Horticulture and Plant Diseases

What to remember

• Horticulture is the culture of fruits, vegetables, flowers, spices, plantation crops and medicinal plants. Its main propagation tools are cuttings, layering, budding and grafting.
• Plant diseases are caused by fungi, bacteria, viruses, phytoplasmas and nematodes. The disease triangle needs a susceptible host, a pathogen and a favourable environment.
• Control is integrated: resistant varieties, clean nursery practice, cultural methods, and carefully chosen chemicals such as Bordeaux mixture.

Classification of horticultural crops

Fruit types by climate: tropical (mango, banana, papaya), subtropical (citrus, guava), temperate (apple, pear).

Ripening behaviour: climacteric fruits (mango, banana, papaya, apple, guava) show a rise in respiration and ethylene at ripening and can ripen after harvest; non-climacteric fruits (citrus, grapes, pineapple) ripen only on the plant and show no such rise.

Andhra Pradesh is well known for horticulture: mango (the Banganapalli variety is a famous local type), citrus, banana, papaya, cashew, coconut in the coastal delta region, oil palm, tomato, chilli and flowers.

Propagation methods

Sexual: seed. Seedlings vary, so trees from seed are used mainly for rootstocks, forest trees and breeding. Mango and citrus can have polyembryony (many embryos in one seed), where nucellar seedlings are true to the mother and used as rootstocks.

Asexual (vegetative): produces plants identical to the parent.

Terms: rootstock (stock) is the lower part with the roots; scion is the upper part; cambium layers of both must touch for union. The inter-stock is a middle piece. Grafting is best done when the stock and scion are in active growth or when the scion is dormant and the stock is active, depending on method. Veneer grafting and softwood grafting are widely used in mango nurseries.

Plant growth regulators: auxins (IBA, NAA, 2,4-D) for rooting and weed control; gibberellins (GA3) for stem elongation and bigger seedless grapes; cytokinins for cell division and shoot growth; ethylene (supplied by ethephon) for ripening and flowering in pineapple; abscisic acid for dormancy and stress.

Orchard and nursery operations

• Site, layout and spacing vary: square, rectangular, triangular, hexagonal, contour. Example: for a square system, plants per hectare = 10,000 / (row spacing × plant spacing in m). For mango at 10 m × 10 m, 10,000 / 100 = 100 plants per ha.
• Pruning removes parts to shape the tree; training directs growth (open centre, central leader, modified leader).
• Manuring: nitrogen, phosphorus, potassium (NPK) with organic manure; micronutrient sprays when needed; split doses.
• Irrigation: drip irrigation saves water and suits orchards; mulching conserves moisture.
• Intercropping in young orchards with legumes or vegetables.
• Harvest and post-harvest: harvest at the right maturity, grade, pack, cool and transport; ripen climacteric fruits with ethylene or traditional methods; pre-cooling extends storage.
• Forest nurseries use the same concepts: raised beds, mother beds, polybag filling, hardening and shade.

Plant pathogens and disease terms

• Disease: a harmful change in normal function, shown by symptoms; sign is the visible pathogen structure (for example fungal growth); symptom is the plant's reaction.
• Inoculum is the pathogen that starts infection; host the plant attacked; vector the carrier (insects carry viruses and phytoplasmas).
• Koch's postulates show that a particular organism causes a disease: isolate, grow pure, inoculate healthy plant, reproduce disease, re-isolate.
• Tobacco mosaic virus was the first virus discovered in plants.

Important diseases (horticulture and forestry)

Disease management

Management uses combined methods (integrated disease management):

• 1. Exclusion: quarantine and healthy planting material.
• 2. Cultural: clean cultivation, crop rotation, removal of diseased parts, balanced fertilizer, drainage, sun-drying and soil solarisation of nursery beds.
• 3. Resistant varieties and rootstocks.
• 4. Biological: Trichoderma, Pseudomonas fluorescens, neem products.
• 5. Chemical:
• Bordeaux mixture: copper sulphate + lime (quick lime) + water, commonly 1% strength (1 kg copper sulphate + 1 kg lime in 100 litres water); safe for many horticultural crops.
• Mancozeb: protectant fungicide. Carbendazim: systemic fungicide. Sulphur: for powdery mildew. Thiram: seed treatment.
• Systemic products move inside the plant; contact (protectant) products stay on the surface.
• 6. Integrated Pest Management (IPM): use thresholds, natural enemies and the least harmful chemicals.

Worked example: to prepare 200 litres of 1% Bordeaux mixture, take 2 kg copper sulphate and 2 kg lime. Dissolve each separately in water and pour the copper sulphate solution into the lime solution (not the reverse) while stirring.

Nursery media and plant health basics

Good horticultural nurseries use a light, well-drained potting mixture of soil, sand and farmyard manure, often in equal parts, sterilised or sun-dried to kill pathogens. Seed is treated before sowing. Mother plants for scions must be healthy, true to type and free from virus. Hardening before sale prepares plants for field conditions. Mineral deficiency can look like disease: nitrogen shortage gives pale older leaves; iron shortage gives yellow young leaves with green veins (chlorosis); zinc shortage causes little leaf in some fruit trees. Always separate nutrient problems from pathogen problems before spraying.

Exam traps

• Rootstock vs scion: the root part is rootstock; the shoot piece is scion.
• Budding vs grafting: budding uses one bud; grafting uses a shoot with several buds.
• Cutting vs layering: layering roots while attached to the parent.
• Climacteric vs non-climacteric: banana and mango are climacteric; citrus and grape are not.
• Sign vs symptom: sign is the pathogen; symptom is the plant reaction.
• Bordeaux vs Burgundy mixture: Bordeaux uses lime; Burgundy uses sodium carbonate.
• Phytoplasma vs virus: sandal spike is phytoplasma, not a virus.
• Contact vs systemic: mancozeb is protectant; carbendazim is systemic.

One-liners

• 1. Polyembryony in mango and citrus gives nucellar seedlings.
• 2. Air layering is also called gootee.
• 3. T-budding is common in citrus and rose.
• 4. In approach grafting both plants stay on their own roots until union.
• 5. Ethephon releases ethylene.
• 6. Bordeaux mixture is copper sulphate plus lime.
• 7. Damping-off is a seedling disease of nurseries.
• 8. Late blight of potato is caused by Phytophthora infestans.
• 9. Citrus canker is bacterial (Xanthomonas).
• 10. Sandal spike is caused by a phytoplasma.
• 11. Tobacco mosaic virus was the first plant virus found.
• 12. Root-knot disease is caused by Meloidogyne nematodes.

Practice questions

1. Many embryos in one seed, as in mango and citrus, is called:

   1. Parthenocarpy
   2. Apomixis of leaves
   3. Vivipary
   4. Polyembryony
   Answer

   D. Polyembryony

   Nucellar embryos are true to the mother plant.

2. Air layering is also known as:

   1. Chip budding
   2. Whip grafting
   3. Marcotting by roots only
   4. Gootee
   Answer

   D. Gootee

   Used in litchi, guava and pomegranate.

3. T-budding (shield budding) is commonly used in:

   1. Teak seed
   2. Citrus and rose
   3. Paddy
   4. Banana and ginger
   Answer

   B. Citrus and rose

   A single bud is inserted under bark cut in a T shape.

4. In approach grafting, the stock and scion:

   1. Are both cut from the roots at once
   2. Remain on their own roots until the union forms
   3. Are joined as seeds
   4. Are grown in sterile media only
   Answer

   B. Remain on their own roots until the union forms

   The scion is severed from its parent only after union.

5. Ethephon is used in horticulture because it releases:

   1. Ethylene
   2. Gibberellin
   3. Cytokinin
   4. Auxin
   Answer

   A. Ethylene

   Ethylene promotes ripening and, in pineapple, flowering.

6. Bordeaux mixture is a combination of:

   1. Zinc sulphate and gypsum
   2. Copper sulphate and lime
   3. Sulphur and soap
   4. Mancozeb and urea
   Answer

   B. Copper sulphate and lime

   It is a protectant fungicide, usually used at 1%.

7. Damping-off is mainly a disease of:

   1. Seedlings in nurseries
   2. Stored grain
   3. Mature mango fruits
   4. Timber logs
   Answer

   A. Seedlings in nurseries

   Pythium and other fungi rot seedlings at the collar.

8. Late blight of potato is caused by:

   1. Xanthomonas
   2. Colletotrichum
   3. Phytophthora infestans
   4. Fusarium oxysporum
   Answer

   C. Phytophthora infestans

   It caused the Irish famine.

9. Citrus canker is caused by a:

   1. Nematode
   2. Fungus (Pythium)
   3. Virus
   4. Bacterium (Xanthomonas)
   Answer

   D. Bacterium (Xanthomonas)

   It forms corky lesions with yellow halos.

10. Spike disease of sandal is caused by a:

    1. Nematode
    2. Fungus
    3. Bacterium (Xanthomonas)
    4. Phytoplasma
    Answer

    D. Phytoplasma

    Insect vectors spread it.

11. The first plant virus discovered was:

    1. Potato virus Y
    2. Tobacco mosaic virus
    3. Banana bunchy top virus
    4. Cucumber mosaic virus
    Answer

    B. Tobacco mosaic virus

    Studied by Ivanovsky and Beijerinck.

12. Root-knot disease in vegetables is caused by:

    1. Xanthomonas
    2. Pythium (fungus)
    3. Meloidogyne (nematode)
    4. Phytoplasma
    Answer

    C. Meloidogyne (nematode)

    It forms galls on roots.

13. A climacteric fruit is:

    1. Banana
    2. Grape
    3. Citrus
    4. Pineapple
    Answer

    A. Banana

    Banana shows a rise in respiration and ethylene at ripening.

14. A non-climacteric fruit is:

    1. Mango
    2. Papaya
    3. Banana
    4. Citrus
    Answer

    D. Citrus

    It ripens only on the plant.

15. Panama wilt of banana is caused by:

    1. Phytophthora infestans
    2. Oidium mangiferae
    3. Fusarium oxysporum f. sp. cubense
    4. Xanthomonas
    Answer

    C. Fusarium oxysporum f. sp. cubense

    It is a soil-borne fungal wilt.

16. Powdery mildew of mango is caused by:

    1. Colletotrichum falcatum
    2. Oidium mangiferae
    3. Pythium
    4. Meloidogyne
    Answer

    B. Oidium mangiferae

    White powder on panicles; sulphur sprays help.

17. The lower part of a graft that carries the root system is the:

    1. Rootstock
    2. Scion
    3. Cambium
    4. Inter-bud
    Answer

    A. Rootstock

    The upper shoot part is the scion.

18. The upper part of a graft that forms the tree crown is the:

    1. Stolon
    2. Scion
    3. Sucker
    4. Rootstock
    Answer

    B. Scion

    The scion supplies the desired variety.

19. Which growth regulator is most used to help cuttings form roots?

    1. Abscisic acid
    2. GA3
    3. IBA
    4. Ethephon
    Answer

    C. IBA

    Indole-butyric acid is an auxin used for rooting.

20. GA3 sprays on grapes are mainly used to:

    1. Elongate bunches and enlarge berries
    2. Kill mildew
    3. Fix nitrogen
    4. Raise soil pH
    Answer

    A. Elongate bunches and enlarge berries

    Gibberellins improve berry size in seedless grapes.

21. In plant pathology, a 'symptom' is:

    1. The fungicide used
    2. The soil pH
    3. The pathogen's structure seen on the host
    4. The plant's visible reaction to disease
    Answer

    D. The plant's visible reaction to disease

    The visible pathogen growth is a 'sign'.

22. Koch's postulates are used to:

    1. Prove which organism causes a disease
    2. Measure soil salinity
    3. Calculate fungicide dose
    4. Graft trees
    Answer

    A. Prove which organism causes a disease

    The organism is isolated, inoculated and re-isolated.

23. Carbendazim is a:

    1. Herbicide
    2. Contact insecticide
    3. Plant growth hormone
    4. Systemic fungicide
    Answer

    D. Systemic fungicide

    It moves inside the plant.

24. Mancozeb is best described as a:

    1. Systemic insecticide
    2. Protectant (contact) fungicide
    3. Nitrogen fertiliser
    4. Rooting hormone
    Answer

    B. Protectant (contact) fungicide

    It stays on the plant surface.

25. A main advantage of drip irrigation in orchards is:

    1. Weed growth between rows
    2. Higher evaporation
    3. Water saving and direct supply to roots
    4. Soil erosion
    Answer

    C. Water saving and direct supply to roots

    Water is delivered close to the root zone.

26. Nucellar seedlings in citrus are valued as:

    1. Variable breeding material
    2. Ornamental plants
    3. Uniform rootstocks true to the mother
    4. Seedless fruit sources
    Answer

    C. Uniform rootstocks true to the mother

    They arise from nucellar tissue, not from fertilisation.

27. Phytoplasma diseases such as little leaf of brinjal are spread by:

    1. Insect vectors such as leafhoppers
    2. Wind-borne spores only
    3. Irrigation water only
    4. Contaminated seed coats only
    Answer

    A. Insect vectors such as leafhoppers

    The insects carry the pathogen between plants.

28. Soil solarisation of nursery beds is done to:

    1. Add nitrogen
    2. Reduce soil pathogens by heat
    3. Raise pH
    4. Increase soil salts
    Answer

    B. Reduce soil pathogens by heat

    Clear plastic traps heat that kills many pathogens.

29. Damping-off in a nursery is reduced by:

    1. Using diseased seed
    2. Dense sowing
    3. Heavy irrigation at night
    4. Raised well-drained beds, thin sowing and seed treatment
    Answer

    D. Raised well-drained beds, thin sowing and seed treatment

    These reduce humidity and inoculum.

30. Yellow young leaves with green veins usually indicate shortage of:

    1. Nitrogen in old leaves only
    2. Phosphorus
    3. Iron
    4. Calcium oxalate
    Answer

    C. Iron

    This pattern is iron chlorosis.

31. Mango planted in a square system at 10 m x 10 m has how many plants per hectare?

    1. 50
    2. 200
    3. 1,000
    4. 100
    Answer

    D. 100

    10,000 / 100 = 100.

32. Plants per hectare at 10 m x 5 m spacing:

    1. 500
    2. 200
    3. 400
    4. 100
    Answer

    B. 200

    10,000 / 50 = 200.

33. To prepare 200 litres of 1% Bordeaux mixture, copper sulphate required is:

    1. 2 kg
    2. 20 kg
    3. 1 kg
    4. 4 kg
    Answer

    A. 2 kg

    1% = 1 kg per 100 L; 200 L needs 2 kg.

34. For 500 litres of 1% Bordeaux mixture, lime required (equal to copper sulphate) is:

    1. 5 kg
    2. 50 kg
    3. 1 kg
    4. 2.5 kg
    Answer

    A. 5 kg

    1 kg per 100 L x 5 = 5 kg.

35. Urea contains 46% nitrogen. To supply 92 kg N, urea needed is:

    1. 100 kg
    2. 250 kg
    3. 150 kg
    4. 200 kg
    Answer

    D. 200 kg

    92 / 0.46 = 200 kg.

36. To supply 69 kg N per hectare using urea (46% N), the quantity is:

    1. 100 kg
    2. 120 kg
    3. 150 kg
    4. 200 kg
    Answer

    C. 150 kg

    69 / 0.46 = 150 kg.

37. 500 grafts are made and 80% succeed. Successful grafts:

    1. 350
    2. 400
    3. 450
    4. 80
    Answer

    B. 400

    500 x 0.8 = 400.

38. A fungicide at 2 g per litre is sprayed in 500 litres. Fungicide required:

    1. 100 g
    2. 1 kg
    3. 2 kg
    4. 500 g
    Answer

    B. 1 kg

    2 g x 500 = 1,000 g = 1 kg.

39. A 0.2% solution is needed. For 50 litres, the chemical needed is:

    1. 100 g
    2. 20 g
    3. 1 kg
    4. 10 g
    Answer

    A. 100 g

    0.2% is 2 g/L; 2 x 50 = 100 g.

40. Consider the statements: 1. Budding uses a single bud as scion. 2. Grafting uses a shoot piece with several buds as scion. Which of the statements is/are correct?

    1. 1 only
    2. 2 only
    3. Both 1 and 2
    4. Neither 1 nor 2
    Answer

    C. Both 1 and 2

    Both statements are correct.

41. Consider the statements: 1. Bordeaux mixture is copper sulphate plus lime. 2. Bordeaux mixture is made with sodium carbonate instead of lime. Which of the statements is/are correct?

    1. 1 only
    2. 2 only
    3. Both 1 and 2
    4. Neither 1 nor 2
    Answer

    A. 1 only

    Burgundy mixture uses sodium carbonate.

42. Consider the statements: 1. Banana and mango are climacteric fruits. 2. Citrus is climacteric. Which of the statements is/are correct?

    1. 1 only
    2. 2 only
    3. Both 1 and 2
    4. Neither 1 nor 2
    Answer

    A. 1 only

    Citrus is non-climacteric.

43. Consider the statements: 1. Sandal spike is caused by a phytoplasma. 2. It is spread by insect vectors. Which of the statements is/are correct?

    1. 1 only
    2. 2 only
    3. Both 1 and 2
    4. Neither 1 nor 2
    Answer

    C. Both 1 and 2

    Both statements are correct.

44. Consider the statements: 1. Carbendazim is a contact fungicide. 2. Mancozeb is a systemic fungicide. Which of the statements is/are correct?

    1. 1 only
    2. 2 only
    3. Both 1 and 2
    4. Neither 1 nor 2
    Answer

    D. Neither 1 nor 2

    Carbendazim is systemic; mancozeb is a protectant.

45. Consider the statements: 1. Nucellar seedlings from polyembryonic seeds are true to the mother. 2. Seedlings from seed are always identical to the parent in all fruit trees. Which of the statements is/are correct?

    1. 1 only
    2. 2 only
    3. Both 1 and 2
    4. Neither 1 nor 2
    Answer

    A. 1 only

    Sexual seedlings usually vary.