The Fusarium Head Blight Situation

In 1993, 1994 and 1995 Fusarium Head Blight (FHB or scab) epidemics caused widespread and severe losses in grain yield and quality resulting in millions of dollars in losses to barley producers and processors. At present FHB is the greatest single limiting factor in barley production in Minnesota and threatens the livelihoods of small grains produces in a manner not seen since the wheat rust epidemics of the 1950's. In response to this the University of Minnesota with financial assistance from the state legislature and the grain industry organizations has responded with a research initiative which aims to address the control and management of FHB in barley and wheat. Much of the focus of the barley pathology program has been directed to this effort.

Current Research

Emphasis has been placed on the control of FHB through the use of host resistance. In order to identify sources of resistance and screen breeding material for resistance to FHB effective screening procedures need to be developed. Considerable effort has been directed to the development of generating artificial epidemics in the field, scoring material in breeding nurseries and in the development of an efficient greenhouse screen for adult plants.

In nearly all cases fungi of the genus Fusarium cause FHB in barley. The principal pathogen is Fusarium graminearum and its teleomorph Gibberella zeae and inoculum may be either in the form of macroconidia or ascospores. Studies have been initiated to examine the most effective way of generating an artificial FHB epidemic in screening nurseries.

Plots of two barley varieties were planted in a randomized complete block with four replicates. Plots were 10 feet long and 10 rows at 7 inch row spacings. The plots were separated by a minimum distance of 7 ft by winter rye borders. Inoculum was prepared using five F. graminearum isolates. The isolates used were isolated from barley and wheat heads showing visible symptoms of FHB and collected in Minnesota in 1994. Four methods for generating and applying inoculum to plots were evaluated for their ability to induce FHB in Robust (susceptible) and Chevron (moderately resistant) in 1995. Spore suspensions were produced utilizing two methods. One method consisted of culturing the pathogen on potato dextrose agar (PDA) and mung bean agar (MBA) in petri dishes for 7-10 days. After plates were fully colonized by the fungus they were combined (10 plates PDA, 10 plates MBA) and comminuted in a blender. A second method consisted of culturing the pathogen in 4 L mung bean broth (MBB) which was also comminuted prior to inoculation. Both spore suspensions were made up to 5 L with water prior to application. Four applications of spore suspensions were made to the plots with a backpack mist sprayer. The first application was made at heading and subsequent inoculations were made at 5-7 day intervals. Alternatively inoculum was prepared by colonizing autoclaved wheat seed with the pathogen for 14 days and then dried. Colonized seed was placed in plots in mesh bags suspended at head height or by dispersing them on the soil surface. About ten days after being placed in the plots the colonized seed showed evidence of the development of perithecia and ascospores. Colonized seed inoculum was applied to the field prior to heading to allow ascospore development to occur. Non inoculated plots were also maintained to facilitate observation of disease development in comparison with background inoculum levels.

Significant differences in disease severity were observed among the four inoculation techniques. Disease severity was assessed on a one to four scale (0 = 0%; 1 = <25% ; 2 = 25-50%; 3 = 50-75%; 4 = 75%) in each of 30 heads per plot and then averaged to provide a plot mean. Plots that were inoculated with suspensions of conidia and hyphal fragments had plot averages of 3.2 and 3.4. Plots inoculated with colonized wheat seed were 1.4 to 1.6. The disease severity readings for Chevron averaged 1.4 across treatments and was significantly less than Robust at 3.0.

One hundred kernels were randomly sampled from each plot, assessed for kernel discoloration and plated onto PCNB agar. Kernels were also rated for the presence or absence of Fusariumspp. and tested for viability. The mean number of kernels infested with Fusarium spp. were significantly (P<0.05) higher in plots that were inoculated with suspensions of macroconidia than where seed inoculum was spread. Seed from plots inoculated with macroconidia suspensions were also more discolored and had lower viability than the kernels from plots inoculated with the seed inoculum. It may be that earlier application of the seed inoculum may improve the efficacy of the inoculum by allowing more time for the development if perithecia and ascospores and further studies on the generation of seed inoculum will be conducted in 1996.

Efforts to develop an effective adult plant greenhouse screening have been promising. A technique for the preparation of macroconidial inoculum, free of hyphal debris has allowed us to use an airbrush to inoculate adult plants in the greenhouse. The use of a large capacity (300 plants) temperature controlled adult plant dew chamber has provided for more uniform infection levels. Good differentiation between lines in the greenhouse has been observed in consecutive screens suggesting that a screen measuring the response of plants to a single inoculation will provide useful information on varietal resistance and allow screening of breeding germplasm through the winter.