Cooperative Extension: Maine Wild Blueberries

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.

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.

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