How to prevent and control rice neck blast? Master the best application timing and judgment methods
According to recent field monitoring results from rice blast rural monitoring sites across Heilongjiang Province, combined with comprehensive analysis of rice planting varieties, resistance, field growth, cultivation management and meteorological conditions, it is expected that rice neck blast in Heilongjiang Province this year has a risk of severe recurrence. So how to prevent and control rice neck blast? What is the best application timing? How to judge?
Rice neck blast is a common, airborne major disease of rice. During the flowering and grain-filling stage, if climatic conditions are suitable, neck blast is very likely to occur. After infection, as the pathogen reproduces and causes damage, the neck node necroses, the nutrient channel transporting to the ear is blocked, thus forming white ears or shriveled-grain ears. In years when neck blast occurs, yield is generally reduced by 10-20%, and in severe fields by more than 30%, or even total crop failure. Once the neck is infected with rice blast, it greatly affects rice yield and quality.
1. Symptoms of damage
Neck blast mainly occurs on the neck or on the rachis and branches. When the neck is affected, the lesion is brown or grayish-black, spreading upward and downward from the neck, and can reach 3-4 cm in length. When onset is early and severe, it causes white ears; when onset is late and mild, it causes reduced 1000-grain weight or blighted grain. Neck blast often causes the ear stem to break. The symptoms on the rachis branches, branches and secondary branches are similar to those on the neck, but only the spikelets above the lesion are damaged, causing spikelet sterility.
2. Transmission routes and occurrence patterns
The pathogen overwinters as conidia and mycelium on rice straw and rice grain. The following year it produces conidia that spread to rice plants by wind and rain, germinate and invade the host, expanding to neighboring cells and causing disease, forming a center of infection. Conidia formed on diseased parts spread by wind and rain for reinfection. Sowing infected seeds can cause seedling blast. Moderate temperature and high humidity, with the presence of rain, fog and dew, favor disease development. Mycelial growth temperature is 8-37°C, optimum 26-28°C. Spore formation temperature is 10-35°C, optimum 25-28°C, relative humidity above 90%. Spore germination requires the presence of water lasting 6-8 h. Suitable temperature and humidity are needed to form appressoria and produce infection pegs, penetrate the rice epidermis, and spread between cells to absorb nutrients. Under conditions of continuous rain, insufficient sunshine, alternating sun and rain, morning and evening clouds and fog, and dew formation, the disease spreads rapidly.
3. Conditions for onset
3.1 Strength of varietal resistance: Resistance differs markedly among varieties. Most resistant varieties currently used in production are vertical-resistance varieties, whose resistance can only be maintained for a certain period depending on physiological race composition. Once a inferior race becomes the dominant race, some varieties lose disease resistance, and the disease occurs. Generally, indica-type varieties have better resistance than japonica-type varieties.
3.2 Amount of inoculum: If seeds carry much pathogen and are not disinfected by seed soaking, especially if diseased rice straw and diseased blighted grain are not destroyed or are incompletely treated, overwintering inoculum is abundant; once suitable temperature and humidity conditions occur, spread is rapid and onset is severe.
3.3 Harsh meteorological conditions: If continuous rain occurs during the tillering stage, with many rainy days and frequent rain, forming conditions of high humidity, low temperature and insufficient light (few sunshine hours), rice disease resistance is reduced, favoring disease occurrence. If continuous rainy weather occurs around the heading stage, with many rainy days and frequent rain, forming high humidity, low temperature and insufficient light conditions, it is unfavorable to rice growth; but it favors pathogen spore production, spore germination and invasion. Among these, high humidity favors spore germination and invasion; while low temperature and insufficient light conditions make neck blast, node blast and grain blast more severe.
3.4 Weak field management: For example, applying too much nitrogen fertilizer, too late, or frequently irrigating with cold water or deep water, reduces rice disease resistance and can aggravate the disease. During the ear stage, resistance is weakest at the initial heading stage. Excessive or over-application of nitrogen fertilizer favors onset. Long-term deep irrigation causes poor root development, weak disease resistance and severe onset.
4. Misconceptions in prevention and control
Neck blast is rice blast occurring on the neck (the junction where the rachis connects to the ear). The neck is the necessary passage for transporting photosynthate produced by rice leaves to the ear, and is also the part of the rice plant most susceptible to rice blast. In recent years, the reason neck blast has been relatively severe is mainly inadequate prevention and control, with misconceptions.
4.1 Applying pesticide only when white ears appear: When the rice blast pathogen first invades, symptoms generally do not appear, and early symptoms are not obvious. Waiting until the neck necroses and white ears appear to prevent and control is too late. To control neck blast, pesticide is generally applied at the breaking-boot initial heading stage; for blast-susceptible or non-resistant varieties or overly vigorous fields, another application is needed at the full heading stage (when about 80% has headed). Rice blast occurrence is greatly affected by the environment. If conditions at the tillering stage are unsuitable, seedling blast may not be severe; if conditions at the ear stage are suitable, susceptibility may appear. For blast-susceptible or non-resistant varieties, if rainy weather occurs at the ear stage, regardless of the severity of seedling blast, it may cause a major epidemic of neck blast. Therefore, for blast-susceptible or non-resistant varieties, or fields with severe rice blast in past years, even if seedling blast is not severe, prevention and control of neck blast must be strengthened, applying pesticide once each at the breaking-boot initial heading stage and the full heading stage.
4.2 Neglecting pesticide for varieties with stronger blast resistance: There is no absolutely blast-resistant variety; resistance is relative. Although resistant varieties have significantly lower incidence than susceptible varieties, when planted in fields with severe annual onset (old blast spots), or with excessive nitrogen fertilizer, overly vigorous vegetative growth, or continuous rainy weather at the heading stage, they may also develop the disease. For fields with annual onset, even resistant varieties should be timely controlled for neck blast; for fields with excessive nitrogen fertilizer and overly vigorous vegetative growth, whether resistant or susceptible varieties, neck blast control must not be neglected.
4.3 Not applying pesticide when rain occurs at the heading stage: Rainy weather at the heading stage may cause severe onset, and in places with large planting areas of blast-susceptible varieties, it may even cause a major epidemic of neck blast. Therefore, to control neck blast, pesticide should be applied once each at the breaking-boot initial heading stage and the full heading stage, and if rain occurs within 8 h after application, it should be reapplied. Even on rainy days, pesticide should be applied in the gaps between rains. For high-quality rice, varieties or combinations planted for many years, fields with relatively severe seedling blast, rice fields with overly vigorous plant growth, cold-waterlogged fields, and fields experiencing rainfall at the heading stage, pesticide should be applied once each at the breaking-boot initial heading stage and the full heading stage.
5. Prevention and control measures
According to the occurrence characteristics of rice neck blast, the prevention and control method of “prevention first, integrated control” should be adopted.
5.1 Select excellent varieties: Choose high-quality, high-yield varieties (combinations) with good resistance; after continuous planting for several years, replace with resistant varieties in a timely manner.
5.2 Thoroughly eliminate inoculum: For fields where rice blast has occurred, promptly remove or burn diseased rice straw in the field to reduce the amount of overwintering inoculum; use prochloraz, strong chlorine essence, etc. for seed soaking and disinfection to kill pathogens carried by rice seeds, which can also concurrently prevent bakanae disease.
5.3 Scientifically manage fertilizer and water: On the basis of dry-raised strong seedlings, promote 30 cm wide-row cultivation. In fertilizer management, increase basal fertilizer, apply heavy leaf fertilizer, and skillfully apply ear fertilizer. Generally, for fields with a yield of 550-650 kg per mu, apply 12.5-13.5 kg pure nitrogen, of which basal fertilizer and leaf fertilizer each account for 35-40%, and ear fertilizer accounts for 20-30%, and the interval between the last leaf fertilizer and ear fertilizer is 30-40 d. For ear fertilizer, on the basis of mid-term nitrogen control, apply fertilizer when yellowing appears, apply early when yellowing is early, apply late when yellowing is late, apply more on thin land and less on fertile land. In water layer management, from transplanting to the effective tillering stage, maintain a shallow water layer, and pay attention to appropriate drying and replacing with fresh water; drain the field at the appropriate time during the ineffective tillering stage; maintain a shallow water layer during the young ear differentiation stage, with 2-3 small drainings; maintain a 4-6 cm water layer during the heading and flowering stage; after entering the grain-filling stage, adopt the management method of alternating dry and wet, and frequent shallow irrigation.
5.4 Timely chemical control: For fields with seedling blast and leaf blast, susceptible varieties, and old disease areas in low-lying land soaked by floods, spray pesticide once each 2-3 d before breaking-boot and at the initial full heading stage, using 75-100 g of 20% tricyclazole per 667 sq m, or 100 g of 40% Fuji 1, diluted with 50-75 kg water to spray the plants thoroughly; alternating the use of appropriate pesticides can improve control effectiveness.
5.5 Precautions for prevention and control: 1. Rice blast control must adhere to the strategy of “prevention first”, strictly grasp the control timing; applying pesticide at the breaking-boot stage is the key to whether there is a harvest, and applying pesticide at the full heading stage is the measure for how much harvest. 2. Avoid applying pesticide during the flowering stage, and try to choose before 10 am and after 4 pm, because spraying during the flowering stage allows the liquid to easily enter the floral organs, damaging or killing pollen, anthers, ovaries, etc., and the liquid will wash off pollen, seriously affecting pollination and increasing the rice sterility rate; 3. Use sufficient pesticide and enough water, spray the ear layer evenly and thoroughly, and spray the plants thoroughly. 4. For two or more control applications, rotate pesticide varieties to avoid developing resistance. 5. Avoid rain and seize clear weather for application; if rain occurs within 8 hours after application, reapply in the gaps between rains. Avoid