P. Jenkinson

Quantification of Trichothecene-Producing Fusarium Species in Harvested Grain by Competitive PCR To Determine Efficacies of Fungicides against Fusarium Head Blight of Winter Wheat

ABSTRACT We developed a PCR-based assay to quantify trichothecene-producingFusarium based on primers derived from the trichodiene synthase gene (Tri5). The primers were tested against a range of fusarium head blight (FHB) (also known as scab) pathogens and found to amplify specifically a 260-bp product from 25 isolates belonging to six trichothecene-producing Fusariumspecies. Amounts of the trichothecene-producing Fusariumand the trichothecene mycotoxin deoxynivalenol (DON) in harvested grain from a field trial designed to test the efficacies of the fungicides metconazole, azoxystrobin, and tebuconazole to control FHB were quantified. No correlation was found between FHB severity and DON in harvested grain, but a good correlation existed between the amount of trichothecene-producing Fusarium and DON present within grain. Azoxystrobin did not affect levels of trichothecene-producingFusarium compared with those of untreated controls. Metconazole and tebuconazole significantly reduced the amount of trichothecene-producing Fusarium in harvested grain. We hypothesize that the fungicides affected the relationship between FHB severity and the amount of DON in harvested grain by altering the proportion of trichothecene-producing Fusarium within the FHB disease complex and not by altering the rate of DON production. The Tri5 quantitative PCR assay will aid research directed towards reducing amounts of trichothecene mycotoxins in food and animal feed.

Strategies for the Control of Fusarium Head Blight in Cereals

Fusarium head blight (FHB) is a widespread and destructive disease of small grained cereals caused by a number of Fusarium species and Microdochium nivale. In addition to causing significant reductions in grain yield, FHB can result in the reduction of grain quality, either by affecting grain processing qualities or by producing a range of toxic metabolites that have adverse effects on humans and livestock. Control of FHB can be achieved by a number of cultural, biological and chemical strategies along with the exploitation of host plant resistance. In recent years, much of the research undertaken for the control of FHB has been concentrated on understanding and exploiting the genetic resistance of cereal plants to FHB-causing pathogens. Although, a brief overview of genetic resistance is presented, this review seeks to summarise the significance of FHB and review the effectiveness of cultural, biological and chemical control strategies that have been investigated for the control the disease.

Effect of dose rate of azoxystrobin and metconazole on the development of Fusarium head blight and the accumulation of deoxynivalenol (DON) in wheat grain

Glasshouse studies were undertaken to determine if fungicides used for the control of Fusarium head blight (FHB) result in elevated concentrations of the trichothecene mycotoxin, deoxynivalenol (DON) in harvested wheat grain. Metconazole and azoxystrobin, at double, full, half or quarter the manufacturers recommended dose rate, were applied to ears of wheat (cv. Cadenza), artificially inoculated with conidia of either Fusarium culmorum or F. graminearum. Metconazole demonstrated high activity against both pathogens, reducing significantly the severity of FHB and the DON concentrations at each of the four dose rates tested when compared to untreated controls. Applications of azoxystrobin significantly reduced FHB and DON compared to unsprayed controls. However, their effectiveness was significantly less than that of metconazole and no dose rate response was observed. Quantification of the amount of trichothecene-producing Fusarium present in harvested grain was determined using a competitive PCR assay based on primers derived from the trichodiene synthase gene (Tri5). Simple linear regression analyses revealed strong relationships between the amount of trichothecene-producing Fusarium present in grain and the DON concentrations (r2=0.72–0.97). It is concluded that fungicides, applied for the control of FHB, affect DON concentrations indirectly by influencing the amount of trichothecene-producing Fusarium species present in wheat grain. There was no evidence that fungicide applications directly increase the concentration of DON in grain.

Effect of artificial culture media on germination, growth, virulence and surface properties of the entomopathogenic hyphomycete Metarhizium anisopliae

Culture media influenced the germination of conidia, appressorial development and mycelial growth of Metarhizium anisopliae. Although the addition of KC1 to Sabouraud dextrose agar (SDAM) significantly reduced germination of the conidia of three isolates in vitro and lowered germination and appressorial development on the cuticles of Myzus persicae and Meligethes aeneus , conidia grown on SDAM and minimal medium (MM) were more aggressive than conidia derived from SDA or yeast extract agar. No relationship was found between either germination or appressorial development and LT 50 values, suggesting that virulence depends on the speed of infection. Calcofluor staining and lectin binding showed that culture conditions also influenced the adhesion of Metarhizium anisopliae to insect cuticles through alteration of surface carbohydrates such as 1–4-β-glucans. Dark, pigmented conidia from MM bound less lectins and calcofluor whereas the paler conidia from the other media fluoresced more intensely. Conidia from SDAM fluoresced 20-fold brighter and adhered more readily to insect cuticle compared with conidia derived from MM. Although inocula from both media showed elevated pathogenic activity against aphid and pollen beetle, MM-grown conidia were more aggressive since high mortality was induced by comparatively fewer conidia. When fluorescent intensity of calcofluor-stained conidia was compared to the number of conidia adhering to the insect cuticles, the relationship was significant, indicating that this dye has practical implications for the assessment of fungal strains in screening programmes for commercial development.

Effect of Timing of Fungicide Application on the Development of Fusarium Head Blight and the Accumulation of Deoxynivalenol (DON) in Winter Wheat Grain

The effect of different timings of fungicide applications on Fusarium head blight severity and mycotoxin accumulation in wheat grain was investigated in two field experiments. The fungicides metconazole, tebuconazole, azoxystrobin and mixtures of metconazole + azoxystrobin and tebuconazole + azoxystrobin were applied either, 5 days pre-, 2 days pre-, 2 days post-or 5 days post-inoculation of wheat ears with Fusarium spp. and Microdochium spp. at GS 65. Fungicides applied 2 days pre-or 2 days post-inoculation were most effective at reducing Fusarium head blight severity and DON concentration in grain. Metconazole and tebuconazole applied alone within two days of inoculation were most consistent in their effects on Tri5 DNA and DON in harvested grain.