PLANT DISEASE MANAGEMENT FOR Tropical CROPS:
ENIKUOMEHIN, O. A. (Prof)
DEPARTMENT OF CROP PROTECTION
UNIVERSITY OF AGRICULTURE, ABEOKUTA
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,
mulching, over watering, over fertilization etc.
over
- Pesticides use e.g. herbicide, fungicide and insecticide
damage.
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.
Common rust
Leaf blight
Curvularia leaf spot
Grain mould
Symptoms of fungal diseases include leaf spots, blights, wilts,
root and/or crown rots, abnormal growth, seed discolouration
and malformation etc.
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 blight
Bacterial leaf stripe
Bacterial disease symptoms are similar to fungal disease
symptoms but the surfaces and/or periphery of bacterial
infected tissues are usually watery.
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
Phythium damping-off
Method of application
1.
Moringa oleifera
2.
Allium cepa
Alternaria tenuis,
Fusarium spp.,
Helminthosporium spp
Ground 50g
onions/litre of water
as field spray
3.
Azadirchta indica
4.
Zingiber officinale
Wide-array of foliar
diseases
Powdery mildew,
Rhizoctonia solani,
Sclerotium rolfsii
Foliar application of
plant extract
20 g/L water sprayed
thrice at intervals of 15
days
5.
Aspilia africana
6.
Chromolaena odorata
7.
Tithonia diversifolia
7.5 g/L water as foliar
spray every fortnight
and as Seed dressing
(soaking seeds for 3
mins)
8.
Delonix regia
Leaf spot and leaf
blight of sesame
Leaf spot and leaf
blight of sesame
Leaf spot and leaf
blight of sesame
Sclerotium rolfsii and
crown rot of cereals
* 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)
Incorporate leaves into
soil 1-wk prior to
planting
Mixing wood ash with
soil at planting
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.
THANK YOU