A new strain of Plasmopara halstedii, a form of mildew that attacks sunflowers

Since the identification of downy mildew species 710 and 703 in France in 1988 and 1989 systematic monitoring of the sunflower crop has led to the identification, in 1995, of two further species, 300 and 700, and, since 1994, of other strains resistant to metalaxyl. These discoveries have not affected the control of downy mildew by means of mildew resistance, as varieties of sunflower are used that are resistant even to these new strains. in the spring of this year, two strains were identified in two fields in the South of France, which behaved differently from previously known strains. Tests using the different international sunflower varieties [1] indicated that both strains belonged to species 304 (table). The resistant American varieties of sunflower, HA335, HA336, HA337, HA338 and HA339, widely used by French growers do not show effective resistance to species 304. Fortunately there are a number of other varieties available which do show resistance (particularly RH274, RHA340, XRQ). In addition, at this time the extent of the dissemination of species 304 is extremely limited it was discovered, very quickly, thanks to monitoring involving the entire sunflower growing community. it should be possible to avoid any significant crop losses from this particular strain of downy mildew by the use of appropriate resistant varieties.