What pesticide to use for corn stalk rot? Detailed control methods
Corn stalk rot is a disease caused by a variety of pathogens infecting alone or in combination, mainly damaging the stems and leaf sheaths of corn plants. Generally speaking, if the diseased part rots but has no foul odor, it is mostly caused by saprophytic fungi; if it is accompanied by a foul odor, it is mostly caused by bacteria, or by a mixed infection of saprophytic fungi and bacteria. So what pesticide should be used when corn gets stalk rot? What are the control methods?
Symptoms and cause analysis of corn stalk rot
Stalk rot generally begins to occur from the grain-filling stage of corn, with the milk-ripe to dough stage being the peak period of symptoms. The pathogen invades from the root system and spreads and expands within the plant. In most diseased plants, the primary roots and secondary roots become necrotic, turning red or green; especially after rain followed by sudden clear weather, wilting and green wilt become more obvious.
Corn stalk rot appears as basal stalk rot at the seedling stage and as green wilt at the maturity stage, so it is also called corn green wilt.
1. Corn basal stalk rot (corn green wilt)
(1) Symptoms. Corn basal stalk rot mainly infects the roots and base of the stem of corn. The pathogen invades from the root system and spreads and expands within the plant. When roots or the base of the stem are cut open and examined at the seedling stage, the xylem is found to have turned brown; in severe cases, the primary roots and secondary roots become necrotic and rot, causing the roots and stem base to rot, leading to lodging or death of the whole plant. The aboveground part shows yellowing and browning at the edges of the middle and lower leaves, and in severe cases the leaves of the whole plant wither and turn yellow. Corn basal stalk rot has its peak symptom period from the late milk stage to the dough stage. Diseased plants first show wilting, the leaves of the whole plant suddenly fade, lose luster, and lose water from bottom to top, turning grayish green and drying up, showing green wilt symptoms. Some diseased plants show acute symptoms, that is, the whole plant suddenly wilts green at the late milk stage or dough stage, without an obvious process of gradual development from bottom to top; this situation is more commonly seen after rain followed by sudden clearing. From the first appearance of diseased plants to symptoms on the whole plant generally takes about 1 week, as short as only 1-3 d, and as long as more than 15 d. The roots and stem base of diseased plants are hollow and softened, and are prone to breaking and falling in wind; after splitting open, the pith tissue of the stem can be seen to have turned brown and decomposed, with fewer fibrous roots, and the plant is easy to pull up; the ear droops, the ear stalk is tough and not easy to snap off, the kernels are lusterless and grayish, loosely arranged and easy to thresh.
(2) Cause analysis. Corn basal stalk rot is an important soilborne fungal disease caused by several species of Fusarium or several species of Pythium that damages corn roots and the stem base. It is a systemic infection disease mainly carried by soil and infecting primarily the main roots. The pathogen overwinters on diseased residues in the soil and invades the plant through stomata or wounds the following year. Disease at the seedling stage is mainly due to no-till sowing of corn into stubble, which leads to soil compaction and poor permeability, or due to poor fertility, overly dry or overly wet weather, or excessive herbicide residues in the soil, all of which easily cause poor root development of seedlings, weak growth vigor, and reduced disease resistance; in the middle growth stage of corn, growth is rapid and tissues are tender, making it also susceptible to disease; the occurrence of green wilt near corn maturity is related on the one hand to the disease resistance of the variety itself, and on the other hand closely related to the climatic conditions at the time. In years with high temperature and abundant rain, especially when heavy rain suddenly turns to clear weather, the disease may become epidemic. This is because from the middle grain-filling stage to the dough stage, continuous overcast rain, insufficient light, and heavy overcast followed by sudden clearing are the most favorable conditions for the occurrence of green wilt; high humidity from water accumulating between leaf sheaths at the stem base after rain also makes disease occurrence easy; excessive nitrogen fertilizer application, excessive dense planting, field closure, poor ventilation and light transmission, and various injuries to plants all aggravate the disease; in continuous corn cropping fields, the amount of pathogenic bacteria accumulated in the soil is large, and the disease is severe; low-lying waterlogged corn fields are also extremely prone to disease occurrence.
2. Corn bacterial stalk rot
(1) Symptoms. Corn bacterial stalk rot begins to occur when corn has more than 10 leaves, before the large trumpet stage. First, irregular water-soaked lesions appear on the leaf sheaths and stems of the middle and lower parts of the plant. During infection of the stem and young leaves, the pathogen causes necrosis and rot of the growing point tissue, the young leaves lose green and wilt and are easy to pull out, and finally the tissues of the diseased plant soften and rot and give off a fishy foul odor. Diseased plants easily break at the diseased part and cannot tassel or set seed; generally the incidence rate is equivalent to the loss rate.
(2) Cause analysis. Corn bacterial stalk rot is a highly damaging disease newly discovered in recent years, caused by heavy rain followed by sudden clearing. Corn bacterial stalk rot is a destructive disease caused by bacterial infection. The pathogenic bacteria overwinter on diseased residues in the field and become the main source of primary infection the following year. In the field, the pathogen spreads with airflow and wind and rain. The pathogen mainly invades through wounds and between leaf sheaths. High temperature, high humidity, continuous overcast rain, hot-water irrigation, or sultry weather after rain are favorable conditions for the occurrence and spread of the disease. The disease begins to occur at 28-30°C and relative humidity above 80%, spreads fastest at 34-35°C, and stops developing when the temperature drops below 26°C. In early to mid-July, the weather in our city was hot and rainy, humid and sultry, with water accumulating in leaf sheaths, which was most favorable for pathogen infection. When corn is 60 cm tall, the tissues are tender and this disease is likely to occur; wounds caused by pest damage also favor pathogen invasion. In addition, pests carry the pathogen while also playing a role in transmission and inoculation; for example, when populations of pests such as corn borer and cotton bollworm are large, the disease is severe. Continuous corn cropping fields have severe disease due to the presence of large numbers of bacteria in the soil. Low-lying land or poor drainage, excessive density, poor ventilation, excessive nitrogen fertilizer application, and many wounds lead to severe disease. In fields with crop rotation, good drainage, and an appropriate ratio of nitrogen, phosphorus, and potassium fertilizers, plants are robust and the incidence rate is low.
Control methods for corn stalk rot
In the face of corn stalk rot, we should adopt comprehensive control measures such as selecting resistant and tolerant varieties, strengthening cultivation management, and focusing on pesticide protection.
1. Remove diseased plant residues in the field. For seed-production corn, promptly pull out diseased female and male plants at the tasseling stage. After corn harvest, thoroughly remove diseased plants and burn them in a centralized manner or bury them deeply in combination with deep plowing of the land.
2. Crop rotation. Implement rotation of corn with other non-host crops to prevent the accumulation of soil pathogens. Severely affected plots can be rotated for 2-3 years with rice, potato, or vegetable crops.
3. Rational fertilization. At the jointing stage or booting stage of corn, increase potassium fertilizer or use phosphorus and nitrogen fertilizers in combination. In potassium-deficient plots, generally apply 7.5-10 kg of potassium chloride per mu, or 25-30 kg per mu of ternary compound fertilizer. Among cultivation factors, rational application of nitrogen, phosphorus, and potassium can enhance plant growth and improve disease resistance; potassium chloride in particular can reduce the occurrence of stalk rot.
4. Rational dense planting. Maintaining field ventilation and light transmission and good soil permeability can reduce or delay disease occurrence. When stalk rot occurs, promptly remove the lowest 2-3 leaves of the stem, clear away diseased plants and old leaves, and burn them in a centralized manner.
5. Strengthen cultivation management. Open drainage ditches well, lower the groundwater level, drain promptly in the rainy season, and achieve no standing water after rain stops; hill up soil in stages, cultivate and loosen soil promptly, and avoid various injuries.
6. Control pests in time. Control aphids, small brown planthoppers, corn borers, and underground pests well, eliminate the routes by which pests transmit viruses and pathogens, and prevent pathogens from entering through pest wounds and harming plants.
7. When disease occurs at the stem base, promptly scrape away the soil hilled around the stem base to reduce humidity and reduce infection, and after the peak disease period has passed, hill the soil back up. Also promptly pull out severely diseased plants, burn them or bury them deeply to avoid spread.
8. Chemical control. In the early stage of disease, spray for control with 50% carbendazim wettable powder at 500 times dilution, or 65% mancozeb wettable powder at 500 times dilution, or 70% chlorothalonil wettable powder at 800 times dilution, or 20% triadimefon emulsifiable concentrate at 3000 times dilution, or 50% benomyl wettable powder at 1500 times dilution. In the middle stage of disease, drench the roots with 98% hymexazol at 2000-3000 times dilution.