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Rice bacterial blight and Citrus canker
According to Julius Kuhn, plant disease is an abnormal change in physiological processes which disturbs the normal activity of plant organs.
- Plant disease are important due to the economic loss they cause to the grower.
- The loss occurs in the field during storage at any time between sowing and consumption of the harvest.
- Standing crops can be attacked by a disease and plants start dying and to yield satisfactorily reduced.
- E.F Smith is the main contributor to discoveries of plant diseases and called the father of Phyto bacteriology due to his discoveries in 1905-1920.
Symptoms and the type of diseases caused by bacteria are
- leaf spots, Rots
- vascular wilts
List of diseases and their causative agents:
|Canker of citrus||Xanthomonas compestris pv. citri|
|Leaf spot and black arm of cotton||X.compestris pv. malvacearum|
|Leaf blight of rice (BLB)||X. oryzae pv. oryzae|
|Fire blight of apple and pear||Erwinia amylovora|
|Vascular wilt and bulb spot pf red beat||Corynebacterium betae|
|Red stripe of sugar cane||Pseudomonas rubrillineans|
|Little leaf of Eggplant||Mycoplasma like organisms|
Bacterial leaf blight of rice
- Blight: Means burnt appurtenance.
- Blight is the sudden death of plant affected tissues due to extensive necrosis in that area results in scorched or burnt appearance.
- Affected part will turn brown or black and may disintegrate soon.
- The first time reported in Japan in 1881 as an endemic disease.
- Yield loss was reported 25-35 % and in some cases, it was 60 %.
- Also, in China, India, Australia, Malaysia, Pakistan.
- In Pakistan it first time reported in Kala Shah Kaku Rice Research institute and nearby farmers’ fields in 1977.
- Keller belt the best quality rice-producing area is now in danger for this disease.
Source of transmission of an agent
- Primary inoculum from infected seeds, straw, crop.
- In naturally infected leaves bacteria survive 4C and 25-28 C but not at 35-37C.
- Secondary spread by wounds, natural openings like stomata, raindrop splashes, irrigation water, rainwater coming from infected fields, contact between diseased healthy plants.
There are two distinct symptoms of the disease
Leaf Blight phase
- Mild phase, visible very early in a temperate region. Damage is due to a partial or total brightening of leaves. Wilting of affected tillers leads to unfilled grains.
- Occurs at heading or tillering (tiller the grain bearing portion) stage after 4-6 weeks of transplantation.
- The optimum temperature for disease development is 21-30 C
- Characterized by linear, yellow to straw-colored stripes with wavy margins on both edges of leaves.
- Stripes development starts from leaf tip (water pores present or wound) cause twisting of the leaf tip.
- Then it moves crosswise and lengthwise to infect the whole leaf surface. In occasional cases, stripes also occur on lamina along the midrib.
- It then extends towards leaf sheath and culm (mother stem) and necrosis res results in the killing of whole tiller or culm.in dry weather turbid drops of bacterial ooze can be seen on the leaf surface in the morning which converts to yellow beads, washed by rainwater and act as a source of transmission of disease.
Kresek or Wilt phase: (kresek means sound of dead leaves)
- A devastating form of disease mostly occurs in tropics.
- Effect the xylem portion results in the complete drying of crop. it occurs before seed maturation and optimum conditions are 64-84 % humidity, a combination of temperature 24-26 C and 30-35 C, high rainfall
- Appears after 1-2 weeks of transplantation and it results in complete rolling of leaves. Leaves turn grayish-green and tillers become withered.
- Weed host Leersia hexandra present in rice-growing season carry the bacterium without sowing blight symptoms.
- Infected seeds, straw also introduce pathogens in the nursery beds.
- When seedlings appear, pathogen accumulates in root surface, move towards the crown region of the plant by utilizing metabolites.
- First, infect a lower portion of the plant (lower leaves).
- Bacterial ooze in this area could contamination water.
- It then multiplies in the parenchyma of plant cells and produces phytotoxins such as isovaleric acid, 3-methylthiopropionic acid.
Management of disease
- Soak seed in 0.025% agrimycin soln. and 0.05% wettable ceresan for 12 hours. Now again soak seeds in hot water at 52-54 C for 30 min.
- Streptocycline is the best antibiotic to use
- Increased supply of calcium also reduces the risk of disease.
- Cultivate resistant strain of rice cause low ascorbic acid and high peroxidase production leads to activation of phenols which activates quinones act as antimicrobials.
- such as TCM-3 resistant to kresek phase of the disease.
- It originated from China and then spread to Europe in 1910.
- It was eliminated from the USA in 1949 but reappeared again in 1984.
- It is a bacterial disease having worldwide distribution also reported in Japan, India, etc.
- 90 % of national production is done in Sargodha region of Punjab Pakistan.
- Plant hosts are lime, grapefruit, oranges.
- It is economically important because of the downgrade appearance of fruits and premature fruit drop.
- Xanthomonas campestris pv. citri is a gram-negative, aerobic, rod-shaped bacterium.
- It can be isolated on Beef agar.
- The disease occurs on leaves, twigs, fruits.
- Leaf lesions are small, round, watery translucent spots that are raised and turn to yellowish-brown.
- Lesions first appear on lower leaf surface than on both surfaces.
- Spots or lesions finally rupture in center and cork cell produced.
- This cork cell also called Necrotic canker which is composed of yellow-green margins (living cells, millions of pathogenic bacteria) and watery yellow halo ((center of dead cells).
- On fruits, canker appearance is more prominent but the injury is only skin deep does not penetrate to pulp and fruit is safe for consumption.
- But due to lesions on the skin, it cannot be a salad.
- Lesions can increase in size from 1mm to 1cm.
- Young leaves at a very early stage of formation and fruits of about 2-4cm in diameter are more prone to disease.
- Early spring is favorable weather for disease development.
- The bacterium is not soil-borne because it cannot survive in the soil for a longer time due to antagonistic actions of other microbes in the soil.
- The twigs, leaves of standing trees, bearing old lesions is a source of transmission.
- Affected fallen leaves and an insect Citrus Leaf Minor which carry bacterium on its body also a source of transmission.
- Along with rain splashes in nurseries or field agents from these lesions infect healthy plants.
- Chances od disease with citrus leaf minors are 26-48 % but without it 3-10%.
- Man is the main contributor to the transmission of disease through infected stock and tools.
- Bacteria enter to stomata, the wound which opens mesophyll tissues and multiplies in intercellular spaces, middle lamella between cell walls.
- Then establishes in the cortical region.
- Mild temperature of 20-30 C and wet weather is favorable for disease to occur.
- The pressure of free moisture for 20 minutes is enough for a successful infection.
Control of disease
- Proper care about citrus leaf minors.
- Quarantine regulations of infected stock.
- Cultivate disease-free nursery stock
- Streptomycin sulfate spray should be done which is absorbed by leaves and translocated, act as a systemic bactericide
- The spray of neem cake at a rate of 80kg/acre