Genetic Resistance of Lactuca spp. against Fusarium oxysporum f. sp. lactucae Race 1

Fusarium wilt of lettuce is caused by the pathogen Fusarium oxysporum f. sp. lactucae (Fol) and is a growing threat to global lettuce production. Fol was first detected in Florida in 2017 and was subsequently confirmed as race 1. Management strategies for this long-persisting soil pathogen are limited, time-consuming and expensive, and they may lack efficacy. Identifying diverse sources of genetic resistance is imperative for breeding adapted cultivars with durable resistance. The objectives of this study were to identify sources of resistance against a race 1 isolate of Fol in Florida, delineate the relationship between foliar and taproot symptoms, and investigate the inheritance of resistance and partial resistance in two F2 populations. Thirteen experiments were conducted in greenhouse and field locations to characterize the diversity of genetic resistance in the genus Lactuca. Leaf cultivars Dark Lollo Rossa and Galactic; romaine breeding lines 43007, 60182, and C1145; and iceberg breeding line 47083 consistently exhibited low foliar and taproot disease symptoms. Resistance was not identified among the wildtype Lactuca or primitive plant introductions (PI) in this study based on taproot symptoms. An additional test was conducted to study the segregation pattern of Fol resistance between one resistant and one susceptible accession (R × S) and one partial resistant and one susceptible accession (PR × S). The F2 population from ‘60182 × PI 358001-1’ fit the expected segregation ratio for a single recessive locus model, whereas the ratio for ‘Dark Lollo Rossa × PI 358001-1’ did not fit either recessive or dominant single locus models. These sources of resistance are potential candidates for developing commercial cultivars with multiple resistance loci against Fol race 1, especially for the Florida lettuce production system.

Plant breeding for resilient pastures

Plant breeding has had, and continues to have, an important role in providing farmers with resilient pastures. Early breeding relied on improvement of ecotype populations and this was accelerated by crossing with selected introduced germplasm. The primary traits under selection have targeted speed of establishment, total and/or seasonal dry matter (DM) yield, nutritive value or feed quality, flowering time and reduced aftermath heading, disease resistance, persistence and seed yield. Continued improvement through plant breeding to meet environmental concerns and tolerances to both biotic and abiotic stresses will be achieved through ongoing plant introductions, exploiting heterosis, speed breeding, genomic selection, improvements in phenotyping, metabolomics, improved compatibility with beneficial microbes, and potentially the use of transgenic and gene editing technologies.

Classification methods and identification of reniform nematode resistance in known soybean cyst nematode resistant soybean genotypes

Plant parasitic nematodes are a major yield-limiting factor of soybean in the United States and Canada. It has been indicated that soybean cyst nematode (SCN, Heterodera glycines Ichinohe) and reniform nematode (RN, Rotylenchulus reniformis Linford and Oliveira) resistance could be genetically related. For many years fragmentary data has shown this relationship. This report evaluates RN reproduction on 418 plant introductions (PIs) selected from the USDA Soybean Germplasm Collection with reported SCN resistance. The germplasm was divided into two tests of 214 PIs reported as resistant, and 204 PIs moderately resistant to SCN. The defining and reporting of RN resistance changed several times in the last 30 years, causing inconsistencies in RN resistance classification among multiple experiments. Comparison of four RN resistance classification methods was performed: (1) ≤10% as compared to the susceptible check, (2) using normalized reproduction index (RI) values, and transformed data (3) log10 (x) and (4) log10 (x+1), in an optimal univariate k-means clustering analysis. The method of transformed data log10 (x) was selected as the most accurate for classification of RN resistance. Among 418 PIs with reported SCN resistance, the log10 (x) method grouped 59 PIs (15%) as resistant, and 130 PIs (31%) as moderately resistant to RN. Genotyping of a subset of the most resistant PIs to both nematode species revealed their strong correlation with rhg1-a allele. This research identified genotypes with resistance to two nematode species and potential new sources of RN resistance that could be valuable to breeders in developing resistant cultivars.

Weed seed contamination in imported seed lots entering New Zealand

Imports of seeds for sowing are a major pathway for the introduction of contaminant seeds, and many agricultural weeds globally naturalised originally have entered through this pathway. Effective management of this pathway is a significant means of reducing future plant introductions and helps minimise agricultural losses. Using a national border inspection database, we examined the frequency, origin and identity of contaminant seeds within seed for sowing shipments entering New Zealand between 2014–2018. Our analysis looked at 41,610 seed lots across 1,420 crop seed species from over 90 countries. Overall, contamination was rare, occurring in 1.9% of all seed lots. Among the different crop types, the arable category had the lowest percentage of seed lots contaminated (0.5%) and the forage category had the highest (12.6%). Crop seeds Capsicum, Phaseolus and Solanum had the lowest contamination rates (0.0%). Forage crops Medicago (27.3%) and Trifolium (19.8%) had the highest contamination rates. Out of 191 genera recorded as contaminants, Chenopodium was the most common. Regulated quarantine weeds were the rarest contaminant type, only occurring in 0.06% of seed lots. Sorghum halepense was the most common quarantine species and was only found in vegetable seed lots. Vegetable crop seed lots accounted for approximately half of all quarantine species detections, Raphanus sativus being the most contaminated vegetable crop. Larger seed lots were significantly more contaminated and more likely to contain a quarantine species than smaller seed lots. These findings support International Seed Testing Association rules on maximum seed lot weights. Low contamination rates suggest industry practices are effective in minimising contaminant seeds. Considering New Zealand inspects every imported seed lot, utilises a working sample size 5 times larger than International Seed Testing Association rules require, trades crop seed with approximately half of the world’s countries and imports thousands of crop seed species, our study provides a unique overview of contaminant seeds that move throughout the seed for sowing system.

New Sources of Resistance in Winter Squash (Cucurbita moschata) to Phytophthora Crown Rot and Their Relationship to Cultivated Squash

Butternut squash (Cucurbita moschata) is an important vegetable crop grown and consumed in most states in the USA. Cucurbita moschata lines and interspecific hybrids between Cucurbita species are also used as rootstocks for grafting watermelon and melon. However, currently most commercially available C. moschata squash varieties are highly susceptible to crown and root rot caused by the oomycete pathogen Phytophthora capsici, especially in the southeastern USA. All available plant introductions (PIs) of C. moschata (319 PIs) were evaluated for resistance to crown rot. Four-week-old plants were inoculated with 104 zoospores from a local South Carolina (SC) isolate of P. capsici. Plants were rated for disease severity three weeks after inoculation using a 0-5 rating scale (0=No symptoms and 5=Plant dead). The majority (87%) of the C. moschata PIs were highly susceptible to crown rot in the first evaluation and were rated as 5. Reevaluation of the promising PIs identified several potential new sources of resistance (e.g. Grif 935, PI 442272, PI 442264, PI 512142, PI 438724, PI 438778, PI 442280). Variability in resistance reaction among plants within a PI was also observed, and not all plants exhibited resistance. Further evaluation of S1 generation from the most resistant plants (rated ≤1) demonstrated that highly resistant plants could be selected from these PIs to develop lines for use in breeding programs. These new sources of resistance can be utilized for developing new crown and root rot resistant rootstocks for watermelon grafting and for developing resistant varieties for human consumption.

Assessing the relative importance of human and spatial pressures on non-native plant establishment in urban forests using citizen science

Since 2007, more people in the world live in urban than in rural areas. The development of urban areas has encroached into natural forest ecosystems, consequently increasing the ecological importance of parks and fragmented forest remnants. However, a major concern is that urban activities have rendered urban forests susceptible to non-native species incursions, making them central entry sites where non-native plant species can establish and spread. We have little understanding of what urban factors contribute to this process. Here we use data collected by citizen scientists to determine the differential impacts of spatial and urban factors on non-native plant introductions in urban forests. Using a model city, we mapped 18 urban forests within city limits, and identified all the native and non-native plants present at those sites. We then determined the relative contribution of spatial and socioeconomic variables on the richness and composition of native and non-native plant communities. We found that socioeconomic factors rather than spatial factors (e.g., urban forest area) were important modulators of overall or non-native species richness. Non-native species richness in urban forest fragments was primarily affected by residential layout, recent construction events, and nearby roads. This demonstrates that the proliferation of non-native species is inherent to urban activities and we propose that future studies replicate our approach in different cities to broaden our understanding of the spatial and social factors that modulate invasive species movement starting in urban areas.

Increased rate of introduction of non-native plants in China since 2002 driven by intentional introductions

The history of plant introductions in China is inextricably associated with globalization and trade. There have been two periods where China has opened to the world, punctuated by periods of isolation. Here, we compare historical rates of introduction from 1840–2011 and ask how the rate of introduction of non-native plants has changed and how intentional and non-intentional introductions compare. The highest rate of introductions occurred between 1900 and 1940, where mean annual introduction was 8.7 plant species. Plant introductions post 2002 occurred at the second highest rate, but this was still little over half the earlier period at 4.4 plant species per year. In both periods, intentional introductions contributed the greatest proportion, and intentional and non-intentional introductions shared similar temporal trajectories.

Characterizing Resistance to Soybean Aphid (Hemiptera: Aphididae): Antibiosis and Antixenosis Assessment

Host-plant resistance (HPR) remains a vital tool to manage soybean aphid (Aphis glycines Matsumura), a major pest of soybean in Midwestern United States and southern Canada. HPR can be overcome by virulent biotypes of A. glycines; thus, in order to increase the durability of resistant cultivars, HPR needs to be deployed strategically. To improve the strategic deployment, a complete understanding of HPR in existing resistant germplasm will help ensure HPR success. In this study, we characterized HPR soybean to determine antibiosis and antixenosis categories of resistance to different biotypes of A. glycines. No-choice and free-choice tests were performed on 11 previously reported plant introductions (PIs) possessing resistance to at least one A. glycines biotype (1, 2, and 3). Overall, we found that the PIs manifested differences of a particular resistance category in response to infestation by different biotypes. Our data from no-choice tests indicate that all tested PIs possess antibiosis-based resistance to three biotypes. However, the strength of antibiosis was variable as some PIs showed stronger antibiosis toward a given biotype than others. All tested PIs manifested antixenosis, in addition to antibiosis. Furthermore, detached leaf assays revealed that resistance to A. glycines was not retained in excised soybean leaves. Characterization of resistance in this study can contribute to develop strategies for future deployment of resistant cultivars developed from these PIs.