219-Disease Management Guide for Wild Blueberries

Fact Sheet No. 219, UMaine Extension No. 2000

NOTICE: It is unlawful to use any pesticide for other than the registered use. Read and follow the label on the product container. The user assumes all responsibility for use inconsistent with the label. This fact sheet is to be used only during 2019. Use in subsequent years may lead to improper and illegal use of pesticides. If your version of the guide is out of date, please request an updated version from your Extension office or on the Maine Wild Blueberries website.

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Groundwater is a major natural resource. Pesticides have been detected in the groundwater in all states. A sound pesticide application program including a site-specific selection of pesticides, adherence to label directions, sprayer calibration and mixing accuracy, spill and back-siphon prevention, proper waste disposal, integrated pest management, and judicious pesticide use can prevent groundwater contamination.

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Disease Management

Prevention is the key to effective disease management. Many management practices reduce the incidence of disease-causing organisms. These include regular fire pruning, sanitation of equipment used in the field, and disposal of winnower refuse by composting or burning.

Burn pruning can decrease infected plant debris and fungal overwintering structures and therefore decrease the source for fungal infection of plants in the following year. Burn pruning each crop cycle will produce the largest effect, but burning every few crop cycles will still break up the cycle of fungi surviving on diseased tissue. Blueberry plants near rock walls should also be burned if possible.

Sanitation is key to preventing the spread of disease from one field to the next. Blueberry boxes, harvesters, sprayers and other equipment should be washed, preferably in the field they were used in, before being moved to another field. Washing with pressurized water will remove most plant debris, including weed seeds. Disposal of winnower refuse by composting to complete breakdown or by burning removes a source of future infection.

Early detection and correct identification of diseases is also beneficial to prevent their spread and improve management. Some diseases, such as Valdensia leaf spot, are much easier to control when they are in a small patch. Valdensia leaf spot is an example of a disease that can rapidly spread under suitable conditions to most of a field within a season.

Mummy berry may be decreased by mulching or burning plant debris under clones with high levels of mummy berries on the plants. Mulching with material to a depth of 2” will prevent most mummy berries from germinating in the following years. Spot burning of plant debris under clones with high numbers of mummy berries may also decrease the number to germinate in the next year. To be effective, both treatments need to be done once all of the mummy berries have fallen to the ground. Burning plants once they show disease symptoms in May and June will not prevent the spread of the disease. Once you have noticed the disease symptoms on your plants, many of the fungal spores will have already been spread by pollinators and other insects.

When conditions might permit diseases to become a problem, fungicides can be used to protect plants from potential infection; they will not cure already diseased plants. Early detection by scouting is essential for determining whether fungicide applications are necessary and when fungicides should be applied to be the most effective. The use of fungicides may also be necessary to produce the desired level of control.

For additional information on the identification, life cycles and control of blueberry diseases, please refer to Wild Blueberry Fact Sheets No. 211, Blueberry Diseases 1 and No. 218, The Influences of Pruning Method on Disease and Insect Control.

All publications may be found on the Maine Wild Blueberries website.

Fungicide and pollinators

No fungicides recommended in this document are acutely toxic to honey bees. Precautions to minimize exposure of pollinators to fungicides is recommended since there are some indications that some fungicides may interact with some insecticides to affect pollinators, and some fungicides may decrease the visits of pollinators to flowers.

Try to avoid applying fungicides during bloom. If a fungicide application is absolutely necessary, you should try to make the application after dusk when most pollinators are not active in the field.

Mummy Berry, Monilinia

Primary infections of leaf and flower buds by Monilinia occur for several weeks following bud-break. The fungus is then spread by spores from the dead leaves and flowers to healthy blossoms where the fungus will then infect the developing fruit producing a mummy berry (pseudosclerotia) that overwinters. The fungal spores on dead leaves and flowers do not cause new killing infections of leaves and flowers, so applications of fungicides are not necessarily warranted when dead leaf and flower tissue is seen. Primary leaf and flower bud infections are the most damaging stage of this disease on wild blueberry. Pruning blueberries with fire, using efficient harvesting techniques to reduce the number of infected fruit on the ground, composting, and burning or disposing of winnower refuse can reduce the incidence of this disease.

Primary infections can be controlled by ground applications of a fungicide when greater than 40% of the flower buds are at the crown stage (stage 4:Inflorescence tip visible; in Wild Blueberry Factsheet No. 216, Flower Primordia Development Stage) and mummy berry cups (apothecia) are present in the field. The mummy berry forecast method (refer to Wild Blueberry Fact Sheet No. 217, A Method to Control Monilinia Blight) can be used to predict when there has been an infection period for the fungus. Some fungicides (propiconazole or fenbuconazole) applied within 72 hours of an infection period will kill off fungi that have penetrated into the plant and prevent further infections for up to 7 to 10 days. Fungicides also can be applied before infection periods to protect the plants. When compared to the calendar method (applying fungicides in regular seven to ten day intervals) in 2009 and 2010, the mummy berry forecast method required two fungicide applications to protect plants compared to three applications required for the calendar method to protect plants in the same field for the same length of time. Full coverage around the stems is essential.

Blossom and Twig Blight, Botrytis

Blueberry blossoms are the most susceptible part of the plant and are susceptible to Botrytis just before they open (closed but pink in color at the base) and when open. Botrytis can cause greater disease severity in blossoms with frost damage or during late-bloom. In severe outbreaks, leaves and stems also can become infected. An extended period of wet weather, the length of which is dependent upon the temperature, is required for infection to occur. Fields with regular fog cover are more likely to be infected by Botrytis. Botrytis is an infrequent disease in most blueberry fields and requires scouting of early blossoms to determine if it is present and whether control in necessary. Blossoms can also be killed by Monilinia (mummy berry disease) or frost, so determine the cause of blossom death before applying fungicide. Do not apply fungicides unless you are sure you have Botrytis and it is likely to cause crop loss. Do not apply fungicides while bees are in the field if at all possible. Refer to Wild Blueberry Fact Sheet No. 212, Botrytis Blight Control for Wild Blueberries for more information on controlling blossom blight.

Valdensia leaf spot, Valdensia (formerly Valdensinia)

Leaf spots caused by this fungus are usually round, large (>1/4 inch) brown spots with black margins. Young leaves with a single spot may fall off the plant while still green. Heavy infections can cause complete defoliation which can decrease flower bud formation and yield. Early leaf drop occurring at the end of June and early July is a symptom of this disease. Older leaves do not fall off of the plant and will show more leaf spots. This fungus overwinters in veins of infected leaves from the previous year and will first produce spores after a 3 to 4 day wet period in May to early June. Fallen infected leaves will produce new spores after a couple of days of wetness throughout the growing season. The spores are not carried by wind or water, but can be transported on leaf contaminated equipment, clothing or shoes. There are few registered fungicides for this disease and they will only suppress the disease and will not remove it from your field. The recommended method of control is eradication by a hard burn to destroy all leaf litter of infected plants and all plants within 10 feet of infected areas.

“False Valdensia” (fungus not yet identified”)

This leaf spot is often mistaken for Valdensia leaf spot, and can cause defoliation of heavily infected leaves in some clones in July and August with dry weather. Symptoms start to appear after bloom and appear as large brown patches (1/8 to ½ inch) on the lower and upper surfaces of leaves that may continue to expand during the season. Applications of fungicides after bloom will decrease incidence of this disease and defoliation but do not have a consistent effect upon yield.

Red-Leaf, Exobasidium

Plants with red-leaf disease occur singly, in scattered clumps, or in patches. Infected plants are recognizable by the bright red color in irregular blotches on partially affected leaves. In late June and July, the underside of diseased leaves turn white when spores are produced. In August, infected leaves will shrivel and dry up. Few or no fruit develop on infected stems; some twigs may be killed. The disease overwinters in stems and rhizomes. No fungicides adequately control this disease, but it does not seem to be increasing in importance. The only control recommendation is not to spread spores by walking through diseased areas when spores are being produced. Removing infected plants by directed spraying with an herbicide may kill infected stems but may also kill surrounding unaffected plants by transference of herbicide through rhizome connections.

Septoria Leaf–Spot, Septoria

Leaves and stems become infected during bloom in wet weather. Severe spotting can cause defoliation of some clones in dry weather in July and August, which may result in reduced vigor and decreased yield. Some berries may also fall with severe infections and dry weather. Leaf lesions are small (pin-prick) water-soaked spots on the underside of leaves in mid-June, and the spots become necrotic and appear as small red to brown spots on the top of the leaf by July. If excessive leaf drop has occurred in the past, then fungicide applications may be necessary to reduce the incidence of this disease.

Powdery Mildew, Erysiphe (formerly Microsphaera)

In most years, powdery mildew appears in July and August in both crop and prune fields and is not known to affect yield. Diseased leaves range in appearance from those covered with a white mildew to those showing little or no mildew in the center of large red spots or rings. When the disease is severe, leaves can be shed prematurely. Fungicide treatments may reduce powdery mildew, but treatment is seldom necessary unless substantial amounts of disease and leaf loss have occurred in the past.

Leaf Rust, Thekopsora (formerly Pucciniastrum)

Leaf rust has a complex life cycle that includes several different spore types and the infection of both blueberry and an alternate host, hemlock in one year’s life cycle. Rust infections are typically not evident until mid-July or later, so rust has a minimal effect on cropping fields which are typically harvested in August. It can cause be a problem in non-bearing fields by causing extensive leaf spotting that results in excessive leaf drop. Leaf rust symptoms are consist of small spots with brown centers and diffuse red margins that can be easily distinguished from other diseases by raised yellow to rust colored pustules on the lower leaf surface. Leaf drop can be extensive by early September. There has been no evidence that pruning by burning decreases disease and even fields which do not have hemlock on their perimeter can still be severely affected by rust since the spores can travel for miles by wind.

Stem Blight, Phomopsis or Godronia

Stem blights caused by Phomopsis or Godronia are common diseases found in blueberry. The primary symptom is blighting of the first year’s stems. Death of stems usually occurs scattered in patches of one to a few stems. The infected stem tissue is reddish brown and dead reddish brown leaves remain attached to the stem for some time. The fungus over-winters in blueberry stems infected in the previous year. Conidia are released during rainy periods and are spread by splashing raindrops. Disease levels have not been high enough to require control.

Chemical Disease Control for Wild Blueberries
Disease	Material		Rate / acre Product	Comments
Mummy berry (Monilinia) Primary Infection	Tilt / Bumper / Propimax (propiconazole – Grp 3)		6 oz.	Begin applications when >40% of stems have blossoms at the crown stage. Follow the mummy berry forecast method to time applications or apply every 7 to 10 days following the calendar method Tilt/Bumper/Propimax: Do not make more than 2 sequential applications. Do not apply more than 30 oz/acre per year or within 30 days of harvest. Fontelis: Do not make more than 2 sequential applications or exceed 72 oz. per year. Indar: Do not apply more than 24 oz. of 2F / acre per year or within 30 days of harvest. Quilt Xcel: Do not apply more than 2 consecutive applications before alternating to a non-Group 11 containing fungicide. Luna Tranquility: Do not apply more than 2 consecutive applications before alternating to a non-Group 7 or Group 9 fungicide. Do not apply more than 54.7 oz/a per year or more than 0.446 lbs/a fluopyram or 2.1 lbs/a pyrimethanil per year. Pristine: Label indicates for use only with water as carrier, and can only be tank mixed with captan. Do not apply more than 2 sequential applications. Proline: Do not apply more than 2 applications per year. Do not apply within 7 days of harvest. Quash: Do not make more than 2 sequential applications per year. Do not apply more than 7.5 oz/A per season. Do not apply within 7 days of harvest.
	Fontelis (penthiopyrad – Grp 7)		16 to 24 oz.
	Indar 2F (fenbuconazole – Grp 3)		6 oz. & 1% crop oil concentrate
	Quilt Xcel (azoxystrobin & propiconazole – Grp 11 and 3)		14 to 21 oz.
	Luna Tranquility (fluopyram and pyrimethanil – Grp 7 and 9)		13.6 to 27 oz.
	Pristine (pyraclostrobin and boscalid – Grp 11 and 7)		18.5 oz.
	Proline (prothioconazole – Grp 3)		5.7 oz.
	Quash 50% WDG (metaconazole – Grp 3)		2.5 oz.
	Chemical Disease Control for Wild Blueberries
	Organic materials
	Procidic (citrus extract)		2 to 2.5 oz. / 10 gal. water	Apply at bud break and repeat every 7 to 10 days, Spray solution must be pH 5.5-6, low salt concentration and un-chlorinated. Use higher rate during high humidity. Do not mix with salt based or high alkali reaction products. Also bactericidal and may be used post-harvest. 0 day PHI.
	Serenade ASO (Bacillus subtilis – Grp 44)		2 to 4 qts.	For suppression, begin application at bud break and repeat every 7 to 10 days as needed.
Blossom and Twig Blight (Botrytis)	CaptEvate 68 WDG (captan & fenhexamid – Grp M4 and 17)	3.5 to 4.7 lbs.		Do not apply fungicides when pollinators are in the field, if at all possible. CaptEvate: Apply at 10% bloom, repeat every 7-10 days through petal fall. Do not apply more than 2 consecutive applications or apply greater than 21 lbs / acre per season Elevate: Begin application at 10% bloom, continue every 7-10 days, when conditions favor disease development. Do not make more than 2 consecutive applications or apply more than 6 lbs / acre per season. Switch: Make application prior to disease development. Additional applications may be applied at 7 to 10 day intervals, if conditions are favorable for disease. Do not make more than 2 consecutive applications or apply more than 56 oz/acre per year.
	Elevate 50 WDG (fenhexamid – Grp 17)	1.5 lbs.
	Switch 62.5 WG (cyprodinil & fludioxonil –Grp 9 and 12)	11 to 14 oz.
	Organic materials
	Procidic (citrus extract)	2 to 2.5 oz. / 10 gal. water		Repeat every 5 to 7 days as needed.
	Serenade ASO (Bacillus subtilis – Grp 44)	2 to 4 qts.		Begin application prior to disease development and repeat on a 7 to 10 day interval or as needed.
Leaf-spot (Septoria) Leaf Rust (Thekopsora)	Aprovia (benzovindiflupyr – Grp 7)		10.5 oz.	Aprovia: No more than one application per year. Do not apply more than 10.5 fl oz/acre/year. Apply only in prune year. Equus: Do not apply more than 10.9 lbs. of DF or 12 pt / acre per season of 720 SST. Minimum interval is 10 days. Apply only in prune year for these diseases. Bravo: Maximum of 12 pt. / acre per year (or 9 lbs. active ingredient). Minimum interval is 10 days. Apply only in prune year for these diseases. Proline: Do not apply more than 2 applications per year. Min interval 7 days. Do not apply within 7 days of harvest.
	Equus DF (chlorothalonil – Grp M5)		2.7 to 3.6 lbs
	Equus 720 SST (chlorothalonil – Grp. M5)		3 to 4 pt. in 20 to 100 gal. water/acre
	Bravo Weather Stik (chlorothalonil – Grp M5)		3 to 4 pt. in 20 to 100 gal. water / acre
	Proline (prothioconazole – Grp 3)		5.7 oz.
Valdensia Leaf Spot (Valdensia)	Proline (prothioconazole – Grp 3)		5.7 oz.	Eradication by hard burn of all leaf litter and infected plants is the preferred method of control of this disease. NOTE: Valdensia control is not listed on the Pristine label. Proline: For suppression ONLY. Do not apply more than 2 applications per year. Do not apply within 7 days of harvest.

Information in this publication is provided purely for educational purposes. No responsibility is assumed for any problems associated with the use of products or services mentioned. No endorsement of products or companies is intended, nor is criticism of unnamed products or companies implied.

Prepared by Seanna Annis, Associate Professor of Mycology, David E. Yarborough, Extension Blueberry Specialist, and Greg Kornelis, Research Assistant, The University of Maine, Orono, ME 04469. Revised February 2019.

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