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The objectives of the lab researches are:
- Etiology and epidemiology of bacterial and fungal pathogens causing diseases on common bean in Bulgaria.
- Searching of domestic and introduced bean genotypes for disease reaction.
- Breeding dry bean for diseases resistance.
Common bacterial blight (Xanthomonas campestris pv.phaseoli) presents a considerable problem for dry bean production in Bulgaria. The lack of resistant cultivars and appropriate certificate programs are the main reason for the wide distribution of the disease during the last years.Since 1991, over 2 500 domestic and introduced accessions from DAI-Phaseolus collection have been screened for CBB disease reaction in greenhouse and field experiments. Over 25 resistant bean genotypes were selected and included in the CBB resistance breeding program. The cultivars Oreol, G.N.Jules, G.N.Star, HR 45 and VAX 3-6 are the main resistant donors at the present.
Two strain variants of X.c.pv.phaseoli (normal and fuscous) are distributed in the country. Different metabolic capacity between these variants have bean established in BIOLOG GN-microplate experiments. A high variability in virulence between and in each of the strain variants was observed under greenhouse and field conditions.
Methodology
Leaves, pods and seeds with CBB symptoms are collected to watch the pathogen variability in North and North-Eastern Bulgaria (South Dobroudja), the main dry bean production region. The pathogen is isolated on MXP medium and stored in 15% glycerol at -20C or maintained on YDC at 4C. Several tests are conducted for identification of purified strains. The virulence variability is observed by inoculation of appropriate resistant donors. Two methods for inoculation are used depending on plant organs- multiple needle method (MN)for leaf and hypodermic syringe with needle (HS)for pod. These methods are applied in field screening of breeding lines.
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Halo Blight
Halo blight, caused by bacterium Pseudomonas syringae pv. phaseolicola is of limited importance for dry bean productions in Bulgaria. The reason is resistance of commercial cultivars to the distributed races. Five races of the pathogen were identified by a standard set of 8 differential genothypes. These races were derived from previously reported old races 1 (races 1, 7 and 9) and 2(races 2 and 6). Race 6 is predominant in South Dobroudja. Recently, pathotypes different from those of the races referred above were observed.
Methodology
The pathogen is isolated from diseased seeds and plant organs on semi-selective medium MSP.Purified strains are stored in 15% glycerol at -20C or on YDC at 4C.Pathogenicity, phaseoli toxin production and metabolic profile are investigated. Race affiliations are determined by inoculation of standard differential set. A hypodermic syringe is used for infiltration of bacterial suspension (106 cfu.ml) in primary leaves. The resistance of the breeding lines are evaluated under field conditions.
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Bacterial Wilt
Bacterial wilt caused by Curtobacterium flaccumfaciens pv. flaccumfaciens has limited efficacy on dry bean production in the country. Nevertheless, most of the commercial cultivars are susceptible to the pathogen. A higher virulence variability in the pathogen populations has been established.
Methodology
The bacterium is isolated from diseased plant organs on CNS medium. Purified strains are stored in 15% glycerol at -20C or on YDC at 4C. A hypodermic syringe is used for injecting a bacterial suspension in plant cotylidonory node. This technique is applied in field and greenhouse screening of breeding lines.
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Anthracnose
In spite of its rare occurrence in Bulgaria, bean anthracnose ( Colletotrichum lindemuthianum) can cause severe crop damages under environmental conditions appropriate for the fungus development.Three races of the pathogen were identified in the country by inoculation of a set of 12 differential genotypes. Race 81 is a widely distributed pathotype in the bean production regions, and the other two races (2 and 130) are spread in the mountains regions. In 2002 a new race ( race 23) was isolated from seeds sample with unidentified origin. All Bulgarian commercial cultivars are susceptible to races 81 and 23, and most of them are resistant to races 2 and 130.
Methodology
The fungus is isolated from plants and seeds with anthracnose symptoms. Purified, monosporic cultures are maintained on PDA or lyofilized. A set of 12 standard differential genotypes is used for race determination. Disease reaction of breeding lines is evaluated under greenhouse conditions in V2 plant stage. The plants are sprayed with spore suspension and immediately transferred to wet chamber for 72h.
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White Mold
Rotation of dry bean with sunflower is the main reason for the crop looses caused by fungus Sclerotinia sclerotiorum in Bulgaria.. At the last few years several common bean accessions with physiological resistance to white mold has been selected. Part of these accessions were included in DAI bean breeding program for resistance to the pathogen.
Methodology
The pathogen is isolated from sclerotia formed in diseased plants on PDA medium. The straw method is used to determine the physiological resistance of breeding lines under greenhouse and fields conditions.
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Rust
Bean rust fungus Uromyces appendiculatus can cause sever crop losses in case of earlier development of the disease in Bulgaria. Seven races were identified in the country by the set of 19 standard differential cultivars. Some of commersial cultivars are resistant to the identified pathotypes.
Methodology
Mobile nursery of standard differential genotypes is used to study virulence variability in Bulgarian rust populations. The uredinias of the pathogen are collected from diseased leaves and stored on silicagel at -20C. The set of differential cultivars is inoculated with purified uredinia cultures for race determination. The resistance of breeding lines is studied in fields and greenhouse.
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