Bacterial Diseases of Wheat
1. Bacterial Lead Blight of Wheat:
Causal Organism:
Pseudomonas syringae pv. syringae is a bacterium that causes bacterial leaf blight, which can affect both dicots and small grains. This microbe is prevalent worldwide.
Symptoms:
- After plants reach the boot stage, symptoms of bacterial leaf blight appear on the topmost leaves.
- Small water-soaked lesions, less than 0.04 inches in size, gradually enlarge and merge together.
- The affected areas become necrotic, and their color changes from gray-green to tan-white.
- In humid conditions, slimy droplets may form on the leaves.
- Although the entire leaves may die, the heads and glumes usually do not show any symptoms of the disease.
Life Cycle of Pathogen:
Bacterial leaf blight is a rare occurrence in small grains within the High Plains region, typically linked to sprinkler irrigation during cool (60 to 77ºF), humid, and wet conditions. The bacteria can survive in seeds, plant debris, and pathogenically on alternate hosts. They can also exist epiphytically on alternate hosts and weeds. The pathogen can spread quickly through irrigation water.
Management strategies:
- To prevent the onset of bacterial leaf blight, it is recommended to use the pathogen-free seed for planting.
- Overhead irrigation, especially during cool and humid weather, should be avoided.
- Reusing irrigation tail water should also be avoided. It is advisable to irrigate less frequently to maintain a low relative humidity level.
- Planting locally adapted cultivars with moderate to high resistance levels is also a good approach to avoid the disease.
2. Bacterial mosaic of wheat:
Causal organism:
The cause of this disease is a Gram-positive bacterium known as Clavibacter michiganense subsp. tessellarius, which was previously called Corynebacterium michiganense. The pathogen is identifiable by its characteristic orange, mucoid colonies with smooth borders on particular media. Additionally, the pathogen can infect triticale as well.
Symptoms:
- Bacterial mosaic disease in wheat is identified by an irregularly spread foliar mosaic.
- The yellow lesions with indefinite margins are evenly distributed throughout the foliage, resulting in the mosaic effect.
- In ideal moist conditions, the flag leaves can be affected, leading to reduced yield.
Inoculum and conditions:
The bacteria surviving in host residue, glumes, and seeds serve as the primary source of inoculum for the disease. Seed infection can be detected through seeding kernels on specific media in the laboratory. Moist conditions provide favorable conditions for the growth and spread of the pathogen.
Control:
- To minimize the risk of bacterial mosaic disease, it is recommended to use pathogen-free seeds for planting.
- Some cultivars may exhibit resistance to the disease, but no completely resistant cultivars are currently available.
- Nonetheless, it's worth considering cultivar-associated responses while making planting decisions.
3. Bacterial sheath rot of wheat:
Causal organism:
Bacterial Sheath Rot is a disease of wheat caused by the bacterium Pseudomonas fuscovaginae.
Symptoms:
- During the booting stage of bread wheat, durum wheat, and triticale, angular blackish-brown lesions, measuring 10-20 cm in length, can be observed on the leaf sheaths.
- These lesions are surrounded by a 1-2 mm wide angular area that appears purple-black and has a grayish center.
Transmission:
The pathogen responsible for this disease can survive in infected seeds, plant residues (such as straw and stubble), soil, water, and even weeds under favorable environmental conditions. The fungus can infect plants at various growth stages, entering through stomata or wounds, and causing significant damage to the plant, particularly after the booting stage, although it can infect other growth stages as well. Insects and mites can damage the plant, facilitating the pathogen's entry, while other pathogens can also weaken the plant. It is suspected that secondary infections may occur through injured tissues and be wind-borne. Although less is known about the seed-borne transmission of the disease, yield losses can range from 20% to 85%, depending on the specific pathosystem conditions.
Management Practices:
To minimize the risk of this disease, it is recommended to take the following measures:
- Clean the field immediately after harvest and avoid off-season cultivation of a crop. Remove plant litter and re-growths.
- Adjust the sowing time to avoid low temperatures.
- Use seedlings that are 20-30 days old, rather than older seedlings.
- Use clean seeds to minimize the risk of seed-borne transmission.
- Treat seeds with hot water at 65°C to further reduce the risk of seed-borne transmission.
4. Basal Glume Rot of Wheat:
Causal organism:
Basal Glume Rot is caused by the gram-negative bacterium Pseudomonas syringae pv. atrofaciens (also known as Pseudomonas atrofaciens).
Symptoms:
- Initial symptoms consist of small water-soaked lesions that are dark green in color and begin at the base of the glume.
- Diseased glumes appear translucent when held up to the light.
- Eventually, glume lesions turn dark brown to black, typically covering the lower third of the glume but may extend to the rest of the glume, kernels, and rachis under favorable conditions.
- Symptoms on affected grains vary from light brown to charcoal black at the germ end.
- Other symptoms that have been linked to the disease, such as dark discolorations on culms, streaks on glumes, and irregular water-soaked lesions on leaves, may be coincidental and not directly associated with the pathogen.
- Some of these symptoms resemble those of black chaff (also bacterial in origin), false black chaff (genetic melanism), glume blotch (Septoria nodorum blotch), and frost damage.
Inoculum source and condition:
The pathogen can be transmitted through infected seeds, as well as by surviving on crop residues, soil, and various grass hosts. It can be dispersed through windblown dust, rain splash, and insects. Wet and mild weather conditions during the heading and grain-filling stages promote disease development and spread. The pathogen can persist on the plant surface until conditions become favorable for infection.
Management:
- To reduce the incidence of the disease, it is recommended to avoid overhead irrigation during the heading stage and minimize moisture.
- Planting clean seeds can help decrease the source of inoculum, and incorporating crop residues through plowing and rotating with non-host crops can also contribute to reducing the source of inoculum.
5. Bacterial strip or black chaff:
Causal organism:
Symptoms:
- Black chaff affects the glumes, while bacterial stripe primarily affects the leaves and leaf sheaths.
- The initial symptoms of both diseases are narrow chlorotic lesions or stripes that appear waterlogged.
- In rainy or dewy conditions, yellowish exudate droplets may be visible, which eventually dry to form crusty droplets or a translucent film on affected tissues.
- Severe infections can cause sterility or death of leaves or spikes, particularly if the plant is infected early in its growth cycle.
- The film on affected tissues may also crack and appear scaly.
Disease development:
The bacterium responsible for bacterial stripe and black chaff can persist on crop residues in the soil and be transmitted through seeds. It is able to survive in both warm and freezing temperatures. In order for the disease to spread, free moisture is necessary. Infection occurs through stomata or damaged epidermal tissue and can be spread through splashing rain, contact between plants, and insect vectors.
Condition for disease outbreak:
Several factors can increase the susceptibility of crops to bacterial stripe and black chaff, including warm temperatures, wet conditions with high humidity, frost damage, and ice nucleation. Physical injury from sources such as hail or sandblasting can also make plants more vulnerable. In addition, root rot and leaf spot diseases caused by fungi, as well as insects, can cause injury and potentially transmit the bacterium to the plant.
Management:
- The optimal way to manage black chaff is by using certified, pathogen-free seeds.
- Seed producers are encouraged to test their seeds for black chaff before planting.
- To reduce primary inoculum, it's also recommended to control volunteer cereals and grassy weeds.
- Proper irrigation management is crucial to creating an unfavorable environment for disease development and spread.
- It's important to ensure that the plant canopy is completely dry between irrigation cycles.
- Currently, highly resistant wheat cultivars are not available; however, highly susceptible cultivars should be avoided.
Note:
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