Fruit Crops: A Summary of Research 1998

Research Circular 299-99

Evaluation of an Empirical Model for Predicting Sooty Blotch and Flyspeck of Apples in Ohio

Michael A. Ellis, Laurence V. Madden, and L. Lee Wilson

Introduction

Sooty blotch (SB) of apples, a disease complex caused by Peltaster fructicola Johnson, Sutton & Hodges, Leptodontium elatius (G. Mangenot) De Hoog, Geastrumia polystigmatis Batista and M. L. Farr, and other fungi (7, 8), and flyspeck (FS), caused by Schizothyrium pomi (Mont. and Fr.) Arx. (anamorph: Zygophiala jamaicensis E. Mason), are the most common summer diseases of apple (Malus x domestica Borkh.) in Ohio. Infections by fungi that cause these diseases may occur soon after petal fall (mid to late May to early June) in Ohio; however, symptoms generally do not appear until late July or August. The incidence and severity of sooty blotch and flyspeck are dependent upon moisture and temperature conditions (1, 6, 7, 10, 13). Brown and Sutton (2) reported that, in the mountainous apple-growing areas of North Carolina, the average daily temperatures from petal fall to harvest rarely fall outside the optimum range for the SB and the FS fungi. Therefore, symptom development is most likely dependent on the frequency of moisture and, to a lesser extent, on temperature. Similar temperature patterns occur in Ohio from petal fall to harvest.

Models have been developed for several apple diseases to warn that infection has occurred (4, 9). These models are based on measures of moisture (leaf wetness, rainfall, or relative humidity) and temperature. However, there are fewer models that predict the appearance of symptoms (11, 14). Brown and Sutton (2) have developed a model that predicts when symptoms of SB and FS first appear in the orchard. Their model is based on cumulated hours of wetting of four hours duration or greater, beginning with the first rain 10 days after petal fall. They suggest the use of a threshold of 200 or 225 hours of accumulated wetness to initiate a fungicide program for controlling SB and FS. The model does not consider temperature effects and has been validated at several locations in North Carolina (2) and in Kentucky (5).

In Ohio, SB and FS generally require a 10- to 14-day protectant fungicide program throughout the summer to provide effective control. During dry growing seasons, the spray interval can generally be extended; however, there is no scientific basis to determine how far spray intervals can be extended. By predicting when symptoms first appear in the orchard, Brown and Sutton's model could serve as a guide for initiating fungicide applications for controlling these diseases. In dry growing seasons, the use of such a model could result in reduced fungicide use. The objective of this study was to evaluate the model under Ohio conditions.

Materials and Methods

Validation studies were conducted during three growing seasons (1993, 1994, and 1995) in three orchards located at the Ohio Agricultural Research and Development Center's Snyder Farm on The Ohio State University's Wooster, Ohio, campus. One orchard (Mark orchard) consisted of four-tree blocks of the cultivars 'Delicious,' 'McIntosh,' 'Golden Delicious,' and 'Rome,' in that order from east to west, planted 3 m apart within the row with 9 m between rows. The wide spacing between rows was to facilitate the use of the orchard for fungicide screening trials. All trees were planted on Mark rootstock in 1986.

The second orchard (M.9 orchard) consisted of six-tree blocks of 'Liberty' alternated with 'McIntosh,' planted on M.9 rootstock in a high-density system trained to a three-wire trellis. Trees were spaced with 2.1 m between trees and 3.7 m between rows. The orchard was planted in 1989.

The third orchard was a 30-year-old abandoned orchard of the cultivar 'Cortland' on seedling rootstock. This orchard is situated on the northeast corner of the farm and is located approximately 500 m from the other test orchards. Unsprayed fruit in the abandoned orchard were also examined for SB and FS symptom development on the same schedule as the test orchards. The date of first symptom development was recorded for each disease in each orchard.

Cumulative hours of leaf wetness of four hours duration or greater, beginning with the first rain 10 days after petal fall, were recorded using a Belfort leaf-wetness recorder manufactured by Belfort Instruments Company, 727 S. Wolfe Street, Baltimore, Maryland 21231-3513. The instrument uses a hemp string as a sensing element and functions in a similar manner to the de Wit leaf wetness meter (12) that was used by Brown and Sutton to develop the SB and FS model (2). Changes in the length of string in response to wetting are recorded on a revolving seven-day chart. The instrument was placed near the center of the Mark orchard. It was placed on the north side of a tree, within the drip line, approximately 1.5 m above ground. An hour was considered wet when there was a 50% or greater deflection in the recording pen from the dry to wet position. Fruit on unsprayed trees of the cultivars 'Golden Delicious,' 'Liberty,' and 'Cortland' in the Mark, M.9, and abandoned orchards, respectively, were examined every other day for symptoms of SB and FS beginning at 10 days after petal fall.

During the 1993 and 1994 growing seasons, fungicide applications were made in the Mark orchard only for SB and FS control in response to various numbers of accumulated wetness hours. In 1993, two treatments in the annual fungicide evaluation trial received applications of Captan 50WP at 4 lb/A at inch green, followed by Nova 40WP at 5 oz. per acre on a 10-day schedule, from tight cluster to second cover for control of apple scab. Nova is known to have little efficacy for control of SB and FS. Treatments consisted of four, single-tree replications of 'Golden Delicious' arranged in a randomized complete block design. Fungicides were applied dilute (300 gal/A) with a handgun at 450 PSI. Petal fall occurred on May 13, and second cover occurred on May 27.

Accumulation of wetness hours was initiated at 10 days after petal fall on May 23. Fungicide applications in these plots were suspended after second cover until wetness hours reached 200 and 250 hours, respectively. Foliar applications of Benlate 50W 12 oz. per acre plus Captan 50W 4 lb. per acre were applied to each plot when the designated number of hours were reached. Additional cover sprays were then maintained on a 14-day schedule until harvest. Percentage of fruit infected by SB and FS was determined on 25 fruits per tree (replication) for each treatment (200 and 250 accumulated wetness hours) and an untreated control at harvest.

In 1994, two treatments were repeated as previously described except that 300 and 325 wetness hours were used to initiate cover sprays after second cover. Petal fall occurred on May 18, and accumulation of wetness hours was initiated on May 28 (10 days after petal fall). Second cover was made on June 2.

Results

During all three years of testing, the number of wetness hours required until the first symptoms of SB and FS were visible exceeded the 200- to 225-hour threshold in the Mark and M.9 orchards (Table 1). In 1993, conditions were very dry, and a total of only 277 hours of wetness was accumulated for the entire season. Although the authors initiated fungicide treatments at 200 (Aug. 3) and 250 (Sept. 8) hours of wetness, no SB or FS developed. By initiating cover sprays at 200 and 250 hours, a total of four and six sprays were saved, respectively (Table 2). For the 250-hour treatment, no additional sprays were made after second cover. In the abandoned orchard, symptoms of SB were observed at 225 hours and FS at 240 hours. Although disease incidence in the abandoned orchard was 100%, disease severity was low (less than 5%).

In 1994, similar results were observed. Due to the lack of symptom development in 1993, the authors extended the threshold period to 300 and 325 hours. First symptoms of both diseases were observed at 392 hours on July 25 in the Mark and M.9 orchards. A total of 657 hours accumulated for the total season (ending Aug. 30). At harvest, the incidence of SB and FS was 100% and 79%, respectively, on unsprayed 'Golden Delicious' fruit in the Mark orchard; however, severity was less than 2% for each disease. By initiating fungicide sprays at 300 hours (July 9) and 325 hours (July 12), two fungicide applications were saved and disease control was still 100%. In the abandoned orchard, first symptoms of SB and FS were observed after 230 hours (June 30) and 244 hours (July 2), respectively. At harvest, disease incidence and severity were 100% and 85%, respectively.

In 1995, SB and FS first appeared in the Mark and M.9 orchards after 489 hours of wetness on July 28. Symptoms of both diseases first appeared after 241 hours of wetness in the abandoned orchard.

Discussion

Conditions for SB and FS development in the Mark and M.9 orchards were not highly conducive. Both orchards have well-pruned (3) dwarf trees and are in an open area that promotes good air movement. In addition, the orchards are approximately 500 m from the nearest woods. There were no wild brambles within the orchard to serve as a source of inoculum. In contrast, the abandoned orchard, which has not been pruned for more than 25 years, contains a large population of wild blackberries and raspberries (alternate hosts) and consists of very large trees on seedling rootstock. The authors believe that the differences in orchard management and inoculum level for both diseases account for the large differences in the number of accumulated wetness hours required for first symptom development in these tests.

In the well-managed orchards with very low inoculum levels, the number of accumulated wetness hours greatly exceeded the 200- to 225-hour threshold suggested by Brown and Sutton (2). However, in the abandoned orchard, first symptoms always appeared in less than 250 hours over three years of testing. Thus, under conditions that are highly conducive for disease development, a threshold of 200 to 250 hours may be satisfactory for use within Ohio. In orchards where conditions are not highly conducive for disease development, a much higher threshold may be required.

In order to determine a precise threshold for well-managed orchards with low inoculum levels in Ohio, a great deal of research is required; however, use of a conservative threshold such as 225 to 250 wetness hours could be valuable. Use of a threshold of 250 wetness hours for well-managed, low-inoculum orchards, especially during dry growing seasons, could allow growers to adjust the summer disease-spray program. Growers may wish to cut sprays completely until the threshold is reached, increase spray intervals to 21 days instead of 14, or use only a protectant fungicide such as Captan in cover sprays, and include a benzimidazole fungicide such as Benlate or Topsin-M only when the 250-hour threshold is reached.

Acknowledgment

The authors wish to thank the Ohio Fruit Growers' Society for partial support of this research.

Literature Cited

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