Preparing and using mummy berry observation stations in blueberries

Visual monitoring of mummy berry germination and apothecial development in the spring is critical for optimal control of mummy berry in blueberries. Mummy berry observation stations facilitate monitoring.

Mummy berry, caused by the fungus Monilinia vaccinii-corymbosi, is a common and widespread disease of blueberries in Michigan, particularly in wetter sites and in susceptible cultivars such as Blueray, Bluehaven, Rancocas, Rubel and Jersey. The fungus survives the winter on the ground as small, black, pumpkin-shaped fungal structures (sclerotia) in mummified fruit. These germinate in the spring to form small, trumpet-shaped mushrooms (apothecia) that release ascospores into the air. Ascospores cause primary infections called “shoot strikes” on young blueberry leaves and shoots in the spring.

The disease is notoriously variable from year-to-year. Most of the unpredictability is due to variability in the rate and timing of germination of the mummies, which are greatly influenced by soil moisture and soil temperature over the winter and early spring. The number of mummies overwintering on the ground from the previous season will also determine disease pressure in the current year. A snow cover usually helps protect mummies from desiccation and muffles temperature extremes. While mummies can withstand very cold temperatures, they do need a certain number of chilling hours below 7 degrees Celsius (45 degrees Fahrenheit) before they can germinate. This ensures synchronization of the mummy berry pathogen and its blueberry host. However, with sufficient soil moisture and warm spring temperatures, the fungus may be early in relation to the blueberry plant, whereas a dry, cold start of the spring may delay mummy berry germination. In that case, rains in April and May can result in later flushes of apothecia.

Infection of young blueberry shoots in the spring is directly correlated with the number of apothecia on the ground below blueberry bushes. Since 2012 was a dry year with relatively low mummy berry pressure, there are not as many overwintering mummies to be found this year, making observation even more difficult at this time. In order to facilitate spring monitoring of mummies that are well-camouflaged due to their brown color, small size and growth underneath leaf litter, it is helpful to set up mummy berry observation stations or “nurseries” in fields with a history of the disease. These stations will help you determine the percentage germination, the size of the apothecia as they develop (the larger they are, the more ascospores are released and the higher the infection risk), and when the apothecia collapse or dry up, ending the primary infection period. A hard frost at ground level may also injure apothecia, reducing their capacity to produce spores.

For optimal management of mummy berry, Michigan State University Extension advises each grower to monitor mummy berry germination on his or her own farm as germination and disease pressure tend to be site-specific. A mummy berry observation station consists of a small concentrated area of mummified berries in one or more locations on a farm. Multiple locations will help cover variability in soil type and moisture. Having the mummies concentrated in known locations will make it easier to find them and monitor them in the spring. If you are concerned about having a concentrated area of mummies in your field, don’t worry; you can spray the bushes in this area a little extra or use the site to also monitor shoot strike development.

The observation sites are selected based on conditions that are expected to be conducive to mummy berry germination, such as moist, low-lying areas, and areas along a woods edge such as areas that are normally prone to mummy berry infection on your farm. Also, you want to place them where you can have easy access to them and where they are not likely to be disturbed by farm machinery, like between two bushes at the end of a row. Sometimes mummy germination is highest in the rut next to the row, which may be a good place to set up a nursery as well. Mark the site with a colorful stake or flag so you can easily find it again in the spring. Because it is fairly easy to see the pale pink to light-purple mummies as they fall from the bushes around harvest time, late summer or early fall is the best time of the year to set up the observation stations.

Clear an area of about 1 square foot of weeds and leaves or other debris. To contain the mummies, use a barrier, such as the rim of a 5-gallon, light-colored plastic bucket. Cut the rim off about one-third down the bucket, leaving enough of the bucket to push into the ground to keep the rim in place. Leave about 2 inches of the bucket rim above the soil line. Don’t use a pan or anything with a bottom as that would change the moisture content of the soil where the mummies are located; the soil should drain naturally. Metal rings are not a good choice either due to reflection of sunlight and heating of the rim, leading to faster development or possible desiccation of apothecia near the rim.

Collect 50 to 100 mummies from areas within the field. Mummies can also be collected from the culls from blueberry sorting lines. Sprinkle 50 to 100 mummies evenly on the ground within the rim. You may press them lightly to ensure good soil contact, but don’t bury them. Then, treat the area like any other area on your farm, allowing rain and leaves to fall on the site. Weeds may make it difficult to see them in the spring, so an occasional weeding may be helpful. However, don’t disturb the mummies after October-November.

If an observation station was not established in the fall, one can still be improvised in the spring to facilitate observation. At this time, the mummies are very sensitive to being moved and apothecial initials can break or desiccate easily, especially if they are dislodged from the surrounding soil. Also, apothecia grow towards the light, so if the mummy is placed upside down, the apothecia will stop developing and will die. Carefully scoop up germinated mummies with surrounding soil and place them in indentations within the rim without changing their orientation if possible. Make sure the site is as moist as where the mummies came from and set them carefully, causing as little disturbance as possible. This may help in observing apothecium size and development.

For the first infections to occur, three conditions must be met:

  1. Apothecia must be present and have an opening of at least 2 mm.
  2. Green leaf tissue should be visible.
  3. There must at least four to six hours of moisture on the leaves.

Infection can occur at temperatures as low as 36 F, but the leaves must be wet for about eight to 10 hours in that case. Do keep in mind that under warm conditions, apothecia can go from a pin-prick opening to 2 mm in a day.

Dr. Schilder's work is funded in part by MSU's AgBioResearch.

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