Phytopathology: Bacterial diseases

This ppt illustrates and describes the two bacterial diseases included in the BSc Hons Program Syllabys Core Course III or DSC 3- Citrus canker and angular leaf spot of cotton

1. Dr Rita Som Paul Associate Professor Siliguri College Siliguri BSc Hons Program ;Core Course III : Mycology and Plant Pathology Unit 9 -: Phytopathology ; Bacterial Diseases

2. I. Angular Leaf Spot or Black Arm Disease or boll rot of cotton

3. Causal organism Xanthomonas axonopodis pv . malvacearum (Smith) Vauterin . Class : Schizomycetes Order : Pseudomonadales Family : Pseudomonadaceae Aerobic, rod-shaped (bacillus)- singly or in pairs, is capsulated but forms no spores, and is motile by one polar flagellum. G ram negative .

4. Symptoms

5. Symptoms:   i )Small water-soaked spots appear on the under surface of cotyledons, which may dry and wither. Such spots also appear on the leaves. They become angular bound by veinlets and turn brown to black in colour . ii)Several small spots may coalesce in wet weather. In dry weather the leaves fall off and the plants remain barren. iii ) Elongated, sunken and dark brown to black lesions appear on stem, petioles and branches. iv)The young stems may be girdled and killed in the black arm phase. The stem in this phase turns black from which the symptom name “ blackarm ”. v)Sunken black lesions may be seen on the bolls. Young boll may fall-off. The attacked stem becomes weak. Bacterial slime is exuded on the brown lesions. Discolouration of lint may take place. The seeds may become contaminated and lint is totally destroyed. Hence the name” boll rot”.

6. Disease Cycle

7. Disease cycle : Mode of entry : The bacterium does not enter the vascular bundles but are confined to parenchyma. From the leaf-stem lesions the bacteria are disseminated by wind –blown rain on the bollswhere spots are formed. In the infected capsules the bacteria penetrate the developing seed and form slimy masses around the hairs surrounding micropyle .

8. The primary infection takes place through seed-borne bacteria. On germination of the contaminated seeds the bacteria infect the cotyledons as they emerge from soil and formelongated marginal lesions.Thereafter , the leaves and stems are infected.  

9. Dissemination: The pathogen can remain as slimy mass inside the seed or on the fuzz. The disease may be carried over through infected leaves, bolls and twigs on the soil surface. The secondary infection is through water, wind.

10. Predisposing factors are heavy rains accompanied by strong wind, high relative humidity and temperature.

11. Management: 1 . Field sanitation. 2. (a) Externally seed borne infection can be eradicated by delinting the seed with Cone H 2 SO 4  for 5 minutes, wash with lime solution to neutralise the effect and finally washing with running water to remove the residue and drying seeds. (b) Internally seed borne infection can be eradicated by soaking seeds overnight in 100 ppm streptomycin sulphate or Agrimycin . 3. Secondary spread of the disease can be controlled by spraying the crop with streptomycin sulphate 100 ppm + Copper oxychloride (0.25%) at an interval of 15 days.  

12. II. Citrus canker

13. Causal organism/pathogen Xanthomonas campestris pv citri ; Gram negative rod with a single polar flagellum Systematic position Class – Schizomycetes Order : Eubacteriales Family : Xanthomonadaceae

14. Symptoms: Infection may occur on all aerial parts (leaf, stem fruits, thorns) either through the wounds or stomata (indirect entry). The characteristic symptoms first appear on leaves on both the leaf surfaces as small translucent spots, which are darker green in colour than the healthy tissues and sometimes surrounded by persistant yellow halos. The surface of the lesions are raised and as they enlarge, they become light- coloured and rupture to expose a spongy tissue within a typically depressed crater-like center. Depending on the species infected, epidermal remains may be seen around the margin of the lesions. This may vary in appearance on different Citrus species

15. Symptoms… The exposed spongy tissue later becomes brown and corky with age and the region rough to touch. Fruit lesions resemble those on the leaves, but are without the characteristic halo and may coalesce and cause a scabby appearance. The fruit lesions do not extend through the rind ( ie not deep seated) The market value of the fruits is reduced due appearance of the scabby lesions.

16. Disease cycle : When the leaves are wet from rain or dew, the bacteria exhude out of the canker lesions and these are disseminated by wind, rain or insects The older parts of the plant are infected primarily through wounds including that caused by the citrus leaf miner ( Phylocnistic citri , an insect) ( It enters the leaf and wades through it , making a channel below the cuticle).

17. Disease cycle…. Free moisture is required for spread of bacteria at temperatures 20-35 o C . Strong winds favour the spread of bacteria Fruits and leaves nearing maturity are more resistant than young leaves. The symptoms do not always appear immediately after infection and the causal bacteria may survive for considerable time in bark lesions. However, bacteria do not survive for more than a few days in unsterilized soil, but may remain viable for 50 dys in sterilized soil.

18. Disease cycle…. Infected young shoots are more important than leaves as source of primary inoculums, although bacteria may persist over winter as latent infection in the upper leaves. Rainfall does not affect the incubation period, but it alters the appearance of the canker lesion, which enlarges rapidly under moist conditions Secondary spread of the disease may also occur during grafting and pruning

19. Control measures : Eradication by by prurning the diseased trees in place and extreme sanitation including clothing of workers. Strict exclusion of imported Citrus from countries where the disease prevails (quarantine) Planting of wind-breaks (plants taller than citrus)

20. Control… Stimulation of foliage growth and foliage pruning during dry months Copper fungicides should be sprayed during the period of early fruit formation. Spraying streptomycin sulphate @ 500 ppm Bordeaux mixture spray checks bacterial infection and reduces disease to some extent.