Plant Disease

One of the devastating early season diseases of cotton is Fusarium wilt caused by Fusarium oxysporum f. sp. vasinfectum (Fov). Recent emergence of highly virulent Fov race 4 (Fov4) and its aggressiveness toward Gossypium barbadense (pima) cultivars are raising significant concerns for the US cotton industry. One of the key challenges in studying Fov4 virulence and cotton Fusarium wilt pathogenesis is establishing a disease assay strategy that can help researcher overcome several technical challenges, including efficient infection and highly reproducible and consistent symptom development. Here, we have developed a small-scale, soil-free Fusarium wilt disease assay that can complement conventional assays with faster symptom development and high reproducibility in infected pima cotton seedlings. Our data showed statistically significant differences (p<0.0001) between Fov4-infected and non-infected pima cotton at 4 and 6 days post inoculation (dpi) when compared to control experiments. At 6 dpi, longitudinal observations under magnification showed Fov4 colonization in primary xylem of infected plants, which is a common symptom observed in Fov4 triggered Fusarium wilt in pima cotton. While this is an artificial assay system, this soil-free disease testing strategy can offer another strategy to supplement current assays when studying pathogen-host interaction in soil-borne diseases.

Verticillium stripe disease is an emerging threat to canola production in Western Canada. Accurately assessing verticillium stripe resistance in breeding germplasm collections is crucial for identifying sources of genetic resistance. Currently, field phenotyping is the most widely used method for evaluating verticillium stripe resistance; however, achieving uniform disease pressure under field conditions presents significant challenges. Here we report a novel controlled environment (CE) soil-less assay for the rapid evaluation of verticillium stripe resistance in canola. The CE results were validated in a field trial which showed a strong correlation between the field data and the CE results. This cost-effective and time-efficient assay enables accurate assessment of verticillium stripe symptoms in a one-month testing cycle.

Members of the genus Trichoderma are widely recognized as beneficial fungi and frequently applied as biocontrol agents. However, recent studies have identified Trichoderma afroharzianum as a pathogenic species, capable of infecting maize, wheat, and barley under greenhouse conditions. Its pathogenic potential in small-grain cereals under field conditions, however, has been so far unexplored. To evaluate its pathogenic potential, two-year field trials (2023 and 2024) were conducted using ten wheat, five barley, two rye, and one triticale cultivar. Artificial inoculation was carried out at full flowering using a spray inoculation with pathogenic T. afroharzianum isolates obtained from maize and water as control. Three fungicides were applied: untreated control, foliar treatment before inoculation, and ear application after inoculation. Disease development was assessed based on colonization rate of harvested kernels, and thousand kernel weight (TKW). The results showed significant differences in susceptibility between and within cereal species. T. afroharzianum was able to colonize wheat and barley ears under field conditions, with some cultivars showing significant reduction in TKW and high colonization rate. In contrast, rye and triticale exhibited much lower infection rates. Fungicide treatments had varying levels of efficacy. Ear application after flowering was most effective in reducing colonization and preventing yield loss, whereas early foliar treatment showed limited effect. These findings indicate that T. afroharzianum may represent a relevant pathogen in cereal production adding a new ear disease in cereals and requiring monitoring and enhanced fungicide applications in the future, unless less susceptible cultivars are identified and utilized in practice.

Beyond the significant impact of Cassava witches broom disease (CWBD), caused by the fungus Rhizoctonia (syn. Ceratobasidium) theobromae on cassava cultivation in French Guiana and Brazil, this disease also poses a potential threat to cacao trees in the region, since the fungus is responsible for Vascular Streak Dieback (VSD) of cacao in South East Asia. Cross-pathogenicity trials were conducted in several cassava fields in French Guiana by planting young cacao plants adjacent to diseased cassava plants. Vascular necrosis was observed in some cacao plants, and the presence of R. theobromae in the cacao tissues was confirmed through PCR diagnostics using primers specific to the fungus. Sequence analysis indicated 100% similarity between samples from both hosts and 97.53 to 99.74% identity with R. theobromae isolates previously reported from cassava in the Americas and Southeast Asia. Additionally, symptomatic cacao in a mixed cacao-cassava farm yielded R. theobromae-positive PCR results, suggesting a natural infection. Ongoing work includes artificial inoculations and controlled cross-pathogenicity trials under screenhouse conditions to attempt reproduction of the symptoms. While current data do not yet establish definitive causality, the findings indicate potential host jump and warrant rapid communication to researchers, policy makers, and farmers to safeguard cacao production and Theobroma biodiversity in the Amazon region.

Fusarium oxysporum Species Complex (FOSC) includes important soilborne pathogens of a range of crops worldwide. In Victoria (VIC) and New South Wales (NSW), Australia, FOSC has been identified as the cause of stunting and poor growth in processing tomato fields, resulting in significant yield losses. A total of 40 FOSC isolates were obtained from symptomatic tomato plants, irrigation water samples and culture collections, which were confirmed to be FOSC by multi-gene phylogenetic analyses based on four genomic loci: beta tubulin, calmodulin, the second largest subunit of nuclear RNA polymerase II, and translation elongation factor one-alpha. There was a high level of genetic diversity among isolates, with multiple phylogenetic lineages detected. Glasshouse bioassays demonstrated that all isolates were pathogenic to processing tomato, resulting in significant reductions in plant growth, with above ground height reduced by 11 to 26% and root dry weight by 44 to 83% compared with the control (p < 0.05). Although aggressiveness varied among isolates, growth reduction occurred irrespective of their phylogenetic placement. Moreover, the shared genetic background of isolates from irrigation water and plant samples highlights the role of irrigation water as a potential source of inoculum in Fusarium epidemics, underscoring its significance for disease management in Australian processing tomato systems.

Whitebark pine (Pinus albicaulis), a wide-ranging high-elevation conifer in western North America, is listed as threatened in the U.S. and as endangered in Canada. A major threat to whitebark pine is the non-native, invasive white pine blister rust disease, caused by the fungal pathogen Cronartium ribicola. In many pathosystems (including white pine blister rust), seedling inoculation trials are used to identify parent trees with genetic resistance. However, many of these trials use only one spore density for inoculation, and little information exists on the effectiveness of quantitative disease resistance (QDR) under varying spore densities and the corresponding implications for field performance. In this study, we examine the levels of infection and survival present within six whitebark pine seedling families previously rated for QDR (three susceptible and three resistant families) under six widely varying inoculum densities. The susceptible families showed very high infection and mortality at all inoculum densities, while performance of the resistant families varied with spore density treatment. The information gathered from the study will be useful in updating the projections of the future of whitebark pine populations under field conditions in areas of different rust hazard. The results also serve as a caution to those working in other pathosystems where seedling inoculation trials based on one spore density level are used to rate the resistance level of parent trees and their associated progeny.

Fusarium wilt of banana (FWB), caused by soilborne Fusarium spp., is a major global threat to the cultivation of bananas. In addition to persistent chlamydospores, weeds are a reservoir of the causal agents. However, it remains unclear whether other Zingiberales species, which are grown in the same geographic regions, also can serve as hosts for Fusarium spp. that cause FWB. Greenhouse assays were conducted to investigate whether Fusarium phialophorum (Race 1; pathogenic to Gros Michel banana) and Fusarium odoratissimum (TR4; pathogenic to Cavendish banana) can infect three Heliconia species, two ornamental banana species or Musa textilis (abaca). Heliconia latispatha, Musa balbisiana, and Musa coccinea displayed external symptoms after inoculation with TR4, while inoculation with Race 1 caused symptoms in H. latispatha, H. psittacorum, M. coccinea, and M. velutina. Isolates recovered from distinct organs of all studied plant species were characterized and re-isolated strains caused FWB symptoms in Gros Michel and Cavendish banana plants, and their rhizome discolored area scores were similar to the reference strains. The susceptibility of some ornamental species and the presence of Fusarium strains as asymptomatic endophytes in others, with remaining pathogenicity, call for a revision of the race nomenclature and the current containment protocols for FWB.

Fusarium is one of the most important fungal genera of plant pathogens that affect the cultivation of a wide range of crops. Agricultural losses caused by Fusarium oxysporum f. sp. cubense (Foc) have a direct effect on the income, subsistence and nourishment of thousands of smallholder farmers worldwide. In addition, also commercial growers are strongly affected. For Viet Nam, predictions on the impact of Foc for the future are dramatic with an estimated loss in banana production area of 8% within the next 5 years and up to 71% within the next 25 years. In the current study we applied a combined morphological - molecular approach to assess the taxonomic identity and phylogenetic position of the different Foc isolates that were collected in northern Viet Nam. In addition, we aim to estimate the proportion of the different Fusarium races that are infecting bananas in northern Viet Nam. The morphology of the isolates was investigated by growing the collected Fusarium isolates on four distinct nutritious media (PDA, SNA, CLA, and OMA). Molecular phylogenetic relationships were inferred by sequencing partial rpb1, rpb2 and tef1a genes and adding the obtained sequences into a large phylogenetic framework. The present study showed that Foc Race 1 is the most common strain in northern Viet Nam, causing 74% of all the infections. A more in-depth molecular characterization shows that of the Foc Race 1 infections, 92% are caused by Fusarium tardichlamydosporum and 8% by F. duoseptatum. Compared to Foc Race 1, Foc TR4 (represented by F. odoratissimum) account for only 10.5% of the Fusarium wilting in northern Viet Nam demonstrating that Foc TR4 is not yet a dominant strain in the region. Foc Race 2 infections of Vietnamese bananas also account for 10.5% of the Fusarium wilting in northern Viet Nam. One of the isolates cultured from diseased bananas collected in northern Viet Nam was phylogenetically not positioned within the F. oxysporum species complex (FOSC), but in contrast fell within the Fusarium fujikuroi species complex (FFSC). As a result, it is possible that a new pathogen for bananas has been found. Besides being present on several ABB Tay banana, Foc Race 1 was also found as pathogen on wild Musa lutea, showing the importance of wild bananas as possible sink for Foc.

Field Pennycress (Thlaspi arvense) is gaining attention in the US Midwest as a potential oilseed cover crop for their corn-soybean systems. Field research with breeding trials have shown that pennycress is susceptible to major fungal diseases affecting Brassica crops. In this study, we identified two pathogens: Alternaria japonica and Sclerotinia sclerotiorum which cause Alternaria black spot and Sclerotinia white mold diseases, respectively. Both fungi infect pennycress leaves and pods, with S. sclerotiorum also able to infect stems of the two pennycress accessions tested. We found accession 2032 to be more susceptible than MN106. Traditional visual methods to estimate disease severity could not capture the differential progression of Alternaria black spot in the two pennycress accessions. To address this, we developed a cost-effective DNA-based qPCR-based assay that can detect differential growth of both A. japonica and S. sclerotiorum on leaves and pods of two pennycress accessions. This method also provided more precise quantification of A. japonica and S. sclerotiorum at early infection stages than was possible visually. This assay could be helpful in evaluating various pennycress cultivars and other crops affected by these pathogens.

The genus Nigrospora has been identified as the primary causative agent of leaf spot disease in banana plantations in Mindanao, the Philippines. The symptoms caused by Nigrospora species are similar to those of black leaf streak disease (black sigatoka), which has made managing the disease challenging. Furthermore, development of rapid and precise molecular diagnostic methodologies for leaf spot disease has remained elusive. In the present study, a polymerase chain reaction (PCR)-based detection system was developed. The system targets a Nigrospora-specific genomic region, which was identified through comparative analysis of the genus Nigrospora and related genera. The newly designed primer set nigF/nigR specifically detected six pathogenic Nigrospora species associated with banana leaf spot disease (N. chinensis, N. lacticolonia, N. nigrocolonia, N. singularis, N. sphaerica, and N. vesicularifera), while no amplicon was observed for 15 non-Nigrospora species, including Pseudocercospora fijiensis isolated from banana leaves. The results indicate that the primers enable clear differentiation from other genera, including the important pathogen P. fijiensis. The primers also detected diverse Nigrospora isolates (N. banbusae, N. chinensis, N. musae, N. oryzae, N. sphaerica, and N. rubi) obtained from different hosts and regions within Japan, demonstrating broad applicability. Sensitivity testing demonstrated the capacity to detect as minimal as 10 pg of genomic DNA. Additionally, a pathogenic Nigrospora isolate was detected successfully from infected banana tissue. This study presents a novel molecular diagnostic tool for Nigrospora species, offering a rapid, specific, and practical approach, which could enhance evidence-based disease management in epidemiological investigations and banana production systems.

1.Wheat stem rust (Puccinia graminis f. sp. tritici) is a major global threat to wheat production, driven by rapid shifts in virulence and race diversity. This study aimed to identify physiological races of stem rust pathogens in Ethiopias irrigated wheat-growing areas. Surveys and race analyses were conducted during the 2020/21-2023/24 large-scale wheat expansion periods, alongside a systemic review of rust dynamics from 2012 to 2022. Results revealed significant shifts in stem rust races, with an increasing dominance of virulent races over the last decade. TKTTF dominated from 2012 to 2016, succeeded by TTTTF in 2017, and TKKTF in 2019-2020. By 2021-2022, TTKTT and TTKTF races were prevalent, affecting 90% of wheat fields. From 2020 to 2024, five major races were identified: TTKTT, TTKTF, TTTTF, TKKTF, and TKTTF, with TTKTT, a virulent Ug99 mutant, emerging as the most dominant. This race exhibits a 95% virulence spectrum, overcoming resistance genes such as Sr24, widely used in commercial cultivars. The study highlights the urgent need to prioritize resistance breeding against virulent races, particularly in irrigated wheat regions, which serve as hotspots for pathogen evolution. Strategic rust intervention and improved screening are essential for protecting both irrigated and rain-fed wheat crops, ensuring sustainable wheat production in Ethiopia and the globe under climate change. 1. AbstractWheat stem rust (Puccinia graminis f. tritici) is a significant concern for farmers and agricultural communities around the world. It is essential to monitor the changes in virulence among these infections, as understanding these shifts can help prevent unexpected epidemics that impact livelihoods and food security. ObjectiveThis study seeks to compassionately uncover the physiological races of stem rust pathogens present in irrigated wheat-growing regions of Ethiopia, recognizing the vulnerability of local farmers to these challenges. Materials and MethodsWe conducted thorough surveys and surveillance, accompanied by detailed physiological and genetic race analyses. The insights gained from this research will be instrumental in guiding the large-scale demonstration and expansion of irrigated wheat from 2020/21 to 2023/24. Furthermore, we conducted comprehensive reviews of the dynamics of stem and yellow rust races affecting wheat production in Ethiopia over the past decade, from 2012 to 2022. Results and DiscussionOur findings reveal that changes in stem rust pathogen races, with a wider pathogenicity spectrum, are prevalent in both rain-fed and irrigated wheat-growing areas of Ethiopia. The rise of virulent races in affected fields has understandably raised concerns among farmers, with frequencies reaching up to 50%. Over the last ten years, the dynamics of wheat rust races illustrate a troubling shift toward more virulent strains. For example, the TKTTF race, virulent to resistance gene 85, was predominant from 2012 to 2016 but was subsequently replaced in 2017 by the more aggressive TTTTF race, which is harmful to gene 90. In 2018, we observed that TTTTF, along with TKTTF and TKKTF (virulent to gene 80), became equally common, creating additional challenges for our farming communities. The TKKTF race held dominance in 2019 and 2020, while from 2021 to 2022, TTKTT (virulent to gene 95) and TTKTF (virulent to gene 85) became more widespread, affecting around 90% of wheat fields.During the years 2021/22 to 2023/24, we carefully collected and analyzed 51 field rust samples to understand the situation better. We identified seven Pgt pathotypes in irrigated wheat in the 2021/22 season: TTKTT, TTKTF, TKKTF, TTRTF, TTTTF, TKTTF, and TKPTF. The emergence of the TTKTT race, a pathogenic Ug99 mutant, particularly in regions like Jimma, Buno Bedale, West Arsi, and East Shoa, is particularly concerning for the local farmers who depend on stable crops. In 2020/21, five stem rust races TTKTT, TTKTF, TTTTF, and TKTTF were identified, with TTKTF and TTKTT showing dominance in geographic distribution. Many popular varieties are now susceptible to these races, adding to the anxiety of farmers who rely on these crops for their livelihoods. The TTKTT race has been particularly alarming, with a 49% prevalence in Ethiopia for the first time, and it has shown varying reactions ranging from susceptible to moderately susceptible among different wheat types. Importantly, the resistance gene Sr24, which many farmers have relied upon in their cultivars, has unfortunately been rendered ineffective against this race. ConclusionTo develop durable resistance, it is crucial to introduce a transgene cassette containing five resistance genes into bread wheat as a single locus within the wheat production area. Integrating robust resistance genes from wild grass relatives with modern scientific breakthroughs is essential. Consequently, breeding programs should focus on identifying additional sources of resistance to counter the more virulent races of the pathogen. Special attention must be given to irrigated wheat production, which harbors virulent and genetically diverse races, threatening the belg and main season wheat crops, as well as the green bridge. The background data collected from this study will aid in strategic rust intervention, screening, and guidance for resistance breeding among wheat breeders and seed technology multiplication units in the area addressing the climate change.

Xanthomonas wilt (XW) of banana caused by Xanthomonas vasicola pv. musacearum (Xvm) does not spread to all plants physically interconnected through the rhizome when one or a few are diseased. However, the factors behind this incomplete systemic spread of Xvm are not fully known yet could inform XW management. This study explored the effect of Xvm inoculum amounts, number and size of suckers, sucker positioning on mother plant corms and other mother plant corm attributes on sucker colonization. A shorter (p <0.05) incubation period (17.9 vs 21.1 days) and higher (p<.001) cumulative number of symptomatic leaves (5.2 vs 1.6 leaves) was observed when all (high inoculum) compared to two leaves (low inoculum) were inoculated. Xvm was recovered in corms at 29 days post inoculation (dpi) in both treatments with no differences (p >0.05) in proportions of corms with Xvm between the treatments. However, Xvm was recovered earlier and at a higher frequency in suckers when all leaves were inoculated. Lower Xvm recoveries occurred in the lower corm sections to which most suckers were attached relative to the middle and upper corm sections. Xvm incidence in corms increased with the number of attached maiden suckers, and the dpi while it declined with increasing mother plant and corm height. Thus, Xvm spread within mats is influenced by the amount of inoculum and the physiological stage (e.g., height) of the plant and attached suckers. The position of suckers, predominantly at the bottom of corms also protects them from infection. Measures that reduce Xvm inoculum build-up in mats are thus crucial for minimizing within mat XW spread.

Foot rot is a devastating disease of faba bean crops globally, including in the United Kingdom, the worlds third largest producer. To identify the causal agents, we have sampled foot rot-affected plants and soils from faba bean crops across England. We isolated organisms associated with foot rot disease in culture and assessed pathogenicity in vivo to evaluate the infectivity of the isolates on faba bean. We identified the pathogenic isolates using DNA barcoding of the Internal Transcribed Spacer (ITS) and Translation Elongation Factor one (TEF1 ) molecular markers. A total of 113 clonal isolates were obtained from infected plants and soil samples across England. Of these, 60 were pathogenic, inducing mild to severe symptoms on faba bean. Sequencing of the ITS and TEF1 loci and comparison against sequence databases (Genbank and Fusarium_ID) enabled the identification of pathogenic isolates, in decreasing order of frequency, as Fusarium oxysporum (26.6 %), F. vanettenii (25%), F. redolens (15 %), F. solani (11.6%), F. culmorum (8.3 %), F. avenaceum (6.7 %), F. equiseti (1.7 %), F. clavum (1.7 %), Clonostachys rosea (1.7%) and Alternaria alternata (1.7%). F. oxysporum, F. redolens and F. avenaceum induced the most severe symptoms, whilst F. solani induced the least severe symptoms. Determining the most prevalent causal agents of foot rot in UK faba beans will facilitate targeted disease monitoring and intervention for enhanced productivity.

Soft rot Pectobacteriaceae (SRP) are among the most economically important plant pathogenic bacteria and are especially known to be problematic in potato production. The epidemiology of disease transmission has been investigated for almost a century, and several aspects have been highlighted as plausible infection routes. However, it is generally accepted that the major source of disease is the latently infected mother tuber, but several parameters are still influencing disease prevalence including contaminated equipment, soil water status as well as temperature. Management of the disease is limited to hygiene practices, dry storage and seed certification systems but several studies have also proven biocontrol agents such as bacteriophages (phages) as promising tools. Despite the severity of SRP on potato production, little is known about the genetic diversity of SRPs in Denmark, and since only few isolates are available, the possibility to design a broadly effective phage cocktail is limited. Here we describe a three-year field study utilizing an agri-citizen science approach where Danish farmers provided symptomatic potato plants or tubers, together with metadata such as date, location, potato variety and origin. By using whole genome sequencing (Illumina and Nanopore) together with metadata we were able to investigate and monitor the epidemiological disease spread across the country using 103 complete genomes, sampled across all three years. In this study we provide epidemiological evidence of disease origins and a suite of phages that could be used as a biocontrol tool for early disease intervention. Our results revealed several clonal clades across diverse locations (SNPs < 20) which strongly indicate common origin. A total of 17 Pectobacterium phages were tested and did target > 80% of clonal clades. Based on the clonality across the soft rot isolates we propose the possibility to set in early on using phages targeting strains relevant for soft rot development, with the possibility of a surveillance program together with customizing the phage preference.

Meloidogyne enterolobii is an aggressive root-knot nematode (RKN) species that has emerged as a significant pathogen of sweetpotato in the Southeastern United States. M. enterolobii is spread primarily through the movement of infected seed sweetpotatoes used for propagation. The RKN resistance in commercially grown sweetpotato cultivars has proven ineffective against this nematode. Detecting RKN in sweetpotato by eye is unreliable, and further distinguishing M. enterolobii from other RKN species that infect sweetpotato is labor intensive; relying on molecular tests conducted on individual nematodes dissected out of host roots by trained technicians. Here, we have developed a high-throughput survey method to collect skin samples and extract total DNA from batches of sweetpotato storage roots. Combining this method with species-specific PCR assays allowed for quick and sensitive detection of M. enterolobii and other RKN species infecting sweetpotatoes. We tested this method using batches of infected storage roots at varying levels of M. enterolobii infection. We also inoculated skin samples with varying numbers of individual M. enterolobii eggs to determine the methods detection threshold and used this method to conduct surveys for RKN on fresh market sweetpotatoes. Our results show that this method can consistently and reliably detect M. enterolobii in sweetpotato batches at levels as low as 2 eggs per 10 mL skin sample. This method will be a useful tool to help screen for the presence of M. enterolobii in seed sweetpotatoes before they are replanted, thereby helping to slow the spread of this nematode to M. enterolobii-free sweetpotato growing operations.

The characterization of virulence frequencies has to be regularly updated to identify which genes are currently efficient and use this information to advise gene deployment by choosing varieties depending on the current composition of local pathogen population. In L. maculans on Brassica napus, because different genes were characterized by different teams, because new interactions are continuously identified and seed of differentials are difficult to obtain, we today still lack isolates characterized on all current resistance genes. On the one hand, we assembled a set of 12 isolates characterized on 13 of the 17 described resistance genes, having clearly compatible and clearly incompatible isolates for each interaction. This set can be used to characterize the L. maculans - B. napus interaction at cotyledon stage. Expanding the set of isolates with clearly virulent ones allowed us to detect inconsistent behaviour or intermediate (avirulent) phenotypes. On the other hand, we used this set of isolates as controls to identify virulence frequencies in a current French L. maculans population sampled in 2018 at Le Rheu. We provide the current status for 13 avirulence frequencies, including LepR1, LepR2 and LepR3 available in near isogenic lines of spring canola but not yet documented in France. Avirulence frequencies on the genes Rlm1, Rlm2, Rlm3, Rlm4, Rlm7, Rlm9 and LepR3 were low, indicating the lack of efficacy of these genes against the current population. In the opposite, all or most of isolates were avirulent for the genes Rlm5, Rlm6, Rlm10, Rlm11, LepR1 and LepR2. An optimistic point of view could conclude that there are ample resources for oilseed rape breeding. However, as compared to previous studies, so far all the resistance genes used on significant acreage without additional management practices have lost efficacy and only avirulences corresponding to resistance genes not deployed in France retain efficacy. While the call to wisely manage the available host resistance genes is not recent, it is still relevant. Adding, management practices to the deployment of resistance genes in order to reduce inoculum carry-over from one growing season to the next and to lower population sizes is key to maintain their efficacy over time.

Ascochyta Blight (AB), caused by Ascochyta rabiei (syn Phoma rabiei), is the major endemic foliar fungal disease affecting the Australian chickpea industry, resulting with potential crop loss and management costs. This study was conducted to better understand the risk posed by the Australian A. rabiei population to current resistance sources and to provide informed decision support for chemical control strategies. Recent changes in the pathogenicity of the population were proposed based on disease severity and histopathological observations on a host set. Controlled environment disease screening of 201 isolates on the host set revealed distinct pathogenicity groups, with 41% of all isolates assessed as highly aggressive and a significant increase in the proportion of isolates able to cause severe damage on resistant and moderately resistant cultivars since 2013. In particular, the frequency of highly aggressive isolates on the widely adopted PBA HatTrick cultivar rose from 18% in 2013 to 68% in 2017. In addition, isolates collected since 2016 caused severe disease on Genesis 090, another widely adopted moderately resistant cultivar and on ICC3996, a commonly used resistance source. Of immediate concern was the 10% of highly aggressive isolates able to severely damage the recently released resistant cultivar PBA Seamer (2016). Histopathology studies revealed that the most aggressive isolates were able to germinate, develop appressoria and invade directly through the epidermis faster than lower aggressive isolates on all hosts assessed, including ICC3996. The fungal invasion triggered a common reactive oxygen species (ROS) and hypersensitive response (HR) on all assessed resistant genotypes with initial biochemical and subsequent structural defence responses initiated within 24 hours of inoculation by the most highly aggressive isolates. These responses were much faster on the less resistant and fastest on the susceptible check host, indicating that speed of recognition was correlated with resistance rating. This will inform fungicide application timing so that infected crops are sprayed with prophylactic chemistries prior to invasion and with systemic chemistries after the pathogen has invaded.Competing Interest StatementThe authors have declared no competing interest.View Full Text

Tar Spot continues to threaten U.S. corn production (Telenko et al., 2022). A reliable assay under controlled conditions is needed to study the epidemiology and management of Tar Spot. Researchers have reported controlled environment infections, but incidence and severity were low and the ability to screen germplasm has not yet been reported. In this paper, we describe a controlled environment assay that reliably achieves Tar Spot symptoms on corn plants allowing differentiation of susceptible and resistant germplasm.

Cassava witches broom disease (CWBD) has emerged as a significant threat to cassava production in the Oiapoque region of Amapa, Brazil. Diseased plants exhibit stunted growth, vascular necrosis, abnormal shoot proliferation, and distinctive broom-like appearance. This study aimed to characterize the disease and identify its causal agent(s). The assessment by high-throughput sequencing of total nucleic acids (DNA and RNA) from affected cassava tissues revealed Rhizoctonia theobromae (syn. Ceratobasidium theobromae) associated with CWBD. PCR-based detection using species-specific primers confirmed the presence of R. theobromae in 74% of symptomatic samples. Genetic diversity analysis based on the Ca2+/calmodulin-dependent protein kinase gene showed low variability among Brazilian isolates compared with Asian populations, suggesting their recent introduction. This first report of R. theobromae causing CWBD in Brazil follows a recent report of the disease from French Guiana and highlights the urgent need for effective measures to prevent the further spread of this emerging pathogen, which poses a deadly threat to cassava cultivation in the rainforest, a significant risk to cassava production in Brazil, and regions with similar eco-climatic conditions.

From sowing in late summer until harvest in following summer, oilseed rape can be infected by several fungi, which foliar symptoms (leaf spots) coexist on the crop. Training an expert at their identification is quick for the typical symptoms with characteristic appearance. However, in many cases the size, colour and morphology are similar and for the atypical symptoms, there is a risk of confusion or in-decidability. Also, scouting the fields for expert training is not possible at all seasons and all diseases might not be seen in all years and all places. The aim of our study was to produce large sets of pictures annotated by several experts, from which tables illustrating the diversity of symptom appearance were chosen. These tables will enable assistance to diagnostic and expert training.