Now is a good time to monitor for mummy berry in blueberries
It’s mummy berry season again. The mummy berry fungus enjoyed the snow cover this winter, which provided sufficient moisture for mummies to germinate. However, the rate of germination is lower than last year, with a maximum of 6 percent mummies germinated at this time. The extended cold weather seems to have delayed their development. In addition, some sites are very wet and many mummies may actually be submerged. It is not known how well they can survive waterlogging, but wet soils in general are conducive to disease. At this time, some mummies are showing small finger-like extensions (apothecial initials) and some have small trumpet-like mushrooms (apothecia) ranging from having pin-prick size openings to about 1-2 mm in diameter. At 2 mm (1/12 inch) in diameter, they can start to release ascospores. However, the most spores are released when apothecia are 5-10 mm in diameter (1/4 to 2/5 inch). If there is no leaf tissue on the bushes, it does not matter since infection cannot take place without green tissue being visible. The mummies typically germinate over several weeks to a month, depending on temperature and soil moisture, so there may be more waves of germinating mummies ahead.
What to look for
Blueberry growers should be monitoring for mummies with trumpet-shaped mushrooms (see pictures). The number of germinated mummies (specifically the number of visible apothecia) is a better predictor of disease than simply the number of mummies under a bush, since germination is prerequisite for ascospore release and disease development. Mummy berry occurs primarily at wetter sites and in poorly drained areas; therefore scouting should target those sites. Dry, sandy sites may not have any mummies at all. The mummy berry fungus shoots ascospores out of the apothecial cup as soon as the cup diameter is about 2 mm (1/12 inch) wide. Ascospore release continues until the cup collapses. Longevity of the mushrooms is affected by temperature close to the ground, e.g., at 70ºF, the mushrooms may live for less than a week, whereas at 50ºF, they can last two to three weeks, and at 40ºF up to four weeks. At higher temperatures, the mushrooms expand more quickly (they can almost become dime-sized) and release more spores per day than at lower temperatures. A severe freeze may damage the cups, but research shows that they can partially recover their ability to shoot ascospores after exposure to temperatures of 22ºF and above. The ascospores are windborne and can travel fairly long distances (supposedly up to a mile). So even if you don’t have any mummy berry in your field, there is a chance that ascospores can drift in from other fields or nearby woods with wild or escaped blueberries.
Stages of infection
There are two stages of infection. First the developing shoots are infected by the ascopores released from the mummy berry apothecia. Shoot strike symptoms appear approximately two weeks after infection. Shoots are susceptible from bud break until they are about 2 inches in length. Sometimes flower clusters may also become blighted; these are called flower strikes. Both shoot and flower strikes are characterized by drooping/wilting symptoms and a layer of gray spores (conidia) on the surface. These conidia are spread by insects (primarily bees), wind and rain. Bees are attracted to the shoot and flower strikes due to their UV light pattern (a nifty trick of the pathogen) and pick up the conidia on their legs and bodies. Bees then inadvertently deliver the conidia to the flowers where infection takes place.
The conidia infect the flower stigma followed by colonization of the developing fruit, which eventually mummifies and drops to the ground. Flowers are susceptible for about four days after they open. The more shoot strikes there are and the better the weather for pollination, the greater the risk of flower and fruit infection. Cultivars such as Berkeley, Bluetta, Blueray, Earliblue, Jersey, Nelson, Patriot and Weymouth are susceptible whereas Bluecrop, Duke and Elliott are moderately resistant to the disease. Some cultivars are more susceptible to shoot strikes and less susceptible to fruit infection, whereas others are just the opposite.
While there are multiple fungicides registered for mummy berry control, Indar consistently has outperformed other fungicides for both the primary and secondary phases of the disease in Michigan. Indar is a sterol inhibitor fungicide and therefore prone to resistance development in target fungi. It is recommended to limit the number of sprays of Indar to a maximum of two or three per season (five are allowed per the label). Orbit (propiconazole) and PropiMax (propiconazole), which are in the same chemical class as Indar, now both have a supplemental label for blueberries. Indar, Orbit and PropiMax all have a 30-day PHI. In small plot trials in Michigan, we found that Orbit was similar to Indar in the control of shoot strikes, but did not perform as well as Indar for control of fruit infection. PropiMax has not been tested in Michigan, but is expected to behave similarly to Orbit.
For fungicide resistance management, it is important to alternate SI’s with fungicides in different modes of action, such as Bravo (fair to moderate efficacy), Captevate (moderate to good efficacy) Topsin M + Captan or Ziram (moderate efficacy), and Serenade (moderate to good efficacy). Systemic fungicides such as Indar and Topsin will likely provide better coverage of the flower parts (the stigma specifically). Cabrio and Abound have shown poor to fair efficacy in past trials in Michigan. While Pristine did not perform particularly well for control of mummy berry shoot strikes in small-plot trials in Michigan, it provided good control of fruit infection. We suspect that the activity of Pristine is better at higher temperatures; it therefore may be a good option during bloom as it also controls anthracnose, Botrytis and Phomopsis twig blight and canker.