Screening for resistance to and pathogenic specialization of Fusarium oxysporum f. sp. phaseoli, the causal agent of bean yellows.

RIBEIRO, R. de L. D., and D. J. HAGEDORN. 1979. Screening for resistance to and pathogenic specialization of Fusarium oxysporum f. sp. phaseoli, the causal agent of bean yellows. Phytopathology 69:272-276. Bean cultivars were root-dip inoculated with suspensions of but completely susceptible to that containing 106 conidia/ml. The differmicroconidia of Fusarium oxvsporum f. sp. phaseoli and placed in ential response of bean cultivars and lines inoculated with strains from chambers at 20, 24, and 28 C. Although optimum temperature for growth of Brazil, the USA, and The Netherlands allowed the characterization of two the fungus in vitro was about 28 C, a variable proportion of plants of a pathogenic races of F. oxvsporum f. sp. phaseoli. The European and North susceptible cultivar (Bush Blue Lake 274) did not develop symptoms of wilt American strains appeared to represent a single race, and the Brazilian when incubated at 28 C after inoculation. All plants developed symptoms at strains a different race. The strains from the USA and Europe were highly 20 C even with the lowest inoculum level. The cultivar Preto Uberabinha virulent on Phaseolus coccineus, whereas the Brazilian strains failed to behaved as resistant regardless of the test conditions employed, whereas the induce wilting symptoms on this species. cultivar Tenderette was resistant to inoculum containing 104 conidia/ ml, Additional key words: Phaseolus vulgaris, Fusarium wilt, host reaction. A vascular disease of beans (Phaseolus vulgaris L.), first of dry and snap beans collected in 1973-74 from Rio de Janeiro, described in the USA in 1929 (9) and later called bean yellows, is Brazil; ATCC 18131 isolated originally by G. M. Armstrong and J. caused by Fusarium oxy'sporum (Schlecht) f. sp. phaseoli Kendrick K. Armstrong in South Carolina; and WAG-76 from runner beans and Snyder (12). In Brazil, bean yellows was reported in 1966 (4) in The Netherlands and obtained from N. Hubbeling (Institute of and has become an important disease, particularly for snap bean Phytopathological Research, Wageningen, The Netherlands). crops in certain southeastern areas (5,6,8). There is evidence for Seedlings were grown in vermiculite until the primary leaves more than one race of the pathogen in Brazil (3). Limited tests in the were one-third expanded (usually 7 days old); then they were pulled USA failed to detect differences in pathogenicity of strains from and their root system was washed, uniformly clipped, and California and South Carolina (1). In England (7) and The inoculated. On the day seeds were planted, the fungus Netherlands (N. Hubbeling, personal communication), a similar (monoconidial cultures) was transferred from storage in soil tubes disease occurs on Phaseolus coccineus, but apparently it never has to plates of potato-dextrose agar and incubated at 24 C. On day 4, been reported on local cultivars of P. vulgaris. It was suggested that mycelial disks taken from these plates were added to flasks of the disease of the runner bean described in England might be potato-dextrose broth. The flasks were placed on a rotary shaker incited by a race of F. oxyisporum f. sp. phaseoli different from the for 3 days at room temperature (about 22 C). For the preparation one occurring in the USA (1). of conidial suspensions, contents of the flasks were filtered through In a Rhode Island experimental field plot, differences in susceptia double layer of cheesecloth and the filtrates were centrifuged (10 bility to bean yellows were noticed among bean cultivars under min at 8,000 rpm), followed by resuspension of the conidial pellets conditions of natural infection (11). Cruz et al (6) found resistance in sterile distilled water. Finally, the concentration of conidia was in local dry bean and exposed snap bean cultivars to strains of the adjusted with a haemacytometer or by optical transmittance (600 fungus isolated in Slto Paulo, Brazil. nm) in a Bausch & Lomb Spectronic-20 colorimeter. Roots were The importance of inoculation procedures and environmental dipped momentarily in such suspensions (3) and the inoculated conditions when screening for resistance to Fusarium wilts has seedlings were planted individually in 10-cm diameter pots been demonstrated in several instances (10,17,18). In a recent study containing a mixture of compost, muck soil, and sand (3:3:1, v/v). on the Fusarium wilts of cucurbits, incubating temperature and Uninoculated, susceptible controls were included. Results were inoculum level were included among factors that affect the interrecorded after 30 days and the reaction of individual plants was pretation of results of pathogenicity tests (2). scored according to a disease severity rating (DSR) that ranged The purposes of this paper are: to (i) establish a suitable and from 1.0 (plants showing vascular discoloration, but no external reproducible technique for testing the reaction of beans to F. symptoms) to 5.0 (plants dead). Plants with DSR = 1.0 to 2.0 oxysporum f. sp. phaseoli, and (ii) to compare the virulence of (mild external symptoms followed by recovery) were rated resisstrains of the pathogen from different geographical origins towards tant. Plants with DSR 4.0 (severe leaf dropping, no recovery, bean cultivars and accessions. Preliminary reports have been plants dying) to 5.0 were rated susceptible. Plants with DSR = 3.0 published (14,15). (stunting and chlorosis, limited leaf dropping, plants surviving inoculation) were considered intermediate in reaction. MATERIALS AND METHODS To study the effect of incubation temperatures and inoculum dosages upon the reaction of beans to the yellows organism, The following strains of F. ox'isporum f. sp. phaseoli were used: seedlings were kept in controlled environment chambers (12-hr day three strains (2107-A, 2107-B, and 2116) from diseased specimens and 10,000 lux) at 20, 24, and 28 C. Plants were subjected to approximately 104, 10% and 106 conidia/ml. To compare the 00031-949X/79/000049$03.00/0 virulence of strains of the fungus, a collection of more than 50 bean ©1979 The American Phytopathological Society cultivars and accessions (including 41 Brazilian dry bean intro-