Bacterial diseases in plant

Pathogenic bacteria can cause numerous serious diseases in crops and vegetables. They don't enter the plant directly through tissue instead they penetrate through wounds or stomata or hydathodes. Wounds are the result of damage caused by insects, pathogens, and handling during operations such as picking and pruning.

Bacteria only become active and cause disease when the environmental condition is in favor of them to divide and reproduce. Bacteria have the ability to multiply in large colonies. Some factors like high humidity, high moisture poor air circulation, stress caused by over-irrigation, water deficit, irregular altering conditions, poor soil health, and excess or deficient nutrients required by the plant to grow.

Bacteria diseases can be classified into four broad groups depending on the severity of damage to the plant tissue and the severe symptoms that these diseases cause. The symptoms include vascular wilt, necrosis of the leaf, leaf curl, soft rot, and tumors.

Vascular wilts result from an invasion of the vascular system of plants by bacterial infection. Some common examples of vascular wilts are sweet corn, alfalfa, tobacco, cucurbits and tomato, and black rot of crucifers.

Necrosis is a cell injury resulting from premature cell death. The pathogen can cause necrosis by releasing toxins in the plant. The symptoms are the formation of leaf spots, cankers, or stem blights.

The pathogen can cause Soft rot diseases by secreting enzymes that are capable of degrading cell walls and membranes thus destroying the composition and texture of plant tissue. Hence plant tissue becomes macerated i.e. Soft and has fluid. Soft rot is commonly found on vegetables like potatoes, eggplant, carrot, squash, and tomato.

Tumor diseases are caused by a pathogen that facilitates the uncontrolled division of plant cell leading to into tumor cell that forms abnormally large bulbous structure.

Generally, it is difficult to detect which pathogen has affected the plant.

Transmission and infection

For a bacterium to infect a plant, it is important for the bacteria to first invade the plant tissue and divide. The bacteria enter plants through wounds, scrapes, lesions, caused by adverse weather conditions, humans, machines, other organisms, and nematodes or through natural ways such as stomata, lenticels, hydathodes, nectar, and leaf scars.

Humans also let bacteria into plants through cultivation, grafting, pruning, or contact of healthy plants with diseased plants.

Animals like insects and mites can also transmit disease to plants.

When the conditions are unfavorable for the growth and reproduction of the bacteria, they remain inactive or stay inside the plant tissue. Temperature and moisture are the key influencers of bacterial diseases in plants. However, only a few degrees of change in temperature can lead to the development of bacterial disease. Most of the time, moisture act as an important carrier for an infection to develop on the plant surface.

Survival of the bacteria

Bacterial pathogens survive in either soil or crop debris, seeds, and other plant parts. Bacterial diseases are grow best on weed crops. They act as storage reservoirs for bacterial diseases. They transmit through infected seeds, propagating seeds and crop waste, through water spray and rain. They also transmit through equipment that gets contaminated by worker's hands.

Spread of bacterial disease

The spread of bacterial disease is also favored by overhead irrigation.

The development of some bacterial diseases is also favored by hot, moist weather. While some of the pathogens are favored by cold, moist weather.

Management of bacterial diseases in plants

The management of disease aims straight to favor the growth of the host plant and its development while attacking the stages of the pathogen where they remain vulnerable. Also, to prevent or restrict the development of the pathogen. The key factors to manage bacterial diseases in plants include.

• Use of transplants that is healthy
• Monitor and regulate plants regularly and use predictive models
• Lower the levels of pathogen risk by crop rotation
• Elimination or eradication of pathogen by quarantine and pathogen-tested seeds. Also, use propagated methods to prevent the attack.
• Plant protection using pant varieties that are resistant to that particular pathogen.
• Lower mechanical or physical damage to crops and destruction by insect pest like Bacillus thuringenesis
• Avoid going into the field when they are wet
• Use of bactericidal spray when weather conditions into the disease development to prevent infection
• Use optimum temperature and proper packing conditions during storage and transport.

CONTROL

1. Host resistance by genetics

• The most significant control method is implementing the resistant varieties, cultivars or hybrids.

2. Cultural methods

• Employ bacteria free seeds and propagules.
• Preventing surface wounds that allow the entry of bacteria into the inner tissue.
• Culture only healthy and bacteria free nursery seeds.
• However, long exposure to air, heat and sunlight can result in the death of bacteria in plant material.

3. Sanitation

• Employing disinfestations of pruning tools

4. Chemical applications

• Use of antibiotics like streptomycin or tetracycline may also result in death or can suppress the plant pathogenic bacteria before the infection and reduce the spread of the disease, but they will not cure plants that are already infected.
• Antibiotics are also used to treat diseases caused by fastidious vascular bacteria. Antibiotics particularly tetracycline can also have an effect on Phytoplasmas and They can be used to treat pear trees with decreased pear disease.
• The application of chemicals in the early stage of infections is more effective than in the later stages of disease development.

4. Using biological techniques

• The application of antagonistic or biological control measures may also be effective for managing bacterial diseases of plants.

5. Implementation of Government regulations

• The implementation of strict quarantines that eliminate or prevent the introduction or transmission of pathogen or infected plant material.

Some pathogen that cause infection in plants

1. Causative agents - Pseudomonads syringae pv. Lachrymans

Distribution - spread through the world.

Symptoms

This occur mainly in most cucurbits but it occured as a great significance on cucumbers. At beginning, foliar symptoms appear as small, the water absorbed areas on the underside of the leaf, which later develops as an angular appearance due to restriction by the small leaf veins.

Under humid conditions, a milky latex may appear from the water soaked areas on the lower leaf surface. As the latex dry out, a white outer surface appear at the end when the latex get soaked up. The spots on leaves appear brown and may develop yellow haloes.

The center of the spot may eventually degrade, giving the tattered appearance. Infection on stems, petioles and fruits started appearing as water soaked spots, which give out milky exudates under the humid conditions and eventually give out a white crust surface.

Infection of young fruit may lead to deformation of fruit t maturity. Infected fruit may develop secondary soft rots.

Common bacterial diseases and crops that get affected by these pathogens

2. Causal agent - Acidivorax citrulli

Distribution - Worldwide

Symptoms - different symptoms appear in different fruits

Watermelon

At initially, the disease may first appear on cotyledon as irregular water socked tissue which later turns into brown black scraps.

On young growing leaves, small diagnostic brown lesions may appear on leaf veins. Other symptoms may also occur on seedlings including chlorosis, sharp-pointed lesions veinal or interveinal necrosis and damping off.

In the field, these lesions may appear on the veins of leaves and that eventually dry and may appear as reddish brown to black lesion.

Bulbous blotches may get develop on the tissue of fruits in association with soil. They often associated with fungal infection. As the disease spread, infected areas on the fruit may break or rupture releasing the content.

Control

The main focus is to prevent the spread of the disease than curing the plant. The control measures include:

Use seed that is healthy and resistant to Acidovorax citrulli tested with a valid seed health testing method.

Introduce crop residues to enhance the breakdown of debris and other healthy seedlings.

Crop rotation for a minimum of three years must be done and implement a sanitation program for better cultivation and crews.

The use of copper-based products in transgenic plants and throughout the growing season can help to reduce disease outbreaks and their spread.

3. BACTERIAL FRUIT ROTS

Causal agents - Pectobacterium carotovorum subsp. carotovorum, Pseudomonas spp. and several other bacteria cause Soft rot diseases in fruits.

Pantoea ananas cause brown rot diseases in plants

Distribution - all across the world

Symptoms

Soft rot appear as a water soaked tissue of the fruit which develops very quickly to complete softening and result into the collapse of fruit tissue. Brown spots has been seen on cantaloupe and honeydew types of melon. The lesion of these diseases are generally smooth, firm nd yellow to brown in color. Lesion may appear on the epidermis and extend upto one to two millimetres. Symptoms are less common on netted types.

Condition for disease development

Soft rot appear most commonly under warm and wet or moist conditions. Other diseases like angular leaf spots, blossom end rot can lead a soft rot bacteria to enter fruit. Soft rots cab lso develop into wounds created during harvest or packaging of fruits. Brown spot may also develop similarly like bacterial soft rot.

Brown spot develops under similar conditions as bacterial soft rot.

Control

Avoid bruising, tearing or punching or other mechanical destruction to fruit during harvest and packing.

The application of chlorinated fruit sprays in store or packing house has been shown to reduce the chance of soft rot.

Store fruits at an optimum temperature and humidity to prevent condensation on the surface of fruits.

4. BACTERIAL LEAF SPOT OF CUCURBITS

Causal agent - Xanthomonas cucurbitae

Distribution - Across the world

Symptoms

The symptoms arise on the dorsal side of the leaf as water-soaked scrapes, that appear mostly angular in shape but sometime it might appear round in shape. Leaf veins do not usually appear to show the shape of the lesion in all cases. Yellow spots appear on the upper surface of the leaf. These yellow spots eventually change to brown or may become translucent retaining the bright yellow halo.

The foliar symptoms may appear similar to those of angular leaf spot caused by Pseudomonas syrinae. These foliar leaf spots are caused by Xanthomonas cucurbitae are smaller at initial than angular leaf spots.

The fruit may also bear lesions of different sizes and appearances. By maturation, these lesions accumulate enough amount of moisture surrounding it. Symptoms generally begins with small, rounded, deep lesions with center surrounded by dark nimbus-like scab. By progression, lesions may become deep leading to the breaking of the rind and eventually the fruit rot in the storage house.

Condition for Development of disease

The pathogen Xanthomonas cucurbitae is seen to be associated with seed and can overwinter in crop debris. The infection is promoted by a high temperature of about 25-30°C and high relative humidity. The disease occurs mainly in heavy rains, dew, or excess irrigation.

Control

Avoid the use of over-irrigation.

Avoid entering the field when the plants are wet.

The application of copper-based sprays before the infection may help to restrict the spread.

Destroy infected crop debris by either plowing into the soil or burning the crops.

Crop rotation for at least two years.