According to Julius Kuhn, plant disease is an abnormal change in physiological processes which disturbs the normal activity of plant organs.

Significance

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

Blight

leaf spots, Rots

Cankers

vascular wilts

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.

History

• 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.

Disease cycle

• 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

1. 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.
2. Streptocycline is the best antibiotic to use
3. Increased supply of calcium also reduces the risk of disease.
4. Cultivate resistant strain of rice cause low ascorbic acid and high peroxidase production leads to activation of phenols which activates quinones act as antimicrobials.
5. such as TCM-3 resistant to kresek phase of the disease.