PLANT DISEASE MANAGEMENT FOR Tropical CROPS :

PLANT DISEASE MANAGEMENT FOR Tropical CROPS:

DEPARTMENT OF CROP PROTECTIONDEPARTMENT OF CROP PROTECTION

UNIVERSITY OF AGRICULTURE, ABEOKUTAUNIVERSITY OF AGRICULTURE, ABEOKUTA

ENIKUOMEHIN, O. A. (Prof)ENIKUOMEHIN, O. A. (Prof)

INTRODUCTION A plant disease is any disturbance that

prevents a plant’s normal development and

reduces its economic or aesthetic value.

Diseases can be caused by biotic (living)

and abiotic (non-living) agents.

Disease as a natural phenomenon.

AGENTS OF PLANT DISEASE

ABIOTIC AGENTS (factors) of plant disease include:

- Environmental factors e.g. cold, heat, wind or lightning injuries, excessive moisture, drought etc.

- Mechanical factors e.g. cultivation damage, pruning damage, etc.

- Cultural factors e.g. improper planting, over mulching, over watering, over fertilization etc.

- Pesticides use e.g. herbicide, fungicide and insecticide damage.

BIOTIC AGENTS BIOTIC AGENTS of disease includes:

FUNGI – These are the most common cause of plant

disease. They are microscopic organisms that feed on living plants and/or dead organic matter.

They propagate thorough air-borne, water-borne, spores or drought resistant and tolerant sclerotia and/or mycelia. Insects, birds, man and equipments can facilitate dissemination of the fungal propagules (spores/sclerotia/mycelia).

A wound on a plant readily helps the invasion of a healthy plant by fungi.

Some fungi however, have the ability to infected intact (unwounded) plants.

Symptoms of fungal diseases include leaf spots, blights, wilts, root and/or crown rots, abnormal growth, seed discolouration and malformation etc.

Common rust Leaf blight

Grain mould

Curvularia leaf spot

BACTERIA –

These are single-celled microscopic organisms. Some attack plants and cause diseases.

Bacteria can be carried from plant to plant in droplets of water or by wind, rain splash, insects and equipments.

Bacteria often survive between seasons on crop residue, seeds, cuttings or weeds.

In warm and wet conditions, bacteria reproduce rapidly.

Bacterial disease symptoms are similar to fungal disease symptoms but the surfaces and/or periphery of bacterial infected tissues are usually watery.

Bacterial blight

Bacterial leaf stripe

VIRUSES –

These are the smallest parasites causing plant disease and can only seen through the electron microscope.

Most viruses are spread by certain types of insects (vectors).

The conditions that favour large number of insect vectors often lead to severe virus outbreaks.

Most viruses survive between seasons in live plant tissues (weeds, seeds, cuttings).

Symptoms of viral infections include stunting, colour changes in leaves, growth distortions and malformations.

Principles of Plant disease management

Exclusion (Quarantine) – This means keeping out any material (plant) or objects (equipments) that are contaminated with pathogens or diseased plants and preventing them from entering the production system.

Avoidance – This means ensuring that crop production is undertaken at a time and/or place that makes the plant escape (avoid) the disease.

Protection – This means treating a healthy plant before it becomes diseased.

Eradication – This involves elimination, destruction or inactivation of existing inoculum or vector (agent/carrier) of disease.

Resistance – This is the use of crop varieties that have inherent genetic make up that forestall or reduced disease development.

Common Practices for disease management in organic systems Seed sorting – Hand sorting of diseases seeds before

planting and the use of salt density in seed sorting (where applicable).

Use of seeds dressed with organic biocides for planting to reduced pre and post-emergence disease as well as disease transmission to adult plants.

Destroying residue of previous crop by tilling as far in advance of planting of the new crop as possible.

Appropriate planting date (time), population density (spacing), crop mixtures (intercropping) must be done to facilitate disease escape, reduce inoculum build-up and transfer.

Monoculture promotes build up of pathogen population, so, it is important to follow a good rotation sequence. That is to avoid planting the same crop year after year. This use of non-host crops in the rotation sequence or intercrop provides time and space respectively, for decline in inoculum.

Use of resistant varieties and disuse of known susceptible ones.

Good crop sanitation such as weeding and wide boarder rows.

Solarisation – To reduced disease caused by soil-borne pathogens cover soil with transparent polythene for up to 15 days. This reduces the amount of infective fungal inoculum in the soil.

Removal (Roughing) of disease crops or plant parts early to forestall being a foci of infection of other crops or plant parts.

Application of organic protection agents or plant products (ash, extract, etc.) to reduced disease incidence and severity. Insect vectors can be deterred by intercropping with plants with adverse olfactory effect on the insect species.

Harvesting at optimum periods to reduce inoculum build-up on mature crops (seed) and at the ideal physiological maturity to enhance storability.

Ensure balanced soil fertility status via application of cured and verified manure/compost. Avoid high salt accumulation, excessive manure or organic waste application, water logging and drought

Plants with potentials for use in disease management

S/No Plant Disease/pathogen managed

Method of application

1. Moringa oleifera Phythium damping-off Incorporate leaves into soil 1-wk prior to planting

2. Allium cepa Alternaria tenuis, Fusarium spp., Helminthosporium spp

Ground 50g onions/litre of water as field spray

3. Azadirchta indica Wide-array of foliar diseases

Foliar application of plant extract

4. Zingiber officinale Powdery mildew, Rhizoctonia solani, Sclerotium rolfsii

20 g/L water sprayed thrice at intervals of 15 days

5. Aspilia africana Leaf spot and leaf blight of sesame

7.5 g/L water as foliar spray every fortnight and as Seed dressing (soaking seeds for 3 mins)

6. Chromolaena odorata Leaf spot and leaf blight of sesame

7. Tithonia diversifolia Leaf spot and leaf blight of sesame

8. Delonix regia Sclerotium rolfsii and crown rot of cereals

Mixing wood ash with soil at planting

* Seed sorting with salt density - Rice and other similar grains; 10% salt concentration (100g/L) - Sesame; 5% salt concentration (50g/L) - Cowpea; 15% salt concentration (150g/L)

CONCLUSION

It is important to correctly diagnose a disease before proffering management options.

Diagnosis, being the process of determining the cause of a problem requires the attention of an expert.

Although, experience and practice are very useful tools in plant disease diagnosis, it is advisable to employ the input of an expert at first instance prior to application of available management options.

REFERENCES

Enikuomehin, O. A. (2005) Cercospora leaf spot disease management in sesame (Sesamum indicum L.) with plant extracts. J. Trop. Agric. 43(1): 19-24.

Enikuomehin, O. A. (2008) Seed sorting of sesame (Sesamum indicum L.) by salt density and seedborne fungi control with plant extracts. Arch. Phytopath. Plant Prot. D0I 10.1080/03235400801940175.

Baniecki, J. F. (2009) Plant disease facts. www.wvu.edu/agexten/ipm/disease/pHdisfc.htm(view11/3/2009)

Koike, S. T., Gaskell, M., Fouche, C., Smith, R. and Michell, J. (2009) Plant disease management for organic crops. Organic vegetable production in California series. Publication 7252 6pp (available on www.sfc.ucdavis.edu).

Stoll, G. 2001. Natural crop protection in the tropics. Letting information come to life. F & T. Mullerbader Fildstadt Publishers, Germany, 208p.