Puccinia sorghi Virulent on Sweet Corn with the Rp1-D Gene in Southern France

The Rp1-D gene, which conveys a chlorotic-fleck resistant reaction to Puccinia sorghi, effectively controlled common rust on sweet corn in North America for nearly 15 years. Biotypes of P. sorghi virulent on plants with the Rp1-D gene were widespread in North America for the first time in 1999 and again in 2000 (1,2). Many Rp-resistant sweet corn hybrids that are developed and grown in North America also are grown in Europe, including France where virulence against the Rp1-D gene has not been reported previously. In September 2000, uredinia of common rust were observed on and collected from sweet corn hybrids with the Rp1-D gene in commercial fields and hybrid trials in the Landes and Pyrénées Atlantiques departments of the Aquitaine region of southwestern France. Severity of rust generally was below 5% on these plants except for a few hybrids for which severity was about 20 to 30%. Common rust was not observed on hybrids with the Rp-G gene. Urediniospores were increased as a bulk population on the susceptible sweet corn hybrid Sterling in a greenhouse. Plants with each of 10 single Rp genes (Rp1-A, Rp1-C, Rp1-D, Rp1-E, Rp1-F, Rp1-I, Rp1-K, Rp1-L, Rp1-N, and Rp-G) or each of six compound rust resistance genes (Rp1-D5, Rp1-JC, Rp1-JFC, Rp-GDJ, Rp-GFJ, and Rp-G5JC) were assayed for reactions to this population of P. sorghi. Two to six different sources of seed of each single Rp gene and two different sources of seed of each compound rust resistance gene were replicates with a single pot of 6 to 18 plants grown from a specific seed source. Plants were inoculated three times on successive days by placing 2 or 3 ml of a urediniospore suspension in the whorl of two- to four-leaved seedlings. Reactions were rated 10 days after the last inoculation. Plants without symptoms or with chlorotic-fleck resistant reactions were inoculated again and rated 10 days later. Uredinia did not form on plants with compound rust resistance genes. Plants with the genes Rp1-E, Rp1-I, Rp1-K, and Rp-G also were resistant although a few, very small uredinia (i.e., type-1 uredinia) were observed on a few plants. Plants with the genes Rp1-A, Rp1-C, Rp1-D, Rp1-F, Rp1-L, and Rp1-N were fully susceptible. This pattern of virulence is the same as that observed during the past two years in North American populations of P. sorghi virulent against Rp1-D. Rp-resistance currently available in most sweet corn hybrids will not be effective in France if these virulent biotypes become prevalent. References: (1) J. K. Pataky et al. Plant Dis. 85:165, 2001. (2) M. C. Pate et al. Plant Dis. 84:1154, 2000.