Common diseases of Cassava and diseases assessment in west Africa

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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

SpeciesCultivarAge

Temp, RH, wetness


Symptoms and sign

Symptoms:Expression of a disease by plant

SignVisible presence of a pathogen




Virus symptoms


Bacterial symptoms


Symptom on Stem

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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 reported in East Africa in 1894 it is the most common cassav a disease in Africa.The etiology of cassava mosaic was confirmed beyond doubt in 1983; though it was postulated in 1906 that the causal organism was a virus.

The causal organism, CMV, is a geminivirus (paired or bonded virus particles) averaging 20X30 nanometers.VECTOR: Whitefly (Bemisia tabaci)


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 )


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).


Cassava Anthracnose Disease (CAD)


EVALUATING FOR CAD1

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


EVALUATING FOR CGM1

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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