Mum Powdery Mildew Symptoms: Treating Powdery Mildew On Chrysanthemums

By: Teo Spengler

If your chrysanthemum plants grow in a sunny, well-drainedsite in your garden and get adequate water, they probably are blooming andhealthy. But when that’s not the case, your plants may suffer from fungaldiseases, including powdery mildew. Powdery mildew on chrysanthemums is one ofthose diseases that can usually be avoided with good cultural care. Read on forinformation about mum powdery mildew symptoms and effective chrysanthemumpowdery mildew control.

White Spots on Mums

Chrysanthemums are popular garden flowers.They are hardy perennials that thrive in mild or even cool climates. Thespecies flowers are yellow, and the name comes from the Greek words for goldand flower. Today, however, chrysanthemum blooms come in a large range of shapes and colors includingwhite, purple and red.

If you see white spots on mums that look like pale powder,don’t just hope they will go away. These are mum powdery mildew symptoms.

Powderymildew is a fungal disease. The ashy growths can show up on leaves, flowerparts or on stems. The leaves pucker and distort and many will ultimatelyshrivel and die. In severe cases, the entire plant is covered.

Often, you’ll first see the white spots on lower leaves. Intime, the disease spreads upward. You might spot tiny black round spheresinside the white spots late in the season.

Powdery mildew attacks plants in during hot, humid weather.Standing water is not necessary as long as humidity is high.

Chrysanthemum Powdery Mildew Control

You can go a long way toward preventing powdery mildew onchrysanthemums by planting the shrubs correctly. Space the plants far enoughapart to allow for good air circulation. Be sure they get sufficient water indry weather and are planted in sunlight.

If you see powdery mildew on chrysanthemums in your yard,you can fight the fungal disease with fungicides. Regular foliar fungicideapplications will control this disease.

When you see the first symptoms, apply fungicides with oneor more of the following list of active ingredients:

  • Copper
  • Azoxystrobin
  • Pyraclostrobin
  • Fludioxonil
  • Triflumizole
  • Myclobutanil
  • Triadimefon
  • Propiconazole
  • Sulfur
  • Potassium Bicarbonate
  • Thiophanate Methyl

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Read more about Chrysanthemums

How to Grow Hardy Chrysanthemum (Garden Mum)

The Spruce / Letícia Almeida

Garden mums (Chrysanthemum spp.) are herbaceous perennials in the daisy family and are stalwarts of the flowering autumn garden. When garden centers sell blooming potted mums in the fall, they are usually used as annuals and discarded when the blooms fade. And when gardeners try to transplant these mums into the ground late in the season, chances are they won't make it through winter and become perennial.

However, there are varieties that are truly perennial in most climates when planted in the early spring or in the fall several weeks before the first frost. Their hardiness, plus their ability to be pinched back during the summer so they won't bloom until fall, make these jewel-toned beauties a welcome splash in the garden at a time when most summer flowers have faded. These plants grow fast, and you should have flowers in the first growing season. Bloom times vary with variety and climate from early September through mid-October.

Botanical Name Chrysanthemum spp.
Common Name Garden mum, garden chrysanthemum, hardy chrysanthemum, hardy mum, mum
Plant Type Herbaceous perennial
Mature Size 4 to 36 inches tall and 12 to 36 inches wide (size varies depending on the variety)
Sun Exposure Full sun
Soil Type Rich, humusy, moist, well-draining
Soil pH 6.5 to 6.7
Bloom Time Late summer and fall
Flower Color Gold, white, off-white, yellow, bronze (rust), red, burgundy, pink, lavender, and purple
Hardiness Zones 3 to 9
Native Area Asia, Europe
Toxicity Toxic to dogs, cats, horses


This post was contributed to by Drs. Sarah Jandricic and Chevonne Dayboll.

Summer is in full swing, and so too are garden mums! Although generally an easy crop, there several tweaks you can make to help save headaches AND money.

This post has information to help you optimize your irrigation, fertilizer and pest management programs in garden mums.

Irrigation Method Matters!

Given the large acreage often occupied by garden mums, your watering strategy is one place you can look to save money.

There are plenty of options for irrigation in potted outdoor crops, but not all are created equal when it comes to maximizing water efficiency. Overhead irrigation by boom, or sprinkler is not efficient if your pots are not spaced tightly. Canopy sizes in the later months of production may make this impossible, especially if you choose to go with final spacing when pots first move outside. These methods of irrigation can also lead to pots that are too dry (not watered) or too wet (over watered). Plants can only use water that makes it into the pot, so low volume drip line or tape is a more effective way to delivering usable water to your outdoor crops.

Drip line irrigation can be a more efficient way of delivering water and nutrients to outdoor crops.

Remember drip line only reduces lost irrigation volumes if it is used properly! A “set it and forget it” approach doesn’t work. Look for kinked lines and clogged emitters, and make sure connections are tight. Know your application volumes and irrigate based on crop needs and weather patterns, not a set schedule.

Interested in improving your water use efficiency? Check out this post on outdoor mum and hydrangea production that highlights how to calculate volumes used and applied in an easy way.

A garden mum on low volume drip irrigation. The pot below measures water not captured by the plant, giving the grower an idea of water use efficiency.

Fertilizer Choices:

There are a few schools of thought when it comes to fertilizer types for garden mums. Some growers prefer to have more control over crop nutrition and choose water soluble mixes. The benefit of this is that mixes can be quickly changed if needed. Others choose to use controlled release fertilizers incorporated into the growing media and then irrigate with water only. Both have their pros and cons.

As with irrigation, its important to remember that nutrients that don’t stay with the plant can’t be used by it. The amount of nutrients staying with the plant are usually highest when controlled release fertilizers (CRFs) are used with carefully monitored irrigation. They are usually lowest when water soluble fertilizers are applied in high volumes (resulting in a lot of irrigation running through pots) and irrigation efficiency is low. To keep the fertilizer where you want it, it’s important to irrigate only enough to saturate the pot. Both formulations have their pros and cons, so make sure whatever you’ve chosen is easily managed.

Looking for more information on water and fertilizer use in outdoor crops? Check out research summaries from Flowers Canada and the Soil Research Group.

Pest Issues:

The good news is that garden mums grown outdoors simply don’t get the same pest pressure as pot mums. Why? A lot of this has to do with natural enemy populations from surrounding agricultural lands that seem to keep a lot of pests in check – like the cool praying mantis I found in a mum crop pictured at the top of this post! To conserve these free biocontrol agents, avoid spraying if you can. If you DO spray, do yourself a favour and chose something that’s compatible with beneficials so you don’t end up with a bigger problem on your hands from secondary pests.

Some pest issues you may see (and their solutions) are:

-Thrips. Rarely a problem outside, western flower thrips – and even onion thrips – can affect garden mums grown indoors. Unfortunately, this species of chrysamthemum generally does NOT tolerate the usual recommendation of oil dips for cuttings. A better bet to reduce incoming thrips on this crop, which you should consider “sensitive”, is dips in BotaniGard (see more details in this post on spring crops). After sticking, predatory mite sachets (1 per pot) are your best bet for long term protection for spaced plants. Amblyseius cucumeris (rather than A. swirskii) is the more economical choice for this crop. Given how quick the crop grows, penetration of the soil with drenches of nematodes may not be feasible within a few weeks, so a good secondary measure is foliar applications of Beauveria-containing biopesticides (BotaniGard, BioCeres), if needed. Growers also use a high density of large mass trapping cards in this crop, to help avoid thrips infestations that come from fly-ins in summer.

Large, yellow mass trapping cards can help manage thrips, especially those entering the greenhouse from surrounding agricultural lands in summer. A density of 8 cards/1000 square ft is suggested for floriculture crops (approximately 4 per bench).

– Aphids. We sometimes see these pop up in garden mums. If they do become a problem, it seems to be later in the season (late Aug/Sept), so wait to apply pesticides until you actually see them (this also helps avoid unnecessary applications!). Beleaf (flonicamid) is a good option as it’s a) soft on beneficials and b) can be applied via drench through irrigation lines, and c) is cheap! Altus is a newer registration that is also a good option for aphids and other sucking pests.

One mite-sachet per pot provides protection against thrips when garden mums are grown inside. Remember to tuck your sachets within the plant canopy to provide the right humidity for optimal performance.

-Tarnished Plant Bug (also known as Lygus bugs) can be an issue in August, once buds form. This pest is especially damaging, as their bud feeding causes severe flower deformation. Frequently, one variety or one side of the field gets hit first. Make sure to walk the crop regularly and look for adult bugs and aborted petals on open flowers (pictured below). TPB can be controlled with pesticides applied for other sucking insects (e.g. aphids).

Top Left: Tarnished plant bug (TPB) adult. Right: Garden mum flowers showing the aborted petals that are characteristic of TPB feeding. Bottom Left: a TPB adult feeding on a bud.

-Leafhoppers. These can fly in from the same surrounding agricultural lands as your free natural enemies, and can seem like an issue since they jump around plants a lot as you pass. The good news is they don’t seem to do any real crop damage. Resist the urge to spray, and simply make sure your workers give plants a good shake to dislodge any adults before packing.

– Japanese beetle. While generally not a problem on garden mums, recall that all plants produced outside from June 15 to September 30 need to be treated for JB if you plan to export or ship to a JB-free zone. See this post for details.

-Diseases. Chrysanthemum white rust – a quarantinable disease – is theoretically possible, but has only been detected in Ontario once in the last decade thanks to regulations requiring plant material be sourced from white rust-free facilities. Still, many growers treat as a precaution with Nova (mycobutanil), especially when exporting. Other diseases, such as bacterial blight, Fusarium, Pythium and Rhizoctonia are more common, but generally aren’t big issues in Ontario production. If problems develop, a good guide to disease symptoms and cultural management in garden mums can be found here.

Downy Mildew

Downy mildew is similar in appearance to powdery mildew, but germinates when conditions are wet and cool, usually in temperatures ranging from 40 to 60 degrees. This mildew is very host-specific, so it won't necessarily spread to nearby plants if it gets out of control. When downy mildew appears, it will usually be on the undersides of leaves as white, lavender or purple patches of spores that follow leaf veins. The upper leaf surface may develop yellow or brown patches that correspond to the spores underneath. Prevent downy mildew infestation by watering your plants at the base, never on the leaves, as early in the day as possible. Germination takes place in standing water in as little as eight hours. Once your plant is infected, few chemicals will help, but mancozeb is generally effective if mixed at a rate of 1/3 fluid ounce per gallon and applied with a hand-held or pressurized sprayer weekly. The infected plant's foliage should be coated until the chemical runs off.

Kristi Waterworth started her writing career in 1995 as a journalist for a local newspaper. From there, her meandering career path led to a 9 1/2 year stint in the real estate industry. Since 2010, she's written on a wide range of personal finance topics. Waterworth received a Bachelor of Arts in American history from Columbia College.

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