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Correlation of Signs and Symptoms of Mummy Berry in Highbush Blueberry

T.D. Miles and A.C. Schilder Department of Plant Pathology Michigan State University 107 Center for Integrated Plant Systems East Lansing, MI 48824 USA

Keywords: Monilinia vaccinii-corymbosi, Vaccinium corymbosum, integrated pest

management, apothecia, shoot strikes

Abstract As part of a larger project aimed at demonstrating the benefits of scouting for

management of insects and diseases in highbush blueberry in Michigan, mummy berry (Monilinia vaccinii-corymbosi) signs and symptoms were quantified. Characteristic symptoms include blighting of young shoots and flower clusters and mummified berries. In this study, four field sites in southwest Michigan (cultivars Rubel, Jersey, and Blueray) were evaluated each week from April to August 2007. At the beginning of the season, one field had low, two fields had intermediate and one field had high numbers of overwintering mummies. The numbers of overwintered mummies, germinated mummies, apothecia, shoot strikes, and newly infected berries were visually determined at ten locations per field. Regression analysis was conducted to determine which variable would best predict the number of primary and secondary infections. The amount of overwintered inoculum at the beginning of the season was positively correlated with incidence of shoot and fruit infection. The number of apothecia was generally a better predictor of disease incidence than the number of overwintered mummies or the number of germinated mummies. Inoculum and disease levels varied between bushes and rows within a field; therefore it is important to scout several bushes in multiple rows.

INTRODUCTION

Mummy berry is a serious disease of highbush blueberries in Michigan and is prevalent in most blueberry growing regions throughout the United States. Mummy berry can cause moderate to severe crop losses, depending on inoculum level, cultivar susceptibility, and environmental conditions (Hildebrand et al., 1995). The disease is caused by the fungus Monilinia vaccinii-corymbosi and is characterized by blighting of young shoots and, to a lesser extent, flower clusters. Fruit infection leads to the shriveling and mummification of berries, hence the name mummy berry (Hildebrand et al., 1995). Most mummified berries fall to the ground before harvest, but some make it into harvested lugs. There is a zero tolerance for mummified berries in processed fruit (Scherm and Copes, 1999).

In spring, mummies germinate by producing small, brown, goblet-shaped apothecia that forcibly discharge ascospores into the air (Hildebrand et al., 1995) (Fig. 1A). The percentage germination varies with environmental conditions, with dry winters and sites generally leading to lower germination rates. Ascospores infect developing shoots and lead to wilting and browning of the midribs and lateral leaf veins (Fig. 1B). Blighted shoots are called shoot strikes and are the first symptom of mummy berry disease on the plant. Under humid conditions, tan to gray, powdery spore masses (conidia) form on infected shoots. Blighted flowers (flower strikes) occur occasionally. Conidia are disseminated by insects and wind to open flowers, and infect the developing fruit through the stigma and style (Hildebrand et al., 1995). As the fruit approaches maturity, infected berries turn pink or light brown, shrivel up and take on a whitish cast (Fig. 1C). Mummified berries fall from the bush and overwinter on the ground (Fig. 1D).

Proc. IXth IS on Vaccinium Eds.: K.E. Hummer et al. Acta Hort. 810, ISHS 2009

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Several studies have explored the relationship between environmental parameters and ascospore dispersal. Ramsdell et al. (1975) detected ascospores in the air between April 3 (before bud break) and May 8 (prebloom), 1973. Peak ascospore release occurred when the highest quantity of apothecia were present on the ground. Monitoring apothecia may be a good alternative to the time-intensive monitoring of ascospore release. Disease monitoring is a component of integrated pest management and helps to develop need-based fungicide recommendations. Since mummies overwinter on the ground below bushes, they can be observed and quantified to confirm the presence of the disease in a field and potential disease pressure at the beginning of the season. The objective of this study was to evaluate signs and symptoms of mummy berry in Michigan blueberry fields and examine their correlation in order to improve the efficiency of disease scouting and management.

MATERIALS AND METHODS Scouting procedures

Four blueberry fields [A=Covert (‘Jersey’), B=Grand Junction (‘Blueray’), C=Holland (‘Blueray’), and D=West Olive (‘Rubel’)] in western Michigan were evaluated once per week from April through August 2007. Fields received several fungicide sprays according to the grower’s standard program. At the beginning of the season, one field (A) had low, two fields (C, D) had intermediate and one field (B) had high numbers of overwintered mummies. At each site, two rows of five bushes (spaced five bushes apart) were scouted once a week throughout the season for signs and symptoms of mummy berry. Throughout the season, the number of overwintered mummies, germinated mummies, apothecia, shoot strikes, and newly mummified fruit were counted. Overwintered mummies and apothecia were monitored by visually examining a 1-m2 area of the soil surface at the base of each bush. Shoot strikes were identified by the brown oak leaf pattern along the veins of wilting leaves and a layer of gray powdery spores on the upper part of the leaf and petiole. Flower strikes also were observed occasionally, but were not counted due to the difficulty in distinguishing them from other blossom blights. The growth stage of each bush was noted each week. New fruit infections were identified by the tan or brown discoloration of berries in clusters on the bush and by visually examining a 1-m2 area underneath the bush for new, purplish white mummies. Statistical analysis:

The incidence of mummy berry signs and symptoms (averaged over 10 bushes) was plotted against the respective scouting dates for each site using Microsoft Excel (Microsoft Corporation, Redmond, Washington). Since our goal was to determine the single best disease variable to scout for, simple linear regressions were run in Statgraphics Plus (StatPoint, Inc., Herndon, Virginia) using the number of newly mummified fruits or the number of shoot strikes per bush versus the number of overwintered mummies, number of germinated mummies, and number of apothecia at these respective sites. The number of newly mummified berries was also regressed against the number of shoot strikes. In each case, the peak number was used for the regression analysis. Regression equations and coefficients were calculated using Excel. RESULTS AND DISCUSSION

At field sites A-D, mature apothecia were observed on overwintered mummies from April 13 until May 11 (shoot expansion). The maximum germination of mummies was 12% at site A, 26% at site B, 29% at site C and 33% at site D. A dry period occurred between May 1 and May 8 which caused apothecia to desiccate. Shoot strikes were observed from May 11 to June 8 (Fig. 2). Newly infected berries were observed during fruit ripening from July 6 to July 20. There was considerable variability in disease incidence among sites, although the timing of apothecium and disease development were

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similar in all fields. At site A, which was fairly dry, only two apothecia were found per 10 bushes, and shoot strike and fruit infection incidence remained relatively low throughout the season. Site B, a very wet site, started out with high numbers of overwintered mummies with apothecia, and had the highest number of shoot strikes and a moderate number of newly mummified fruits. Site C and D were intermediate in the number of apothecia, shoot strikes, and newly mummified fruits through the season (Fig. 2). Fungicide applications made by the growers may have affected disease development but in general were not very effective at controlling the disease.

Regression analyses indicated that the number of apothecia was a better predictor of shoot strike symptoms than the number of overwintered mummies or the number of germinated mummies on sites. Apothecia are somewhat easier to spot than mummies, since mummies are dark and often partially buried under leaf debris. There also was a significant (p<0.05) positive correlation of the number of newly formed mummies with the number of apothecia at the beginning of the season in three of the four fields and with the number of shoot strikes in two of the four fields. The correlations were strongest in field C (Fig. 3) and weakest in field A, where disease pressure was very low. CONCLUSIONS

This study indicated that the amount of overwintering inoculum at the beginning of the season was positively correlated with the amount of disease later in the season. The number of apothecia was generally a better predictor of disease incidence than the number of overwintered mummies or the number of germinated mummies, and can be used for monitoring and making management decisions. ACKNOWLEDGMENTS

The authors gratefully acknowledge funding from the EPA Pesticide Environmental Stewardship Program and the USDA Crops at Risk Program. The authors would also like to thank Karlis Galens, Larry Bodtke, Douglas Wassink and Robert Carini for graciously allowing use of their farms. We also thank Rufus Isaacs and Keith Mason for advice and Christine Bates for technical assistance. Literature Cited Hildebrand, P.D., Milholland, R.D. and Stretch, A.W. 1995. Mummy berry. p. 11-12. In:

F.L. Caruso and D.C. Ramsdell (eds.), Compendium of Blueberry and Cranberry Diseases. APS Press, St. Paul, MN, USA.

Ramsdell, D.C., Nelson, J.W. and Myers, R. 1974. An epidemiological study of mummy berry disease of highbush blueberries. Phytopathology 64:222-228.

Ramsdell, D.C., Nelson, J.W. and Myers, R.L. 1975. Mummy berry disease of highbush blueberry: epidemiology and control. Phytopathology 65:229-232.

Scherm, H. and Copes, W.E. 1999. Evaluation of methods to detect fruit infected by Monilinia vaccinii-corymbosi in mechanically harvested rabbiteye blueberry. Plant Dis. 83:799-805.

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Figures

Fig. 1. Signs and symptoms of mummy berry disease that can be monitored in highbush

blueberry: A) Mature, germinated mummy with light brown apothecia resembling small goblets (Photo by Phillip Wharton). B) Shoot strike with a typical “oak leaf” pattern of necrosis along the midrib (Photo by Phillip Wharton). C) Infected blueberries with a whitish cast in cluster of healthy blueberries. D) Mummified berries on the ground below blueberry bush after harvest.

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Fig. 2. (A-D) Average incidence (number per bush) of apothecia, shoot strikes and newly mummified fruits in blueberry fields during the 2007 growing season. The field sites were located in A) Covert (‘Jersey’), B) Grand Junction (‘Blueray’), C) Holland (‘Blueray’), and D) West Olive (‘Rubel’), Michigan.

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Fig. 3. Correlation of disease variables for mummy berry in blueberry (‘Blueray’, site C) in Holland, Michigan in 2007. A) Number of shoot strikes per bush as a function of the number of apothecia below the bush. B) Number of newly formed mummified fruits as a function of the number of apothecia below the bush. C) Number of newly formed mummified berries as a function of the number of shoot strikes on the bush.

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