Search

Twenty-eight bacterial strains isolated from Chinese herb extracts, beer fermentation waste, and raw oyster shells were evaluated for their efficacy in controlling cucumber anthracnose. Four bacterial strains, namely TG01, TG02, LJS06, and LJS08, were found to effectively reduce the mycelial growth of Colletotrichum orbiculare COC3 on PDA media. Spraying or drenching LJS06 spore suspension before inoculation significantly p < 0.05 reduced disease severity; thus, LJS06 was subject to further characterization. On the basis of the morphological, physiological, and biochemical characteristics and a multilocus sequence analysis of partial 16S rRNA, atpD , rpoB , and trpB genes, LJS06 was identified to be Streptomyces griseorubiginosus (Ryabova and Preobrazhenskaya) Pridham et al. Physiological and biochemical tests revealed that S. griseorubiginosus LJS06 can produce amylase, cellulase, chitinase, protease, siderophore, polyamines, and indole-3-acetic acid. Thus, a culture filtrate of LJS06 (specifically SL06) was formulated and evaluated for its efficacy against conidial germination, appressorium formation, and anthracnose management. Diluted SL06 was found to significantly (p < 0.05) inhibit conidial germination and appressorium formation, which can be attributed to impaired membrane integrity, accumulated reactive oxygen species (ROS), and impaired energy metabolism in the conidia. In addition, the spraying and drenching of diluted SL06 before inoculation consistently and significantly (p < 0.05) reduced anthracnose severity. These results jointly suggest that S. griseorubiginosus LJS06 can aid in the management of cucumber anthracnose by directly inhibiting conidial function and priming the plant defense system. [ABSTRACT FROM AUTHOR]

The North Georgia Mountains are the southernmost region along the United States East Coast where European wine grapes (Vitis vinifera) are grown commercially. Epidemics of downy mildew, caused by Plasmopara viticola, are frequent and severe, but little is known about the epidemiology and population biology of the pathogen in this region. Disease monitoring in an experimental vineyard from 2015 to 2017 indicated that times of disease onset and progress rates were highly variable across years and cultivars. Oospores were observed microscopically, and simulation with a process‐based model indicated presence of conditions favourable for oospore germination in the spring and early summer each year. A total of 409 P. viticola isolates collected over three years were genotyped with seven microsatellite markers, revealing very high genotypic diversity, which when combined with the observation of oospores is indicative of a sexually reproducing population. Among the 409 isolates, 225 multilocus genotypes (MLGs) were identified, of which 164 were detected only once and 61 were repeated (clonal). Eight MLGs (represented by 28 isolates) were detected across years, suggesting the possibility of asexual overwintering of P. viticola in this region. Across sampling dates, the percentage of isolates belonging to nonrepeated (unique) MLGs ranged from 27.3% to 63.2%. Even towards the end of the annual epidemic, the percentage of isolates in nonrepeated MLGs was still relatively high, around 30%. These MLGs may have originated from oospores germinating late during the growing season, although incomplete sampling at earlier dates and contribution by immigration cannot be fully excluded. [ABSTRACT FROM AUTHOR]

A spectrophotometric method for determining the viability of sporangia and zoospores of the oomycete Plasmopara viticola (causal agent of grapevine downy mildew) is described and evaluated to overcome the limitations of currently available methods for assessing propagule viability. Sporangia produced on leaf discs in the laboratory were harvested at different days after the initiation of sporulation (DAS) to yield differences in sporangium viability. Sporangia were suspended in sterile water, the suspensions were placed in a cuvette, and sporangium germination was monitored in a spectrophotometer (λ = 600 nm) at 2‐ to 3‐min intervals for 5 hr. Absorbance started to increase after sporangia were suspended in water for ~30–60 min followed by major peak(s) for younger sporangia (1–3 DAS), whereas low to no increase in absorbance was observed for senescent sporangia (>7 DAS). Microscopic observation confirmed that the increase in absorbance corresponded to the release and active swimming of zoospores, whereas absorbance decreased when zoospores encysted and settled. A positive correlation (r =.839, p =.0365) was observed when the time to the initial increase in absorbance was plotted against the age of sporangia. The time to the absorbance peak (marking the time of maximum zoospore movement) was shortest for immature sporangia (0 DAS), longest for young sporangia (2 DAS) and decreased for mature and senescent sporangia. A similar pattern was observed for the standardized area under the absorbance curve (indicating the overall quantity of zoospores released), for which values were lowest for immature and senescent sporangia, highest for young sporangia and intermediate for mature sporangia. Consistent patterns obtained across two independent experiments suggest that the method is reproducible and may be further developed for other zoospore‐releasing pathogens. [ABSTRACT FROM AUTHOR]

Previous phylogenetic studies of the grape downy mildew pathogen, Plasmopara viticola, revealed five cryptic species in eastern North America that differed in their host range and geographic distribution. Preliminary comparative studies also documented differences in temperature responses during infection between certain cryptic species, indicating the biological relevance of knowing which cryptic species of the pathogen are present in a given region. However, limited information is available regarding the presence, prevalence and dynamics of cryptic species of P. viticola in the southeastern United States. Here, 301 P. viticola isolates obtained from cultivated grape species in five distinct grape‐growing regions of Georgia and Florida were subjected to cleaved amplified polymorphic sequence analysis and multilocus sequencing (internal transcribed spacer region of the rDNA, actin and β‐tubulin) to identify cryptic species and infer phylogenetic relationships. Three cryptic species, P. viticola clade aestivalis (Pva), clade vinifera (Pvv) and clade vulpina (Pvu), were identified in Georgia, whereas two, Pva and Pvv, were found in Florida; all three cryptic species are reported here for the first time in Georgia, whereas Pva is reported for the first time in Florida. Pva was the most prevalent cryptic species (72.1% of isolates) and was distributed widely from the North Georgia Mountains to Mid‐Florida, whereas Pvv (27.2%) and Pvu (0.7%) were found only in the Coastal Plain region of the two states. Interestingly, Pvu was obtained from French American hybrid Blanc du Bois and could be subcultured on Vitis vinifera Chardonnay, suggesting a broader host range than only the wild species Vitis vulpina reported previously. [ABSTRACT FROM AUTHOR]