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Occurrence of Smut Caused by a Ustilago sp. on Dry-Beans

In September 1997, after the tropical winter season and following a long warm (28 to 33°C) and rainy (27 mm) period, smut symptoms were observed on two dry-bean (Phaseolus vulgaris) fields in Brazil. Brownish to blackish powdery masses of spores were observed on stems, pods, and roof green and senescent plants. No galls were observed. Hyphae in infected tissue were intercellular; no clamp connections were observed. Typical globose teliospores were observed that measured 7 to 11 μm. Spores were uninucleate or binucleate; predominantly brownish with flattened poles and equatorial band; the exospore wall was thick and surface ornamented (echinulate-type). Teliospores incubated in water drops generated transversely septate promycelia, unbranched, producing terminal and lateral basidiospores. The fungus did not grow well on potato dextrose agar (PDA) medium, generating yeastlike colonies. To confirm pathogenicity of a Ustilago sp., spores collected from field-infected plants were inoculated on 8- and 20-day old seedlings of dry-bean cultivars Rosinha and Perola. Spore suspensions of 10⁵ teliospores were sprayed all over the plants. After inoculation, plants were kept in a moist chamber (relative humidity [RH] 100%) for 2 days. At the third day, inoculated plants were covered with clear polyethylene bags and transferred to the greenhouse (RH 50%, temperature 25 ± 2°C). Noninoculated controls were included for comparisons. Within 2 weeks, all inoculated plants reproduced field symptoms of darkened tissues covered by profuse sori containing teliospores and basidiospores. Eight-day-old plants died 2 weeks after inoculation. Noninoculated plants did not develop any symptoms. Teliospores obtained from dead plants were identical to those used for inoculation, thus completing Koch's postulates. Fischer (1), and later Fischer and Shaw (2), proposed a species concept for Ustilago based on morphology (primarily of the teliospore), symptomatology, and host specialization at the host-family level. Although the teliospores found on dry-beans, in Brazil, are much like those of Ustilago maydis, the fungus did not cause galls on plants, was able to infect roots and kill seedlings. and is very well adapted to a legume plant. Cross inoculation tests indicated that spores from dry-beans cannot cause disease symptoms on maize. We therefore regard the bean smut as a new species. We are not aware of any other Ustilago sp. being reported on a legume host.

References: (1) G. W. Fischer. 1953. Manual of the North America Smut Fungi. (2) G. W. Fischer and C. G. Shaw. Phytopathology 43:181, 1953.