Early Detection of Fusarium Basal Rot Infection in Onions and Shallots Based on VOC Profiles Analysis.

Gas chromatography ion-mobility spectrometry (GC-IMS) technology is drawing increasing attention due to its high sensitivity, low drift, and capability for the identification of compounds. The noninvasive detection of plant pests and pathogens is an application area well suited to this technology. In this work, we employed GC-IMS technology for early detection of Fusarium basal rot in brown onion, red onion, and shallot bulbs and for tracking disease progression during storage. The volatile profiles of the infected and healthy control bulbs were characterized using GC-IMS and gas chromatography-time-of-flight mass spectrometry (GC-TOF-MS). GC-IMS data combined with principal component analysis and supervised methods provided discrimination between infected and healthy control bulbs as early as 1 day after incubation with the pathogen, classification regarding the proportion of infected to healthy bulbs in a sample, and prediction of the infection's duration with an average R2 = 0.92. Furthermore, GC-TOF-MS revealed several compounds, mostly sulfides and disulfides, that could be uniquely related to Fusarium basal rot infection.

First report of Fusarium oxysporum f. sp. lactucae Race 1 causing lettuce wilt in Northern Ireland.

In the UK, lettuce is produced both in the field and in greenhouses or polytunnels. In summer 2022, wilt symptoms were observed for the first time on lettuce (cv. Amica) grown in soil in part of a single 0.55 ha greenhouse in County Armagh, Northern Ireland (NI). Initial presentation of symptoms was stunting of plants, followed by wilting and yellowing of lower leaves in approx. 12% of the plants. Orange-brown discoloration of vascular tissue in the tap root of affected plants was also observed. To isolate the causal pathogen, sections (0.5 cm2) of symptomatic vascular tissue from 5 plants were surface sterilized in 70% ethanol for 45 s, washed twice in sterile water and placed on potato dextrose agar (PDA) amended with 20 µg mL-1 chlortetracycline. Plates were incubated at 20°C for 5 days and fungal colonies sub cultured onto PDA. Isolates from all five samples displayed morphology typical of Fusarium oxysporum and were cream to purple in colour with abundant microconidia and occasional macroconidia. DNA was extracted from 5 isolates and part of the translation elongation factor 1- α (EF1-α) gene amplified by PCR and sequenced as described by Taylor et al. (2016). All EF1-α sequences were identical (OQ241898) and matched with F. oxysporum f. sp. lactucae race 1 (MW316853.1, isolate 231274) and race 4 (MK059958.1, isolate IRE1) with 100% sequence identity using BLAST. Isolates were then identified as FOL race 1 (FOL1) using a race-specific PCR assay (Pasquali et al. 2007). Next, pathogenicity and race identity of one isolate (AJ773) was confirmed using a set of differential lettuce cultivars (Gilardi et al. 2017); Costa Rica No. 4 (CR; FOL1 resistant), Banchu Red Fire (BRF; FOL4 resistant) and Gisela (GI; FOL1 / FOL4 susceptible). Plants were inoculated with AJ773 (this study), ATCCMya-3040 (FOL1, Italy; Gilardi et al. 2017) and LANCS1 (FOL4, UK; Taylor et al., 2019). Roots of 16-day-old lettuce plants (8 replicates per cultivar/isolate) were trimmed and soaked in a spore suspension (1 x 106 conidia mL-1) for 10 min before transplanting into compost in 9 cm pots. Control plants for each cultivar were dipped in sterile water. Pots were placed in a glasshouse (25C day, 18C night). Inoculation with AJ773 and FOL1 ATCCMya-3040 resulted in typical symptoms of Fusarium wilt for BRF and GI 12-15 days after inoculation, while for FOL4 LANCS1, wilting was observed in CR and GI. Thirty-two days after inoculation, plants were cut longitudinally and vascular browning was observed in all plants where wilt was present. All uninoculated control plants, CR inoculated with FOL1 ATCCMya-3040 or AJ773, and BRF inoculated with FOL4 LANCS1 remained healthy. These results confirm the identity of isolate AJ773 from NI as FOL1. Koch's postulates were fulfilled by consistent re-isolation of F. oxysporum from BRF and GI plants and identifying as FOL1 using the race-specific PCR. No FOL was re-isolated from control plants of any cultivar. Fusarium wilt in England and the Republic of Ireland first reported by Taylor et al. (2019) was identified as FOL4 and has been confined to indoor lettuce production with further outbreaks caused by the same race. However, FOL1 was recently identified in Norway in a soil-grown glasshouse crop (Herrero et al. 2021). The presence of both FOL1 and FOL4 in neighbouring countries within the UK is a serious risk to lettuce production and is of particular importance to growers who rely on knowledge of cultivar resistance to specific FOL races to make decisions on which variety to plant.

Preliminary Studies on Detection of Fusarium Basal Rot Infection in Onions and Shallots Using Electronic Nose.

The evaluation of crop health status and early disease detection are critical for implementing a fast response to a pathogen attack, managing crop infection, and minimizing the risk of disease spreading. Fusarium oxysporum f. sp. cepae, which causes fusarium basal rot disease, is considered one of the most harmful pathogens of onion and accounts for considerable crop losses annually. In this work, the capability of the PEN 3 electronic nose system to detect onion and shallot bulbs infected with F. oxysporum f. sp. cepae, to track the progression of fungal infection, and to discriminate between the varying proportions of infected onion bulbs was evaluated. To the best of our knowledge, this is a first report on successful application of an electronic nose to detect fungal infections in post-harvest onion and shallot bulbs. Sensor array responses combined with PCA provided a clear discrimination between non-infected and infected onion and shallot bulbs as well as differentiation between samples with varying proportions of infected bulbs. Classification models based on LDA, SVM, and k-NN algorithms successfully differentiate among various rates of infected bulbs in the samples with accuracy up to 96.9%. Therefore, the electronic nose was proved to be a potentially useful tool for rapid, non-destructive monitoring of the post-harvest crops.

Identification and Expression of Secreted In Xylem Pathogenicity Genes in Fusarium oxysporum f. sp. pisi.

Fusarium oxysporum is a soilborne fungal plant pathogen responsible for causing disease in many economically important crops with "special forms" (formae speciales) adapted to infect specific plant hosts. F. oxysporum f. sp. pisi (FOP) is the causal agent of Fusarium wilt disease of pea. It has been reported in every country where peas are grown commercially. Disease is generally controlled using resistant cultivars possessing single major gene resistance and therefore there is a constant risk of breakdown. The main aim of this work was to characterise F. oxysporum isolates collected from diseased peas in the United Kingdom as well as FOP isolates obtained from other researchers representing different races through sequencing of a housekeeping gene and the presence of Secreted In Xylem (SIX) genes, which have previously been associated with pathogenicity in other F. oxysporum f. spp. F. oxysporum isolates from diseased United Kingdom pea plants possessed none or just one or two known SIX genes with no consistent pattern of presence/absence, leading to the conclusion that they were foot-rot causing isolates rather than FOP. In contrast, FOP isolates had different complements of SIX genes with all those identified as race 1 containing SIX1, SIX6, SIX7, SIX9, SIX10, SIX11, SIX12, and SIX14. FOP isolates that were identified as belonging to race 2 through testing on differential pea cultivars, contained either SIX1, SIX6, SIX9, SIX13, SIX14 or SIX1, SIX6, SIX13. Significant upregulation of SIX genes was also observed in planta over the early stages of infection by different FOP races in pea roots. Race specific SIX gene profiling may therefore provide potential targets for molecular identification of FOP races but further research is needed to determine whether variation in complement of SIX genes in FOP race 2 isolates results in differences in virulence across a broader set of pea differential cultivars.

Phenology of the Diamondback Moth (Plutella xylostella) in the UK and Provision of Decision Support for Brassica Growers.

In the UK, severe infestations by Plutella xylostella occur sporadically and are due mainly to the immigration of moths. The aim of this study was to develop a more detailed understanding of the phenology of P. xylostella in the UK and investigate methods of monitoring moth activity, with the aim of providing warnings to growers. Plutella xylostella was monitored using pheromone traps, by counting immature stages on plants, and by accessing citizen science data (records of sightings of moths) from websites and Twitter. The likely origin of migrant moths was investigated by analysing historical weather data. The study confirmed that P. xylostella is a sporadic but important pest, and that very large numbers of moths can arrive suddenly, most often in early summer. Their immediate sources are countries in the western part of continental Europe. A network of pheromone traps, each containing a small camera sending images to a website, to monitor P. xylostella remotely provided accessible and timely information, but the particular system tested did not appear to catch many moths. In another approach, sightings by citizen scientists were summarised on a web page. These were accessed regularly by growers and, at present, this approach appears to be the most effective way of providing timely warnings.