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© 2013 Plant Management Network.
Accepted for publication 17 June 2013. Published 30 September 2013.


Fungicide Spray Volume and Pruning Impact Performance of a Warning System for Sooty Blotch and Flyspeck on Apple


Jean C. Batzer, Department of Plant Pathology, Iowa State University, Ames, IA 50011; Patricia S. McManus, Department of Plant Pathology, University of Wisconsin, Madison, WI 53706; and Mark L. Gleason, Department of Plant Pathology, Iowa State University, Ames, IA 50011


Corresponding author: Mark L. Gleason. mgleason@iastate.edu


Batzer, J. C., McManus, P. S., and Gleason, M. L.. 2013. Fungicide spray volume and pruning impact performance of a warning system for sooty blotch and flyspeck on apple. Online. Plant Health Progress doi:10.1094/PHP-2013-0930-01-RS.


Abstract

The sooty blotch and flyspeck (SBFS) fungal complex degrades apples by blemishing the surface, reducing the fresh market value up to 90%. In the Upper Midwest United States, growers apply 3 to 8 fungicide sprays starting shortly after petal fall until harvest. The Brown/Sutton/Hartman SBFS warning system uses the duration of daily wet periods to extend the interval between the fungicide spray that is applied 10 days after petal fall (“first cover spray”) and the subsequent spray (“second cover spray”) to until a 175-h cumulative threshold is reached. When we tested this warning system in the Upper Midwest, SBFS incidence was significantly higher in the warning system treatments than in the calendar-based controls in 12 of 28 demonstration trials in commercial orchards. The causes for failure of the warning system were uncertain, but may have been related to pruning and spray volume practices by individual growers. In this study, we compared the effects of pruning and spray volume on the performance of the Brown/Sutton/Hartman warning system in Wisconsin and Iowa. Results of six orchard-year replicated trials and nine non-replicated demonstrations in commercial orchards suggested that both annual pruning and spray volume of at least 100 gal/acre enhance the success of this SBFS warning system and these practices were more beneficial with semi-dwarf than fully dwarf trees.


Introduction

In the United States Upper Midwest, apple growers apply three to eight fungicide sprays from shortly after petal fall until harvest, aimed primarily at managing summer diseases. This practice is not only expensive but may not be sustainable. For example, thiophanate-methyl, the most widely used fungicide for management of summer apple diseases, suppresses predator mites and can thereby exacerbate damage caused by European red mite (14). Resistance of several important apple pathogens to thiophanate-methyl has also been documented (16,20). There are also possible deleterious effects to human health. For example, oral ingestion of thiophanate-methyl caused growth retardation, decreased spermatogenesis, and hyperthyroidism in rats (22). Captan, another fungicide used to control summer apple diseases, is classified in Group B2, probable human carcinogen (23). Thus there are compelling reasons for apple growers to adopt strategies to reduce the number of fungicide applications needed for apple disease control.

The sooty blotch and flyspeck (SBFS) disease complex is the predominant apple disease in the Upper Midwest during summer months. Although the complex is comprised of nearly 80 fungal species worldwide that are mostly in the Dothideomycetes order Capnodiales (13), most apple orchards in the eastern US harbor 2-15 SBFS species (11). The SBFS fungi grow on the waxy cuticle of fruit, causing dark blemishes that reduce fresh market value. Incidence of SBFS can approach 100% if left unchecked (13). Prolonged periods of wetness and high relative humidity are conducive to germination of conidia and mycelial growth of SBFS species (17). Accumulated leaf wetness hours (LWD) and hours of relative humidity (RH) have both been correlated with the time of appearance of SBFS colonies on apples (7,12).

The Brown/Sutton/Hartman SBFS warning system (8,15,19) extends the period between the fungicide spray that is applied 10 days after petal fall (“first cover spray”), which in the Upper Midwest is targeted primarily against apple scab, and the following fungicide spray, which is targeted primarily against summer diseases (“second cover spray”), until a threshold of 175 h of LWD has accumulated. This timing for the second cover spray is in contrast to the conventional practice of spraying every 10-14 days regardless of weather conditions (1). In validation trials in Illinois, Iowa, and Wisconsin, spraying according to the Brown/Sutton/Hartman warning system was effective in replicated university-farm trials and saved up to four sprays per season compared to a conventional, calendar-based spray timing (2). In commercial orchards up to seven sprays per season were saved by using this system, but SBFS incidence was greater than in the conventional-timing treatment in 12 of 28 trials (2). We hypothesized that these control failures resulted from low spray volume or excessively dense tree canopies, since poor spray coverage and slow drying are conducive to SBFS development (23).

The effects of pruning on the incidence and severity of SBFS were noted in 1920, when Colby observed that disease was more severe in mature trees that were not pruned than in young, pruned trees (9). Multiple-year studies in abandoned and commercial orchards in North Carolina, where no fungicides were applied after petal fall, were used to demonstrate that pruning significantly reduced sooty blotch severity in 2 of 3 years, but pruning was not a factor for flyspeck (18). Another North Carolina study showed that a significant increase in incidence and severity of sooty blotch occurred when fungicide spray volume was reduced by 20% using a 14-day spray schedule that was discontinued 7 weeks before harvest (6). Based on these reports, we evaluated the effects of spray volume and pruning on the performance of the Brown/Sutton/Hartman SBFS warning system in replicated trials over 4 and 2 years at university research farms in Iowa and Wisconsin, respectively, and also cooperated with commercial apple growers in on-farm demonstration trials in each state. This is the first study to explore the influence of orchard management factors on performance of a SBFS warning system.


Replicated Trials

Experiments were conducted in apple orchards at the Iowa State University Horticulture Research Station (ISU) from 2006 to 2009 and at the University of Wisconsin West Madison Agricultural Experiment Station (UW) in 2006 and 2007 (Table 1). Mature, semi-dwarf, ‘Chieftain’ trees at ISU were used in a factorial combination of two pruning regimes (pruned or not pruned during the previous winter) and four fungicide spray volumes (0, 48, 100, and 200 gallons per acre), replicated four times in a randomized complete block design with four trees per replicate. Fully dwarf, mature ‘Golden Delicious’ trees at UW included a factorial combination of two pruning regimes (pruned or not pruned during the previous winter) and three fungicide spray volumes (0, 30, and 100 gallons per acre), replicated four times in a randomized complete block design with six trees per replicate. The designated spray volumes were applied for only the first and second cover sprays; subsequent cover sprays were applied at 10- to 14-day intervals until harvest using a spray volume of 200 gal/acre at ISU (Table 1). Wetness duration was monitored at both sites with two WatchDog Data Loggers (Spectrum Technologies, Plainfield, IL) placed adjacent to each other at 4.5-ft height in the orchard canopy, facing north, and positioned at 45 degrees from vertical. At ISU, the second-cover fungicide spray was delayed until approximately 175 h of leaf wetness had accumulated since the first cover spray was applied, counting only blocks ≥4 h leaf wetness duration (8). At UW, only 156 and 123 h LWD had occurred by 31 August in 2006 and 2007, respectively; as a result, no sprays were applied after first cover. The incidence of SBFS was assessed by inspecting 50 fruit per tree within 1 week before harvest in mid October. For the combined Wisconsin and Iowa data set, incidence of SBFS (% fruit with visible SBFS colonies) was used to assess the effects of spray volume and pruning treatments via a two-way analysis of variance (GLM function in Minitab v. 15, State College, PA). Since growers are sometimes able to market apples with a one to a few colonies of SBFS, the ISU study also assessed the number of marketable apples based on USDA standards, under which Extra Fancy grade has <5% of the fruit surface covered with SBFS (3,21). For the Iowa data, the percentage of Extra Fancy grade apples was used to determine treatment effects (PROC MIXED, SAS 9.2 SAS Inc. Cary, NC).


Table 1. Orchard history and fungicide applications to replicated trials at Iowa State University (ISU) and University of Wisconsin (UW).

Site

Year

Cultivar,
rootstock,
spacing
Spray
description
x

Date(s)

Fungicidex and
rate per acre
ISU 2006 Chieftain,
M.7,
25×12 ft
Tight cluster 20 Apr    Rally 40WSPf 6 oz +
Polyram 80DF 3 lb
Bloom 26 Apr    Rally 40WSP 6 oz +
Polyram 80DF 3 lb
Petal fall 4 May    Rally 40WSP 12 oz +
Polyram 80DF 6 lb
First cover 14 May    Topsin M-70WP 20 oz + Captan 50WP 6 lb
Second cover 1 Sep    Topsin M-70WP 20 oz + Captan 50WP 6 lb
Third cover 14 Sep    Topsin M-70WP 20 oz + Captan 50WP 6 lb
ISU 2007 Tight cluster 28 Apr    Rally 40WSP 6 oz +
Polyram 80DF 3 lb
Bloom 8 May    Rally 40WSP 5 oz +
Polyram 80DF 3 lb
Petal Fall 17 May    Rally 40WSP 5 oz +
Polyram 80DF 3 lb
First cover 6 Jun    Topsin M-70WP 20 oz + Captan 50WP 6 lb
Second cover 8 Aug    Topsin M-70WP 20 oz + Captan 50WP 6 lb
Third cover 22 Aug    Topsin M-70WP 20 oz + Captan 50WP 6 lb
ISU 2008 Tight cluster 4 May    Rally 40WSP 6 oz +
Polyram 80DF 3 lb
Pink 8 May    Bayleton 50DF 8 oz +
Polyram 80DF 3 lb
Bloom 14 May    Bayleton 50DF 8 oz +
Polyram 80DF 3 lb
Petal Fall 28 May    Rally 40WSP 4 oz +
Captan 50WP 6 lb
First Cover 4 Jun    Topsin M-70WP 15 oz +
Captan 50WP 6 lb
Second cover 22 July    Topsin M-70WP 15 oz +
Captan 50WP 6 lb
Third cover 8 Aug    Topsin M-70WP 15 oz +
Captan 50WP 6 lb
Fourth cover 15 Aug    Topsin M-70WP 15 oz +
Captan 50WP 6 lb
Fifth cover 22 Aug    Topsin M-70WP 15 oz +
Captan 50WP 6 lb
Sixth cover 5 Sep    Topsin M-70WP 15 oz +
Captan 50WP 6 lb
Seventh cover 23 Sep Topsin M-70WP 15 oz +
Captan 50WP 6 lb
ISU 2009 Tight cluster 3 May    Rally 40WSP 5 oz
Bloom 12 May    Rally 40WSP 5 oz
Petal fall 19 May    Rally 40WSP 5 oz
First cover 4 Jun    Topsin M-4.5 FL 15 fl oz +
Captan 50WP 6lb
Second cover 11 Jul    Topsin M-4.5 FL 15 fl oz +
Captan 50WP 6 lb
Third cover 17 Jul    Topsin M-4.5 FL 15 fl oz +
Captan 50WP 6 lb
Fourth cover 24 Jul    Topsin M-4.5 FL 15 fl oz +
Captan 50WP 6 lb
Fifth cover 10 Aug    Topsin M-4.5 FL 15 fl oz +
Captan 50WP 6 lb
Sixth cover 28Aug    Topsin M-4.5 FL 15 fl oz +
Captan 50WP 6 lb
Seventh cover 11 Sep    Topsin M-4.5 FL 15 fl oz +
Captan 50WP 6 lb
UW 2006 Golden Delicious,
M.26,
12×12 ft
Green tip
through
petal fall
Rotatedz    Captan 50 WP 1.5 lb
Sovran 50WP 1.6 oz
First cover 2 Jun    Sovran 50WP 1.6 oz +
Topsin M-70WP 6 oz
UW 2007 Green tip
through
petal fall
Rotated    Captan 50 WP 1.5 lb
Sovran 50WP 1.6 oz
First cover 17 May    Sovran 50WP 1.6 oz +
Topsin M-70WP 6 oz

 x Phenological stages at which apple fungicide sprays were applied (1). First cover spray is defined as the spray that occurs approximately 10 days after petal fall. Based on the Brown/Sutton/Hartman SBFS warning system, whereby the second-cover fungicide spray is applied after a threshold of 175 cumulative hours of leaf wetness since the date of application of the first-cover fungicide spray is reached. Wetness hours are counted only for periods ≥4 h in duration (8). Cover sprays following the second-cover spray were applied every 10 to 14 days at 200 gal/acre at ISU. No cover sprays were applied after first cover at UW because the warning-system threshold was not reached.

 y Common names and manufacturer for trade names are as follows: Rally 40WSP = myclobutanil Dow; Polyram 80DF = metiram Loveland; Topsin M = thiophanate-methyl United Phosphorous; Captan 50WP = captan Arysta; Bayleton 50DF = triadimefon Bayer CropScience; Sovran 50WP = kresoxim-methyl BASF.

 z Captan and Sovran sprays were rotated an unknown number of times between green tip and petal fall at UW.


Increased spray volume significantly (P ≤ 0.006) reduced SBFS incidence in five of six site-years, with UW in 2007 as the sole exception (P = 0.102) (data not shown). This exception was apparently due to low disease pressure and high variability among replicates, since no differences were observed between the non-sprayed controls and sprayed treatments. Pruning did not impact SBFS incidence, except at UW in 2006, when a lower incidence was observed in pruned trees (P = 0.005). However, both higher spray volume and pruning significantly (P < 0.0001) increased percent Extra Fancy grade apples for the Iowa trial site during 2006-2009. Because a significant (P = 0.0008) interaction of spray volume and pruning on percent Extra Fancy grade apples was detected, the effectiveness of treatment combinations was compared (Table 2). Differences among treatments were the greatest during 2007. Spray volumes at ≥48 gal/acre provided equivalent control (P > 0.05) when trees were pruned. When trees were not pruned the previous winter, however, differences in spray volume had a significant (P < 0.05) impact on the percent of Extra Fancy grade apples in all 4 years. When Iowa data from all years were combined, the 200 gal/acre spray on pruned trees provided the highest percentage of Extra Fancy grade apples (85%) and the non-pruned trees sprayed at 48 gal/acre had the lowest percentage (78%), excluding the non-sprayed controls.


Table 2. Percent of marketable apples from treatment combinations of spray volume and pruning regimes at Iowa State University Horticulture Research Station in 2006, 2007, 2008, and 2009 using the Brown/Sutton/Hartman SBFS warning systemv.

Treatment combinationw

2006 2007 2008 2009 All years
Canopy gal/acre Percent marketable applesx
Prunedy 200  46az 79ab 91a 100a 85a
Pruned 100 48a 80ab 92a 99a 83ab
Pruned 48 48a 78ab 90a 99a 82ab
Pruned 0 39b 74a-c 90a 93b 77d
Non-pruned 200 45a 84a 94a 100a 78cd
Non-pruned 100 47a 81ab 94a 100a 80bc
Non-pruned 48 46a 69c 83ab 99a 78cd
Non-pruned 0 32c 74bc 76b 84c 69e
LSD (P ≤ 0.05) 6.1 8.6 9.9 3.7 3.1

 v During 2006 and 2007 the orchard received 3 cover sprays and during 2008 and 2008 the orchard received 7 cover sprays.

 w An interaction (P = 0.0008) between canopy and spray volume required that treatment combinations be assessed individually.

 x Marketable apples (USDA Extra Fancy) defined as apples with <5% of surface covered with SBFS (21).

 y Canopy pruned previous winter.

 z Different letters indicate significant differences (P = 0.05) among treatments within column.


On-farm Demonstration Trials

To complement the replicated trials, we conducted demonstration trials in cooperation with commercial apple growers. In Wisconsin during 2006 and 2007, cooperators at two locations applied fungicides at either 60 or 100 gal/acre spray volume according to the SBFS warning system to a group of trees that had been pruned the previous winter and to a group of trees of the same cultivar and age that had not been pruned for at least 3 years. In Iowa during 2008 and 2009, cooperators at three locations compared the effects of thiophanate-methyl + captan on blocks of five to ten trees according to the following treatments: (i) applied at 40 gal/acre according to the SBFS warning system; (ii) applied at 80 gal/acre according to the SBFS warning system; and (iii) applied at 40 gal/acre at approximately 2-week intervals from first cover until a few weeks prior to harvest (i.e., “calendar-based” control). At all sites, incidence of SBFS was assessed immediately prior to harvest, in the same manner as the replicated trials. Because treatments were not replicated in these demonstration trials, statistical analyses were not conducted. In Wisconsin, the incidence of SBFS blemishes was lower in pruned trees (Table 3). In Iowa the warning system saved an average of 1.6 sprays per orchard-year. The warning system failed in 1 of 5 orchard years (Jefferson in 2009). In the warning system treatments the higher volume (80 gal/acre) spray usually had lower incidence of SBFS than the lower volume (40 gal/acre) spray (Table 4).


Table 3. Effect of pruning on percentage of apple fruit with sooty blotch and flyspeck in demonstration trials in commercial orchards in Wisconsin.

Location Year Canopy No.
sprays
x

Incidence (%)w

Sooty blotch Flyspeck
Beloity 2006 Pruned 2 27 55
Not pruned 3 66 86
Beloity 2007 Pruned 2 55 40
Not pruned 2 97 78
Barabooz 2007 Pruned 4 14 39
Not pruned 4 22 64

 w Signs were evaluated when apples were at physiological maturity within 1 week of commercial harvest.

 x Sprays applied according to the sooty blotch and flyspeck warning system, in which second cover spray was applied after at least 175 h of accumulated leaf wetness.

 y Cultivar was Golden Delicious at Beloit. Spray volume was 100 gal/acre.

 z Cultivar was Cortland at Baraboo. Spray volume was 60 gal/acre.



Table 4. Effect of spray timing and spray volume on percentage of apple fruit with sooty blotch flyspeck (SBFS) in demonstration trials in commercial orchards in Iowax.

Location Year Spray
threshold
Spray
volume

(gal/acre)
Number
of cover
sprays
SBFS
incidence

(%)
Pella 2009 WSy 40 5 4
WS 80 5 3
Calendarz 40 6 2
Fort
Dodge
2008 WS 40 6 1
WS 80 6 0
Calendar 40 7 0
2009 WS 40 2 6
WS 80 2 3
Calendar 40 6 0
Jefferson 2008 WS 40 4 2
WS 80 4 4
Calendar 40 4 1
2009 WS 40 3 19
WS 80 3 10
Calendar 40 6 0

 x SBFS signs were evaluated when apples were at physiological maturity within 1week of commercial harvest. Cultivar was Golden Delicious at all locations.

 y Warning system: second cover spray was applied after at least 175 h of accumulated leaf wetness.

 z Calendar: sprays applied every 10 to 14 days.



Discussion and Recommendations

Pruning annually can enhance the effectiveness of the Brown/Sutton/Hartman SBFS warning system in suppressing SBFS in the US Upper Midwest, particularly on mature, semi-dwarf trees. In replicated trials on university farms, pruned trees at ISU had a higher percentage of Extra Fancy grade apples than trees that had not been pruned the previous winter, but differences in the incidence of SBFS between pruning regimes was not consistently observed at UW. These inconsistencies between locations may be attributed to the differences in tree size, the UW orchard fully dwarf versus the semi-dwarf ISU orchards. Our results for semi-dwarf trees are commercially relevant; although fully dwarfing rootstocks are the standard for most new orchards worldwide, including the Midwest US, a substantial percentage of trees in Midwest orchards continue to be semi-dwarf. In demonstration trials in Wisconsin using semi-dwarf trees, where the warning system was used to time sprays, there was much lower SBFS incidence on trees that had been pruned than those that had not been pruned for the previous 3 years (Table 3). Our data support previous findings that summer pruning can be used to reduce sooty blotch and flyspeck (10,18). The benefits of pruning are heightened when the warning system delays application of the second-cover spray by several weeks.

A spray volume ≥100 gal/acre when using the warning system is especially critical for SBFS control in trees that have not been pruned in recent years, but also increases control in trees that were pruned the previous winter. In general, the 100 gal/acre spray volume was as effective as the 200 gal/acre spray volume in controlling SBFS using the Brown/Sutton/Hartman warning system, whereas both of these spray volumes (100 and 200 gal/acre) were more effective than lower spray volumes (30 or 48 gal/acre).

Warning system failures probably can be partially attributed to poor fungicide coverage caused by both dense foliage in the tree canopy and low spray volumes. Adequate coverage is essential when reducing the number of fungicide sprays during use of the Brown/Sutton/Hartman warning system.

In this study, warning system control failures (Iowa 2006 and Jefferson 2009) were associated with a dry growing season that delayed the second cover spray into September, effectively resulting in no sprays after the first-cover spray. Since similar control failures of the warning system were observed in previous studies in the Upper Midwest (Gleason and Batzer, unpublished data), we suggest that a second-cover spray should be applied in late August even if the warning system’s threshold has not been reached. A rationale for this “fail-safe” spray strategy is that fruit that are starting to mature in August are likely to have higher levels of leakage of sugars to the fruit surface (24), which can spur growth of SBFS fungi (4,5). An additional rationale is that the risk of fruit rot caused by other pathogens rises late in the season and requires an additional protectant strategy.

This study supports previous findings evaluating suitability of the Brown/Sutton/Hartman warning system in the Upper Midwest (2) in that spray programs directed by the warning system had fewer fungicide sprays but usually provided SBFS control equivalent to a calendar-based spray schedule. However, we showed that using a spray volume of at least 100 gal/acre in conjunction with annual pruning can further reduce the occurrence of control failures when using the warning system.


Literature Cited

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