Mummy berry is one of the most serious diseases of blueberries in the Northeast. According to grower accounts and Extension reports over the past few years, it appears to be on the increase.
The fungus that causes mummy berry can lead to big losses. If left unmanaged in an infected field, the disease can cause a third or more of the berries to become discolored and shriveled up -- like mummies. Itís important to understand the rather complicated lifecycle of this fungus if you want to keep it under control.
Pay attention if the mummies show up. Like a lot of other problems, once you find that mummyberry has started in a planting, itís likely to increase if nothing is done about it. Some farmers that ignored the problem when it was at low levels have found it can develop into a major headache. Early identification and ongoing management of the disease are important.
Mummies are just the beginning: primary infection. The mummies are only part of the picture. True, the causal fungus overwinters in these infected berries that have fallen to the ground. In the spring, these mummies Ďsproutí small, mushroom-like structures, with little cups on the end. These are called apothecia, and they produce spores -- 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 or days for Ďprimary infectioní 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 sequel: secondary infection. If only it stopped there, but it doesnít. 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 doesnít 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 take their sweet time wreaking havoc. They germinate with the pollen and only slowly infect the fruit as it develops. Thatís why you canít tell that the berries are infected until later in the season, when they shrivel and turn pinkish instead of ripening. Later these berries fall to the ground, where they wait to start the cycle all over again next year.
Monitor your mummies. After the snow cover is gone, look closely to see if and when the apothecia appear. Mummified berries themselves are usually partially covered by soil, mulch and/or leaf litter, so they may be hidden or well camouflaged. The apothecia are often just barely visible above the surface. You might have to move some surface residues aside in order to see them.
Once you locate some apothecia, keep an eye on them. When their cups open up about an eighth of an inch, spores will start to shoot out, and they can keep coming for a week or two, until the cup collapses. A single cup can release over a million spores in less than a week. Knowing when spores are being released is very important if you plan to apply a fungicide to protect healthy plant tissue against infection.
Cultural practices can help. 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 Ďburn offí any exposed apothecia. The application has to be well timed to be effective, and thatís difficult since not all the apothecia emerge at once.
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 Ďgreen tipsí 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 donít provide very good protection against either. Two of the more effective materials, Indar and Orbit, are not labeled for use on blueberries; however, many states have applied for and been granted a temporary registration by the EPA on an annual basis. Check with your Extension small fruit specialist or your stateís Agriculture Department.
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 itís applied properly.
Plan ahead to avoid mummies. If you havenít planted your blueberries yet, proper site selection is one way to minimize your potential mummy berry problem. Avoid wet and poorly drained areas. They are not suitable for optimal blueberry growth anyway, but they also promote the growth of apothecia in the spring. Picking the right varieties can also help. Berkeley, Bluetta, Blueray, Earliblue, Jersey, Nelson, Patriot, and Weymouth are susceptible to mummyberry. Bluecrop, Duke and Elliott are less susceptible.
For more information on mummyberry disease, including current fungicide
recommendations and links to other publications, see the New York tree
and berry pathology site http:www.nysaes.cornell.edu/pp/extension/tfabp