Abstract
Anthracnose caused by Colletotrichum lindemuthianum is an important disease of common bean, resulting in major economic losses worldwide. Genetic diversity of the C. lindemuthianum population contributes to its ability to adapt rapidly to new sources of host resistance. The origin of this diversity is unknown, but sexual recombination, via the Glomerella teleomorph, is one possibility. This study tested the hypothesis that Glomerella strains that are frequently recovered from bean anthracnose lesions represent the teleomorph of C. lindemuthianum. A large collection of Glomerella isolates could be separated into two groups based on phylogenetic analysis, morphology, and pathogenicity to beans. Both groups were unrelated to C. lindemuthianum. One group clustered with the C. gloeosporioides species complex and produced mild symptoms on bean tissues. The other group, which belonged to a clade that included the cucurbit anthracnose pathogen C. magna, caused no symptoms. Individual ascospores recovered from Glomerella perithecia gave rise to either fertile (perithecial) or infertile (conidial) colonies. Some pairings of perithecial and conidial strains resulted in induced homothallism in the conidial partner, while others led to apparent heterothallic matings. Pairings involving two perithecial, or two conidial, colonies produced neither outcome. Conidia efficiently formed conidial anastomosis tubes (CATs), but ascospores never formed CATs. The Glomerella strains formed appressoria and hyphae on the plant surface, but did not penetrate or form infection structures within the tissues. Their behavior was similar whether the beans were susceptible or resistant to anthracnose. These same Glomerella strains produced thick intracellular hyphae, and eventually acervuli, if host cell death was induced. When Glomerella was co-inoculated with C. lindemuthianum, it readily invaded anthracnose lesions. Thus, the hypothesis was not supported: Glomerella strains from anthracnose lesions do not represent the teleomorphic phase of C. lindemuthianum, and instead appear to be bean epiphytes that opportunistically invade and sporulate in the lesions.
Highlights
Anthracnose, caused by the fungus Colletotrichum lindemuthianum (Sacc. & Magn.) Scribn., is one of the most important diseases on common bean (Phaseolus vulgaris L.) worldwide, and causes significant losses in Brazil [1,2]
Anthracnose is generally managed by the use of resistant cultivars, but the extreme genetic diversity of the pathogen population in Brazil contributes to frequent failure of resistance sources [3,4,5,6,7,8]
In the conidial B group the conidia were scattered over the surface of the agar while in conidial A the conidia were formed in clumps
Summary
Anthracnose, caused by the fungus Colletotrichum lindemuthianum (Sacc. & Magn.) Scribn., is one of the most important diseases on common bean (Phaseolus vulgaris L.) worldwide, and causes significant losses in Brazil [1,2]. Anthracnose is generally managed by the use of resistant cultivars, but the extreme genetic diversity of the pathogen population in Brazil contributes to frequent failure of resistance sources [3,4,5,6,7,8]. It is unclear how diversity arises, but genetic recombination during sexual reproduction is one possibility. The contribution of sexual recombination to pathogen population diversity in C. lindemuthianum remains unclear
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