December 2, 2004
Source: American Phytopathological Society, Plant Disease Notes [edited]

First report of Downy Mildew caused by Plasmopara helianthi Races 700 and 703 of sunflowers (Helianthus annuus) in Italy
L. Tosi and A. Zazzerini, Department of Arboriculture and Plant Protection, Borgo XX Giugno, 74, 06121 Perugia, Italy. Plant Dis. 88:1284, 2004; published on-line as D-2004-0903-01N, 2004. Accepted for publication 27 Jul 2004.

Sunflower (_Helianthus annuus_ L.) plants showing systemic downy mildew (_Plasmopara helianthi_ Novot.) symptoms (stunting and leaf chlorosis) were collected during May and June 2001-2003 in fields located in four regions of central Italy (Umbria, Tuscany, The Marches, and Emilia-Romagna). Generally restricted, a low disease incidence (2-12 percent) was observed in fields planted with commercial hybrids. In the surveyed areas, higher field incidences (20-30 percent) of downy mildew were observed only where suitable climatic conditions occurred.

To determine pathogen race variability of _P. helianthi_, 18 isolates (one for each field) were recovered directly from infected leaves showing sporulation or after 24 h of incubation in a humid chamber at room temperature. After increasing initial inocula on the susceptible cv. Ala, race identification of all isolates was determined by the whole-seedling immersion technique (2) on 3 sets of 9 differential sunflower lines (3 lines per each set): HA-304, RHA 265, RHA 274; PMI-3, PMI-17, 803-1; and HAR-4, QHP-1, HA 335 using triplet coding (1). All differential lines were tested twice with 40 seeds per replicate (3 replicates per line).

12 days after inoculation, plants were placed in a chamber maintained at 20 deg C and 100 percent relative humidity for 24 h and then evaluated for a susceptible (sporulation on cotyledons and/or 1st true leaves) or
resistance (absence of sporulation) reaction.

Races 700 and 703 were identified during the 3-year survey. Race 700 was recovered frequently (56 percent of isolates) from the Italian regions except Emilia-Romagna. Race 703 was not detected in Tuscany. Races 700 and
703 have been previously isolated in other European countries (3,4), but to our knowledge, this is the 1st report of the occurrence of either race in Italy.

Our results confirm previous investigations and suggest that the restricted presence of downy mildew should be attributed to several factors: (i) lack of any source of resistance to both races in commercial hybrids; (ii) insufficient seed dressing with metalaxyl as observed on remnants of commercial hybrid treated seeds tested in laboratory analyses; and (iii) short rotation of cereals (often only 2- to 3-year interval) with sunflower crops.

Although greenhouse tests showed most _P. helianthi_ isolates were not controlled by metalaxyl seed treatment at the registered rate (2.1 g/kg) loss of fungicide efficiency in fields has not yet been observed indicating that appropriate phytosanitary measures and proper seed treatment can provide control of sunflower downy mildew.

References:

(1) T. J. Gulya. Pages 76-78 in: Proc. 17th Sunflower Res. Workshop, Fargo, ND, 1995.
(2) T. J. Gulya et al. Helia 14:11, 1991.
(3) M. L. Molinero-Ruiz et al. Plant Dis. 86:736, 2002.
(4) F. Virany and T. J. Gulya. Plant Pathol. 44:619, 1995.

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[Sunflower downy mildew [DM], caused by _Plasmopara halstedii_ [Ph], is found in most parts of the world wherever sunflower is grown, although it is absent from Australia. Disease loss can be severe. Epidemics have been regularly observed in Bulgaria, Romania, Moldavia, Hungary and in the Mediterranean and western parts of the European continent. In China, yield losses have ranged from 5 to 80 percent, and losses in Iran have been
recorded up to 40 percent. DM has spread throughout the entire American continent, and is now in Europe, including Russia where cultivars with high oil content have been developed.

The classification of Ph races have been recently reorganized. The major ones are 300, 700, 377, 730, 770, 330, 710 and 403 correspond to the former races 1, 2, 3, 4, 5, 7, A, B, respectively. French races 700 and 300 were
formerly named C and D, respectively. Race 100 is found in some countries of Europe and Africa but not in USA, while Race 300 is primarily found in USA, Africa and France. Moreover, extensive exchanges between scientists in
Europe and North America have contributed to distribution of Ph isolates.

Unfortunately, Ph oospores remain viable in soil for up to 8-10 years, making it almost impossible to eradicate the fungus. Hybrid races evolve, overcoming resistant cultivars, with the result that yields are depressed. Strategies for disease management include planting of pathogen-free seed produced on fields free of Ph, seed treatment with recommended fungicide (metalaxyl and oxadixyl or related compounds), monitoring of fields
previously seeded for 2-3 years and roguing of infected plants. Any plants found should be tested for race identification and, if found to be a non-indigenous, the field should not be used for sunflower production.

Links: <http://www.ext.nodak.edu/extpubs/plantsci/rowcrops/eb25w-6d.htm>
<http://www.eppo.org/QUARANTINE/fungi/Plasmopara_halstedii/PLASHA_ds.pdf>
<http://www.ars.usda.gov/research/publications/Publications.htm?seq_no_115=155267>
<http://www.bspp.org.uk/icpp98/6/135.html>
<http://www.inra.fr/Internet/Produits/HYP3/pathogene/6plahel.htm>
- Mod.DH]