Transmission of Spinach Downy Mildew via Seed and Infested Leaf Debris.

Spinach downy mildew, caused by the obligate oomycete pathogen Peronospora effusa, is a worldwide constraint on spinach production. The role of airborne sporangia in the disease cycle of P. effusa is well established, but the role of the sexual oospores in the epidemiology of P. effusa is less clear and has been a major challenge to examine experimentally. To evaluate seed transmission of spinach downy mildew via oospores in this study, isolated glass chambers were employed in two independent experiments to grow out oospore-infested spinach seed and noninfested seeds mixed with oospore-infested crop debris. Downy mildew diseased spinach plants were observed 37 and 34 days after planting in the two isolator experiments, respectively, in the chambers that contained one of two oospore-infested seed lots or seeds coated with oospore-infested leaves. Spinach plants in isolated glass chambers initiated from seeds without oospores did not show downy mildew symptoms. Similar findings were obtained using the same seed lot samples in a third experiment conducted in a growth chamber. In direct grow out tests to examine oospore infection on seedlings performed in a containment greenhouse with oospore-infested seed of two different cultivars, characteristic Peronospora sporangiophores were observed growing from a seedling of each cultivar. The frequency of seedlings developing symptoms from 82 of these oospore-infested seed indicated that approximately 2.4% of seedlings from infested seed developed symptoms, and 0.55% of seedlings from total seeds assayed developed symptoms. The results provide evidence that oospores can serve as a source of inoculum for downy mildew and provide further evidence of direct seed transmission of the downy mildew pathogen to seedlings in spinach via seedborne oospores.

Understanding the Genotypic and Phenotypic Structure and Impact of Climate on Phytophthora nicotianae Outbreaks on Potato and Tomato in the Eastern United States.

Samples from potato fields with lesions with late blight-like symptoms were collected from eastern North Carolina in 2017 and the causal agent was identified as Phytophthora nicotianae. We have identified P. nicotianae in potato and tomato samples from North Carolina, Virginia, Maryland, Pennsylvania, and New York. Ninety-two field samples were collected from 46 fields and characterized for mefenoxam sensitivity, mating type, and simple sequence repeat genotype using microsatellites. Thirty-two percent of the isolates were the A1 mating type, while 53% were the A2 mating type. In six cases, both A1 and A2 mating types were detected in the same field in the same year. All isolates tested were sensitive to mefenoxam. Two genetic groups were discerned based on STRUCTURE analysis: one included samples from North Carolina and Maryland, and one included samples from all five states. The data suggest two different sources of inoculum from the field sites sampled. Multiple haplotypes within a field and the detection of both mating types in close proximity suggests that P. nicotianae may be reproducing sexually in North Carolina. There was a decrease in the average number of days with weather suitable for late blight, from 2012 to 2016 and 2017 to 2021 in all of the North Carolina counties where P. nicotianae was reported. P. nicotianae is more thermotolerant than P. infestans and grows at higher temperatures (25 to 35°C) than P. infestans (18 to 22°C). Late blight outbreaks have decreased in recent years and first reports of disease are later, suggesting that the thermotolerant P. nicotianae may cause more disease as temperatures rise due to climate change.

Phytophthora capsici, 100 Years Later: Research Mile Markers from 1922 to 2022.

In 1922, Phytophthora capsici was described by Leon Hatching Leonian as a new pathogen infecting pepper (Capsicum annuum), with disease symptoms of root rot, stem and fruit blight, seed rot, and plant wilting and death. Extensive research has been conducted on P. capsici over the last 100 years. This review succinctly describes the salient mile markers of research on P. capsici with current perspectives on the pathogen's distribution, economic importance, epidemiology, genetics and genomics, fungicide resistance, host susceptibility, pathogenicity mechanisms, and management.

First Report of Bacterial Blight on arugula (Eruca vesicaria subsp. sativa) caused by Pseudomonas cannabina pv. alisalensis in New York.

Bacterial blight of arugula (Eruca vesicaria subsp. sativa cv. Standard) was observed in a commercial crop being grown in a high tunnel under overhead irrigation in Argyle, NY in January 2021. Approximately 80-100% of the plants were affected. Symptoms started as small, angular, water-soaked lesions visible on both sides of the leaves, then expanded, coalesced and later dried and turned tan. Fluorescent pseudomonads from five different plants were isolated on King's medium B agar amended with boric acid, chloramphenicol, and cycloheximide (Schaad et al. 2001) from surface disinfested symptomatic leaf tissues macerated in phosphate buffer (10mM, pH 7.0). Five representative isolates from each of the five plants produced levan and were negative for arginine dihydrolase and oxidase. They did not rot potatoes but were able to induce a hypersensitive reaction on tobacco (Nicotiana tabacum L. cv Glurk) within 24 h, thus demonstrating that the isolates belonged to LOPAT group 1 (Lelliott et al. 1966). DNA fragment banding patterns of these isolates generated by repetitive extragenic palindromic sequence PCR (repPCR) with BOXA1R primers were compared to the pathotype strains of Pseudomonas cannabina pv. alisalensis and Pseudomonas syringae pv. maculicola, known fluorescent pathogens of the Brassicaceae. The repPCR banding pattern of all isolates matched the pattern of Pseudomonas cannabina pv. alisalensis. Bacterial inoculum for pathogenicity experiments was prepared from 48 h KBBC agar cultures suspended in phosphate buffer (10mM, pH 7.0) and adjusted to 0.6 optical density at 600nm yielding approximately 108 CFU/ml. Five-week-old arugula plants were sprayed until run-off with one of the three isolates from arugula, a negative control (sterile buffer), or a positive control (P. cannabina pv. alisalensis) that is pathogenic to crucifers and also reported to cause disease on arugula in California (Bull et al. 2004). Experiments consisted of three replications of each treatment and two independent experiments were conducted. Small water-soaked spots resembling the original symptoms developed on all plants inoculated with the three representative isolates and P. cannabina pv. alisalensis. Moreover, reisolates from the symptomatic tissues were fluorescent on KBBC and had identical repPCR banding pattern as inoculated strains, demonstrating Koch's postulates. To our knowledge, this is the first report of bacterial blight on arugula caused by Pseudomonas cannabina pv. alisalensis in the Northeastern US. It was previously reported in California, Nevada, and Minnesota (Bull and du Toit 2009; Bull et al. 2004). This report may have significance for all brassica leafy green growers in the Northeast as P. cannabina pv. alisalensis has a broad host range including members of the Brassicaceae and oats which are commonly used as cover crops in mixed vegetable production.

Emergence of Groundnut ringspot virus and Tomato chlorotic spot virus in Vegetables in Florida and the Southeastern United States.

Groundnut ringspot virus (GRSV) and Tomato chlorotic spot virus (TCSV) are two emerging tospoviruses in Florida. In a survey of the southeastern United States, GRSV and TCSV were frequently detected in solanaceous crops and weeds with tospovirus-like symptoms in south Florida, and occurred sympatrically with Tomato spotted wilt virus (TSWV) in tomato and pepper in south Florida. TSWV was the only tospovirus detected in other survey locations, with the exceptions of GRSV from tomato (Solanum lycopersicum) in South Carolina and New York, both of which are first reports. Impatiens (Impatiens walleriana) and lettuce (Lactuca sativa) were the only non-solanaceous GRSV and/or TCSV hosts identified in experimental host range studies. Little genetic diversity was observed in GRSV and TCSV sequences, likely due to the recent introductions of both viruses. All GRSV isolates characterized were reassortants with the TCSV M RNA. In laboratory transmission studies, Frankliniella schultzei was a more efficient vector of GRSV than F. occidentalis. TCSV was acquired more efficiently than GRSV by F. occidentalis but upon acquisition, transmission frequencies were similar. Further spread of GRSV and TCSV in the United States is possible and detection of mixed infections highlights the opportunity for additional reassortment of tospovirus genomic RNAs.

Basil Downy Mildew (Peronospora belbahrii): Discoveries and Challenges Relative to Its Control.

Basil (Ocimum spp.) is one of the most economically important and widely grown herbs in the world. Basil downy mildew, caused by Peronospora belbahrii, has become an important disease in sweet basil (O. basilicum) production worldwide in the past decade. Global sweet basil production is at significant risk to basil downy mildew because of the lack of genetic resistance and the ability of the pathogen to be distributed on infested seed. Controlling the disease is challenging and consequently many crops have been lost. In the past few years, plant breeding efforts have been made to identify germplasm that can be used to introduce downy mildew resistance genes into commercial sweet basils while ensuring that resistant plants have the correct phenotype, aroma, and tastes needed for market acceptability. Fungicide efficacy studies have been conducted to evaluate current and newly developed conventional and organic fungicides for its management with limited success. This review explores the current efforts and progress being made in understanding basil downy mildew and its control.

Stable isotopic composition of perchlorate and nitrate accumulated in plants: Hydroponic experiments and field data.

Natural perchlorate (ClO4-) in soil and groundwater exhibits a wide range in stable isotopic compositions (δ37Cl, δ18O, and Δ17O), indicating that ClO4- may be formed through more than one pathway and/or undergoes post-depositional isotopic alteration. Plants are known to accumulate ClO4-, but little is known about their ability to alter its isotopic composition. We examined the potential for plants to alter the isotopic composition of ClO4- in hydroponic and field experiments conducted with snap beans (Phaseolus vulgaris L.). In hydroponic studies, anion ratios indicated that ClO4- was transported from solutions into plants similarly to NO3- but preferentially to Cl- (4-fold). The ClO4- isotopic compositions of initial ClO4- reagents, final growth solutions, and aqueous extracts from plant tissues were essentially indistinguishable, indicating no significant isotope effects during ClO4- uptake or accumulation. The ClO4- isotopic composition of field-grown snap beans was also consistent with that of ClO4- in varying proportions from irrigation water and precipitation. NO3- uptake had little or no effect on NO3- isotopic compositions in hydroponic solutions. However, a large fractionation effect with an apparent ε (15N/18O) ratio of 1.05 was observed between NO3- in hydroponic solutions and leaf extracts, consistent with partial NO3- reduction during assimilation within plant tissue. We also explored the feasibility of evaluating sources of ClO4- in commercial produce, as illustrated by spinach, for which the ClO4- isotopic composition was similar to that of indigenous natural ClO4-. Our results indicate that some types of plants can accumulate and (presumably) release ClO4- to soil and groundwater without altering its isotopic characteristics. Concentrations and isotopic compositions of ClO4- and NO3- in plants may be useful for determining sources of fertilizers and sources of ClO4- in their growth environments and consequently in food supplies.

Genetic Variation within Clonal Lineages of Phytophthora infestans Revealed through Genotyping-By-Sequencing, and Implications for Late Blight Epidemiology.

Genotyping-by-sequencing (GBS) was performed on 257 Phytophthora infestans isolates belonging to four clonal lineages to study within-lineage diversity. The four lineages used in the study were US-8 (n = 28), US-11 (n = 27), US-23 (n = 166), and US-24 (n = 36), with isolates originating from 23 of the United States and Ontario, Canada. The majority of isolates were collected between 2010 and 2014 (94%), with the remaining isolates collected from 1994 to 2009, and 2015. Between 3,774 and 5,070 single-nucleotide polymorphisms (SNPs) were identified within each lineage and were used to investigate relationships among individuals. K-means hierarchical clustering revealed three clusters within lineage US-23, with US-23 isolates clustering more by collection year than by geographic origin. K-means hierarchical clustering did not reveal significant clustering within the smaller US-8, US-11, and US-24 data sets. Neighbor-joining (NJ) trees were also constructed for each lineage. All four NJ trees revealed evidence for pathogen dispersal and overwintering within regions, as well as long-distance pathogen transport across regions. In the US-23 NJ tree, grouping by year was more prominent than grouping by region, which indicates the importance of long-distance pathogen transport as a source of initial late blight inoculum. Our results support previous studies that found significant genetic diversity within clonal lineages of P. infestans and show that GBS offers sufficiently high resolution to detect sub-structuring within clonal populations.