Common corn diseases, corn disease prevention and control

How should we prevent and control diseases if we often encounter them when growing corn?

1. Extent of damage and loss

Occurrence: The occurrence area of ​​corn ear rot in Zhuanglang County varies with the planting area of ​​susceptible varieties and the year. In 2003, the diseased area was 3500 hm2, and the diseased ear rate was 46%; in 2004, the diseased area was 3000 hm2, and the diseased ear rate was 26.5%; in 2005, the diseased area was 4500 hm2, and the diseased ear rate was 44.3%; in 2006, the diseased area was 5300 hm2, and the diseased ear rate was 23.9%.

Harmfulness: The diseased kernels are dark brown or reddish brown, the weight of 100 kernels is reduced by more than 1/2, and the quality deteriorates. It can cause poisoning reactions when consumed by humans and animals [5]. Experimental observations show that when 20% of the diseased kernels are mixed into the raw grain, the human taste is bitter and sour, and excessive consumption can cause weakness in the limbs, fever, nausea, vomiting, abdominal distension, abdominal pain, dizziness, etc.; when fed to livestock such as horses, mules, donkeys, and pigs, diarrhea, refusal to eat, and slow growth rate can occur; when fed to sheep and chickens, the appetite decreases, and the egg production rate of hens decreases; if raw grains contain diseased kernels, the commercial value decreases by 0.2-0.4 yuan/kg, and if seeds contain diseased kernels, the sowing amount increases by 100%.

Loss degree: In order to determine the economic losses caused by ear rot, Shendan 10 was classified according to the size of the ear and the degree of disease, and the diseased ears were divided into 5 levels. The length of the ear at each level, the weight of the disease-free ear (w), and the weight of the healthy grains in the diseased ear (y) were measured. The loss of each level was calculated using the formula loss rate (%) = [(wy)/w] × 100. The calculation results showed that the loss rates of the 0, 1.2.3.4 levels after the ears were damaged were 0, 18.4%, 27.3%, 45.0%, and 68.2%, respectively.

Symptoms: Corn ear rot can occur in the field from seedling to maturity. The most typical symptoms are seed rot, weak seedlings, stem rot, and ear rot. Among them, ear rot causes the most serious economic losses:

Seed rot and weak seedlings: The pathogens contaminate the seeds and adhere to their surface. After sowing, those that are severely damaged will fail to germinate and will rot, resulting in missing seedlings and broken ridges. Those that are less damaged will grow slowly and thin after germination, forming weak seedlings.

Ear rot: In the early stage of field infection, the filaments of the ear are dark brown and water-soaked, the top of the ear axis and the grains turn yellow-brown, pink or dark brown, and spread to 1/3-1/2 of the ear. When it is rainy or humid, it can spread to the entire ear. The surface of the diseased grains has a grayish white or light red mold layer, white flocculent or velvety, the ear is soft, the ear axis is dark brown, the pith is light yellow or pink, and the vascular tissue is exposed when broken.

Pathogen: Through microscopic examination of different parts of diseased corn ears collected in Zhuanglang County, and isolation and microscopic examination of pathogens according to the isolation and culture methods of fungi [5.9], it was found that the pathogens were typical Fusarium conidia. Compared with relevant literature data, comparative identification [1-8], it belongs to the class of Ascomycetes, the family of Tuberosporea, the genus Fusarium, and the wheat fusarium (F.graminearum Schw). The sexual stage belongs to the class of Ascomycetes, the order of Hypocreales, and the genus Gibberella. It is a facultative parasite with a wide host range. It harms wheat, corn and other grass plants, causing seedling blight, stem rot, base rot and ear rot.

Disease pattern: The pathogen can invade and cause damage from the seedling stage to the seed storage stage of corn, and the loss of mildew and rot occurs during the harvest and drying of the ears. The pathogen overwinters in the form of mycelium, conidia or ascospores attached to seeds, corn stubble, stalks, cobs and other plant diseased residues. In the following year, under rainy and humid conditions, the ascospores mature and fly away, landing on the corn silks as facultative parasites, and then invading the cobs and grains through the silks to cause ear rot. The incidence of ear rot is affected by many factors such as variety, climate, corn borer damage, agronomic activities, and storage conditions of ears (raw grains, seeds).

There are great differences in disease resistance among corn varieties: Zhongdan No. 2, Jiudan No. 4, and Jiudan No. 2, which are widely planted in Zhuanglang County, have strong disease resistance, with a diseased ear rate of 0-3%, while Shendan 10 (combination Q1261/Shen137) and Shendan 16 (combination 137/K12) are not disease resistant, with a diseased ear rate of up to 31.2%-46%. Inbred line 478 (combination 8112/5003) and its hybrids Yedan 12 and Yedan 13 are highly susceptible to the disease, with a diseased ear rate of about 50%. It was observed that the susceptible variety "green stalk matures" has many filaments in the ear, long and thick bracts, closely arranged grains, high water content in the cob, and slow water loss; while the resistant variety has few filaments in the ear, thin bracts, and exposed grains on the top. It has matured and drooped before harvest, and it is not easy for rainwater to enter. In addition, the incidence of the 2nd and 3rd ears is significantly higher than that of the 1st ear, with a high disease level and a large degree of loss.

The rainfall and rainy days from September to October are the key to the disease: due to the continuous autumn rain during the corn harvest period and after the harvest, farmers cannot harvest, peel and dry the ears in time, which provides good humidity conditions for the occurrence and development of the pathogen. The relationship between the disease severity (y) and rainfall (x) of Shendan 10 from September to October 1999 to 2005 was analyzed by regression statistics: y=-0.751+0.311x, and the correlation coefficient r=0.908>P0.01=0.874, indicating that there is a very significant correlation between x and y.

Temperature conditions are suitable for the growth and development of pathogens: According to the data [2.4-5]: The growth and development temperature of wheat fusarium is 4-32℃, and the relative humidity of the air is 80%-100% for the best development; when the humidity conditions are suitable, conidia can be produced at around 8℃, and the fastest production speed is at 25℃. When the temperature is below 4℃, the germination speed is extremely slow, and it takes at least 1 day; the minimum temperature for the formation of ascospores is 9-10℃, and the temperature range for ascospore germination is 4-35℃, with 25-30℃ as the most suitable temperature. In Zhuanglang County, from the end of September to the beginning of November, the corn matures and the ears are dried. The average temperature in this period is 12.6-6.5℃ every ten days, which can meet the growth and development of pathogens. If the ears are piled together, the heat generated during the respiration process is more conducive to the reinfection of pathogens and the expansion of the disease.

The plots seriously damaged by corn borers have serious disease: In the survey, it was found that in the fruit ears or stalks damaged by corn borers, ear rot and stalk rot occurred simultaneously, resulting in heavy economic losses. According to statistical analysis, the incidence rate of ear rot (%, y) and the rate of corn borer plants (%, x) showed a y=axb power function curve relationship [12.13], y disease = 1.3712x0.9549 insects, F = 323.82**> F0.01 = 10.04, and the regression relationship was extremely significant; the incidence rate of ear rot (%) between corn borer control and no control was tested by t, t = 5.589**> df0.01 = 3.106, which was extremely significant. This is because the excrement discharged by corn borers contaminated the stalks and ear axes, providing a favorable place for the growth of pathogens.

The degree of disease is related to the planting method and sowing period: According to the survey, the disease rate of Shendan 16 planted with mulch film was 18.5% in the first ear and 31.3% in the second ear, which were 16.1 and 19.2 percentage points lower than those in the open field respectively; the disease rate of Shendan 10 planted before "Grain Rain" and with full seedlings once was 21.7%, which was 21.6 percentage points lower than that of seedlings planted after "Beginning of Summer". In summary, the reason for the low disease rate is that mulch film covering and early sowing at the right time can advance the maturity of corn and reduce the moisture content of the cobs and grains of susceptible varieties.

Prevention and control measures

The initial infection sources of corn ear rot are wide, and humidity is the key. Therefore, the prevention and control strategy must be based on agricultural measures, make full use of disease-resistant (tolerant) varieties, improve storage conditions, and use comprehensive prevention and control measures that combine pesticide heart injection with spraying protection.

1. Select disease-resistant parents (seed production) or varieties

It is recommended that scientific research institutions and seed production enterprises select parents and combinations with excellent resistance to ear rot, establish disease-free seed production bases, and cultivate healthy seeds. At the same time, county and city seed companies and agricultural technology centers should list resistance to the disease as the focus of selection in the introduction test of new corn varieties, and varieties with poor resistance should not be introduced; Zhuanglang County should actively introduce new high-yield and disease-resistant varieties on the basis of promoting disease-resistant varieties Zhongdan No. 2, Jiudan No. 2, and Jiudan No. 4.

2. Mulch film covering and early sowing at the right time

Full or half coverage with mulch and early sowing at the right time can make corn mature earlier, reduce the moisture content of the cobs and grains of susceptible varieties, and effectively reduce bacterial infection during harvest and storage.

3. Remove the bracts in time to prevent rain and moisture

There is a lot of autumn rain during the corn harvest period. The harvested ears should not be piled too thickly. The husks should be peeled off early, tied into bunches and hung in a ventilated sunny place to dry. The ears that cannot be tied into bunches should be spread out thinly for drying and turned over frequently to prevent heat and disease. If it rains, cover them in time to prevent rain. Break off the tops of diseased ears to prevent the spread of ear rot.

During the process of peeling the husks, if diseased ears are found, the top of the rotten ear should be broken off at the junction of the diseased and healthy parts to prevent the disease from spreading further and increasing losses. According to experimental observations, if the ears are exposed to rain and the husks are not peeled off and piled for 3 days, the length of the rotten top of the ear will expand from 3cm to 7cm; if the rotten part of the top of the diseased ear is not removed after peeling the husks, the ear rot will expand from 5cm to 15cm in the early stage of threshing; if the rotten part of the ear is not removed thoroughly, the ear rot will expand by 2-5cm in the early stage of threshing; if it is removed thoroughly, the ear rot will not develop.

4. Early threshing and mildew prevention

After harvest, the diseased ears should be picked out, threshed as soon as possible, and dried in the sun or in a pit to prevent the grains from further infection by pathogens and mildew.

Process corn stalks to reduce the source of initial infection.

The accumulation of corn stalks, cobs, and stubble is a favorable place for Fusarium and corn borer to overwinter. Therefore, corn stalks, cobs, and stubble must be thoroughly treated in a timely manner by feeding (feeding livestock), ammoniation (ammoniation of forage), powder (crushing and feeding pigs), fermentation (fermentation or as biogas filler), and burning (burning pits for cooking) to reduce the initial infection source of pests and diseases.

5.Seed selection and coating

Because the survival time of pathogens on the surface of corn seeds is more than one year[5], production and operation units must carefully select seeds before supplying them, remove small and diseased seeds, and coat them with 20% Fok seed coating agent[14], with a dosage of 444.4-800g per 100kg of seeds, or with 30% Do-ke Fok seed coating agent, with a dosage of 214-300g per 100kg of seeds.

6. Chemical control

During the corn bell stage, 5kg of washed river sand with a diameter of about 2mm, 8-10ml of 20% cypermethrin, and 50g of 50% carbendazim WP are mixed evenly to make granules. The dosage of 60-75kg per hm2 is poured into the center of the corn heart leaf and the gap between the 4-5 leaves that make up the heart leaf cluster. Avoid applying the medicine when dew condenses and leaves curl. According to the test, the heart filling not only has a 100% control effect on corn borers, but also has a control effect of more than 90% on corn ear rot, armyworms, and aphids. At the same time, 50% carbendazim WP 1000 times diluted is applied to the fruit about 15 days before corn harvest.