Ground Cover Supplement : GC Supplement - Foliar fungal diseases
Ground covEr FoliAr FunGAl disEAsEs 10 disease identiFiCation trapping disease on the wind A combinAtion oF old And nEw tEchnoloGy is hElPinG PAtholoGists ProvidE EArly disEAsE wArninGs And lEArn morE About thE movEmEnt oF AirbornE disEAsEs mounted on a rotating drum, and any spores, insects, dust particles and so on carried by the air stick to the tape. as the volume of air and the period of rotation for the drum are known, a profile can be produced of what is in the air, and when. traditionally, analysis of the tape meant many hours of microscope work, carried out by experts able to differentiate between pathogens and pollen grains and between spores of different fungal species. now a PhD student at the cooperative research centre for national Plant Biosecurity (crc Plant Biosecurity), Bonny Vogelzang, is developing and testing a system that combines spore traps and molecular diagnostics to identify disease spores rapidly. Under the supervision of Jenny Davidson and Kathy Ophel-Keller, both from the South australian research and Development institute (SarDi), and eileen Scott of the University of adelaide, the new 0 20 40 60 80 100 120 140 160 180 200 13Jun20Jun27Jun4Jul11Jul18Jul25Jul1Aug8Aug15Aug22Aug29Aug5Sep12Sep19Sep26Sep3Oct10Oct17Oct24Oct31Oct7Nov Average lesions per plant Trap plants 500m from peas Trap plants in pea canopy FIGURE 1 SECONDARY SPREAD OF BLACKSPOT SPORES IN FIELD PEAS, KINGSFORD, SA, JUNE TO NOVEMBER 2007 sur vey highlights bean disease SPOre traPS tO identify the presence of fungal spores in the atmosphere have been used since the 1950s. air drawn into the trap strikes an adhesive tape in 2007, cercospora leaf spot was identified as the most prevalent disease of faba beans in the southern region. however, the good news is that the effect of cercospora on yield appears to be less dramatic than established diseases such as ascochyta blight and chocolate spot. these were two key findings from a disease survey carried out by south Australian research and development institute (sArdi) plant pathologist rohan kimber, with assistance from district consultants in winter 2007. many faba bean growers may be surprised at the prevalence of cercospora, but mr kimber reports that misdiagnosis of this disease as ascochyta blight is common in most districts. cercospora tends to develop early in the season, appearing five to eight weeks after sowing in fields with an intensive history of faba bean production, and is less likely to occur in fields that have not been sown to faba beans for the past six years or more. there are several characteristics that distinguish between ascochyta and cercospora, and mr kimber expects his research will assist growers in correct identification and management of cercospora leaf spot. “All commercial varieties of faba bean are susceptible to cercospora,” mr kimber says. “but paddocks that are in short to medium-term rotations of faba beans are most at risk from this soil-borne disease.” control of cercospora in trial plots (in 2005) resulted in a seven per cent yield increase over plots exhibiting high disease levels. strategically targeted treatments showed that early applications (5 to 8 weeks after sowing) of the fungicides tebuconazole or carbendazim significantly reduced disease establishment. in situations of high disease, this offers an economically justified option for control, especially if the fungicide replaces an ineffective fungicide (for example, mancozeb) applied as a result of misdiagnosis of disease. however, a caution is issued on the ‘over use’ of carbendazim, particularly if applications to control chocolate spot later in the season are scheduled. Prior to this survey, which was carried out in south Australia and the victorian wimmera, ascochyta blight had been regarded as the most prevalent disease in faba beans. Cercospora has been identified as the prevailing faba bean disease in the southern region, but rohan Kimber (left), with faba bean breeder Jeff paull, reports its impact on yield loss is less dramatic than ascochyta and chocolate spot.
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