Comparative testing of the resistance of bean accessions to white mold (Sclerotinia sclerotiorum) by a direct and indirect method

Abstract

White mold, caused by Sclerotinia sclerotiorum, is a key disease of common beans in temperate and subtropical climates. For the climatic conditions of Bulgaria, the infection occurs mainly as a result of mycelogenic development of the sclerotia, which is why the disease is observed mostly in varieties with a prostrate type of the habitus. Due to the wide range of hosts of the pathogen and long-term preservation of sclerotia in the soil, agrotechnical measures have little effect in controlling the fungus. Therefore, breeding resistant varieties are the most effective measure to prevent the disease. The selection of appropriate and verified methods for testing the source and breeding materials is of primary importance for the success of breeding programs to increase resistance to the pathogen. The present study aimed to compare the response of bean accessions to S. sclerotiorum, by applying a direct and indirect evaluation method with a view to their parallel use in the breeding process. The studies were conducted under greenhouse conditions, and 29 bean accessions were included in them. The accessions were grown in pots (45x30x10 cm) with a soil-sand mixture. In the first trifoliate leaf, the stem was cut at 3 cm from the node of the primary leaves (STAW test). A single-sided closed plastic straw containing an agar disc from the periphery of 3-day-old cultures of four isolates cultivated on PDA medium (five plants per isolate), was placed on the cut. The reaction of the accessions was recorded 7 days after inoculation on a 9-point scale. The cut plant parts (stem and trifoliate leaf) were immersed in oxalic acid solution (20 mM brought to pH=4.0 with 1N NaOH), five plants per sample. The cut plant parts immersed in sterile water adjusted to pH=4.0 with 1N HCl were used for the control. The reaction of the accessions was recorded after 19h on a 9-point scale. The data were processed with the SPSS stat. ver. 19 software. Significant differences were observed in the response of the accessions concerning the isolates used. Weak positive or negative correlations were found between the oxalate test and the STRAW test, and the direction was related to the aggressiveness of the isolates. It was established that for accessions A195, ‘GTB Scythia’, ‘Izabel’, ‘Rhodopeia’ and ‘Pirina’ the results of the oxalic test are comparable to those of the STRW-test. The obtained results give us reason to recommend the application of the oxalate test in the initial generations of the breeding materials, after a preliminary comparative analysis of the parental forms by both methods.