The infection of soybean leaves by Phakopsora pachyrhizi during conditions of discontinuous wetness.

The ability of Phakopsora pachyrhizi to cause infection under conditions of discontinuous wetness was investigated. In in vitro experiments, droplets of a uredospore suspension were deposited onto the surface of polystyrene. After an initial wetting period of either 1, 2 or 4 h, the drops were dried for different time intervals and then the wetness was restored for 11, 10 or 8 h. Germination and appressorium formation were evaluated. In in vivo experiments, soybean plants were inoculated with a uredospore suspension. Leaf wetness was interrupted for 1, 3 or 6 h after initial wetting periods of 1, 2 or 4 h. Then, the wetting was re-established for 11, 10 or 8 h, respectively. Rust severity was evaluated 14 days after inoculation. The germination of the spores and the formation of the appressoria on the soybean leaves after different periods of wetness were also quantified in vivo by scanning electron microscopy. P. pachyrhizi showed a high infective capacity during short periods of time. An interruption of wetness after 1 h caused average reductions in germination from 56 to 75% and in appressorium formation from 84 to 96%. Rust severity was lower in all of the in vivo treatments with discontinuous wetness when compared to the control plants. Rust severity was zero when the interruption of wetness occurred 4 h after the initial wetting. Wetting interruptions after 1 and 2 h reduced the average rust severity by 83 and 77%, respectively. The germination of the uredospores on the soybean leaves occurred after 2 h of wetness, with a maximum germination appearing after 4 h of wetness. Wetness interruption affected mainly the spores that had initiated the germination

Draft genome sequence of the keylime (Citrus × aurantiifolia) pathogen Colletotrichum limetticola

Many species belonging to the genus Colletotrichum are causal agents of plant diseases, generally referred to as anthracnose, in a wide range of hosts worldwide. Colletotrichum spp. are responsible for impacting numerous economically important crops on a global scale. This genus comprises approximately 257 distinct species, which are further organized into at least 15 major phylogenetic lineages known as species complexes (Talhinhas and Baroncelli 2021). Virtually every crop grown in the world is susceptible to one or more species of Colletotrichum (Baroncelli et al. 2014). Among these, the Colletotrichum acutatum species complex stands out as a diverse group of closely related plant pathogenic fungi within the genus (Baroncelli et al. 2017). Members of the Colletotrichum acutatum species complex have a wide host range in both domesticated and wild plant species, and their capability to infect insects has also been described (Damn et al. 2012, Marcelino et al. 2008). In this species complex, Colletotrichum limetticola (formerly known as Gloeosporium limetticola; Clausen 1912) was initially described in 2012 as a species predominantly associated with wither tip symptoms on sour lime (Citrus aurantiifolia) in Cuba and the USA during the 1910s (Damm et al. 2012). Later descriptions associated the disease with strains of C. gloeosporioides (Brown et al. 1996) or C. acutatum (Peres et al. 2008). Recent findings in Brazil have revealed the presence of C. limetticola causing Glomerella leaf spot on apples, although its prevalence remains low while displaying high virulence (Moreira et al. 2019). To the best of our knowledge, no further occurrences of C. limetticola have been documented, despite the presence of other known Colletotrichum species that infect citrus and apples (Talhinhas and Baroncelli 2021). This raises concerns regarding the conservation status of C. limetticola considering the scarcity of records on its original hosts and the occurrence of cross-infections

Preservation of Phakopsora pachyrhizi Uredospores.

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Localization of Pantoea ananatis inside lesions of maize white spot disease using transmission electron microscopy and molecular techniques.

The etiological agent of maize white spot (MWS) disease has been a subject of controversy and discussion. Initially the disease was described as Phaeosphaeria leaf spot caused by Phaeosphaeria maydis. Other authors have suggested the existence of different fungal species causing similar symptoms. Recently, a bacterium, Pantoea ananatis, was described as the causal agent of this disease. The purpose of this study was to offer additional information on the correct etiology of this disease by providing visual evidence of the presence of the bacterium in the interior of the MWS lesions by using transmission electron microscopy (TEM) and molecular techniques. The TEM allowed visualization of a large amount of bacteria in the intercellular spaces of lesions collected from both artificially and naturally infected plants. Fungal structures were not visualized in young lesions. Bacterial primers for the 16S rRNA and rpoB genes were used in PCR reactions 10 amplify DNA extracted from water-soaked (young) and necrotic lesions. The universal fungal oligonucleotide ITS4 was also included to identify the possible presence of funga! structures inside lesions. Positive PCR products from water-soaked lesions, both from naturally and artificially inoculated plants, were produced with bacterial primers, whereas no amplification was observed when ITS4 oligonucleotide was used. On the other hand, DNA amplification with ITS4 primer was observed when DNA was isolated from necrotic (old) lesions. These results reinforced previous report of P. ananatis as the primary pathogen and the hypothesis that fungal species may colonize lesions pre-established by P. ananatis.200

Localization of Pantoea ananatis inside lesions of maize white spot disease using transmission electron microscopy and molecular techniques.

The etiological agent of maize white spot (MWS) disease has been a subject of controversy and discussion. Initially the disease was described as Phaeosphaeria leaf spot caused by Phaeosphaeria maydis. Other authors have suggested the existence of different fungal species causing similar symptoms. Recently, a bacterium, Pantoea ananatis, was described as the causal agent of this disease. The purpose of this study was to offer additional information on the correct etiology of this disease by providing visual evidence of the presence of the bacterium in the interior of the MWS lesions by using transmission electron microscopy (TEM) and molecular techniques. The TEM allowed visualization of a large amount of bacteria in the intercellular spaces of lesions collected from both artificially and naturally infected plants. Fungal structures were not visualized in young lesions. Bacterial primers for the 16S rRNA and rpoB genes were used in PCR reactions 10 amplify DNA extracted from water-soaked (young) and necrotic lesions. The universal fungal oligonucleotide ITS4 was also included to identify the possible presence of funga! structures inside lesions. Positive PCR products from water-soaked lesions, both from naturally and artificially inoculated plants, were produced with bacterial primers, whereas no amplification was observed when ITS4 oligonucleotide was used. On the other hand, DNA amplification with ITS4 primer was observed when DNA was isolated from necrotic (old) lesions. These results reinforced previous report of P. ananatis as the primary pathogen and the hypothesis that fungal species may colonize lesions pre-established by P. ananatis

Novel insights into the early stages of infection by oval conidia of Colletotrichum sublineolum.

Anthracnose, caused by Colletotrichum sublineolum Henn. ex Sacc. & Trotter, is one of the most important sorghum [Sorghum bicolor (L.) Moench] diseases in Brazil. This fungus exhibits conidial dimorphism, producing either falcate or oval conidia on solid and liquid media, respectively. We compared patterns of the initial infection events by these two types of conidia on sorghum leaves using light microscopy and scanning electron microscopy. The infection events during the first 24 h were similar for both oval and falcate conidia. Globose and melanized apressoria were formed at 24 h after inoculation (hai) regardless of the conidia type. Dense mycelium and oval conidia developed from germinated falcate conidia at 32 hai. Hyphal mass displaying acervuli filled with falcate conidia and surrounded by setae, developed from germinated oval conidia at 48 hai. Oval conidia were as capable as falcate conidia of infecting sorghum leaves. The inherent ability to grow faster and the easeness with which oval conidia can be produced in vitro as compared to falcate, make the former a preferred choice for studies on the C. sublineolum-sorghum interaction. It would be instructive to further investigate the potential role of the oval conidia in epidemics.201