Mummy Berries in Blueberries
Mummy berry, scientific name Monilinia vaccinii-corymbosi, is a fungal disease of major importance in northern and southern highbush blueberry regions. It causes considerable damage to the fruit and can cause a grower to lose 1/3 or more of his crop. In addition, it causes severe blighting of the leaves, shoots, and flower buds. Initially in rabbiteye blueberries, mummy berry was limited to blighting of leaves and shoots, but it did not cause fruit rot. Now, we are seeing fruit rots as well as the shoot blighting symptoms in Macon County not only on highbush blueberries, but also on rabbiteye. Rabbiteye blueberries are known to get the fruit rot phase in Georgia and pathologist think there are more than one species of mummy berry.
How do you know if you have mummy berry?
In the early spring, you will see dieback of shoots and leaves and sometimes flower clusters. Blighting of new shoot tips and blossoms can be easily mistaken for frost damage. That’s the first symptom. Later, infected berries may get nearly full size and then turn tan or gray and shrivel into hard mummies, which drop to the ground at or before harvest. If not removed, these mummies can survive in the field for two or three years. So, dead shoots and grayish mummies in the bush or on the ground are what you see.
How do blueberry plants get the disease?
It’s difficult to say specifically where a disease that has not been seen in an area before comes from. With mummy berry, the spores of the first stage of the disease are airborne and can move miles on air currents. The second stage is more often moved by insects. Mummies from infected plants can be moved from other areas when plants are bought and sold. The mummified fruit may be hidden in leaf debris in a potted blueberry plant brought in from an infected area. If mummyberry disease is present in an area, it can move from infected cultivated plants or infected wild blueberries. The fungus overwinters in infected berries, or “mummies” on the soil under bushes. Under moist conditions in early spring, fruiting structures or apothecia, begin to form from mummified fruit remaining on the soil surface. The fruiting structures are like tiny mushrooms. They produce spores – called ascospores, – lots of them.
The spores spread by wind over fairly long distances, even between fields. When they land on leaf buds and young shoots, it only takes a few hours to a day for ‘primary infection’ to occur, depending on leaf wetness and temperature. Green tissue has to be present for infection. The ideal condition for infection is cool and wet (a common scenario in early spring). About a week or two after becoming infected, the leaves and shoots wilt and turn brown. This damage can easily be mistaken for frost injury. If flowers are present they can sometimes become blighted, too. The blighting of the shoots is considered the primary infection.
Then comes the secondary infection of the berries. When the humidity gets high enough, the fungus in the infected shoots produces another kind of spore, called conidia. The conidia move from the blighted shoots to nearby flower blossoms via rain, bees, or other insects. The grayish-tan layer of conidia at the end of blighted shoots doesn’t look like much to us, but it is attractive to bees because it reflects ultraviolet light and gives off a sugary scent.
Once inside the flowers, conidia germinate with the pollen and only slowly infect the fruit as it develops. That’s why you can’t tell that the berries are infected until later in the season, when they shrivel and turn pinkish instead of ripening. These “mummy berries” become filled with fungus, and have a hard grayish white center. They fall to the ground, shrivel up becoming pumpkin-shaped, and turn dark brown or black. These serve as an inoculum source the following spring when apothecia form and disease cycle begins again.
Are there resistant varieties?
Highbush types: Berkeley, Bluetta, Blueray, Earliblue, Jersey, Nelson, Patriot, and Weymouth are susceptible to mummyberry. Bluecrop, Duke and Elliott are less susceptible.
Rabbiteye types: As I mentioned earlier, severe blighting of the leaves, shoots, and flower buds of rabbiteye cultivars Delite, Southland, Premier and Tifblue has occurred in North Carolina plantings but the fruit rot has not been reported. But I’m very concerned that we are now seeing all stages of the disease in rabbit eye.
How can a homeowner control the disease?
Cultural practices are necessary: Remove the mummies as they form on the plant. There are several ways you can try to reduce primary infection in the spring, without using fungicide. Raking under the bushes and shallow cultivation between the rows is one strategy for destroying the apothecia before they release spores. Another tactic is to apply 2 or more inches of new mulch to bury the fallen mummies and deter the production of apothecia in the spring. A third technique is to apply urea, lime sulfur, or a concentrated liquid fertilizer under the bushes to try and ‘burn off’ any exposed apothecia. The application has to be well timed to be effective, and that’s difficult since not all the apothecia emerge at once. Apothecia production begins around first of March and continues for several weeks.
Fungicides might be necessary. Cultural practices may not do the job, particularly if you have susceptible varieties or high disease pressure. In that case you may want to use a fungicide, either conventional, or organically approved. For these to be effective they have to be applied at the proper time. Applications aimed at primary infection should begin early in the spring, at the bud-break stage, or when the very first ‘green tips’ of growth are visible. New growth remains susceptible until shoots are about two inches in length, so fungicides need to be re-applied at their recommended interval, usually every week, depending on the material.
There are nearly a dozen fungicides registered for use on mummyberry disease in blueberries. Some materials are good against primary infection; others are better against secondary infection (remember, these are different types of spores). Others don’t provide very good protection against either. Two of the more effective materials are Indar and Orbit. Fungicide options for organic growers (and backyard growers) include a material called Serenade, a bio-fungicide containing a beneficial bacterium. Research at Michigan State University has shown pretty good control with this material if it’s applied properly.
Do blueberries have other problems?
There are a number of blueberry diseases that occur. These include mummy berry, Botrytis blight, powdery mildew, various leaf spots, anthracnose of foliage and Botryosphaeria die back. The most important are mummy berry, Botrytis blight and anthracnose of leaves. Control methods for these are the same as mummy berry, sanitation and sprays of captan.