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Queen's Printer for Ontario, 2005. Reproduced with permission.
Accepted for publication 13 October 2005. Published 29 November 2005.


Integrated Management of Angular Leaf Spot (Phaeoisariopsis griseola (Sacc.) Ferr.) on Snap Beans in Ontario


Michael J. Celetti, Plant Pathologist, Horticulture Crops Program Lead, Ontario Ministry of Agriculture, Food and Rural Affairs, University of Guelph, Guelph, Ontario, N1G 2W1; and Melody S. Melzer, Research Associate, and Greg J. Boland, Professor, Department of Environmental Biology, University of Guelph, Guelph, Ontario, N1G 2W1


Corresponding author: Michael J. Celetti. michael.celetti@omaf.gov.on.ca


Celetti, M. J., Melzer, M. S., and Boland, G. J. 2005. Integrated management of Angular leaf spot (Phaeoisariopsis griseola (Sacc.) Ferr.) on snap beans in Ontario. Online. Plant Health Progress doi:10.1094/PHP-2005-1129-01-RS.


Abstract

Angular leaf spot (ALS) caused by the fungus Phaeoisariopsis griseola (Sacc.) Ferr. was first observed and confirmed on snap beans growing in three commercial fields in southern Ontario during the 2000 growing season. The potential impact of this disease on the bean industry in Ontario is not known but this disease is severe in many other regions. The objective of this study was to develop a disease management strategy for ALS in Ontario by investigating the survival of P. griseola in Ontario, and assessing the influence of bean varieties and fungicides on disease development. P. griseola survived at least one winter on crop debris in Ontario and survived better on the soil surface in comparison to burial in soil at depths of 5 or 25 cm. Fifteen snap bean varieties were compared for susceptibility to ALS in a growth room, and nine varieties were compared in a naturally-infested field from 2001-2003. Most varieties reacted similarly to P. griseola in both environments. For example, the varieties Carlo, Storm, and Bush Blue Lake 47 were least susceptible whereas Gold Rush was most susceptible in field and growth room experiments. Boscalid, pyraclostrobin, pyramethanil, vinclozolin, and thiophanate-methyl were tested for effectiveness in managing ALS under field conditions. Overall, pyraclostrobin was most effective. Results indicate that an effective disease management strategy for ALS in snap bean in Ontario should include burying infested plant debris through deep plowing, crop rotation for two years, growing the least susceptible varieties, and applying a registered effective fungicide.


Introduction

Angular leaf spot (ALS) of common and snap bean (Phaseolus vulgaris L.) is caused by the fungus Phaeoisariopsis griseola (Sacc.) Ferr. and is considered a serious disease in tropical and subtropical bean growing regions (14,18). ALS has been reported in more than 70 countries worldwide, including the northeastern and midwestern United States (2,6,7,8,21). The disease is considered to be of minor importance in most bean producing areas of the northern U.S., however, it can be substantial when favourable environmental conditions occur (8). In Pennsylvania and Wisconsin, yield losses of 10 to 50% (6) and 50% (8), respectively, were reported. In Michigan, one of the largest dry bean-producing states, ALS was not reported until 1982 and 1983 when seed growers observed severe ALS outbreaks on red kidney bean (7). In Ontario, ALS was first observed and confirmed on snap beans growing in three fields in southern Ontario in 2000 (Fig. 1) (12). The impact of this disease on the Ontario snap bean industry is not known. However, ALS was once considered to be of minor importance in Brazil due to its low incidence and late appearance in the growing season. ALS is now one of the most important diseases of dry bean in Brazil (17).


 

Fig. 1. Symptoms of angular leaf spot caused by Phaeoisariopsis griseola on snap bean.

 

The objective of this study was to develop a disease management strategy for ALS in Ontario by investigating the survival of P. griseola in Ontario, and assessing the influence of bean varieties and fungicides on disease development.


Inoculum Survival

Leaves and pods with symptoms of ALS were collected from snap bean variety Gold Mine in September 2001 from one naturally-infested field near Carlisle, Ontario where ALS was observed in snap beans in 2000 (12). Twenty air-dried pod pieces (2 cm) and fifteen air-dried leaves with ALS lesions were placed in 30--30-cm mesh bags, and stored at 4C until burial. Bags containing infected crop debris were left on the soil surface or buried at 5 or 25 cm (Fig. 2) below the soil surface on 10 October 2001 in a commercial bean field. Bags were retrieved on 4 June and 1 October 2002. The retrieved tissues were washed, incubated at high relative humidity (e.g., > 90%) for 2 days, macerated in water, and sprayed on the first trifoliolate leaves of 20-day-old Gold Mine snap bean plants. Inoculated plants were incubated at 21C and 100% relative humidity for 10 to 14 days and then evaluated for the presence of ALS lesions. Symptoms of ALS developed on plants inoculated with suspensions prepared with infested leaf tissue left on the soil surface in fall 2001 and retrieved in spring 2002 (Fig. 3). P. griseola was recovered from diseased plants on V8 media (200 ml of V8 juice, 3 g of CaCO3, 18 g of agar, and 880 ml of distilled H2O). Plants sprayed with suspensions prepared with tissue buried 5 or 25 cm below the soil surface in fall 2001 and retrieved in spring 2002 did not develop symptoms of ALS. Plants sprayed with suspensions prepared with tissues retrieved in fall 2002 did not develop symptoms of ALS.


 

Fig. 2. Snap bean leaves and pod pieces with symptoms of angular leaf spot caused by Phaeoisariopsis griseola buried in screen bags for pathogen survival studies.

 

Fig. 3. Symptoms of angular leaf spot caused by Phaeoisariopsis griseola on snap bean plants inoculated with a suspension prepared from infested leaf tissue that overwintered on the soil surface.


Variety Susceptibility

Growth room. To determine the susceptibility of fresh market snap bean varieties to infection by P. griseola, fifteen varieties of bean were grown in soilless potting mix in 10 cm diameter pots, three plants per pot, at 21C with a 16 h photoperiod at 116 M/m2/s for 20 days. P. griseola, originally isolated from snap beans in Ontario (ATCC MYA-2353), was grown in petri dishes containing V8 juice agar for 5 days at 21C. Cultures were flooded with sterile water and gently scraped to dislodge spores into suspension. The spore suspension was measured with a hemocytometer and adjusted to 1 105 spores per ml before spraying until runoff onto the 20-day-old snap bean plants. Inoculated plants were incubated at 24C and 100% relative humidity for 5 days in mist chambers under the same growth room conditions. All inoculated varieties were arranged in a randomized complete block design (RCBD) with four replications. Plants were rated for disease severity 14 days after inoculation (0 = no disease; 1 = 1 to 10% leaf surface diseased; and 10 = 91 to 100% leaf surface diseased). The experiment was repeated once. Varieties responded similarly in each experiment, so the data were combined and analyzed as main effects. A protected least significant differences test was used to detect differences among means.

The fifteen varieties of snap bean evaluated in the growth room were all susceptible to ALS but significant differences in disease severity were observed (Table 1). The least susceptible varieties included Gold Mine, Storm, and Bush Blue Lake 47.


Table 1. Growth room study of disease severity on
snap bean varieties 14 days after inoculation with
Phaeoisariopsis griseola
.

Variety Disease Severityx
Gold Rush               5.5 ayz
Tema               5.1 ab
Distinction               5.0 abc
Bronco               4.9 abc
Stallion               4.9 abc
Opus               4.4 bcd
Strike               4.4 bcd
Brio               4.0 cde
Eureka               3.8 cde
Carlo               3.8 def
Cloudburst               3.8 def
Grenoble               3.8 def
Gold Mine               3.3 efg
Storm               2.8 fg
Bush Blue Lake 47               2.6 g

 x Disease severity was rated on a scale of 1 to 10
with 0 = no disease, 1 = 1 to 10% leaf surface
diseased, 10 = 91 to 100% leaf surface diseased.

 y Combined data for 2 trials.

 z Values followed by the same letter within the same
column are not significantly different at P < 0.05,
based on a protected LSD test.


Field. Nine of the 15 varieties evaluated in growth room conditions were also evaluated in 2001, 2002, and 2003 in a naturally-infested field where ALS was observed in snap beans in 2000 (12). Experiments were arranged in a RCBD with four replications. Three rows of each variety were planted at a seeding rate of 55 kg/ha, in rows 6 m long and spaced 75 cm apart on 25 June 2001, 25 June 2002, and 4 July 2003. Ten randomly-selected plants in each plot were evaluated for disease incidence and severity (0 = no disease; 1= 1-10% leaf surface diseased; and 10 = 91 to 100% leaf surface diseased) on 4 and 13 September 2001; 29 August, and 5 and 12 September 2002; and 27 August, and 3, 10 and 17 September 2003. Twenty pods were randomly picked from each plot and also evaluated for disease incidence and severity on 13 September 2001 and 26 September 2003. Disease was not observed on pods in 2002. The severity rating scale used for pods was the same as previously described for leaves.

Disease incidence and severity progress curves were constructed from the data collected for diseased leaves in each variety trial in each year. Area under the disease incidence progress curves (AUDIC) and area under the disease severity progress curves (AUDSC) were calculated for each variety in each year and tested for interactions with variety response in other years (19). A protected least significant difference test was used to detect differences among means at a = 0.05. The incidence of diseased pod data collected from the variety field trials was transformed using the arcsine transformation (X + 0.1) to improve normality and additivity. A protected least significant difference test was used to detect differences among the transformed means at a = 0.05; however, actual means are presented.

ALS was first noticed on trifoliolate leaves of snap bean plants in late August to early September (54 to 71 days after planting). Incidence and severity of disease was higher in 2001 and 2003 than in 2002 (data not shown). However, varieties responded similarly in each year tested and no significant interaction between varieties among the three years of the experiment was detected in the ANOVA analysis of AUDIC or AUDSC, so the data were combined and analyzed as main effects (Table 2). All snap bean varieties were susceptible to ALS; however, significant differences in AUDIC and AUDSC, and disease incidence and severity, were detected among the leaf and pod disease ratings, respectively (Table 2). AUDIC values were significantly higher on variety Gold Mine compared with varieties Bush Blue Lake 47, Distinction, Storm, and Carlo. Similarly, AUDSC values were significantly higher on leaves of variety Gold Mine compared with varieties Strike, Cloudburst, Bush Blue Lake 47, Distinction, Storm, and Carlo. Incidence and severity of pod lesions was significantly greater for varieties Gold Mine and Storm compared with varieties Carlo and Bush Blue Lake 47. Although there was significantly less disease progression (AUDIC, AUDSC) on leaves of varieties Distinction and Storm compared with Gold Mine, incidence and severity of diseased pods did not differ significantly on these varieties.


Table 2. The susceptibility of snap bean varieties to angular leaf spot based on area under the disease incidence (AUDIC) and severity (AUDSC) progress curves and the incidence and severity of diseased pods.

Variety Plantsw Podsx
AUDIC AUDSC Disease
% Incidencey Severity
Gold Mine     1568 a     20.7 az        37.5 ab     0.90 ab
Gold Rush     1411 ab     17.9 ab        20.1 bcd     0.79 bcd
Eureka     1439 ab     17.3 ab        28.1 bc     0.81 bc
Strike     1405 ab     16.8 b        23.1 bcd     0.77 bcd
Cloudburst     1340 abc     15.7 bc        21.9 bcd     0.67 cd
Bush Blue Lake 47     1236 bcd     15.5 bc        11.3 cd     0.63 cd
Distinction     1162 cd     15.4 bc        35.6 ab     0.88 ab
Storm     1223 bcd     15.3 bc        48.1 a     1.01 a
Carlo     1063 d     12.7 c        10.6 d     0.61 d

 w Combined data for 2001, 2002, and 2003 field trials.

 x Combined data for 2001 and 2003. There were no symptoms on pods in 2002.

 y % Incidence data were transformed using arcsine (X + 0.1) to improve normality and additivity, however, actual means are presented

 z Values followed by the same letter within the same column are not significantly different at P < 0.05, based on a protected LSD test.


Several varieties that were included in both the growth room and field trials reacted similarly to P. griseola in both environments. For example, varieties Carlo, Storm, and Bush Blue Lake 47 were less susceptible and Gold Rush was more susceptible in the field and growth room studies. However, Gold Mine and Distinction were exceptions and reacted differently in field and growth room studies (Tables 1 and 2).


Fungicide Timing and Efficacy

On 25 June 2002, snap bean variety Strike was planted at 55 kg/ha, in 3-row plots with rows spaced 75 cm apart and 6 m long in a naturally-infested field. Thiophanate-methyl (Senator 70 WP) at 1575 grams active ingredient (g.a.i.)/ha, vinclozolin (Ronilan EG) at 750 g.a.i./ha, or pyraclostrobin (Headline EC) at 100 g.a.i./ha were applied with a sprayer (R&D Sprayers, Opelousas, LA) to the center row of individual 1.5--6-m plots using a 60-cm boom with 3 adjustable cone nozzles propelled with CO2 at 280 kPa using a water volume of 250 liters/ha. Fungicides were applied at 10 to 30% bloom (6 August; 42 days after planting), at 50 to 70% bloom (13 August; 49 days after planting) or at 10 to 30% and 50 to 70% bloom. The treatments were arranged in a RCBD with four replications. Ten randomly selected plants in each plot were evaluated for incidence and disease severity, using the same scale as for variety trials, on 29 August and 12 September.

On 4 July 2003, snap bean variety Strike was planted at 55 kg/ha, in 3-row plots with rows spaced 75 cm apart and 6 m long in a naturally-infested field. Boscalid (Lance 70 WG) at 500 g.a.i./ha, pyrimethanil (Scala SC) at 800 g.a.i./ha, or pyraclostrobin (Headline EC) at 100 g.a.i./ha were applied as in 2002. Fungicides were applied at 10 to 30% bloom (14 August; 41 days after planting), at 50 to 70% bloom (21 August; 48 days after planting), or at 10 to 30% and 50 to 70% bloom. The treatments were arranged in an RCBD with four replications. Ten randomly selected plants in each plot were evaluated for incidence and disease severity, using the same scale as for variety trials, on 28 August, and 4, 11, and 18 September. Twenty pods were randomly picked from each plot and also evaluated for disease incidence and severity on 26 September using the same scale.

Disease incidence and severity progress curves were constructed from the data collected for diseased leaves in each fungicide trial in each year. AUDIC and AUDSC were calculated and analyzed as described for field variety trials. The incidence of diseased pod data collected from the fungicide trials was transformed using the arcsine transformation (X + 0.1) to improve normality and additivity. A protected least significant difference test was used to detect differences among the transformed means at a = 0.05; however, actual means are presented.

Disease incidence and severity were higher in 2003 than in 2002, and disease incidence reached 100% in untreated plots by mid-September 2003. There was no significant interaction between application timing and fungicide treatments; therefore, data were combined and analyzed as main effects.

In 2002, thiophanate-methyl and pyraclostrobin significantly reduced the AUDIC and AUDSC values compared to the untreated check (Table 3). Vinclozolin did not reduce disease progress. In 2003, pyramethanil, boscalid, and pyraclostrobin significantly reduced the AUDIC and AUDSC values compared to the untreated check (Table 3). Overall, pyraclostrobin was the most effective fungicide at reducing the incidence and severity of ALS.


Table 3. Efficacy of fungicides against angular leaf spot on snap bean variety Strike using area under the disease incidence (AUDIC) and severity (AUDSC) progress curves for plant ratings, and severity and % incidence for pod ratings.

Fungicide Plant Pod
2002 2003 2003
AUDIC AUDSC AUDIC AUDSC %
Incidence
y
Severity
Untreated 131.3 a 1.4 ax   1660 a   18.1 a   10.8 a      0.15 a
Boscalid -- -- 1263 b   13.2 b   2.5 b      0.03 b
Pyrimethanil -- -- 1181 b   12.5 b   12.1 a      0.13 a
Pyraclostrobin 8.8 b 0.1 b    452 c   4.5 c   1.3 b      0.01 b
Thiophanate-
methyl
11.7 b 0.1 b    -- -- -- --
Vinclozolin 172.1 a 2.0 a    -- -- -- --

 x Values followed by the same letter within the same column are not significantly different at P < 0.05, based on a protected LSD test.

 y % Incidence data were transformed using arcsine (X + 0.1) to improve normality and additivity; however, actual means are presented.


Angular leaf spot lesions did not appear on pods in 2002 but appeared late in the 2003 season after a period of cool, wet weather. Significantly lower disease incidence and severity were observed on pods from plots treated with pyraclostrobin and boscalid compared with pods from plots treated with pyrimethanil or the untreated check (Table 3).

Applying the fungicide at 10 to 30% bloom, 50% to 70% bloom, or 10 to 30% and 50 to 70% bloom significantly reduced the AUDIC and AUDSC values compared to the untreated check in 2003, but not in 2002 (Table 4). Although there was no significant difference between application timings and frequency, plots receiving early fungicide applications tended to have a lower disease incidence and severity. Timing and frequency of fungicide application did not significantly affect the incidence or severity of disease on pods in 2003.


Table 4. The effect of fungicide application timing and frequency on angular leaf spot on snap bean variety Strike based on the area under the disease incidence (AUDIC) and severity (AUDSC) progress curves.

Application timing 2002 2003
AUDIC AUDSC AUDIC AUDSC
Untreated 131.3 a      1.4 ax 1660 a      18.1 a     
10-30% bloom 29.2 a    0.4 a 785 b      8.2 b     
50-70% bloom 87.5 a    0.9 a 1155 b      12.2 b     
10-30% & 50-70% bloom 75.8 a    0.9 a 957 b      9.9 b     

 x Values followed by the same letter within the same column are not significantly different at P < 0.05, based on a protected LSD test.


Discussion

In North America, ALS has been reported in Mexico, United States (CT, DE, FL, GA, ME, MD, MA, MI, NC, NH, NY, OK, PA, SC, TN, TX, VA, WI) (2,6,7,21), and recently in Canada (12). ALS is not an important disease in most bean producing areas of North America, however, under favorable environmental conditions, disease epidemics can occur (10). Environmental conditions conducive for ALS epidemics can occur during the growing season in Ontario. Optimal temperatures for germination of conidia of P. griseola range between 23 to 27C (11), and infection occurs from 16 to 28C with the optimum for infection and disease development at 24C (4,5). Frequent rain and high humidity are important for the initiation of disease and are considered more important than temperature (9,20).

Mean daily air temperatures recorded at a regional weather station during 2001 to 2003 were near the 30-year means for this region during July, August, and September and fell within the range for infection and disease development (3). In contrast, total precipitation during these same periods was more variable with less precipitation than the 30-year mean in July and August in 2001 and 2002, respectively, with August 2002 receiving only 12.5% of the 30-year monthly total precipitation. Precipitation in July and August 2003 was close to the 30-year means. This variation in rainfall may explain the low incidence and severity of disease observed in 2002 compared to other years.

In addition to conducive environmental conditions, a source of inoculum is required for development of ALS (8,9). Infested seed, crop debris, and volunteer plants were reported as sources of inoculum for ALS (9,18). Seed assays from seed lots used in Michigan in 1982 and 1983, when ALS was severe, revealed low amounts of P. griseola in 8 of 20 (40%) and 6 of 59 (10%) seed lots, respectively (16). The pathogen survived in infested crop debris over two winters in Wisconsin (4) and Michigan (7). Stroma that form in lesions allow the pathogen to remain dormant until environmental conditions are favourable for sporulation (13). In the present study, P. griseola survived one winter in Ontario only in association with crop debris on the soil surface.

Pastor-Corrales et al. (14) screened 22,832 cultivated and wild common bean accessions for resistance to ALS and found most were susceptible, with only 59 showing an intermediate response and 64 showing a resistant response. Similar results were obtained when 13,219 breeding lines were screened, with 89 showing intermediate or resistant responses. However, all of the 64 resistant accessions or 89 intermediate or resistant lines developed disease symptoms, suggestive of a quantitative or multigenic resistance rather than oligo- or monogenic resistance. Similarly, all varieties evaluated in the present growth room and field studies were susceptible to ALS; however, some varieties were less susceptible than others. Varieties Bush Blue Lake 47, Carlo, and Storm were least susceptible whereas Gold Rush was most susceptible.

The benzimidazole fungicides thiabendozole (10) and benomyl, as well as the strobilurin fungicides trifloxystrobin and azoxystrobin, have been shown to provide control of ALS in dry beans (15). In the present study, application of the benzimidazole fungicide thiophanate-methyl and the strobilurin fungicide pyraclostrobin provided significant reduction of the area under the disease incidence and severity progress curves. Although there were no significant differences among the timing and frequency of fungicide application on the AUDIC or AUDSC, the earlier application of an efficacious fungicide tended to reduce disease severity and incidence more than a later application. Under the moderate level of disease observed in the current study, there appeared to be no advantage of two applications of a fungicide compared to one. However, with higher disease levels, two applications may be required for effective disease management. The development of a disease predictive model for ALS may help growers decide on the proper timing and frequency of fungicide application.

ALS is currently considered to be a minor disease in Ontario, even though environmental conditions conducive to development of ALS occur. Over the 3 years of this study, ALS was not observed until late in the growing season. Initially, ALS was considered to be a minor disease of bean in Brazil because it appeared late in the growing season but is now considered a serious disease (17). Recent demonstrations of the presence and survival of P. griseola in southern Ontario suggest that ALS could become more prevalent in the future. The current study identifies disease management tools that can be integrated to reduce the economic impact of ALS on snap bean production in Ontario. Chupp and Sherf (5) reported ALS can be controlled by using disease-free seed, selecting well-drained soil, and using a 2-year crop rotation. Using resistant cultivars, applying an effective fungicide, and removing diseased plant tissue also help control disease (1). Results from the current study suggest that, in addition to planting disease-free seed, deep plowing infested crop residue, rotating crops for at least 2 years, growing the least susceptible varieties, and applying a registered effective fungicide such as pyrachlostrobin, should be considered in an integrated approach for managing ALS in Ontario.


Acknowledgments

Financial support from the Ontario Fruit and Vegetable Growers Association, Seminis Vegetable Seeds Inc., BASF Canada Inc., Bayer Crop Science, and Engage Agro Corp. is greatly appreciated. This project was supported by the Canada-Ontario Research and Development Program, administered by the Agricultural Adaptation Council. Funding was provided by Agriculture and Agri-Food Canada and Ontario Ministry of Agriculture, Food and Rural Affairs under the Canada-Ontario Agricultural Safety Net Management Agreement.


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