Common diseases of Cassava and diseases assessment in west Africa

www.iita.orgA member of CGIAR consortium
COMMON DISEASES OF CASSAVA AND
DISEASES ASSESSMENT IN WEST
AFRICA
Presented
by
Oguntade Oluwole
Laboratory Manager, Germplasm Health Unit
IITA, NIGERIA

Summary of Presentation
 Introduction
 Constraints
Plant diseases and it importance
 Disease symptoms
 Disease Assessment/Rating

Cassava Production
Cassava, Manihot esculenta Crantz (Euphorbiaceae) is a starchy root crop that
is among the most important tropical food crops.
It has the ability to produce economic yields under marginal production
conditions, this crop helps to alleviate problems of hunger and carbohydrate
intake deficiency, and thus its importance in terms of food security in African
continent cannot be over-emphasized.
Cassava constitutes the principal carbohydrate source for more than 800
million people in developing countries. About 80% of the cassava produced is
consumed by humans, while the remaining 20% is used for animal feeds and
agro-industrial purposes.
Introduction

Constraints
 Despite the importance of this crop the production is constraint by
Insect pests and plant diseases which reduce cassava yields
substantially, posing a threat to food security throughout the
developing world.
 While agricultural scientists have recognized these threats,
assessments of cassava pests and diseases can only be done if
the various symptoms caused by the different disease can be
identified.

Is an impairment of the normal state of plant
that is interrupts or modifies Its vital functions
Plant Diseases

Two types of plant disease
• Abiotic diseases/disorder are cause by noninfectious
agents such as weather, stress, nutrient deficiency,
chemical injury, soil factors e.t.c
• Biotic diseases/disorder are caused by infectious
agent such as virus, fungi, bacteria and nematodes

Abiotic disease
• Don’t spread from plant to plant
• Causes
-adverse environment
- chemical injuries
-Adverse soil condition
-Soil structure
-Soil fertility

Biotic/infectious disease
Disease triangle
Pathogens Host
Environment
-Fungi
-Bacteria
-Nematode
-viruses
Species
Cultivar
Age
Temp, RH, wetness

Symptoms and sign
Symptoms:
Expression of a disease by plant
Sign
Visible presence of a pathogen

Virus symptoms

Bacterial symptoms

Symptom on Stem

15
Symptom on root
Wet Rot Complex

Sclerotium tuber rot

Disease incidence (%) = Number of symptomatic plants X 100
Total number of plants
Disease severity = Is the overall of degree damage caused by a disease
symptom to a plant part e.g. leaf stem or root. For example anthracnose disease
severity rating of 1-5 where;
1 = Symptomless leaves,
2 = 1-25% leaf spot/blight/mosaic
3 = 26-50% leaf spots /blight/mosaic
4 = 51-75 leaf spots/blight/mosaic
5 = Plant with severe leaf symptoms and chlorotic, covering more than 75% of the
leaf area.
*Generic scale
Disease rating and assessment of cassava
disease symptoms
DISEASE ASSESSMENT: is the amount of disease that is present. This can be measured as the
proportion of a plant community that is diseased (disease incidence) or as the proportion of
plant area that is affected (disease severity).

To know the prevalence and extent of damage caused by a
disease
Importance of disease assessment and measurement
 To develop effective management strategies
 Help in taken good decision during certification

CASSAVA PESTS AND DISEASES
EVALUATION

Cassava mosaic disease (CMD)
Brief description
First reportedinEast Africa in 1894 it is the most commoncassava disease inAfrica.
The etiology of cassava mosaic was confirmed beyonddoubt in 1983; though it was postulated in1906 that the causal organism was a virus.
The causal organism, CMV, is a geminivirus (paired or bondedvirus particles) averaging 20X30 nanometers.
VECTOR: Whitefly (Bemisia tabaci)

21
1. = No visible symptoms (highly resistant)
2:
3:
4:
5:
=mild chlorotic patterns on entire leaflets or mild
distortion at base of leaflets, rest of leaflets
appearing green and healthy (moderately resistant).
= strong mosaic patterns on entire leaf, and
narrowing and distortion of lower one-third of
leaflets (tolerant)
= severe mosaic, distortion of two-thirds of leaflets
and general reduction of leaf size (susceptible).
= severe mosaic, distortion of four-fifths or more of
leaflets, twisted and misshapen leaves (highly
susceptible)

22
CASSAVA BACTARIAL BLIGHT
(CBB )

23
EVALUATING FOR CASSAVA
BACTERIAL BLIGHT (CBB)
1
2
3
4
5
= No visible symptom (Highly resistant)
= lesion around one point of inoculation on the leaves
(Moderately resistant)
= extension and appearance of new lesions on the stem and
beginning of first leaves to wilt (Tolerant)
= extensive leaf wilt and defoliation and stem die-
back(Susceptible).
= extensive leaf wilt , defoliation, stunting and die back
forming candle stick (Highly susceptible).

24
Cassava Anthracnose Disease (CAD)

25
EVALUATING FOR CAD
1
2
= many deep cankers on stems followed by
distortion (Tolerant)
4 = many oval lesions on stems (Susceptible)
5
= No visible symptoms (Highly Resistant)
= few shallow cankers on woody stems, late in the growing
season (Moderately Resistance).
= many lesions on stems and severe necrosis at leaf
axils, followed by wilting and severe defoliation (Highly
Susceptible)
3

26
CASSAVA GREEN SPIDER MITE

27
EVALUATING FOR CGM
1
2
3
4
5
= apparently no visible symptoms
= slight reduction in leaf size and internode length
= serious reduction in leaf size and internode length
= severe bunch top symptoms; obvious reduction of internode
length and severe reduction in leaf size and leaf area
= candlestick appearance; internode length reduced, young portion
of shoot curved and completely defoliated

Field inspection scoring sheet for quality control in
cassava seeds system
Observation Comments
Field Ref. No.
GPS information
Variety
Type of seed (select on) • Pre-basic
• Basic
• Certified
Date of planting
Total size of the farm
Isolation distance
Spacing between plants
Inspectors
Field details
Note: Maximum thresholds for pests and diseases. Fields exceeding these values for any parameter are rejected.
Parameter Basic seed Certified
Off type 10% 20%
*CMD (mosaic) 10% 20%
*CAD (Anthracnose) 10% 20%
*CBB (bacterial blight) 10% 20%
*CM (mealybug) 10% 20%
*CGM (green mite) 10% 20%
Scale insects 0% 20%
*Refers to percent plants with mean severity score of ≥3

Scoring Sheet
Scring sheet
Name of the evaluator:
Ref. No. Field size
Date/Time Crop stage (Age in weeks)

Location name Varieties
District
Region Intercrops
Agro-ecology
Latitude Crops in neighbouring
fields

Longitude
Altitude (m) Researcher (farmer)
Summary Percent infection: Average severity: Severity range:
Row
number
Number
of plants
Number of
virus infected
plants (Score 1)
%
healthy-
looking
plants
Number of virus
infected plants
(>score 3)
%
infected
plants
Number of
anthracnose ‘blight’
affected plants
(>score 3)
% infected
plants

Thank you

Common diseases of Cassava and diseases assessment in west Africa

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