Identification of New Sources of Root Rot Resistance in Manitoba Grown Dry Bean Cultivars

Identification of new sources of root rot resistance in Manitoba grown dry bean cultivars.

R.L. Conner, D.L. McLaren,A. Hou and P.M. Balasubramanian.

Introduction and Literature review
Recent surveys of commercial fields of dry beans (Phaseolus vulgaris L.) have shown that Fusarium root rot and Rhizoctonia root rot are the most prevalent root diseases in Manitoba (Henriquez et al. 2012, 2013). A complex of several pathogenic fungi is usually responsible for root rot in bean, which can include several Fusarium species and Rhizoctonia solani (Goswami et al. 2010) that flourish under similar environmental conditions.

Early infection by the root rot pathogens often results in seedling blight that can lead to the death of the bean seedlings before or shortly after they have emerged from the soil resulting in uneven plant stands in the field (Harveson et al. 2005), which can create weed control problems due to a lack of competition. Infection of adult plants often will not lead to their death, but instead results in the formation of reddish-brown lesions on the tap roots and hypocotyls, stunting and wilting of plants and poor pod filling (Chaudhary et al. 2006). Badly infected plants are less vigorous than healthy plants (Navarro et al. 2008). Rhizoctonia and Fusarium root rot root each produce reddish-brown lesions on the tap roots, but Rhizoctonia root rot results in the formation of more sunken lesions on the stems and the tap roots near the soil line (Schwartz et al. 2005). Rhizoctonia root rot can weaken the stem or sometimes kill the plant if the lesions completely girdle the stem. Above-ground symptoms of root rot are usually most obvious when the bean crop is subjected to some form of moisture stress, poor drainage or soil compaction (Harveson et al. 2005).

The development of root rot resistant cultivars is generally considered to be the best form of long-term disease management and the most cost-effective component for the integrated control of root rot in dry beans (Tu 1992; Park and Rupert 2000; Abawi et al. 2006). Beebe et al. (1981) report that resistance to Fusarium and Rhizoctonia root rot resistance appeared to be common among bean lines from Latin America possibly because of natural selection for that trait. Root rot resistance has been shown to be more common in Mesoamerican genotypes of beans than it is in Andean genotypes (Schneider et al. 2001; Román-Avilés and Kelly 2005; Nicoli et al. 2011). However, inter gene pool crosses can often lead to complications in the interpretation of the results of genetic studies (Schneider and Kelly 1999).

Partial resistance to specific root rot pathogens has been reported in dry bean cultivars and germplasm lines (Beebe et al. 1981). Black beans in particular have previously been shown to be a good source of root rot resistance (Beebe et al. 1981; Román-Avilės and Kelly 2005; Ronquillo-López et al. 2010) or resistance to damping-off (Peῆa et al. 2013).Burke and Miller (1983) developed several pink bean and pinto bean lines or cultivars that were partially resistant to Fusarium root rot and noted that resistance had the largest contribution to improved yields when it was combined with tolerance to various forms of environmental stress.

This field study was established to assess dry bean cultivars that are currently grown in western Canada, for their reactions to seedling blight and root rot. The dry bean cultivars chosen for this study were selected because they have recently been used as parental material in the bean breeding programs at the Morden Research Station or the Lethbridge Research Centre. The effects of different root rot pathogens either alone or in combination on the yields of dry bean cultivars with different levels of partial resistance were also examined.

Objectives

Conduct a five-year study to evaluate Manitoba dry bean cultivars for their reactions to Fusarium root rot and Rhizoctonia root rot. Carry out a field study on the effects of different levels of root rot resistance in reducing yield losses in dry bean caused by different pathogens.
Methodology

In 2008, a total of 37 dry bean cultivars were evaluated for their root rot reactions following inoculation with either Rhizoctonia solani or Fusarium solani. Testing of 36 bean cultivars for their reactions to F. solani and R. solani was completed in 2010, so these cultivars were evaluated for their reactions to F. redolens and F. acuminatum in 2011 and 2012. The 15 new cultivars that were evaluated for root rot resistance in 2010 were tested for their reactions to all four root rot pathogens in 2011 and 2012. From 2009 to 2012 the study was expanded and 10 inoculated, field experiments were carried out to separately to assess the root rot reactions of 61 dry bean cultivars to R. solani,F. solani, F. redolens and F. acuminatum. Field experiments consisting of approximately 15 dry bean cultivars and nine resistant or susceptible check cultivars were carried out to separately evaluate their reactions to each root rot pathogen.
In a second experiment, three cultivars that displayed different levels of root rot resistance in preliminary field tests and a susceptible cultivar were assessed for root rot severity and yield following inoculation with R. solani, F. solani, F. redolens and F. acuminatum either alone or in various combinations, which were compared to an uninoculated treatment of the same cultivars.
All the field experiments were arranged in a randomized complete block design with four replications. The plots were evaluated for seedling emergence, root rot severity and root nodulation. Yield was only assessed in the experiment on the effects of root rot on the four dry bean cultivars with different levels of root rot resistance.
Results

In each year of the study, differences in seedling emergence and root rot severity were observed among the dry bean cultivars. Throughout the study root nodulation levels were generally low, so differences in nodulation among bean cultivars were seldom detected. Certain cultivars were resistant to the root rot pathogens as the moderately resistant check cultivars. However, most of the dry bean cultivars were susceptible to the root rot pathogens. Partial resistance to the root rot pathogens was not always associated with high rates of seedling emergence. High rates of seedling emergence were observed in dry bean belonging to a number of different bean classes, which indicated that those cultivars were resistant to seedling blight caused by the root pathogens. However, only two cranberry bean cultivars, a navy bean cultivar and two black bean cultivars had significantly lower root rot ratings than the mean of the seven partially resistant check cultivars.

Each year, the experiment on the effects of the four pathogens on root rot severity and yield loss in four cultivars of dry beans went well.Significant differences in root rot severity were observed among the four bean cultivars and the four root pathogens. Root rot severity was lowest in the moderately resistant cultivars Etna and greatest in the Envoy, while the other two cultivars had intermediate ratings.The yields of Etna, Envoy and Maverick were not affected by inoculation treatment. The results showed that inoculation with F. redolens and F. acuminatum either alone or in combination did not affect the yields of any of the cultivars.Inoculation of Morden003 with R. solani and F. solani was the only treatment that reduced the yield (16.3% less than the disease-free check) of this cultivar.

In the root rot resistance and the root rot yield experiments, the pathogens were shown to differ in their effects on seedling emergence and root rot severity. Generally inoculation with R. solani had the most adverse effect on seedling emergence, while inoculation with F. solani consistently resulted in the greatest root rot ratings.
Discussion
There was good agreement between the results of the root rot resistance trials throughout2008, 2009, 2010, 2011 and 2012. This study identified five new sources of root rot resistance. Resistance to root rot has been previously reported in black and navy beans. However, due to their Andean origin, partial root rot resistance was not expected to occur in the two cranberry bean cultivars. The lack of a consistent relationship between resistance to seedling blight and root rot indicates these traits may be inherited independently. The availability of partial root rot resistance in well adapted small-seeded Mesoamerican cultivars (i.e., navy and black beans) and in Andean cultivars (i.e., cranberry beans) should facilitate the transfer of this trait to other cultivars within these two major groups.

Dry conditions in July and August in 2011 and 2012 should have placed the bean cultivars in the root rot yield experiment under moisture stress. Severe moisture stress has been reported to increase the adverse effect of root rot on bean yields. However, only Morden003 had its yield significantly reduced by inoculation with F.solani and R. solani. Abawi and Ludwig (2005) reported that root rot had only a minor impact on the yield of beans. This contrasts with estimates of huge yield losses in other root rot studies of beans (Beebe et al. 1981; Tan and Tu 1995).

Next Steps

A combined analysis of all the data from the field trials has been completed and a scientific manuscript describing the results of the bean root rot resistance study is ready for submission to a scientific journal. Information on the root rot reactions of the bean cultivars used in this study has been provided to the dry bean breeders at AAFC-Morden and AAFC-Lethbridge to enable them to plan their crossing program to enhance root rot resistance in their elite breeding lines.A research proposal entitled “Evaluation of root rot resistance of dry bean cultivars” was supported for funding by MPGA in 2013. This studywill enable the continuingevaluation of additional dry bean cultivars from Manitoba for their reactions to six root rot pathogens. Ongoingresearch on the partial resistance to root rot in evaluation of dry bean cultivars is needed to provide this information to growers to assist them in the selection of the most appropriate cultivars for their farm operations.

Acknowledgements

We thank Waldo Penner and Dennis Stoesz at AAFC-Morden for their technical assistance, and the Manitoba Pulse Growers Association and the Growing Forward I program of AAFC. We also thank Dr. Ken McRae for conducting an in-depth statistical analysis of the results from this study.
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