Characterization of Glomerella strains recovered from anthracnose lesions on common bean plants in Brazil.

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.

GcSTUA, an APSES transcription factor, is required for generation of appressorial turgor pressure and full pathogenicity of Glomerella cingulata.

Glomerella cingulata, which infects a number of different hosts, gains entry to the plant tissue by means of an appressorium. Turgor pressure generated within the appressorium forces a penetration peg through the plant cuticle. A visible lesion forms as the fungus continues to grow within the host. A G. cingulata homolog (GcSTUA) of the genes encoding Asm1, Phd1, Sok2, Efg1, and StuA transcription factors in Magnaporthe grisea and other fungi was cloned and shown to be required for infection of intact apple fruit and penetration of onion epidermal cells. Mobilization of glycogen and triacylglycerol during formation of appressoria by the GcSTUA deletion mutant appeared normal and melanization of the maturing appressoria was also indistinguishable from that of the wild type. However, GcSTUA was essential for the generation of normal turgor pressure within the appressorium. As is the case for its homologs in other fungi, GcSTUA also was required for the formation of aerial hyphae, efficient conidiation, and the formation of perithecia (sexual reproductive structures).

Glomerella truncata sp. nov., the teleomorph of Colletotrichum truncatum.

Anthracnose of lentil, caused by Colletotrichum truncatum is a serious threat to lentil (Lens culinaris) grown in western Canada. The teleomorph stage of this pathogen was induced to form under laboratory conditions. Random pairing of single conidium isolates enabled the identification of fertile isolates. The individual isolates of this fertile pair were crossed with 14 other isolates, and all isolates were also incubated alone. Self-sterility was observed for all 16 isolates tested. Three isolates did not produce perithecia with either tester isolate, and none of the isolates tested produced perithecia with both tester isolates. Perithecia were brown-black, superficial, solitary or in small groups, obpyriform to ovate or ampulliform, 200-520 x 110-320 microm (mean: 350 x 200 microm). Asci were cylindrical, narrowing slightly at the apex, unitunicate, evanescent, 53-142 x 5-14 microm (mean: 90 x 8 microm), and contained eight ascospores. Ascospores were hyaline, aseptate, oblong, 12-20 x 5-8 microm (mean: 15.7-6.7 microm). The characteristics agree with those described for the genus Glomerella, and the species was named G. truncata sp. nov. The morphology of the new species is compared with that of other species in the genus, and future research on G. truncata is described.

Active changes of lignification-related enzymes in pepper response to Glomus intraradices and/or Phytophthora capsici.

The activities of enzymes responsible for lignification in pepper, pre-inoculation with arbuscular mycorrhizal (AM) fungus of Glomus intraradices and/or infection with pathogenic strain of Phytophthora capsici, and the biological control effect of G. intraradices on Phytophthora blight in pepper were investigated. The experiment was carried out with four treatments: (1) plants pre-inoculated with G. intraradices (Gi), (2) plants pre-inoculated with G. intraradices and then infected with P. capsici (Gi+Pc), (3) plants infected with P. capsici (Pc), and (4) plants without any of the two microorganisms (C). Mycorrhizal colonization rate was reduced by about 10% in pathogen challenged plants. Root mortality caused by infection of P. capsici was completely eliminated by pre-inoculation with antagonistic G. intraradices. On the ninth day after pathogen infection, Peroxidase (POD) activity increased by 116.9% in Pc-treated roots but by only 21.2% in Gi+Pc-treated roots, compared with the control, respectively. Polyphenol oxidase (PPO) and Phenylalanine ammonia-lyase (PAL) activities gradually increased during the first 3 d and dramatically decreased in Pc-treated roots but slightly decreased in Gi+Pc-treated roots, respectively. On the ninth day after pathogen infection, PPO and PAL decreased by 62.8% and 73.9% in Pc-treated roots but by only 19.8% and 19.5% in Gi+Pc-treated roots, compared with the control, respectively. Three major POD isozymes (45,000, 53,000 and 114,000) were present in Pc-treated roots, while two major bands (53,000 and 114,000) and one minor band (45,000) were present in spectra of Gi+Pc-treated roots, the 45,000 POD isozyme was significantly suppressed by G. intraradices, suggesting that the 45,000 POD isozyme was induced by the pathogen infection but not induced by the antagonistic G. intraradices. A 60,000 PPO isozyme was induced in Pc-treated roots but not induced in Gi+Pc-treated roots. All these results showed the inoculation of antagonistic G. intraradices alleviates root mortality, activates changes of lignification-related enzymes and induces some of the isozymes in pepper plants infected by P. capsici. The results suggested that G. intraradices is a potentially effective protection agent against P. capsici.