© 2012 Plant Management Network.
Cultivar and Fungicide Effects on Pythium Leak of Snap Bean
John P. Damicone and Jenifer D. Olson, Department of Entomology and Plant Pathology, and Brian A. Kahn, Department of Horticulture and Landscape Architecture, Oklahoma State University, Stillwater, OK 74078-3033
Damicone, J. P., Olson, J. D., and Kahn, B. A. 2012. Cultivar and fungicide effects on Pythium leak of snap bean. Online. Plant Health Progress doi:10.1094/PHP-2012-0418-01-RS.
Pythium leak is a pod decay complex of several pathogens and is a problem in the processing industry where snap beans are harvested in bulk. In Oklahoma, Pythium leak of snap bean is caused primarily by Pythium aphanidermatum and P. ultimum. Fungicide programs alone have not provided adequate disease control. The objective of this study was to evaluate snap bean cultivars for their reaction to Pythium leak with and without fungicide (mefenoxam + copper hydroxide) treatment. Entries were compared to the regional standards of Roma II for flat pod types and Nelson for round pod types. Over two years, disease incidence (DI = plants with symptoms) in untreated plots averaged 22% for Roma II and 30% for Nelson. Among the flat-pod types, entries that had less (P = 0.05) disease and yielded similar to, or greater than, Roma II included Bogota (DI = 13%), Navarro (DI = 12%), Romano 942 (DI = 12%), Cerler (DI = 10%), Tapia (DI = 10%), Primo (DI = 10%), and Ebro (DI = 5%). Among the round pod types, PLS 75 (DI = 14%) and SB 4261 (11%) had less disease than Nelson. However, all of the entries with round pods yielded less than Nelson. Over entries, fungicide treatment reduced disease incidence by an average of 40%, but the effect of fungicide was not significant for entries with low disease incidence such as PLS 75, Cerler, and Ebro. Except for plant height and lodging which were weakly correlated with disease incidence, plant architectural characteristics did not explain observed differences in disease incidence. Results suggest that reaction to Pythium leak is an important characteristic in selecting processing snap bean cultivars.
Snap bean (Phaseolus vulgaris Linn.) is an important vegetable crop grown for processing in Oklahoma and the surrounding states of Missouri and Arkansas. Because snap beans are machine harvested in bulk with non-selective cutters, the crop is intensively managed to be nearly free of blemishes from disease and without detectable levels of weeds, insect feeding damage or parts, and other contaminants. An increasing constraint to snap bean production in the region is Pythium pod rot or leak. Basal pods, particularly those near the soil, develop a wet rot with profuse growth of white, fluffy mycelium (Fig. 1). Severely affected fields are rejected for harvest, resulting in a total economic loss to both the grower and processor. Fields with a low level of Pythium leak may be harvested, but the disease often increases dramatically in bulk containers used for transit and storage prior to processing (12).
Pythium spp. are major pathogens of bean that comprise a disease complex with both subterranean and aerial phases (17). The subterranean phases of the disease complex that include pre- and post-emergence damping off and root rot have received the most attention. P. ultimum, P. myriotylum, and P. aphanidermatum are the most important pathogens depending on the production region (15,17,18). Reduced plant stand occurs from the lethal effects of damping off and root rot on young plants. Reduced growth and yield results from non-lethal root rot, caused by Pythium spp. alone or in combination with other pathogens (17,19). Control of subterranean Pythium diseases has centered on use of fungicide seed treatments to control damping-off and deep plowing to bury inoculum (13,17). Resistance to damping off has been identified in bean germplasm, but it has been difficult to incorporate into commercially acceptable cultivars (14,17).
The aerial phases of bean diseases caused by Pythium spp. affect stems, leaves and pods. P. aphanidermatum, P. myriotylum, and P. ultimum were reported to cause basal stem rot of seedlings and mature bean plants in mid-Atlantic states (9). In Wisconsin, Pythium blight or aerial Pythium is caused by P. debaryanum and P. ultimum (1,11). Infection of lower leaf nodes occurs when infested water is splashed upward and the foliage remains wet for 48 to 72 h. Physical injury to the leaf nodes also increases disease development. The pathogen spreads up and down the plant affecting leaves stems and pods. Cottony leak, a wet rot of snap beans in transit caused by P. aphanidermatum, was first described in 1927 on loads of snap beans from Florida (12). The disease was suspected to originate in the field on pods in contact with the soil in the spring when temperatures warmed. After harvest, cottony leak rapidly increased in bulk containers to cause nested areas of completely decayed pods held together in a mass of cottony mycelium. P. aphanidermatum has since been reported to cause cottony leak on a related Phaseolus host (3). In Oklahoma and Arkansas, Pythium spp. have been identified as the most important causes of pod decay, occurring more frequently than other pod rot pathogens including Sclerotinia sclerotiorum, Phytophthora spp., and Rhizoctonia solani (7,10,21,23).
Management strategies for aerial Pythium diseases are not well developed. A few bean cultivars are resistant to Pythium blight caused by P. debaryanum (2,11). However, little is known about cultivar reactions to cottony leak or Phytophthora pod decay. Post-harvest treatments with hot water and fungicides have been evaluated (24), but have not been adopted by the processing industry. In 2002 and 2003, fungicides with known activity against Oomycetes were evaluated for pre-harvest control of cottony leak of snap bean in Oklahoma (5,6). In 2002 pod decay was caused by P. aphanidermatum, while P. ultimum was the primary pathogen in 2003. Some fungicides reduced disease incidence in 2002, but not in 2003. None of the fungicides provided a high level of disease control compared to results achieved with metalaxyl + copper on downy mildew of lima beans caused by Phytophthora phaesoli (8).
Observations from local commercial fields indicate that Roma II, a cultivar with flat pods that is grown for a specialty product, is highly susceptible to pod decay. Roma II has prostrate canopy architecture and is susceptible to lodging. Because basal pods in contact with the soil are thought to be primary infection courts for cottony leak, we hypothesized that cultivars with upright growth habits and/or that set pods higher on the plant may escape infection. Furthermore, fungicide penetration into the plant canopy may be improved on cultivars with an upright growth habit. The objective of this study was to evaluate snap bean cultivars for their reaction to Pythium leak in the field and to measure their disease response to a fungicide program. A brief report on this research has been published (4).
Plant materials. Snap bean cultivars and breeding lines with round and flat (Romano-type) pods were obtained from several seed companies who submitted bush-type cultivars with characteristics that might result in reduced pod rot and improved mechanical harvest efficiency. These characteristics included an upright plant habit, reduced lodging, and increased pod set height to minimize pod contact with soil. Entries were compared to the local standards of Roma II for cultivars with flat pods and Nelson for cultivars with round pods.
Plot management practices. Field trials testing the combined effects of cultivar and fungicide were conducted from 2004 to 2007 at the Oklahoma Vegetable Research Station in Bixby, OK, in a field of Wynona silty clay loam soil where Pythium leak has been a previous problem. Only data from 2004 and 2007 are presented because of crop failures in 2005 and 2006. In 2005, plants were stunted and did not develop Pythium leak, while in 2006, large areas of plants were killed by root rot before the trial could be completed.
In 2004, the field received 29-29-29 kg/ha N-P-K granular fertilizer prior to planting on 28 April. Weeds were controlled with a pre-emergence application of s-metolachlor at 0.67 kg/ha made immediately after planting. Plots were top-dressed with 38 kg/ha N as ammonium nitrate on 18 May and with 24 kg/ha N as ammonium sulfate on 18 May and 25 May. Insects were controlled with methomyl at 0.67 kg/ha on 12 June. In 2007, the field received granular fertilizer at 30-77-0 kg/ha N-P-K prior to planting on 19 April. Plots were top-dressed with 52 kg/ha N as urea on 16 May and 31 May. Weeds were controlled with a tank mixture of bentazon (0.56 kg/ha), fluazifop-P-butyl (0.21 kg/ha), fomesafen (0.21 kg/ha), and non-ionic surfactant (0.58 liter/ha) applied post-emergence on 17 May. Plots received over twice the N rate of 28 to 62 kg/ha recommended by extension specialists in Oklahoma (16). This was done to reflect the high N rates often used on processing snap beans in the region, and because levels of Pythium leak in previous soil fertility trials were observed to be most severe under high nitrogen fertility.
Experimental design and fungicide application. The experimental design was a split-plot with four randomized complete blocks separated by fallow 1.5-m-wide alleys. Main plots consisted of four 6.1-m-long rows of each cultivar spaced 0.9 m apart. Seeds were spaced approximately 3.4 cm apart within rows. Sub plots consisted of two rows left untreated, and two rows treated with a fungicide pre-mixture consisting of mefenoxam at 0.14 kg/ha and copper hydroxide at 1.68 kg/ha (Ridomil/Gold Copper 65W, Syngenta Crop Protection, Greensboro, NC). The fungicides were applied as a directed spray through three flat-fan nozzles (8002vk, Teejet, Wheaton, IL) per row using a CO2-pressurized wheelbarrow sprayer. The sprayer was calibrated to deliver 402 liters/ha in 2004 and 318 liters/ha in 2007 at 276 kPa. The fungicides were applied on approximately 7-day intervals beginning when pods first developed. Applications were made on 7 June, 14 June, and 25 June in 2004; and on 6 June and 14 June in 2007. Plots were infested with P. aphanidermatum by spreading 50 ml of oat kernels colonized by the fungus over each plot row on 14 June each year.
Plant architecture ratings. To assess the relationship between plant architecture and Pythium leak development, cultivars were evaluated for lodging, plant height, canopy size, and height of the lower pods on 23 June 2004 and 25 June 2007. Lodging was assessed by visually estimating the percentage of plants lodged in each plot. Plant height was determined by measuring the tallest plant in the center two rows of each plot. Canopy size was assessed visually for each plot row with an index of 1 to 4 where 1 = narrow row with large gap between rows, and 4 = wide row with no gap between rows. Pod set height was assessed by examining non-lodged plants nearest to the center of each plot and assessing the position of the lowest pod raceme on a 1 to 5 scale where: 1 = lowest, most pods touching ground; 2 = low, a few pods touching ground; 3 = intermediate, pods set on lower half of plant but not touching ground; 4 = middle, pods set on middle of plant and not touching ground; and 5 = highest, pods on upper plant and not touching ground.
Yield and disease assessment. Plots were harvested on 24 June 2004 and 21 June 2007 by cutting plants from a typical 1-m row segment from the center of each sub-plot and hand-picking the pods. Pods were immediately weighed and graded. Cultivars with round pods were graded by determining the percentage of a 500-g sample at each sieve size. Cultivars with flat pods were graded by measuring the length of the largest seed from 10 large pods per cultivar. Because some grade samples indicated immaturity, plots of the cultivars PLS 75, Cerler, and Nelson were harvested and graded again on 1 July 2004. Disease incidence was measured by counting the number of 15-cm row segments with pod decay in each sub plot on 7 July 2004 and 25 June 2007. The counts were converted to the percentage of row length affected.
Data analysis. Data on disease incidence, yield, and the plant architectural characteristics lodging and plant height were subjected to analysis of variance using the MIXED procedure of SAS (version 9.2, 2008, SAS Institute Inc., Cary, NC). The fixed effects of cultivar, fungicide treatment, and their interaction were evaluated. Trial and block (trial) were considered experimental replications and hence random effects. For the cultivar*fungicide interaction, the effects of fungicide program within cultivar and cultivar within fungicide program were evaluated using the SLICE option of the LSMEANS statement in the MIXED procedure. Least square means over trials were compared using t-tests produced by the PDIFF option when indicated by a significant effect (P ≤ 0.05) in the analysis of variance. Letters were assigned to the mean separation groupings using a freely available macro (20). The plant architectural characteristics row width and pod height, which were evaluated on ordinal scales, were analyzed using non-parametric techniques (22). The data were ranked from lowest to highest using the RANK procedure where the lowest value received a rank of 1 and ties were averaged. The rank transformed variables were analyzed using the ANOVAF option of the MIXED procedure. The ANOVAF option is invoked using freely available macros (22). Least square means of the ranks were compared as described above. The influence of plant architectural characteristics on disease incidence was evaluated by correlation analysis. Unless otherwise indicated, differences reported are significant at P ≤ 0.05.
Cultivar and Fungicide Effects on Pythium Leak
In June when pods developed, average daily temperature was near the normal (30-year average) of 23.5°C and rainfall was above the normal of 120 mm each year. Rainfall in June totaled 155 mm in 2004 and 280 mm in 2007. In general, plants approached maturity before symptoms of pod decay developed. Plots were evaluated 7 to 14 days after harvest in 2004 and 4 days after harvest in 2007 when Pythium leak reached measurable levels. Pythium leak developed each year, but was more severe in 2004 when disease incidence averaged 12.7% compared to 9.7% in 2007. The excessive rains in 2007 stunted plant growth and differences in plant architectural characteristics also were not as great in 2007 compared to 2004. From isolations made from representative symptomatic pods on water agar, P. aphanidermatum was the predominant pathogen isolated each year. In 2007, P. ultimum and Phytophthora parasitica var. nicotianae were also isolated from symptomatic pods.
The effects of cultivar (P < 0.01), fungicide treatment (P < 0.01), and the cultivar fungicide interaction (P = 0.01) were significant on disease incidence. In untreated plots, the reference cultivars Nelson and Roma II had the highest levels of disease, exceeding 20% incidence (Table 1). For the entries with round pods, all had reduced disease incidence compared to the reference cultivar Nelson. For entries with flat pods, all except Moncayo had reduced levels of disease compared to the reference cultivar Roma II. Entries with the lowest disease incidence had less than half the disease of respective reference cultivars and included SB 4261 for entries with round pods, and the cultivars Cerler, Tapia, Primo, and Ebro for entries with flat pods.
Table 1. Disease and yield responses of snap bean cultivars to fungicide (metalaxyl + copper hydroxide) for control of Pythium leak.
v Percentage of row length with symptoms of cottony leak.
w Pod type where f = flat (Romano-type) and r = round.
x Test for treatment effect within cultivar.
y Average of untreated and fungicide-treated sub-plots.
z Values (least square means) in a column followed by the same letter are not statistically different at P = 0.05 according to t-tests obtained from the PDIFF option of SAS.
Treatment with the fungicides cupric hydroxide + mefenoxam resulted in an overall reduction in disease incidence from 14.1% to 8.4%. Entries responded differentially to fungicide treatment (Table 1). Fungicide treatment reduced disease incidence for all entries except for PLS 75, Cerler, Tapia, and Ebro. Except for PLS 75, entries that did not respond to fungicide treatment were among those with the lowest disease incidence (10% or less) in untreated plots. Within fungicide-treated plots, the cultivar effect was not significant (P = 0.07) and numerical differences among most cultivars except Nelson, which had nearly 20% disease incidence in treated plots, were small.
The effect of cultivar (P < 0.01) was significant on yield, but not the effects of fungicide treatment (P = 0.68) or the cultivar × fungicide interaction (P = 0.11). Therefore, yields were averaged over untreated and fungicide-treated subplots (Table 1). Yields varied among the cultivars with round pods. Entry SB 4261 had higher yield compared to the reference cultivar Nelson, while PLS 75 had lower yield. Among the entries with flat pods, all had higher yield compared to the reference cultivar Roma II except, Moncayo, Romano 942, Tapia, and Ebro. Over years, cultivars, and fungicide treatment, plot yields were not correlated with disease incidence (r = < 0.01, P = 0.95).
Except for plant height, there was significant variability among entries for the plant architectural characteristics evaluated (Table 2). The entries Ebro, PLS 75, and Bogota had lower lodging levels compared to the reference cultivars Roma II and Nelson. The cultivars Primo and Romano 942 numerically had the highest levels of lodging (> 60%), but they did not statistically differ from the reference cultivars. The effect of entry on plant height was not significant (P = 0.12) and most of the entries were similar in height to the reference cultivars except for Romano 942, which was numerically the tallest. Both the ANOVA-like test statistics (P < 0.01) and the Wald-type statistic (X ² < 0.01) for the effects of entry on row width and pod set height ranks were significant. Among entries with flat pods, Navarro, Romano 942, and Primo ranked higher in row width compared to Roma II, while Ebro and Bogota ranked lowest. For the entries with round pods, PLS 75 ranked the lowest in row width. Among the entries with round pods, none had ranked higher in pod set height compared to Nelson. For the entries with flat pods, the entries Cerler, Primo, and Romano 942 ranked higher in pod set height compared to Roma II and the other entries. Plant architectural characteristics were only weakly correlated with disease incidence. Disease incidence was positively correlated (P = 0.05) with lodging (r = 0.16) and plant height (r = 0.16), but not with ranks of row width or pod set height.
Table 2. Plant architectural characteristics of snap bean cultivars evaluated for their reaction to Pythium leak.
v Pod type where f=flat (romano-type) and r=round.
w Rating for row thickness where 1 = narrow row with large gap between rows, and 4 = wide row with no gap between rows.
x Height of the lowest pod raceme where 1 = lowest, touching ground, and 5 = highest not touching ground.
y The ordinal ratings of row width and pod height were ranked from lowest to highest using the RANK procedure of SAS where the lowest value received a rank of 1 and ties were averaged.
z Values (least square means) in a column followed by the same letter are not statistically different at P = 0.05 according to t-tests obtained from the PDIFF option of SAS.
Summary and Discussion
Several snap bean genotypes were identified that had partial resistance to development of Pythium leak in the field compared to regionally important reference cultivars. In general, entries with the best resistance had about 50% less disease than their respective reference cultivars with flat or round pods. Fungicide treatment with mefenoxam + copper hydroxide provided an additional level of disease control that reduced disease levels by an average of 40%, but fungicide application was not effective on all entries. Partially resistant entries had yields similar to the reference cultivars and appeared to be commercially acceptable. While significant reductions in disease incidence were observed, none of the cultivar or treatment combinations eliminated Pythium leak or reduced disease incidence below a 5% level, suggesting a need for additional management strategies.
We hypothesized that because Pythium leak mainly affects lower pods near or in in contact with the soil, genotypes with plant architectural characteristics that minimize soil and pod contact might have lower levels of disease or permit better disease control with fungicide by increasing fungicide deposition onto pods. These characteristics included an upright plant habit with less lodging and an increased height of pod set on the plant. The entries Cerler, Primo, and Romano 942 were tall cultivars that had higher pod set heights than the reference cultivars and most of the entries. However, these entries also had the highest levels of lodging. As pods mature on genotypes that set pods high on the plant, the increased weight of the upper plant apparently promotes more lodging than on genotypes with lower pod set heights. Lodging might be responsible for increased pod contact with soil and negate any benefits from an increased pod set height. These interactions among plant architectural characteristics might explain the weak relationships between architectural characteristics and disease incidence as determined by correlation analysis.
Fungicide efficacy was minimally affected by cultivar. Generally, reductions in disease incidence were greatest for the most susceptible cultivars and fungicide did not reduce disease incidence for some of the cultivars such as Ebro and Cerler that had low levels of disease in untreated plots. It also did not appear that any of the entries with different plant architectural characteristics than the reference cultivars, were more conducive to effective disease control with fungicide. For example, it is reasonable to expect that fungicide penetration to pods might be increased in entries with reduced canopy density. However, fungicide performance was not increased in entries with narrow rows such as Ebro, Tapia, Bogota, and PLS 75. Research is needed to assess fungicide deposition to pods in the lower canopy and explore alternative application methods such as increasing carrier volume or using alternative application methods such as chemigation.
Factors affecting outbreaks of Pythium leak on snap beans are poorly understood. The disease did not develop to high levels in the field and methods are needed to enhance disease development in research plots. While Pythium leak exceeded 20% disease incidence for some entries when measured by counting row sections with infested pods, the percentage of pods affected was generally too low to evaluate despite inoculation with oats infested with P. aphanidermatum. The effectiveness of the inoculation is uncertain because non-inoculated plots were not included and pathogens such as P. ultimum and Phytophthora parasitica var. nicotianae were also involved in the disease outbreak in 2007. The disease did not develop until after many of the entries reached maturity levels established for machine harvested snap beans grown for processing. However, maturity alone does not account for the differential levels of disease observed. For example, Nelson, the reference cultivar with round pods that had the highest level of disease each year was less mature at harvest than SB 4261 which had lower levels of disease. Factors other than soil moisture alone must affect Pythium leak development because the high levels of rainfall received on the trial in 2007 produced prolonged periods of saturated soil that should have favored severe levels of disease caused by oomycete pathogens. Yet disease was more severe in 2004 when levels of rainfall were near normal. Factors affecting outbreaks of Pythium leak may be complex and involve interactions of the host and environment.
Results of this research have been partially adopted by the local snap bean processing industry. Cultivars such as Bogota, Navarro, and Tapia displaced Roma II in the production of Romano type snap beans in part because of their partial resistance to Pythium leak. However, undesirable processing characteristics of some cultivars have limited adoption. Conversely, fungicide application has not been adapted as a management strategy for Pythium leak. The economic benefit of disease control with fungicides has not outweighed the increased input costs. In this study, 2 to 3 applications were made which exceeds budgets for processing snap beans. Despite the numerous fungicides registered for control of oomycete diseases in recent years, none have provided a high level of disease control and little fungicide is currently applied to processing snap beans in the region.
Approved for publication by the Director of the Oklahoma Agricultural Experiment Station. This research was funded in part by the Oklahoma Agricultural Experiment Station and Allens, Inc.
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