Effect of Water Flooding on Survival of Leptosphaeria biglobosa ‘brassicae’ in Stubble of Oilseed Rape (Brassica napus) in Central China

Blackleg (Phoma stem canker) caused by Leptosphaeria maculans and L. biglobosa is an economically important disease on oilseed rape and many cruciferous vegetables. Oilseed rape–rice rotation is a routine cultivation practice in central China. This study was conducted to assess the effect of flooding on survival of L. biglobosa ‘brassicae’ in the stubble of winter oilseed rape (Brassica napus). Basal stems with typical blackleg symptoms were collected and cut into small pieces (2 cm) that were either submerged in water at 16 and 20, 20 and 28, 28 and 33, and 33 and 40°C (12 and 12 h) or kept dry at room temperature (control). Moreover, in a field experiment, the stem pieces were placed on the soil surface in a rice field or in a cotton field and either flooded in water or not flooded, respectively. After 1, 2, 4, 6, and 8 weeks, the stem pieces were sampled for retrieval of L. biglobosa ‘brassicae’ on V8-juice agar and for determination of dry weight. Selected L. biglobosa ‘brassicae’ isolates from the stem pieces were identified by polymerase chain reaction (PCR). Results from the two experiments showed that, compared with the controls, flooding for 1 to 2 weeks substantially reduced recovery of L. biglobosa ‘brassicae’ and flooding for 4 weeks resulted in negligible recovery of L. biglobosa ‘brassicae’. All of the 99 selected isolates produced a 444-bp DNA fragment in the PCR, confirming that they belong to L. biglobosa ‘brassicae’. Results also indicated that flooding caused rapid decomposition of the stem pieces. After flooding for 8 weeks, the dry weight of the stem pieces was reduced by 28 to 42% in the laboratory experiment and by 26 to 36% in the field experiment. These results suggest that oilseed rape–rice rotation is probably an efficient way to reduce longevity of L. biglobosa ‘brassicae’ in stubble of winter oilseed rape in central China.