Fusarium Head Blight Situation

Fusarium head blight (FHB or scab) of barley remains a major problem to producers in Minnesota. Following the devastating epidemics of 1993 and 1994, the impact of FHB decreased in 1995 and again in 1996, the disease being neither as severe or widespread in the previous years. Despite the lower disease levels, FHB was still considered the major limiting factor to production for individual producers in regions of the Red River Valley and barley prices suffered as the result of contamination of grain with the mycotoxin deoxynivalenol (DON).

The University if Minnesota, with financial assistance of the Minnesota State Legislature and Barley and Wheat Producers, initiated a collaborative research effort to address the control of FHB in barley and wheat. The development and implementation of field and greenhouse screening techniques to facilitate the identification and incorporation of resistance has been the primary focus of the barley pathology effort.

Field Tests

We have proceeded with the evaluation of field based procedures for generating artificial FHB epidemics in screening nurseries. During the summer of 1995, we evaluated three methods of inoculating 'Robust' and 'Chevron' barley plots to generate epidemics of FHB. The inoculation methods consisted applications of F. graminearum grown in mung bean broth (MBB, hyphal fragments and macroconidia), F. graminearum grown on mung bean and potato-dextrose agar (MBA/PDA, hyphal fragments and macroconidia), and F. graminearum -colonized wheat seed (to generate ascospores). Laboratory plating of seed onto selective media revealed that the percentage of barley kernels infected with FHB, from plots inoculated with MBB and MBA/PDA, was 67.7 and 69.3% (l.s.d.=6.9%, P=0.05) respectively, and were more than double that of plots inoculated with FG-colonized wheat seed. The test was repeated in the summer of 1996 with the addition of two inoculation treatments that consisted of F. graminearum-colonized corn crop residue (to generate ascospores) and F. graminearum macroconidia that were washed from the surface of MBA and sprayed onto plots at heading. Plots inoculated with FG in MBB and plots inoculated with F. graminearum macroconidia had mean FHB severities (visual estimates of infection) of 14.8 and 16.8% (l.s.d.=4.3%, P=0.05) respectively. The data suggested that epidemics were more reliably established in barley plots that were inoculated with FG hyphal fragments and/or macroconidia.

Annual field testing of germ plasm in the breeding program was established in 1993 to determine the level of resistance to FHB in elite material and advanced lines and to identify potential sources of resistance to FHB. In 1996, over 400 lines were screened at the three locations (St. Paul, Morris, and Crookston) where field screening nurseries have been established. Information on the efficacy of various inocula in generating epidemics will be used to ensure continued reliable screening of breeding materials in future years.

Greenhouse Testing

A greenhouse screening method was implemented during the fall and winter of 1995/96 and was repeated in the fall/winter of 1996/97. The greenhouse screening tests were conducted using an airbrush to spray macroconidia of Fusarium graminearum (FG) onto barley heads when they were fully emerged. After allowing the inoculated heads on barley plants to dry for one hour, the plants were then placed in a dew chamber. The dew chamber developed a relative humidity of 100% that was maintained at 22 C for a period of 72 hours with continuous lighting. After the dew period, plants were removed to benches in the greenhouse. Heads of inoculated barley plants were assessed 14 days after inoculation by counting the number of infected florets per total florets per head. From these counts, a percentage of Fusarium Head Blight (FHB) could be calculated for each genotype tested. Analyses of variance and mean comparisons have revealed significant differences among barley genotypes for resistance to FHB. Check cultivars 'Chevron' (FHB resistant), 'Robust', and 'Stander' (FHB susceptible) have been included for comparison in all greenhouse screening procedures and have allowed us to make good comparisons of promising genotypes for reaction to FHB. Percentages of FHB severity have ranged from zero to one-hundred percent with coefficients of variation ranging from 50 to 60% among the tests we have concluded. To date, approximately 500 barley genotypes, including check cultivars, have been evaluated for reaction to FHB. We continue to improve greenhouse screening procedures as we evaluate additional promising breeding lines.

Laboratory Tests

Little information exists concerning the early events of F. graminearum infections in florets on barley heads. Thus, we have been investigating the biology of F. graminearum infections in florets of Robust and Chevron over time. Specifically, we wished to understand when and to what extent the fungus produces trichothecene toxins during the infection process. Macroconidia of three F. graminearum isolates and water (controls) were inoculated separately into the central floret of spikes of Robust and Chevron barley cultivars. Inoculated plants were subjected to a 72 hour post-inoculation (PI) dew period at 22 C and inoculated spikes were sampled at time 0, 24, 48, 72, 120 and at 240 hours PI. Gas chromatography/mass spectrometry analyses of extracts of single inoculated florets revealed that 15-acetyldeoxynivalenol (15-ADON) was detectable at 24 hours PI in Robust and both 15-ADON and deoxynivalenol (DON) were detectable at 48 hours PI in both cultivars. Other trichothecenes, 3-acetyldeoxynivalenol and 7-acetyldeoxynivalenol, were first detected at 120 and again at 240 hours PI in both cultivars. At 48 hours, concentrations of 15-ADON and DON across isolates averaged 7.98 and 1.10 ppm/floret in Robust and 4.31 and 0.88 ppm/floret in Chevron, respectively. At 72 hours, concentrations of 15-ADON and DON across isolates averaged 7.19 and 1.07 ppm/floret in Robust and 18.74 and 5.69 ppm/floret in Chevron, respectively. The early occurrence and concentration of 15-ADON and DON support the hypothesis of trichothecene involvement in pathogenesis and/or virulence by F. graminearum.