Fusarium head blight of small grains in Pennsylvania: unravelling species diversity, toxin types, growth and triazole sensitivity

Fusarium graminearum is the main causal species of Fusarium head blight (FHB) globally. Recent changes in the trichothecene (toxin) types in the North American FHB pathogens support the need for continued surveillance. In this study, 461 isolates were obtained from symptomatic spikes of wheat, spelt, barley, and rye crops during 2018 and 2019. These were all identified to species and toxin types using molecular-based approaches. An additional set of 77 F. graminearum isolates obtained from overwintering crop residues during Winter 2012 were molecularly identified to toxin types. A subset of 31 F. graminearum isolates (15 15ADON and 16 3ADON) were assessed for mycelial growth, macroconidia, perithecia, and ascospore production, and sensitivity to two triazole fungicides. Ninety percent of isolates obtained from symptomatic spikes (n = 418) belonged to F. graminearum, with another four species found at a lower frequency (n = 39). F. graminearum isolates from symptomatic spikes were mainly of the 15ADON (95%), followed by 3ADON (4%), NIV (0.7%), and NX-2 (0.3%) toxin types. All F. graminearum isolates obtained from overwintering residue were of the 15ADON type. Toxin types could not be differentiated based on multivariate analysis of growth and reproduction traits. All isolates were sensitive to tebuconazole and metconazole fungicides in vitro. This study confirms the dominance of F. graminearum and suggests ecological and environmental factors that lead to similar composition of toxin types in Northern U.S. Our results are useful to assess the sustainability of FHB management practices and provide a baseline for future FHB surveys.

Investigating in vitro mating preference between or within the two forms of Pyrenophora teres and its hybrids

Net blotch diseases result in significant yield losses to barley industries worldwide. They occur as net-form and spot-form net blotch caused by P. teres f. teres (Ptt) and P. teres f. maculata (Ptm), respectively. Hybridisation between the forms was proposed to be rare, but recent identifications of field hybrids has renewed interest in the frequency and mechanisms underlying hybridisation. This study investigates the mating preference of Ptt, Ptm and laboratory-produced hybrids in vitro, using 24 different isolates and four different experimental setups. Two crosses in our study produced ascospores during two intervals separated by a 32-35 day period of no ascospore production. For these crosses Ptt isolates mated with isolates of the same form during the early ascospore production interval and produced hybrids during the later interval. Ptm isolates did not mate with isolates of the same form, instead hybridised with Ptt isolates. Analyses based on DArTseq™ markers confirmed that laboratory-produced hybrids, when given the choice to mate with both Ptt and Ptm, mated with Ptt isolates. These results unravel a novel concept that Ptt seems to have a greater reproduction robustness than Ptm, which could lead to increased prevalence of hybrid incidences in vivo.

Modelling Inoculum Availability of Plurivorosphaerella nawae in Persimmon Leaf Litter with Bayesian Beta Regression

Circular leaf spot (CLS), caused by Plurivorosphaerella nawae, is a serious disease affecting persimmon (Diospyros kaki) that induces necrotic lesions on leaves, defoliation and fruit drop. Under Mediterranean conditions, P. nawae forms pseudothecia in the leaf litter during winter and ascospores are released in spring infecting susceptible leaves. Persimmon growers are advised to apply fungicides for CLS control during the period of inoculum availability, which was defined based on ascospore counts under the microscope. A model of inoculum availability of P. nawae was developed and evaluated as an alternative to ascospore counts. Leaf litter samples were collected weekly in L’Alcúdia (Spain) from 2010 to 2015. Leaves were soaked, placed in a wind tunnel, and the ascospores of P. nawae released were counted. Hierarchical Bayesian beta regression methods were used to model the dynamics of ascospore production in the leaf litter. The selected model included accumulated degree days (ADD) and ADD taking into account the vapor pressure deficit (ADDvpd) as fixed effects, and year as random effect. This model had a mean absolute error of 0.042 and a root mean square error of 0.062. The beta regression model was evaluated in four orchards from 2010 to 2015. Higher accuracy was obtained at the beginning and the end of the ascospore production period, which are the events of interest to schedule fungicide sprays for CLS control in Spain. This same modeling framework can be extended to other fungal plant pathogens whose inoculum dynamics are expressed as proportion data.

Modelling Inoculum Availability of Plurivorosphaerella nawae in Persimmon Leaf Litter with Bayesian Beta Regression

Circular leaf spot (CLS), caused by Plurivorosphaerella nawae, is a serious disease of persimmon (Diospyros kaki) inducing necrotic lesions on leaves, defoliation and fruit drop. Under Mediter-ranean conditions, P. nawae forms pseudothecia in the leaf litter during winter and ascospores are released in spring infecting susceptible leaves. Persimmon growers are advised to apply fungicides for CLS control during the period of inoculum availability, which was defined based on ascospore counts under the microscope. A model of inoculum availability of P. nawae was developed and evaluated as an alternative to ascospore counts. Leaf litter samples were collected weekly in L’Alcúdia from 2010 to 2015. Leaves were soaked, placed in a wind tunnel, and released ascospores of P. nawae were counted. Hierarchical Bayesian beta regression methods were used to fit the dynamics of ascospore production in the leaf litter. The selected model, having the lowest values of DIC, WAIC and LCPO, included accumulated degree days (ADD) and ADD taking into account the vapor pressure deficit (ADDvpd) as fixed effects, and year as a random effect. This model had a MAE of 0.042 and RMSE of 0.062. The beta regression model was evaluated in four orchards for different years from 2010 to 2015. Higher accuracy was obtained at the beginning and the end of the ascospore production period, which are the events of interest to schedule fungicide sprays for CLS control in Spain. This same modelling framework can be extended to other fungal plant pathogens whose inoculum dynamics are expressed as proportion data.

Comparative Studies on the Effect of Adjuvants with Urea to Reduce the Overwintering Inoculum of Venturia inaequalis

A 2-year study on the use of organic and conventional adjuvants alone, or mixed with urea, was conducted for management of overwintering inoculum of the apple scab pathogen, Venturia inaequalis. Select adjuvants (LI 700, Bond Max, Latron B-1956, and Organic Wet Betty [OWB]) have the potential to hasten urea-driven leaf litter decomposition and reduce V. inaequalis overwintering inoculum comparable to urea, and that one organic surfactant could perform the same level of leaf decomposition as urea. Combinations of adjuvants with urea significantly improved leaf litter degradation compared with urea alone, concomitant with reducing the number of pseudothecia present and pseudothecium fertility. We demonstrate that the combination of urea with Bond Max or OWB reduced pseudothecia fertility and ascospore production to less than 5% in the remaining pseudothecia, a significantly greater reduction than with urea alone. These results suggest that conventional growers combine urea with Bond Max or OWB to more effectively reduce overwintering inoculum, and that the adjuvant OWB can provide organic growers with comparable performance to urea used in conventional orchards for improved sanitation.

Effects of Temperature and Duration of Preconditioning Cold Treatment on Sclerotial Germination of Claviceps purpurea

Claviceps purpurea is an important ovary-infecting pathogen that replaces seed with sclerotia in Kentucky bluegrass grown for seed. Sclerotia overwinter in the soil and germinate in the spring to produce ascospores that infect grass seed ovaries. To better understand environmental conditions affecting ascospore production, the effects of preconditioning cold treatment and subsequent incubation temperature on germination of sclerotia were determined in growth chambers under controlled conditions. Preconditioning cold treatment was essential for germination only in treatments where the incubation temperature was high (at least higher than 20°C). At lower incubation temperatures (10 to 20°C), preconditioning also played a role in improving sclerotial germination. Preconditioning at 4°C (in darkness) for 4 to 8 weeks followed by incubation at 10 and 20°C (cycle of 12 h each of darkness and light), or constant 15°C (cycle of 12 h each of darkness and light), was optimal for ergot germination. When sclerotia were preconditioned for 4 weeks or longer, number of incubation days required for initiation of germination was not affected by temperature in the range from 10 to 25°C (cycle of 12 h each of darkness and light), although the duration of germination (or the progress speed of germination) was still affected by temperature. A simple model was developed based on laboratory results and validated with historic spore trap data collected from various Kentucky bluegrass fields in Oregon (Willamette Valley, central Oregon, and Grande Ronde Valley). The prediction model could predict ascospore onset well and explained 55% of variation in the data.

Effects of Temperature and Moisture on Development of Fusarium graminearum Perithecia in Maize Stalk Residues

ABSTRACTFusarium graminearumis the predominant component of the Fusarium head blight complex of wheat.F. graminearumascospores, which initiate head infection, mature in perithecia on crop residues and become airborne. The effects of temperature (T) and moisture on perithecium production and maturation and on ascospore production on maize stalk residues were determined. In the laboratory, perithecia were produced at temperatures between 5 and 30°C (the optimum was 21.7°C) but matured only at 20 and 25°C. Perithecia were produced when relative humidity (RH) was ≥75% but matured only when RH was ≥85%; perithecium production and maturation increased with RH. Equations describing perithecium production and maturation over time as a function ofTand RH (R2> 0.96) were developed. Maize stalks were also placed outdoors on three substrates: a grass lawn exposed to rain; a constantly wet, spongelike foam exposed to rain; and a grass lawn protected from rain. No perithecia were produced on stalks protected from rain. Perithecium production and maturation were significantly higher on the constantly wet foam than on the intermittently wet lawn (both exposed to rain). Ascospore numbers but not their dispersal patterns were also affected by the substrate.

Fitness Attributes of Fusarium graminearum Isolates from Wheat in New York Possessing a 3-ADON or 15-ADON Trichothecene Genotype

In all, 50 isolates of Fusarium graminearum from wheat spikes in New York, including 25 isolates each of the 15-acetyl-deoxynivalenol (15-ADON) and 3-ADON genotype, were tested to determine whether 3-ADON isolates are more fit for saprophytic survival and pathogenicity on wheat spikes than are 15-ADON isolates. The isolates were characterized and compared for 14 different attributes of saprophytic fitness and pathogenic fitness on a susceptible wheat variety. Isolates of the two genotypes could not be differentiated for most of these traits. Three principle components—ascospore production on corn stalks, total trichothecene amount in wheat kernels, and incidence of diseased spikelets up from the point of inoculation—accounted for 29.4, 18.9, and 10.8% of the variation among the isolates, respectively. A bootstrapping procedure grouped the isolates into two distinct groups, with 27 and 23 isolates each, with isolates from both genotypes represented in similar proportions (15-ADON/3-ADON, n = 14/13 and 11/12). Within the contemporary population of F. graminearum causing wheat head blight in New York, isolates with a 3-ADON genotype did not possess any detectable advantage over isolates with a 15-ADON genotype in saprophytic fitness or in pathogenic fitness on a susceptible wheat cultivar.