rodale Press, 1996), there are several steps to dealing with black spot on your roses. First, avoid wetting the leaves, and do not handle the plants when foliage is wet. Prune the plants to make sure there is good air circulation. Make sure the roses are in sun, and are not shaded by large shrubs or trees. Avoid using high-nitrogen fertilizers, and only fertilize based on a soil test's indications. If you expect an appearance of black spot (based on past experience), spray plants weekly with sulfur or fungicidal soap. Once you see symptoms, it is hard to control black spot. Remove and dispose of any affected parts of the plant (don't compost). Make a solution of 1 teaspoon baking soda in a quart of water, and spray the infected plants well.

University of California, Davis's Integrated Pest Management website says the following about black spot (Diplocarpon rosae):

"The fungus requires free water to reproduce and grow, so leaves should not be allowed to remain wet for more than 7 hours. (When hosing off aphids, do it in the morning so leaves have a chance to dry by midday.) Provide good air circulation around bushes. Remove fallen leaves and other infested material and prune out infected stems during the dormant season. (...) Miniature roses are more susceptible than other types, although a few varieties are reliably resistant to all strains of black spot.(...) A combination of sodium bicarbonate or potassium bicarbonate plus horticultural oil (as discussed above under "Powdery mildew") or neem oil has also been shown to be effective in reducing black spot."

Brooklyn Botanic Garden has information on natural disease control, including the following:
"Baking soda (sodium bicarbonate) is non-toxic, readily available, and very inexpensive. It can be effective against powdery mildew and somewhat useful against black spot. If you repeatedly spray leaves with bicarbonate, though, it will eventually reach the soil below, where it can accumulate and lead to slower plant growth. Bicarbonate can form insoluble particles with calcium and magnesium ions when it concentrates in the soil, making these important nutrients unavailable to plants. High levels can also prevent plants from absorbing iron and can lead to chlorosis.
Bicarbonate is most likely to build to damaging levels in drought-stressed areas where there is little rain to flush it away. It is also likely to build up when applied in a small area, and when used in conjunction with drip-type irrigation. Garden situations are so complex that it is hard to predict the point at which you will see adverse effects. Stop applying bicarbonate sprays, however, at the first sign of plant damage or lower quality blooms."

Brooklyn Botanic Garden also mentions a beneficial bacterium which may provide some help:
"Preliminary research shows that the beneficial bacterium Bacillus laterosporus (sold as Rose Flora) is as effective at protecting black spot-susceptible rose cultivars as some chemical fungicides. It probably protects against black spot through competition, but this agent is still relatively new and experiments detailing its mode of action have not been completed. As a ground spray, it can help control new sources of black spot infection. As a foliar spray, it seems to be more effective when mixed with the antitranspirant sold commercially as Wilt-Pruf. The powdered formulation can cause eye irritation, so use eye protection when mixing solutions and applying."

About the practice of using banana peels to control black spot on roses, I found the following item on Gardening Folklore from Ohio State University Extension, which suggests the peels might be a good fertilizer, but does not say they will control the fungal problem.
"Placing several banana peels in the planting hole was popular among rose growers in the 18th century, but they had no idea why the peels seemed to yield healthier roses. Today, we know that banana peels contained many useful nutrients, including calcium, magnesium, sulfur, phosphates and sodium. The peels rot quickly which means these nutrients are readily available to the plant."

Some sources recommend using compost tea or milk sprays on black spot-affected leaves, but Washington State University Horticulture Professor Linda Chalker-Scott dismisses these methods as ineffective. She also states in an article in Master Gardener magazine (Spring 2009) that baking soda sprays may only be of limited efficacy in combatting black spot. Studies have shown that it works better when combined with horticultural oil.

To sum up, I would pay attention to the cultural practices (not wetting the leaves, etc.). You can try a baking soda spray (always test on a small area of the plant first), but it may not have lasting power as a treatment. Prof. Chalker-Scott mentions that coarse organic mulch (such as wood chips) reduces incidence of black spot, so you may want to adopt this mulching practice.

Date 2018-08-23
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Plant Answer Line Question

Keywords: Plant diseases--Control, Prunus, Pesticides and the environment, Pesticides

We live in a heavily treed (fir and cedar mostly) condominium complex. Our shrubs are sprayed twice a year by a professional spraying company to protect against fungus and other problems. We are thinking of spraying every other year to save on expenses.

Would we be jeopardizing the health of our shrubs and small cherry trees by doing so?


Unless your shrubs and trees have a history of trouble with diseases, I can't think of any reason they should be sprayed at all. Even if the plants were susceptible to disease, a more sustainable approach than annually applying fungicides and other pesticides would be to select disease-resistant plants that will thrive in your garden's conditions without that sort of intervention.

Spraying, depending on what is being sprayed, can be a hazard to human health and the environment. You may be able to stop your spraying program entirely by instituting good garden practices, like cleaning up debris and providing good air circulation around the trees, and avoiding overhead irrigation.

Examples of nonchemical ways to manage fungal problems that may affect ornamental cherry are provided below, from Washington State University Extension's HortSense website:
"Brown rot is a fungal disease which initially infects the flowers. The petals turn light brown, develop water-soaked spots and may have tan or grayish areas of fungal spores. Infected flowers often remain attached to the plant, spreading the disease to small twigs and branches. Infected twigs and branches are often observed in the summer as flagged, dead leaves and twigs. Infected branches develop cankers which may produce gumming (leaking sap) or may girdle and kill the branch. Most brown rot cankers develop with a dead twig at the center where the initial branch infection occurred. Fruit can also be infected, dry out, and hang in the tree. Tan or gray fungal spores may be found on infected blossoms, fruit, or twig cankers. Ornamental and fruiting stone fruit trees are affected.
Select Non-chemical Management Options as Your First Choice!!

  • Avoid wounding trees.
  • Clean up and destroy fallen flowers and other debris beneath trees.
  • Remove and destroy all infected twigs and branches during the summer, making pruning cuts well below infected tissues."

Similarly, here are their recommendations for managing Coryneum blight or shothole:

  • Avoid overhead watering, as leaves must be moist for infection to occur.
  • Prune and destroy dead buds and cankered twigs if present.
  • Rake and destroy infected leaves.

Again, for cherry leaf spot:

  • Avoid overhead watering. If overhead irrigation is necessary, limit it to times when foliage can dry quickly.
  • Rake and destroy all fallen leaves and debris under trees.
  • Space plantings and prune to provide good air circulation.

Date 2018-10-06
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Garden Tip

Keywords: Plant care, Plant diseases--Control, Insect pests--Control, Attracting wildlife

By November Seattle has usually had a good hard frost and most of our herbaceous (non-woody) perennials have either turned to mush or look a bit tattered. Before you give in to the temptation to cut back everything in sight, consider the advice of natural gardening advocates James Van Sweden, author of Gardening with Nature (Random House, 1997) and Jackie Bennett, author of The Wildlife Garden (David & Charles, 1993):

  • Leaving seed heads and dead stems over the winter gives the garden winter interest, especially if we get some snow
  • Seed heads from Black Eyed Susans, Echinacea, Larkspur and Evening primrose provide bird food
  • Beneficial insects hibernate or over-winter as eggs on plant waste
  • Marginally hardy plants like some salvias and lavenders benefit from the little bit of frost protection from the desiccated stems

On the other hand, sanitation is critical if your apples suffered from codling moth or scab or your roses suffered from black spot. Rake up and dispose of every single diseased leaf or infected fruit. Insect and disease organisms also over-winter on plant debris, so if you had a problem this year, start the treatment now with a thorough clean-up.

Date: 2007-03-26
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Garden Tip

Keywords: Plant diseases--Control, Vaccinium ovatum

If you grow blueberries and found shriveled gray fruit mixed in with normal plump berries your bush is infected with "Mummy Berry" disease. To lessen the severity of the disease in next year's crop, gather all the mummy berries you can find and throw them away. Add mulch in autumn to cover up the infected mummies that fell to the ground, and then cultivate around the bush in early spring to disrupt the fungal life-cycle that starts in the soil. Details and color pictures.

Date: 2007-05-17
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Garden Tip

Keywords: Plant diseases--Control, Tools and equipment, Pruning

Wondering when is it necessary to clean or sterilize your pruning tools, and how to do it? Washington State University Extension horticulturist Linda Chalker-Scott offers excellent "how-to" advice. Make sure there is no excess dirt or debris on your pruners before you clean them. There are quite a few impractical or inadvisable methods (formalin? chlorine bleach? no!); her preference is to use ordinary household cleaners.

Another factsheet discusses when to sterilize your tools. Chalker-Scott says that first you must know which pathogen is causing the disease. Then,

  • "if it's a virus or viroid, disinfect your tools.
  • if it's a vascular fungus or bacteria, and/or forms oozing cankers, disinfect your tools. Avoid cutting active, oozing cankers; wait until they dry.
  • if you are pruning irreplaceable plants, disinfect your tools.
  • choose a disinfectant treatment that has been shown to be effective through published research; I would probably not use alcohol but one of the common household cleaners at full strength."

Date: 2014-10-17
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