Leaf Spot On Mums – Treating Chrysanthemum Bacterial Leaf Spot

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When it comes to easy growing and general disease resistance, few plants can compare to the chrysanthemum. Lighting up the autumn landscape with myriad colors and forms, mums are a welcome addition to any outdoor space, whether in pots or planted in the garden. Unfortunately, the mighty mum has an Achilles heel: chrysanthemum leaf spot disease.

How to Avoid Leaf Spot on Chrysanthemum

Leaf spot of chrysanthemum is caused by the bacteria Pseudomonas cichorii, which is sometimescarried on the leaves of the plant, so even healthy looking specimens may besusceptible when conditions are right. For this reason, it is important toprovide the proper growing conditions and use the appropriate wateringtechnique to avoid bacterial leaf spot on mums.

Bacteria thrives in warm, moist environments, so whenplanting mums, always use adequate spacing between plants to ensure good aircirculation. Water plants at ground level rather than from above to avoidmoisture sitting on the leaves. And lastly, avoid over-feeding, which seems toencourage leaf spot of chrysanthemum.

Identifying Chrysanthemum Leaf Spot Disease

The home gardener’s first line of defense is knowing what tolook for. Characteristics of the disease are dark brown to black, irregularspots on leaves that will dry to a lighter color and crack.

They will usually start at the base of plants, travelling upto cause leaf curl and blight in both buds and flowers. When spots are dark(when they are damp), the bacteria are active, so avoid handling wet plants orsplashing water from infected plants onto healthy ones.

Mum Leaf Spot Control

The use of a copper hydroxide spray may be beneficial in treatingchrysanthemum bacterial leaf spot, as bacterial sprays have been found to beineffective. Make sure to apply the spray as soon as symptoms occur and in away that achieves full coverage of the plant. Badly infected plants should beremoved and destroyed.

There are some chrysanthemum cultivars that are moreresistant than others, so speaking with a local gardening expert or countyextension agent about the best mums to grow in your area may be an optionto avoid planting highly susceptible varieties.

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Article Summary:

Diseases of Chrysanthemum and their Management
Authors: Prativa Anand and Babita Singh
Division of Floriculture and Landscaping
ICAR- Indian Agricultural Research Institute, New Delhi-110012

  1. Root Rot (Pythium sp. and Phythophthora sp.)

  • Rotting of stem cuttings in rooting beds in warm, moist conditions, is most common. The fungus may also enter the established plants through the wounds caused by pinching, etc.
  • The infected parts like roots, stem and leaves wilt suddenly.
Management: Sterilization of propagation media, good drainage, quick removal of infected cuttings and regular drenching with thiram are controlling measures of the disease.

  1. Stem Rot and Wilt (Fusarium oxysporum)

  • Disease develops only after the appearance of flower buds or when they show colour, although infection might have occurred even in rooting beds.
  • Stem near soil becomes dark brown and dries.
  • Lower leaves turn yellow and plants wilt permanently afterwards.
Management: Soil application of Dithane M-45 (0.2%) controls the disease.

  1. Septoria Leaf Spot (Septoria obesa and Septoria chrysanthemella)

  • This is the most common and serious disease of chrysanthemum.
  • Brown spots appear on leaves that grow in size and number in humid conditions.
  • Leaves turn yellow and die afterwards.
Management: Control measures include picking and destroying leaves, providing dry and airy environment and spraying bavistin (0.1%).

  1. Grey Mold (Botrytis cinerea)

  • First girdling of stem takes place, which results in death of upper part.
  • Leaf infection starts from margin and proceeds towards centre and base, showing semi-circular bands.
  • Flower infection starts with brown water soaked spots on lower petals.
Management : Good aeration, adequate planting distance and spraying with bavistin (0.1%) or thiram (3g/l) provides good control.

  1. Ray Blight (Didymella ligulicola)
Symptoms :

  • It affects all above ground parts but is most severe on the shoots and on the flowers which develop reddish petal spots on light coloured cultivars and brownish spots on darker coloured types.
  • The lesions spread rapidly and the flowers collapse with the ensuing decay.
  • On shoots, the disease often begins with the rotting of terminal buds while the leaves are affected by patches of brown black decay.
Management: Destroy diseased plants promptly and avoid high humidity in greenhouse. Spray atleast three times at weekly intervals with zineb or thiram (0.2%) following the appearance of symptoms. Where the disease is suspected or prevalant, dip unrooted cuttings in benomyl suspension.

  1. Powdery Mildew (Oidium chrysanthemi)

  • White powdery patches appear on the upper surface and on the tender shoots.
  • In severe cases, leaves fall off and flower production is reduced.
Management: Keeping the environment dry and spraying with wettable sulphur (0.2%) controls the disease.

  1. Rust (Puccinia chrysanthemi)

  • Yellowish green spots on the upper surface and chest nut browny pustules on the lower surface of leaves.
  • Severely infected plants are stunted and do not bloom properly.
Management: Steam sterilization of soil and pots, keeping humidity below 75 percent, cutting and burning infected plants and spray of mancozeb are some of the control measures.

  1. Verticillium Wilt (Verticillium albo-atrum and Verticillium dahliae)

  • Symptoms appear at the flower bud initiation and plants wilt suddenly.
Management: Selection of healthy stock plants, soil sterilization and spraying with bavistin has been recommended.

  1. Bacterial Leaf Spot (Pseudomonas cichorii)

  • Circular elliptical leaf spots appear, increase and coalesce to form large necrotic areas.
  • The disease appears first on basal leaves and progresses upwards to flower buds which turn dark and die later.
Management: Use of disease free cuttings and spraying in rainy season with copper sulphate controls the problem.

  1. Viral Diseases

  • Mosaic caused by Chrysanthemum mosaic virus, Chrysanthemum virus-B, Tomato aspermy virus shows yellow mottling of leaves and breaking and distortion of flowers.
  • Tomato spotted wilt virus appears as pale mottling and necrotic spots on the leaves.
  • Chrysanthemum stunt virus shows overall reduction in plant size, pale foliage with margins failing to enlarge giving stiff appearance, flowers may open prematurely, red and bronze flowers are often bleached.
Management: Rouging and destruction of diseased plants, use of disease free cuttings, resistant varieties and insecticidal sprays to control vectors are controlling measures of the disease.

About Author / Additional Info:
Working as Scientist, Division of Floriculture and Landscaping, ICAR-Indian Agricultural Research Institute, New Delhi-110012

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Bacterial leaf spots on ornamental crops are typically caused by bacteria in the genera Pseudomonas, Xanthomonas​, and Acidovorax. Some of these bacteria cause disease on a wide host range while others cause disease on specific plant genera. In general, species of Xanthomonas are most active under warm conditions (>80F), but the presence of cooler temperatures does not eliminate the possibility of a bacterial disease being present.

Bacterial leaf spot on geranium.

NCSU Plant Disease and Insect Clinic

Bacterial leaf spot on geranium.

NCSU Plant Disease and Insect Clinic

Bacterial leaf spot on begonia.

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Bacterial leaf spot on begonia.

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Water-soaked lesions on pelargonium.

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Water-soaked lesions on pelargonium.

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Canna lily with lesions caused by Acidovorax sp.

NC State Plant Disease and Insect Clinic

Canna lily with lesions caused by Acidovorax sp.

NC State Plant Disease and Insect Clinic

How to plan when to get your garden mums

You’ll need to consider two elements to determine when to order and receive your mums. First, when you want to have the plants begin budding so you can sell them. Second, what pot size(s) you’ll be using, and how much time you’ll need for vegetative growth prior to budding.

When will you sell the plants?

If you want to begin selling mums in September, you’ll want to order mums that begin budding around that time to offer the best display and longest bloom time to your customers.

Since Garden Mums are photoperiodic, you need to be aware of any light that is reaching your plants. Mums need more daytime light and shorter nights for vegetative growth and less daytime light with longer nights to initiate bud growth. Be aware that any light, whether natural or artificial, can affect growth. Mums need to be grown in full sun, not shade.

Plants will bloom at their regular time under natural day regardless of vegetative growth. This means that a mum that blooms in mid-September will bloom then whether it is an inch tall or three feet tall. Be sure to plan enough time for vegetative growth prior to budding and choose varieties that begin budding around when you’ll display them for sale.

You can use a controlled growth program with black cloth and artificial lights to adjust and control plant and bud growth and grow out of season. You can also use black cloth or grow lights to supplement a natural day approach for appropriate amounts of light off-season or to help your mums bud sooner. However, a natural day approach to growing mums is simpler than one that involves black cloth and can easily be implemented if you order and grow your plants at the right time.

In the North, plan for varieties that bud earlier, before frost, or plan to use black cloth to control flowering and plant your liners with the intended budding date in mind. Southern bloom time will be later. You may need to shift an early October mum to late September in colder climates.

This page will show you varieties by bloom time, so you can choose mums that are appropriate for your needs. The bloom time information available is for southern crops. For northern crop time information, contact us directly, or visit Gediflora’s website. Get more tips and tricks on ordering your Belgian Mum Rooted Cuttings here .

What size pot will you be using?

Pot size will determine how many weeks you’ll need to plan for your mums to have an appropriate canopy size. The umbrella of flowers at the top of the plant should be twice the size of the pot. For example, an 8-inch mum pan should have a 16-inch canopy.

With some fluctuation based on heat and feed rate, mums grow at a rate of about 1.5 inches a week. Plan for an extra week to account for fluctuations. An 8-inch mum pan takes 12 weeks to finish a 9-inch mum pan takes 14 weeks to finish.

Once you know how many weeks you’ll need to finish your mum rooted liners for the appropriate pan size and when your mums will begin blooming, you can calculate when to have your mums shipped.

To finish 9-inch mum pans for blooming in mid-September, you’ll need to plant your liners the last week in May to allow for 14 weeks of vegetative growth, with a week for wiggle room to allow for minor heat and feed rate shifts. Book your mums to arrive a week before you need to plant them to make up for shipment or shipping delays.

Tips to Minimize Late-Stage Foliar Disease Pressure in Garden Mums

By Rick Yates | April 18, 2019

Bacterial Leaf Spot symptoms on chrysanthemum.
Photo by Rick Yates

Late summer is notorious for bringing high humidity and, sometimes, drenching rains to much of the country. All that heat and moisture can lead to increased disease pressure on garden mums. Unfortunately, this can trigger disease outbreak just in time for the beginning of the shipping season.

Two major issues that make up the bulk of the foliar disease threat in garden mums are aerial Rhizoctonia, which sometimes occurs in concert with Botrytis, and a bacterial leaf spot caused by Pseudomonas.

Here’s What To Look For
Aerial Rhizoctonia is a fungal disease favored by wet foliage and high humidity. Sometimes this disease manifests itself as light-brown lesions scattered across the outside canopy of the garden mum. As the crop progresses to the later stages, well-grown garden mums with a thick, tight canopy of foliage will sometimes develop aerial Rhizoctonia down in the plant. The mum looks great from the outside but, using your hands to gently separate the foliage, you may see the wet rot going on down inside the plant canopy.

Pseudomonas cichorii is a bacterial disease that is readily spread throughout a garden mum crop by splashing water. If you are currently using overhead sprinkler irrigation to water your mum crop, this is a good reason to consider making the switch to drip or trickle irrigation.

Rhizoctonia web blight disease on chrysanthemum.
Photo by Rick Yates

Prevention Tips to Maintain Quality Crops
Growers can take steps to minimize the risk of losing quality or even experiencing shrinkage as the garden mum crop approaches the finish line.

First, pushback against these foliar diseases can come in the form of cultural considerations. For crops that are beginning to crack color and are close to sale, reduce fertilizer from 200 ppm to 100 ppm when using a constant liquid feed. Less fertilizer is required at this crop stage and reducing the amount of nitrogen in the tissue can help reduce foliar disease risk. We recommend you don’t go to clear water, however, as nutrient deficiencies can quickly arise and reduce plant quality.

For growers who are beginning to ship their mum crop, as space becomes available, improve air movement through the field by pulling the plants apart. Overhead sprinkler irrigation results in consistent foliage wetting and increased foliar disease risk. Here’s another reason to investigate a transition to drip or trickle irrigation.

When cultural fine-tuning isn’t enough, preventative fungicides may be enlisted. Effective fungicide options for aerial Rhizoctonia appear in Table 1. We only recommend fungicides that control both Rhizoctonia and Botrytis because, as mentioned previously, they often occur together.

Growers have observed variety-based differences in Pseudomonas susceptibility. With the abundance of quality garden mum genetics available, consider replacing disease-prone varieties in your program to reduce the risk. Since most traditional fungicides don’t provide control of bacterial diseases, the list of effective products to choose from is relatively small. Table 2 includes the products we feel are most effective for Pseudomonas bacterial infection on garden mums.

The products mentioned are all intended to be applied as a foliar application and are considered bloom-safe unless otherwise noted. The provided tables are intended as a summary it is up to growers to read and follow the entire pesticide label. Products other than those mentioned may also be safe, legal, and effective. Some of these products may not be registered in certain states. Contact your supplier for further guidance.

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