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ISSN : 1598-5504(Print)
ISSN : 2383-8272(Online)
Journal of Agriculture & Life Science Vol.52 No.2 pp.13-19
DOI : https://doi.org/10.14397/jals.2018.52.2.13

Evaluation of Ascospore Prediction Model for Circular Leaf Spot caused by Mycosphaerella nawae of Persimmon

Choi Okhee1, Jung-Joon Park1,2, Byeongsam Kang3, Yeyeong Lee3, Jiyeong Park3, Jin-Hyeuk Kwon4, Kim Jinwoo1,2,3*
1Institute of Agriculture & Life Science, Gyeongsang National University, Jinju, 52828, Korea
2Department of Plant Medicine, Gyeongsang National University, Jinju, 52828, Korea
3Division of Applied Life Science, Gyeongsang National University, Jinju, 52828, Korea
4Gyeongsangnam-do Agricultural Research and Extension Services, Jinju, 52733, Korea
Corresponding author: Jinwoo Kim Tel: 82-55-772-1927 Fax: 82-55-772-1929 E-mail: jinwoo@gnu.ac.kr
January 20, 2017 April 6, 2017 August 30, 2017

Abstract


Circular leaf spot(CLS) disease causes considerable economic damage to persimmon(Diospyros kaki) in South Korea. Mycosphaerella nawae ascospores are the primary inoculum for CLS epidemics. In this study, we investigated the seasonal fluctuation of spore release and its relationship to environmental factors, based on spore trapping. We evaluated the seasonal pattern of released M. nawae ascospores in South Korea. During three persimmon growing seasons(2010 to 2012), we recorded the release of ascospores in two regions, Jinju and Gimhae, which are major producing regions of sweet persimmon in South Korea. The release of CLS ascospores was from the first week of May to the end of July. The maximum release of spores was observed in late June. A computer model used accumulated degree days to simulate ascospore release. The overall mean accumulated degree days, from 01 May to 50% ascospore release for the observed data(1174°C days), was not significantly different from the predicted value of 1144°C days. The mean differences between predicted and observed release percentages for the sampling periods were not significantly different from zero.



초록


    Ministry of Education
    2015R1A6A1A03031413

    Introduction

    Oriental persimmon, Japanese persimmon, or kaki (Diospyros kaki) is named the “food of the Gods” (from Greek, Dios meaning God and Spyros meaning food). The species seems to have originated in China, several centuries before being introduced to Japan in the 7th century. It was introduced to Korea in the 14th century. Persimmon was introduced to Europe in the 17th century and, by the 18th century, it was grown globally(Kang & Ko, 1997;Yonemori, 1997).

    Circular leaf spot(CLS) disease of persimmon, caused by Mycosphaerella nawae, is one of the most economically important diseases in many persimmonproducing areas of Asia and Europe(Kwon & Park, 2004;Berbegal et al., 2011;Vicent et al., 2012). The CLS disease only occurs on leaves, with no occurrence on fruits or young twigs. Symptoms of CLS include circular necrotic spots with green halos. The diseased leaves resemble red-colored tinted autumnal leaves, and are eventually defoliated rapidly at the end of the summer season. As a result, in Japan and Korea is called spotted leaf casting. The diseased leaf lesions during the late summer cause early maturation and fruit abscission. Infected persimmon orchards might suffer complete yield losses in September, which is two months before the normal time of harvest.

    Mycosphaerella nawae overwinters on dead leaves on the ground as immature pseudothecia, which complete their growth lifecycle in the spring. Ascospores mature as the weather becomes favorable for growth and development of the host(Kwon et al., 1997). Early spring temperatures positively influence the maturation of pseudothecia. Once pseudothecia are mature, release of ascospores is dependent on weather conditions, especially rainfall. Rainfall is also the major determinant of the number of airborne ascospores (Kang et al., 1993). The airborne ascospores attach to leaf surfaces and germinate. New leaves are infected during the spring, without symptom development until late summer, with a long incubation period(Kwon & Park, 2004). In Korea, ascospore release occurs from May to August, with peaks associated with effective disease infection between early June and the middle of July(Kang et al., 1993;Kwon & Park, 2004;Berbegal et al., 2011).

    Elimination of symptomatic leaves could be helpful in reducing disease severity, but the most practical control method is fungicide application(Berbegal et al., 2011). Fungicide application control strategies are based on preventative treatments during the spring (Berbegal et al., 2011). Once symptoms have developed, fungicide applications are no longer effective(Kwon & Park, 2004). A practical method of measuring spores is one of the key factors in disease management (Beresford & Manktelow, 2012;Kwon et al., 2014). By understanding when and how many ascospores are produced during the spring, it should be possible to time fungicide application to the periods of highest risk(Beresford & Manktelow, 2012). Models that use weather parameters to predict the availability of ascospores are important tools for enabling more efficient use of fungicides for disease control(Alves & Beresford, 2013).

    In this work, we used seasonal counts of released M. nawae ascospores to evaluate a model for predicting ascospore release. Use of this model has shown that temperature drives ascospore release, affecting the number of airborne ascospores.

    Materials and Methods

    1. Ascospore release monitoring

    Ascospore release was investigated in two regions, Jinju(35°09′09.18″N, 128°05′47.70″E) and Gimhae (35°17′00.21″N, 128°43′01.32″E), which are major sweet persimmon producing regions of in South Korea. The study was conducted from 01 May to 31 July, during 2010-2012, using the exposed glass slide monitoring method(Kwon et al., 1995;Beresford, 1999). Spore traps, consisting of glass slides(76×26 mm) coated with glycerin jelly(40 g gelatin, 100 ml distilled water, 80 ml glycerin), were vertically positioned 30 cm above the ground. At each site, three replicates of glass slides were exposed for 24 h over CLS-diseased sweet persimmon leaves, which were collected from each orchard block every year, and overwintered at each monitoring site. Glass slides were retrieved and replaced by new slides daily. Glass slides were then mounted with a drop of distilled water and covered by a cover glass(18×18 mm). Spores trapped under the cover glass were counted with a light microscope at magnifications of 100-400×, and the number of spores per square centimeter was recorded. This method provides information on the relative numbers of ascospores available and it could be used to identify the beginning, peak, and end of the ascospore release season(Kwon & Park, 2004;Beresford & Manktelow, 2012;Kwon et al., 2014).

    2. Weather data

    Automatic weather stations at each of the ascospore monitoring sites provided daily records of temperature, relative humidity, and rainfall. Daily mean temperatures for degree day accumulation were calculated from air temperature readings(Beresford, 1999).

    3. Ascospore release model

    Ascospore release was modelled by the sigmoid function, which predicted the cumulative percentage of the season’s ascospore release at any number of cumulative degree days, starting on 01 May. The cumulative percentages of observed ascospores released were plotted against degree-day accumulation. Using the degree day model(Alves & Beresford, 2013), the observed ascospore release was compared with the cumulative percentage of released ascospores (y) predicted:

    Cumulative ascospore release(%)= 100/{1+exp[-(DD-1144)/286.5]} Where DD is the degree day accumulation.

    Where DD is the degree day accumulation.

    Results and Discussion

    1. Ascospore release monitoring

    This study monitored percentages of the season’s total ascospores that were released daily. Actual numbers of ascospores depended on the amount of inoculum that overwintered in individual orchards. Release of the CLS ascospores started during the first week of May and continued until the end of July. By late June, 50% of the spores were released. The overall mean value for the two regions was 1174℃ days. The cumulative rainfall in milliliters for each region was used to compute the number of milliliter days at which 50% release occurred for the observed data. The overall mean values, over the two regions, were 257 milliliter days, for an average of 18 days(Table 1). Overlap of temperature fluctuation and ascospore release counts in the two monitoring regions was determined(Fig. 1). Comparisons of temperature fluctuations and ascospore release counts in two monitoring regions yielded surprising insights into the temperature pattern of the bottleneck period running from the date of initial ascospore release to the date of 50% release(Fig. 1, Table 1). The CLS risk period was designed to detect days on which there was a risk of 20-80% of the ascospores being released(Fig. 1).

    2. Ascospore release model

    To quantify the time at which 50% spore release occurred, based on the observed data in each region, the cumulative curves in Fig. 2 were linearized by logit transformation. The regression parameters for each region were used to calculate the number of degree days at which 50% release occurred for the observed data(Fig. 2). The mean value across both regions 1174℃ days was greater than the value of 1144℃ days used in the model, but the model’s predicted value was contained within the 98% confidence interval of the observed data(Fig. 2).

    Linear regression analyses demonstrated that the model with degree day accumulation starting on 01 May produced the highest coefficient of determination(R2) of 0.8966(Fig. 3). Degree day models, which described ascospore maturation, had been developed previously for black spot(scab) diseases caused by Venturia inaqulis for apple(Gadoury & MacHardy, 1982;Schwabe et al., 1989;Beresford, 1999). The model provided apple growers with knowledge of when the release season begins, when it is likely to peak, and when it ends(Beresford, 1999). An ascospore release prediction model, which used a degree day model in conjunction with rainfall to predict the daily ascospore release, predicted the risk of ascospore release from day to day, rather than just the underlying potential for release according to the stage of the ascospore release season(Beresford, 1999). To estimate the value of a parameter based on units of an environmental factor, it was necessary to select a stage in ascospore release as the starting point for data accumulation.

    In this study, the environmental factor of concern was temperature, expressed as degree days. The linear statistical model of ascospore release was a regression equation that described the relationship between degree day accumulation and ascospore release. A statistical description of the model would prove useful in research, but persimmon growers might find it difficult to understand meaningful values. Persimmon growers could estimate ascospore release from their orchards based on degree day accumulation, if provided with the graph shown in Fig. 2 and a set of guidelines.

    The results from this study appear to be adequate for use in timing fungicide application for CLS disease control. Three ways in which the results from this study could help persimmon growers make decisions on timing fungicide application are: 1)identifying the start of ascospore release to assess the need for early fungicide protection; 2)identifying exactly when the rapid increase in ascospore release occurs so protection using fungicide application could be maintained over the period of highest risk; and 3)identifying the end of the ascospore release so that growers who have achieved good control of ascospore infection could safely reduce fungicide applications(Beresford, 1999;Kim et al., 2016).

    A practical method of measuring these spores is one of the key factors for disease management (Beresford, 1999;Kwon & Park, 2004;Beresford & Manktelow, 2012;Kwon et al., 2014). If we can understand when and how many ascospores are produced during spring, it should be possible to schedule fungicide applications during high-risk periods. We believe that our efforts will facilitate estimation of the commencement, peak, and end of ascospore release, thus allowing fungicides to be appropriately applied.

    Acknowledgement

    This research was supported by Basic Science Research Program through the National Research Foundation of Korea(NRF) funded by the Ministry of Education(2015R1A6A1A03031413).

    Figure

    JALS-52-13_F1.gif

    Overlap of temperature fluctuation and ascospore release counts in two monitoring regions, Jinju and Gimhae, which are major producing regions of sweet persimmon in South Korea, from 01 May to 31 July of 2010-2012. The CLS risk period was designed to detect days on which there was a risk of 20-80% ascospores being released.

    JALS-52-13_F2.gif

    Three years of Mycosphaerella nawae ascospore counts, from Jinju and Gimhae, as cumulative percentages of the ascospores released as observed from glass slide ascospore counts versus degree day accumulation in two regions, from May to July of 2010-2012.

    JALS-52-13_F3.gif

    Differences between modelled and observed percentage ascospore release in two regions from May to July of 2010-2012. Modelpredicted cumulative percentage ascospore release versus observed cumulative ascospore release, with the regression line: y=1.0063x + 0.5379(R2=0.8966).

    Table

    Ascospore release, degree days, milliliter days, and rainfalls to 50% release for observed data in two regions

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