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.
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.
The overall symptoms are very similar in each disease organism. A closer look can help define which disease type is affecting the plant.
End of season cleanup is crucial. There are several cultivars that have been bred with resistance to many of these diseases and include:
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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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.
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:
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:
These are the major fungal fruit rots of blueberries and their symptoms. Once these diseases infect your blueberries, they’re very difficult to eradicate:
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.
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.
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.
These are not generally a problem but be aware that these diseases can be troublesome in certain situations:
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.
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 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.
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.
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.
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.