Early rice diseases and pests are expected to occur at a generally severe level.

It is expected that, overall, rice diseases and pests will be relatively severe this year, with a nationwide affected area of 254 million mu‑times, roughly unchanged from last year. Specifically, brown planthopper, stem borer, and sheath blight are forecast to occur at a severe level, while rice leafroller is expected to range from moderate to severe…

On May 12, the National Agricultural Technology Extension Center convened an online consultation meeting on the 2020 national forecast for the occurrence trends of major early rice diseases and pests. Based on a comprehensive analysis of current pest and disease populations, rice cultivation practices, and climatic conditions, technical personnel from major rice-producing provinces in southern China, along with relevant experts from research and educational institutions, projected that overall early rice disease and pest pressure will be relatively severe this year, affecting 254 million mu across the country—roughly the same as last year. Specifically, brown planthopper, stem borer, and sheath blight are expected to occur at a severe level; rice leafroller is anticipated to range from moderate to severe; blast disease will be moderate; and stem borer is expected to be mild. Compared with last year, rice leafroller and stem borer are forecast to be more severe, while brown planthopper, sheath blight, and blast disease remain at similar levels, and stem borer is slightly less severe.

Trend of occurrence

Brown planthoppers are expected to occur at a severe level in the rice-growing regions of South China and southern Jiangnan, at a moderate level in northern Jiangnan and the middle reaches of the Yangtze River, and at a mild level in the southern parts of Southwest China—levels that are comparable to those of the previous year. The nationwide affected area is estimated at 63 million mu‑times.

The rice leafroller is expected to occur at moderate to severe levels in the rice-growing regions of South China and the Yangtze River Basin, with lighter infestations in the middle reaches of the Yangtze. Overall, its incidence is projected to be higher than last year. The nationwide affected area is estimated at 49 million mu‑times.

The rice stem borer is generally experiencing a severe outbreak, with a clear upward trend. Specifically, it is severely prevalent in parts of the Jiangnan region and the middle reaches of the Yangtze River, moderately prevalent in the northern South China rice-growing areas, and lightly prevalent in other regions. The nationwide affected area totals 46 million mu‑times.

The rice stem borer is occurring at a light to moderate level in southern South China, while in other rice-growing regions it is at a light level, with a clear downward trend. The nationwide affected area totals 4 million mu‑times.

Sheath blight occurred at a relatively severe level across most rice-growing areas in South China, the Jiangnan region, and the middle reaches of the Yangtze River, remaining on par with the previous year. The nationwide affected area totaled 59 million mu.

Rice blast occurred at moderate levels in southern South China, the Jiangnan region, and the middle reaches of the Yangtze River rice-growing areas, with localized outbreaks of moderate to severe intensity in hilly and mountainous regions, historically endemic areas, and fields planted with susceptible varieties—levels comparable to those of the previous year. The nationwide affected area totaled 10 million mu‑times.

Southern rice black-streaked dwarf disease has occurred at a relatively mild level in the rice-growing regions of South China, while other pests and diseases such as rice false smut, bacterial leaf blight, and stem borers have been observed to some extent in certain areas, affecting a total area of 23 million mu.

Basis of the forecast

(1) Most pests and diseases have a baseline incidence level of moderate or higher.

Brown planthoppers began migrating into the rice-growing regions of southern China in early March, 4–8 days earlier than last year and close to the long-term average. In late March, a minor immigration peak was detected in southern Guangxi, with peak densities 1.7 times and 3.2 times those of the previous year and the multi-year average for the same period, respectively. Since late April, sudden surge peaks have been observed in the rice-growing areas of South China and southern Jiangnan, with peak populations exceeding levels seen in recent years at this time. At present, field counts in the southern part of South China generally range from 100 to 700 insects per 100 plants, with some plots reaching 1,000 to 2,000; these conditions provide a sufficient pest population base to support a moderately severe or worse outbreak.

The rice leafroller began migrating into the rice-growing regions of South China and the Yangtze River Basin in early March, with the onset of migration a month earlier than last year. Sharp peaks in adult populations were recorded in mid-March, late April, and early May across South China and southern Jiangnan, with migrant numbers exceeding those of the same period last year. At present, both adult and larval densities remain elevated: in South China, adult counts generally range from 200 to 700 per mu, with localized fields in western Guangdong and southern Guangxi reaching as high as 8,000 to 15,000 adults per mu; larval densities typically stand at 1,500 to 4,000 per mu, occasionally climbing to 8,000 to 12,000 per mu, indicating a pest‑source base sufficient to support medium‑to‑high levels of infestation.

The rice stem borer has a relatively high overwintering population in the Jiangnan region and the middle‑and‑lower reaches of the Yangtze River. In Zhejiang, Hunan, and Jiangxi, the average number of surviving larvae per mu ranges from 6,500 to 7,400, up 109%, 29%, and 6% year on year, respectively. In Fujian, Hubei, and Anhui, the average stands at 1,500 to 4,400 larvae per mu, an increase of 15% to 77% compared with the previous year. At present, egg counts in the fields are also elevated: in Jiangxi and Hunan, the average is 507 and 127 egg masses per mu, respectively, with some localities—such as Huichang, Taihe, and Yongxiu in Jiangxi, and Hengnan in Hunan—reporting as many as 800 to 1,800 egg masses per mu, providing a sufficient overwintering source to support a severe or worse level of infestation.

The causal agents of sheath blight and rice blast are abundant in the field. In northern Vietnam, specifically in Lai Chau Province, the southern rice black-streaked dwarf disease has been observed on 120 mu of winter–spring rice, with infected plant rates ranging from 4% to 10%, indicating a measurable reservoir of the pathogen. Consequently, there is a risk of pathogen proliferation during the mid- to late‑stage growth of early‑season rice.

(2) Rice cultivation practices and the increasing resistance of pests and diseases to pesticides are conducive to their occurrence.

This year, the acreage of early-season rice has rebounded, providing ample food for rice pests such as the “two‑migration” pests and stem borers. The dominant early‑rice varieties are primarily high‑quality, high‑yielding types with generally weak disease resistance, posing a significant risk of widespread disease outbreaks. Farmers tend to overapply nitrogen fertilizers, resulting in vigorous plant growth and dense stands that create favorable microclimatic conditions—warm temperatures and high humidity—conducive to the emergence and spread of rice diseases and pests. Moreover, brown planthoppers and the second‑generation stem borer have developed resistance to most commonly used pesticides, compromising control efficacy and exacerbating pest damage.

(3) Climatic conditions are conducive to the outbreak and spread of pests and diseases.

In January–February 2020, the southern rice-growing regions exhibited distinct warm and humid climatic conditions, with above‑normal temperatures and increased precipitation, which favored the overwintering of rice pests such as stem borers. In March, the early‑rice growing areas experienced abundant rainfall and frequent weather events, creating favorable conditions for the northward migration and subsequent settlement of “two‑migrant” pests, resulting in an earlier and larger influx during the early season. According to forecasts from the National Climate Center, during mid‑to‑late May, temperatures in the southern rice‑growing regions are expected to remain above normal, while precipitation in the South China and Jiangnan rice‑growing areas is likely to exceed the long-term average for this period, thereby promoting the northward migration and reproduction of “two‑migrant” pests and facilitating the occurrence and spread of plant diseases.

Source: National Agricultural Technology Center