Mummyberry of Highbush Blueberry in Northwest Washington
Dalphy Harteveld and Tobin Peever
Department of Plant Pathology, Washington State University
Mummyberry disease of blueberry, caused by the fungus Monilinia vaccinii-corymbosi, is the most economically important disease of northern highbush blueberry in Washington. Disease management options for both conventional and organic growers are available, but are unable to consistently control the disease. Most mummyberry management strategies were developed outside Washington possibly making such strategies less effective under WA conditions. Research at the WSU Mount Vernon Research Center focuses on the mummyberry disease cycle to improve our understanding of the disease in NW Washington and aid in the establishment of improved control strategies for this region. This article reports on our 2015 field data concerning timing of various stages in the mummyberry life cycle and presents some preliminary results on variation in susceptibility of blueberry cultivars grown in NW Washington to mummyberry disease.
Timing of ascospore release
Overwintering of the mummyberry pathogen occurs as blue-black, pumpkin-shaped berries called mummies on the ground under blueberry bushes. In early spring, small mushrooms, called apothecia, develop from the mummies (Fig. 1). The apothecia release ascospores that are then spread to emerging buds on blueberry plants via wind. These ascospores spread within 30 meters and infect emerging green tissue of blueberries breaking bud in the spring. The timing of this ascospore release and plant infection are the most important events in mummyberry disease development. Starting in January 2015, mummies in several fields with a history of severe mummyberry in Whatcom and Skagit counties were monitored for timing of apothecia development. The first observations of apothecial initials were found at the end of January. After developing for a month, the first mature apothecia were found in both counties. To determine the timing of ascospore released from apothecia into the field, spore traps were used. Spore traps capture spores released from the apothecia over a period of time by impacting an airstream against a rotating tape that is coated with petroleum jelly. Ascospores are then detected on the tapes using a microscope in the laboratory. Results from 2015 showed that ascospores were released between the first week of March through to the second week of April in both Skagit and Whatcom counties.
Variation in host susceptibility
The disease cycle of mummy berry has two stages of infection, primary and secondary. Primary infections are caused by ascospores infecting emerging leaf and flower buds and result in what are called shoot and flower strikes. Mummyberry shoot strikes first appear as dark brown discoloration or necrosis of the leaf veins. Over time, necrotic areas enlarge forming an irregular shaped lesion over the length of the leaf blade, which emerges from the mid-vein of the leaf outwards to covering the entire leaf (Fig. 2). Infected tissue is colonized by the fungus and a fluffy, gray growth of the fungus appears on the leaf surface (Fig. 2). Mummyberry flower strikes are characterized by browning of the flowers and gray fluffy growth visible on the flower pedicels (Fig. 3). The gray fluffy growth on leaf or floral surfaces produces a second spore type called conidia in a sticky UV refractive matrix that is attractive to pollinating bees. These pollinators carry the conidia to open flowers where they penetrate the female floral parts mimicking the growth of pollen, eventually invading the ovary and colonizing the developing fruit.
Figure 2: Mummyberry shoot strike on northern highbush blueberry.
Figure 3: Mummyberry flower strike on northern highbush blueberry.
Blueberries are susceptible to infection by ascospores throughout the early stages of leaf and floral bud break. Blueberry cultivars vary significantly in timing of bud break as well as flowering and fruit development and apothecia only produce ascospores for a limited period during the growing season. Some cultivars are more likely to get infected than others depending on how long and how much susceptible tissue is available during the period that apothecia are producing ascospores. In order to determine if cultivars vary in the timing of susceptibility, three fields each of the most popular cultivars growing in WA, Duke, Bluecrop, Liberty and Draper, were selected in Whatcom and Skagit counties. Stages of leaf and flower development were monitored weekly throughout the season. Results show that susceptible tissue was present on all four cultivars in both counties between the first week of March and the second week of April, which also coincided with the ascospore release period. Within this period, Duke and Bluecrop showed the highest numbers of susceptible flower buds earliest, around March 10, whereas Liberty showed the highest numbers about a week later, around March 17, in both counties (Fig. 4).
Figure 4: Percentage susceptible flower buds during ascospore release in four blueberry cultivars in Skagit and Whatcom counties.
The flower development in Draper varied between the two counties and showed the highest number of susceptible flower buds around March 10 in Whatcom and around March 17 in Skagit County (Fig. 4). Peak amounts of susceptible leaf buds were observed earliest on Bluecrop at around March 3 in both counties (Fig. 5).
Figure 5: Percentage susceptible leaf buds during ascospore release in four blueberry cultivars in Skagit and Whatcom counties.
Draper showed the highest number of susceptible leaf tissue around March 24 in Whatcom and a week later in Skagit County. Liberty and Duke varied in their timing expressing susceptible leaf buds, where in Whatcom, both cultivars exposed high numbers of susceptible tissue for a longer period, from mid-March to the end of March, whereas in Skagit peak amounts were observed only in first week of April (Fig. 5).
In order to determine which cultivar showed the highest amount of susceptible tissue during the ascospore release period, the total number of susceptible leaves and flowers were calculated. All cultivars had a larger amount of susceptible flower tissue available compared to leaf tissue (Fig. 6).
Figure 6: Amount of susceptible flower and leaf tissue on blueberry plants during ascospore release in Whatcom and Skagit counties during the 2015 season.
Liberty had the highest number of susceptible flower buds, followed by Draper, Bluecrop and Duke (Fig. 6). This was observed in both counties. In contrast, Bluecrop had the largest number of susceptible leaf buds compared to the other three cultivars. Field-to-field variation was significant in Skagit but not in Whatcom County for both tissue types.
Our findings showed that flower buds may play a more important role in the mummyberry disease cycle compared to leaf buds and indicating that the timing of disease control may need to focus more on floral development rather than leaf development. Based on the timing of susceptible tissue, our results indicate that among the four main cultivars grown in WA, Liberty and Draper may be more susceptible to mummyberry than Duke and Bluecrop. However if ascospores are released early in spring, Duke and Bluecrop may have a higher chance to get infected than later in the season. These findings will be used in further research towards improved control strategies for mummyberry in NW Washington.
Contact information: Dr. Dalphy Harteveld, email: firstname.lastname@example.org, phone: 360-848-6130, address: WSU Mount Vernon Research and Extension Center, 16650 State Route 536, Mount Vernon, WA 98273 and Dr. Tobin Peever, email: email@example.com, phone: 509-335-3754, address: Department of Plant Pathology, WSU, Pullman, WA 99164-6430.
Figure 1: Mummies with emerging apothecia.