Blueberry Leaf Spot Treatment: Learn About Types Of Blueberry Leaf Spot

By: Bonnie L. Grant, Certified Urban Agriculturist

Spotting on leaves may mean more than a cosmetic problem. Blueberries with leaf spot often look like they were injured with chemical sprays or hail, but other signs can help discern fungal diseases from mechanical or environmental injury. Early leaf spot control on blueberry with selected fungicide can help prevent these diseases from taking hold and causing defoliation and reduced vigor.

Types of Blueberry Leaf Spot

Blueberries with leaf spot are common at any point in the growing season. While there may be some signs of disease on flowers, stems or even fruit, the primarily affected part is the leaf. As the disease progresses, the leaves begin to die and fall off. Such defoliation reduces a plant’s ability to photosynthesize. Recognizing disease symptoms is key to designing effective blueberry leaf spot treatment and prevention of disease the next season.

Anthracnose and Septoria are the two main causes of leaf spotting. Each is a fungal organism that overwinters in soil or plant debris and spreads primarily through rain splashing. Alternaria is another common leaf spot fungus which attacks many types of plants. Gloeocercospora leaf spot is also prevalent on blueberry crops but causes little major damage. Valdensinia is a relatively new disease that causes early leaf drop and low plant vigor.

No matter the fungal organism, most types of blueberry leaf spot occur during wet periods. The moisture causes the overwintered spores to flourish and spread. Symptoms may appear as early as three days after infection but, in most cases, take up to 4 weeks to appear.

Most infections occur in early spring when temperatures are warming and rains are most prevalent and attack the newest growth. Mature leaves are rarely affected severely. The best leaf spot control on blueberry is clean up post season. Most disease overwinters in dispelled plant matter, which should be removed and destroyed.

Symptoms on Blueberries with Leaf Spot

The overall symptoms are very similar in each disease organism. A closer look can help define which disease type is affecting the plant.

  • Double Spot – Initial spots are tiny but grow larger in late summer. Spots spread to a classic fan shape with secondary necrosis around the original spot. The necrosis is darker on one edge of the original spot.
  • Anthracnose – Small reddish flecks on leaves and stems. Large brown lesions on leaves which eventually infect stems. Stems of current year growth develop red circular lesions at the leaf scars which progress to the rest of the stem.
  • Septoria – The heaviest infection is from June to September. Small white spots with tan to purplish borders.
  • Gloeocercospora – Large dark brown, circular lesions on leaves in mid-summer. The edges of the lesions become a lighter tan.
  • Alternaria – Irregular to round brown or gray spots surrounded by a red border. Symptoms appear very early in spring after cool, wet weather.
  • Valdensinia – Large round bull’s eye spots. Spots spread rapidly to stems within days and cause early leaf drop.

Blueberry Leaf Spot Treatment

End of season cleanup is crucial. There are several cultivars that have been bred with resistance to many of these diseases and include:

  • Croatan
  • Jersey
  • Murphy
  • Bladen
  • Reveille

Fungicides should be used in areas with leaf spot problems. An early application is recommended followed by treatment every 2 weeks from harvest until August. Benlate and Captan are the two most used fungicides in blueberry production.

Avoid walking around blueberry stands as a single leaf transmitted to an uninfected blueberry can spread infection. In some cases, the disease can move on contaminated machinery, containers and tools. Disinfect each as you move from plant to plant.

Many commercial growers top their plants after harvest, removing old foliage. The new foliage that emerges will nourish the plant and generally is free of disease. The use of resistant cultivars combined with fungicides and good hygienic practices can drastically reduce leaf spot disease and its movement from plant to plant.

Note: Any recommendations pertaining to the use of chemicals are for informational purposes only. Specific brand names or commercial products or services do not imply endorsement. Chemical control should only be used as a last resort, as organic approaches are safer and more environmentally friendly.

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Managing Blueberry Bush Diseases

Although blueberry bushes are disease resistant, some pathogens can infect them. The most common diseases are caused by fungus and bacteria. You can avoid blueberry diseases by giving your plants the conditions they need to remain healthy. It’s usually poor climate and soil conditions that cause blueberry plants to suffer.

What Blueberries Need

Providing your blueberries with optimal conditions will keep them disease free. The following list details the requirements for growing blueberries that produce high yields of berries:

  • Soil requirements – Well-drained, acidic soil with a pH of 4.5 to 5.6., and high organic matter
  • Water – 1 to 2 inches of water every week, being careful not to water-log the roots.
  • Cover crops – Planting cover crops reduces pathogens and controls weeds. These crops include buckwheat, sorghum, millet and rapeseed.
  • Mulch – Apply 4 to 6 inches of mulch with a cover layer of pine bark, peat moss or sawdust compost
  • Nitrogen – Test the soil and add nitrogen each year, if necessary.
  • Prune – Prune any dead or damaged canes while the blueberries are dormant. This allows for air circulation to prevent fungus growth.

Blueberry Diseases

If you gave your blueberry bushes the proper requirements, you probably won’t have to worry about them becoming infected, but sometimes it can happen. These are the most common diseases that blueberries contract:

Fungal Fruit Rots

These are the major fungal fruit rots of blueberries and their symptoms. Once these diseases infect your blueberries, they’re very difficult to eradicate:

  • Ripe rot or Anthracnose – The fungus (Colletotrichum acutatum) causes this disease. Its spores infect the green berries, which begin rotting when they start to ripen. The sticky, orange fungus continues to invade the berries even after harvest.
  • Botrytis blight or gray mold – Cool, wet weather causes gray mold (Botrytis cinerea) to grow on blueberry bushes. The blight overwinters on dead or decomposing plants that are covering the soil. It results in blossom blight, fruit rot and dying green branches.
  • Mummy berry – This fungus (Monilinia vaccinia-corymbosi) creates infected flowers that attract bees. The bees carry the fungal spores to other flowers as they collect pollen, infecting more of the plant. The diseased flowers produce berries filled with the fungus. The pink berries fall to the ground, infecting the soil. The fungus then infects young plant shoots.

Bacterial Diseases

Bacterial diseases are devastating to blueberry bushes. Pruning all the dead and diseased branches helps manage these diseases, but sometimes replacing the bushes is the only solution.

  • Bacterial canker – The bacterium (Pseudomonas syringae) causes this blueberry disease. The succulent young branches of newly planted blueberries are prime candidates for this bacterial canker. The disease enters the plant in the spring through damaged areas caused by frost. Prune any damaged branches immediately.
  • Crown gall – This disease (Agrobacterium tumefaciens) lives in the soil. It infects damaged stems and roots, resulting in the formation of knobby galls. Rain, water, infected garden tools and cuttings spread the disease to other blueberry plants. If any of your blueberries grow galls, pull them up. You have to wait three years before planting in the infected area again.

Many other blueberry diseases caused by fungus, bacteria, viruses and nemotodes can infect your plants. Remember that you can prevent most of these diseases with proper care and maintenance.

Identification Is Key

Blueberry spot characteristics are as varied as the diseases that cause them. Fungal rust diseases, such as from the causal pathogen, Pucciniastrum vaccinii, may cause leaf spots that first appear yellow and then turn reddish-brown on the tops of the leaves and yellowish-brown on the undersides of leaves. Another fungal leaf-spotting disease of significance is caused by the pathogen Exobasidium maculosum, which leaves light green spots on the tops of the leaves and, frequently, white fungal spots on the undersides. This disease may also cause red-ringed spots on the blueberry fruit. Fusicoccum (Fusicoccum putrefaciens) and Phomopsis (Phomopsis vaccinii) cankers leave reddish spots on blueberry stems that form bull’s-eye patterns. Two primary viral diseases infect blueberry -- tomato ringspot and tobacco ringspot -- and cause brown spots on the leaves and stems. Identifying whether a disease is fungal or viral determines the course of action.

  • Some diseases leave indelible marks on blueberry bushes (Vaccinium spp.)
  • Another fungal leaf-spotting disease of significance is caused by the pathogen Exobasidium maculosum, which leaves light green spots on the tops of the leaves and, frequently, white fungal spots on the undersides.

Minor fungal diseases

These are not generally a problem but be aware that these diseases can be troublesome in certain situations:

Double spot

The fungus Dothichiza caroliniana will cause roughly circular leaf spots in early summer that are light brown to gray and are outlined by a dark brown ring. No control is recommended unless disease incidence is high. Fungicides applied to control fruit rot also should aid in reducing double spot.

Leaf rust

The disease is caused by the fungus Pucciniastrum vaccinii, and it can attack all Vaccinium species. The alternate host for the rust fungus in this region is hemlock (western or Pacific hemlock, Tsuga heterophylla, and the mountain or black hemlock, T. mertensiana).

Spores form on the lower surface of the leaf in yellowish-orange pustules that become rusty red with age.

Powdery mildew

Powdery mildew is caused by Microsphaera alni. Because this disease usually develops after most of the fruit is harvested, it has little impact on production except when both disease incidence and severity are very high.

Stem canker

The disease, caused by Botryosphaeria corticis, has not been a problem in the Pacific Northwest, but has been found in Oregon. Reddish conical swellings appear in summer and fall on current-year canes. The lesions enlarge and become fissured in the second year, giving the cane a rough, blistered appearance. Prune out and destroy infected branches.

Twig canker

The fungus Sporocadus lichenicola can infect blueberry plants suffering from winter injury, sunscald, or damage from other sources. The fungus has been found sporadically in the Willamette and Hood River valleys of Oregon. Twigs have multiple grey-white cankers with reddish margins associated with nodes and range from 1 cm to the entire length of the twig. Tactics that minimize winter or other injury to plants are encouraged.

Witches’ broom

This disease is caused by a rust fungus Pucciniastrum geoppertianum. Stems swell, and there is excessive branching, giving the witches’ broom effect. True firs (Abies sp.) are the alternate hosts. Both hosts are required for the fungus to complete its life cycle. Because the fungus becomes systemic and perennial, the only method of control is to remove and destroy infected plants.

Watch the video: why plant leaves turn brown and dry on the ends

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