Rapeseed Pest Area Hits 26.51 Million Mu; Sclerotinia Up 54.3%

At present, winter rapeseed in southwestern China and southern Jiangnan is at full flowering; northern Jiangnan and the middle and lower Yangtze region are mostly between the bud/bolting stage and early flowering, while Shaanxi and Henan are at the bolting stage. Because winter temperatures across most of the country were higher than the long-term average for the same period, rapeseed pests and diseases appeared earlier this early spring and are more severe than in a normal year. As of 5 March, the national affected area of rapeseed pests and diseases was 26.51 million mu, up 15.1% year on year.

I. Occurrence dynamics

(1) Sclerotinia stem rot — national affected area 6.71 million mu, up 54.3% year on year. Affected by higher-than-normal winter and recent temperatures, apothecia germinated early in most regions and disease onset was clearly earlier than usual. All major rapeseed-producing areas have now entered the stage of rapidly increasing leaf infection, and some areas of Anhui have begun stem infection. In Hubei's Jianghan Plain, apothecia were first observed on 31 January, 20 days earlier than usual; in a mid-to-late February survey, 37.5% of fields had apothecia, averaging 9.8 apothecia per square metre, with a maximum of 103; the province-wide weighted average rate of diseased leaves was 2.1%, with a maximum of 21%. In Sichuan, disease occurred in 73 counties (cities, districts) across 17 cities (prefectures) including Guangyuan and Chengdu, 19 more counties than a year earlier, with a cumulative affected area of 988,000 mu. In Chongqing municipality, the average rate of diseased plants was 6.1%, up 1.6 percentage points year on year, and the highest rate in local fields of Zhongxian reached 36%.

(2) Downy mildew — occurring mainly in the southwest, Jiangnan and the middle Yangtze reaches, with a national affected area of 9.18 million mu, unchanged from a year earlier. Sichuan, Hunan and Hubei, the provinces with larger affected areas, each exceeded 1 million mu, with infection more severe on old leaves at the base. In Hunan, the general rate of diseased plants was 4.2%–20.7%, averaging 7.1%, with a maximum of 53.7%. In Hubei, the general rate was 1%–40%, averaging 10.0%, 3 percentage points higher than a year earlier. In Chongqing, the average rate was 7.8%, up 0.6 percentage points year on year, with a maximum of 45% in local fields of Qijiang. In Jiangxi, the rate of diseased plants was generally 2.4%–8.5%, with a maximum of 30%; the rate of diseased leaves was generally 0.6%–5.5%, with a maximum of 15.6%. In Zhejiang the disease was widespread; a survey on 26–28 February found an average rate of diseased plants of 25%, lighter than a year earlier.

(3) Aphids — widespread in the main winter rapeseed-producing areas, with a national affected area of 10.08 million mu, up 11.6% year on year. In Dehong, Yunnan, the highest count reached 28,600 aphids per 100 plants. In Chongqing, the average was 111 aphids per 100 plants, 1.2 times the level a year earlier, with a maximum of 1,890 per 100 plants in local fields of Shizhu. In Jiangxi, aphids were clearly more severe than a year earlier: the rate of infested plants was generally 13%–19%, up to 30%, and the count per 100 leaves was generally 380–640, up to 4,500, an increase of 38.9% year on year. In Anhui, the rate of infested plants was generally 2.7%–19%, with 13–180 aphids per 100 plants, averaging 79.8, an increase of 90% over a year earlier. In Zhejiang, occurrence was lighter but still heavier than a year earlier; in most areas the average was below 40 aphids per 100 plants, while in parts of northern areas such as Hangzhou and Jiaxing the maximum exceeded 3,000 per 100 plants.

(4) Other pests and diseases — viral disease occurred on 250,000 mu in the middle Yangtze reaches, down 16.7% year on year; beetles occurred to some degree in Shaanxi, Hubei, Yunnan and Hunan, with higher counts in parts of Shaanxi, affecting 290,000 mu nationwide, up 81.2% year on year.

II. Occurrence trend

According to the National Climate Centre, over the next 10 days temperatures in most winter rapeseed-producing areas will be above or close to normal, with slightly more rainfall than usual, and weather conditions will be generally favourable for the occurrence of rapeseed pests and diseases. In southern South China, temperatures will be 1–2°C above normal, favouring drought-loving pests such as the rapeseed aphid; in Jiangnan and northern South China, precipitation will be slightly above normal, with 50–70 mm in some areas and 80–120 mm locally, favouring the spread and epidemic of sclerotinia stem rot and downy mildew. Pests and diseases are expected to enter their peak period of damage in mid-to-late March, and sclerotinia stem rot in Jiangnan and the middle and lower Yangtze region is expected to reach moderate or higher levels. Localities should closely monitor local weather conditions, strengthen surveys and monitoring, and do a good job of early warning and forecasting, so as to provide effective information for timely prevention and control.