A Detailed Guide to the Latest Control Techniques and Methods for Corn Head Smut
Corn head smut, also known as ustilago and mute corn, occurs commonly in North China, Northeast China, Central China, Southwest China, South China and Northwest China. Since the disease was first reported in Northeast China in 1919, it has spread rapidly and occurs to varying degrees every year. In China, the northern spring corn region, the southwestern hilly and mountainous corn region and the northwestern corn region are the most severely affected. In ordinary years the incidence rate is 2%-8%, and in individual plots it reaches 60%-70%, causing heavy losses. In the 1980s, corn head smut was basically brought under control, but it is still one of the major diseases of corn production.
Symptoms of corn head smut damage
The typical symptom of corn head smut is deformation of the male floral organs, with the base of the male flowers swollen and filled with a mass of black powder, so that no tassel can form. On affected female ears, the ear becomes shorter and the base thickens; apart from the husk leaves, the entire ear becomes a mass of black powder and scattered filamentous material, seriously affecting corn yield.
1. Seedling-stage symptoms of corn head smut
Corn head smut is a systemic infection disease that invades at the seedling stage. Typical symptoms generally appear at the ear stage, mainly damaging the female and male ears. Severely affected plants may show various symptoms at the seedling stage. Seedlings tiller more and form a clustered shape, the plants are obviously dwarfed, the internodes are shortened, and the leaves are dark green and upright. Farmers describe these symptoms as: "short in height, dense leaves, thick at the bottom, thin at the top, dark leaves, green color, and the body is still bent." On some varieties, yellow-white stripes parallel to the leaf veins appear on the leaves, and on some seedlings the heart leaves are tightly rolled together and curved in a whip-like shape.
2. Adult-plant-stage symptoms of corn head smut
The symptoms on diseased ears at the adult plant stage of corn can be divided into two types, namely smut ears and abnormal deformed ears.
(1) Smut ears. Apart from the husk leaves, the entire ear of a smut-diseased ear becomes a black powder mass, mixed with filamentous host vascular bundle tissue, hence the name head smut. The affected ear is relatively short, thick at the base and pointed at the top, approximately spherical, and does not extrude silks.
(2) Abnormal deformed ears. This is caused by deformation of the tassel floral organs so that no stamens form; the glumes become leafy due to stimulation by the pathogen. The glumes of the female ear may also grow excessively into tubular long spines due to pathogen stimulation, forming a hedgehog-head shape. The base of the long spines is slightly thick and the tip slightly thin, the center is hollow and loose, and the lengths vary; they grow in clusters upward from the base of the ear, making the entire ear deformed.
Transmission routes of corn head smut
The pathogen of corn head smut is Sporisorium reilianum. This fungus mainly overwinters in the soil as teliospores, and some overwinter mixed into manure or adhered to the seed surface. Soil-borne inoculum is the most important source of primary infection, while seed-borne inoculum is the main route of long-distance spread of the disease. Teliospores can survive in the soil for 2-3 years. Teliospores begin to mature during the silking stage of the corn female ear, and large numbers fall into the soil, while some fall onto the seeds (especially at harvest). After sowing, they generally infect the germ when the seed germinates or when the seedling has just emerged, and some infection also occurs at the 2-3 leaf stage (some reports consider the final infection stage to be the 7-8 leaf stage). Teliospores germinate to produce septate basidiospores, and the basidiospores germinate to form infection hyphae, which invade from the germ or radicle and quickly extend to the stem and grow along the growing point. When the flower buds begin to differentiate, the hyphae enter the floral primordium, invade the female and male ears, and finally destroy the male and female flowers. Because the corn growing tip grows relatively quickly while the hyphae spread relatively slowly, the hyphae fail to enter the stem growing point of the plant, which causes the phenomenon that some diseased plants only have diseased female ears while the male ears are disease-free.
The pathogen overwinters in soil, manure or on seeds, becoming the primary infection source for the following year. Seed-borne inoculum is the main route of long-distance spread. Chlamydospores survive in the soil for 2-3 years. Invasion at the seedling stage is a systemic infection disease. When corn germinates after sowing, the overwintered chlamydospores also begin to germinate, and can invade from the white-tip stage to the 4-leaf stage of corn, reaching the growing point and, as the corn plant grows and develops, entering the flower buds and ear parts, forming large amounts of black powder to become head smut and producing large numbers of teliospores to overwinter. Long-term continuous cropping of corn and early sowing of corn result in more severe disease; high, cold and cool plots are prone to the disease. Sandy loam soil has mild disease. Dryland with good soil moisture has mild disease; poor soil moisture has severe disease.
Conditions for the occurrence of corn head smut
1. Large-scale planting of susceptible varieties is one of the factors leading to severe occurrence of head smut. In addition, new physiological races of the pathogen may appear, causing originally resistant varieties to lose their resistance.
2. Long-term continuous cropping causes the soil inoculum level to increase rapidly. According to reports, if the diseased plant rate is used to reflect the inoculum level, then the soil inoculum level can increase by about 10 times each year.
3. Use of undecomposed barnyard manure. According to experiments, the incidence rate in fields treated with pig manure was 0.1%, while the incidence rate in fields with furrow application of inoculum-carrying cow manure was as high as 17.4%-23%, and the incidence rate in fields with broadcast application of cow manure was 10.6%-11.1%.
4. Seed-borne inoculum that has not been disinfected and diseased plant residues that have not been properly treated will both increase the inoculum level in the soil and lead to severe occurrence of the disease.
5. Soil temperature and humidity during the period from corn sowing to emergence are extremely closely related to disease occurrence. Soil temperatures within the range of 15-30°C all favor pathogen invasion, with 25°C being the most suitable. Soil humidity that is too high or too low is unfavorable for pathogen invasion, and the incidence rate is highest under 20% humidity conditions. In addition, the higher the altitude, the deeper the sowing, and the weaker the seed vitality, the more severe the disease. The infection temperature range is 15-35°C, the suitable infection temperature is 20-30°C, and 25°C is optimal. A soil water content lower than 12% or higher than 29% is unfavorable for disease occurrence.