Apr 28, 2016

Mummy berry germinates in Michigan blueberries

Mummy berry is caused by the fungus Monilinia vaccinii-corymbosi and is characterized by blighting of young shoots, which are referred to as “shoot strikes.” Fruit infection leads to the shriveling and mummification of berries, hence the name mummy berry. These mummies overwinter on the ground below blueberry bushes. At the beginning of the growing season, small, trumpet-shaped apothecia develop on the overwintered mummies and start releasing ascospores which infect young leaves.

Previous research has shown that the optimum temperature for formation of apothecia is 50 to 57 degrees Fahrenheit. These temperatures are also very conducive to leaf infection, which requires four to six hours of continuous leaf wetness under those conditions. Leaf wetness can come from rain, dew, fog or overhead irrigation water. Infection can occur at temperatures as low as 36 F, but a wetness period of 10 hours is required for infection at that temperature. The risk of infection is reduced at temperatures of 80-86 F and above.

In general, longer wetness periods lead to more infections, observed as higher numbers of shoot strikes. Also, frost injury increases susceptibility of the shoots to infection. In past trials, fungicide applications within 24 hours of a frost event significantly reduced shoot strike incidence.

At this time in Michigan fields, mummies have germinated and apothecia or apothecial initials are present. We had already started seeing “apothecial initials,” the horn-like structures developing on overwintered mummies in March, indicating that the mummies were ready. The mummy germination rate is 20 to 45 percent this spring, with higher germination rates and more advanced development at wetter locations. This germination rate is pretty typical of most years in Michigan. Spore release starts when the apothecial opening is about 2 millimeters and continues to increase as the apothecia expand.

Open apothecial cups have been reported and now that the blueberries are at green tip or leaf expansion, this indicates a risk of primary infection. In general, the more apothecia that are present and the larger the apothecial cups, the greater the risk of infection. Michigan State University Extension therefore advises scouting in fields with a history of mummy berry to confirm the stage of development of the fungus. Continued scouting will also help determine when infection risk has passed. Remember that there can be several flushes of apothecia depending on soil moisture. Later flushes of apothecia are actually more risky due to the presence of shoot strikes with fresh spores during bloom.

When soils stay cool and moist, apothecia may last for two to three weeks and release spores on a daily basis, especially in the morning as the relative humidity starts to drop. Ascospores are dispersed by wind, which picks up in speed in late morning. Warm and dry soils will lead to quicker deterioration of the apothecia. You can use MSU’s Enviro-weather website to monitor for mummy berry infection conditions at nearby weather stations. There is no specific model for mummy berry, but blueberry growers can use the Multi-Crop Disease Summary tool in the fruit section of Enviro-weather to find the hours of wetness and the average temperature during the wetting period for all the stations in the region.

Fungicides work best when applied preventatively (before infection) because the fungus grows too fast during optimal conditions for post-infection sprays to work well. While the risk of fungicide resistance development appears to be relatively low for this pathogen, it is nonetheless advisable to alternate fungicide chemical classes as indicated by different Fungicide Resistance Action Committee (FRAC) code numbers. If shoot strikes are controlled well – you can scout fields to confirm this – and no shoot strikes are present during the bloom period, the risk of fruit infection is minimal, unless there is a nearby field where shoot strikes are present. Spores produced on the shoot strikes can be carried by bees to nearby fields that are in bloom. Be careful with fungicide applications during bloom; avoid spraying after bee hives have been placed in the field or spray at night when bees are not active.

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

—  Annemiek Schilder and Mark Longstroth, Michigan State University

Source: Michigan State University Extension