Ground Cover Supplement : GC Supplement - Foliar fungal diseases
9 FOLIAR FUNGAL DISEASES GROUND COVER Triple-resistant mungbeans NEW MUNGBEAN VARIETIES OFFER RESISTANCE TO THREE FOLIAR PATHOGENS BY MALCOLM RYLEY THE IMMINENT RELEASE of new mungbean varieties with improved resistance to three foliar pathogens should offer growers substantial improvements in their ability to control disease across the season. Halo blight (caused by Pseudomonas savastanoi pv. phaseolicola) and tan spot (caused by Curtobacterium flaccumfaciens pv. flaccumfaciens) are both seed-borne bacterial diseases that have many alternative hosts. While halo blight is favoured by cool, wet weather, the symptoms of tan spot are more severe in hot, dry conditions. Management options are limited to planting disease-free seed and resistant varieties, and controlling weed hosts. Powdery mildew (caused by the fungus Podosphaera fusca) is not seed-borne but can only survive from season to season on living plants of its many hosts. It is often a serious late-season disease that can be controlled by applications of the fungicide Headland sulphur, but host resistance offers the best long-term solution. The Queensland Department of Primary Industries and Fisheries (QDPI&F) pulse plant-pathology team at Toowoomba and Kingaroy has been collaborating for the last four years with the QDPI&F Mungbean Improvement Program team to identify sources of resistance to all three pathogens. A soon-to-be-released large and shiny-seeded variety, CrystalA (breeding line 45/52-21), is more resistant to all three foliar pathogens than most current commercial varieties, and at least equal to the most resistant of them. On the National Variety Trials (NVT) uniform disease-rating scale this new variety can be classed as moderately resistant -- moderately susceptible (MR- MS) to tan spot and powdery mildew, and moderately susceptible -- susceptible (MS-S) to halo blight. Future mungbean varieties will have even better resistance to tan spot, powdery mildew and halo blight. During the past four years, the pathology and breeding teams have identified germplasm and advanced breeding lines that are resistant -- moderately resistant (R-MR) to tan spot and powdery mildew. Of even greater significance is the identification, after only one season, of a mungbean line with the highest level of resistance (R) to halo blight. Different pathotypes of the halo blight and powdery mildew organisms are known to exist overseas, so potentially the effective life of newly released, disease- BREEDING Management options for halo blight, tan spot and powdery mildew are limited in mungbeans, so growers will welcome the release of new varieties with improved resistance to all three diseases. resistant varieties could be compromised. The QDPI&F pulse-pathology team is undertaking studies to determine if there are pathotypes of the two pathogens in Australia. A set of Phaseolus germplasm lines has been imported from overseas, and when released from quarantine will be used to differentiate pathotypes of the halo blight bacterium. The powdery mildew pathogen cannot be grown in culture, which makes similar work very difficult. However, in an exciting breakthrough, the team has discovered that when the stems of mungbean leaves that have been removed from the plant are placed in moist vermiculite and kept under humid conditions, roots develop from the base of the leaf stem and the leaves remain green for about three weeks. The team expects that they will be able to inoculate such leaves in the field with isolates taken directly from powdery- mildew-infected plants. GRDC Research Code DAQ00108 More information: Dr Malcolm Ryley, principal plant pathologist, QDPI&F, 07 4688 1316, firstname.lastname@example.org Future mungbean varieties will have even better resistance to tan spot, powdery mildew and halo blight.
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