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Whitefly and whitefly-borne viruses in the tropics : Building a ... - cgiar

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considered that <strong>the</strong> disease was most<br />

severe dur<strong>in</strong>g periods of high<br />

temperature. Farmers’ estimates of <strong>the</strong><br />

losses attributable to CMD varied<br />

considerably, rang<strong>in</strong>g from “none” to<br />

“total crop loss”.<br />

Manag<strong>in</strong>g whiteflies <strong>and</strong><br />

<strong>whitefly</strong>-transmitted <strong>viruses</strong><br />

Only 7.3% of farmers attempted to<br />

control CMD; 2.4% rogued, 3.7%<br />

selected for healthy plant<strong>in</strong>g material<br />

at plant<strong>in</strong>g, while some 1.2%<br />

de-topped CMD-<strong>in</strong>fected plants to<br />

reduce plant shoots with disease<br />

symptoms <strong>in</strong> <strong>the</strong> field. Farmers who<br />

selected <strong>the</strong>ir plant<strong>in</strong>g material ma<strong>in</strong>ly<br />

used <strong>the</strong> absence of disease symptoms<br />

as <strong>the</strong> selection criterion. Farmers who<br />

rogued gave three reasons for do<strong>in</strong>g<br />

so—prevention of disease spread (7%),<br />

reduction of disease (7%) <strong>and</strong> removal<br />

of poorly grow<strong>in</strong>g plants (5%). Most of<br />

<strong>the</strong> rogu<strong>in</strong>g was done before <strong>the</strong> crop<br />

was 4 months old. Only 5% of farmers<br />

considered selection <strong>and</strong> 7%<br />

considered rogu<strong>in</strong>g as highly effective<br />

<strong>in</strong> controll<strong>in</strong>g <strong>the</strong> disease. Some<br />

farmers (10%) reported <strong>the</strong> occasional<br />

shortage of clean plant<strong>in</strong>g material,<br />

with 5% report<strong>in</strong>g ab<strong>and</strong>on<strong>in</strong>g<br />

production of <strong>the</strong> crop <strong>in</strong> 1990 <strong>and</strong><br />

1992. This may have been associated<br />

with <strong>the</strong> severe damage caused by <strong>the</strong><br />

cassava mealybug, Phenacoccus<br />

manihoti Matile-Ferrero, at this time.<br />

Improved varieties, primarily developed<br />

for <strong>the</strong>ir superior yield <strong>and</strong> agronomic<br />

characteristics, were grown by only 5%<br />

of <strong>the</strong> farmers, who considered <strong>the</strong>ir<br />

performance to be better than that of<br />

local varieties. Very few farmers (5%)<br />

had received any technical assistance<br />

<strong>in</strong> <strong>the</strong> management of whiteflies.<br />

However, a large proportion (40%) of<br />

farmers were will<strong>in</strong>g to monitor <strong>whitefly</strong><br />

<strong>and</strong> disease problems if it would help<br />

<strong>in</strong> <strong>the</strong> management of <strong>the</strong> disease.<br />

70<br />

Whiteflies <strong>and</strong> <strong>Whitefly</strong>-<strong>borne</strong> Viruses <strong>in</strong> <strong>the</strong> Tropics<br />

Conclusions<br />

Prospects for manag<strong>in</strong>g CMD<br />

successfully are good <strong>in</strong> most locations<br />

sampled, s<strong>in</strong>ce <strong>the</strong> use of diseased<br />

cutt<strong>in</strong>gs was <strong>the</strong> ma<strong>in</strong> source of<br />

<strong>in</strong>fection. However, despite <strong>the</strong> majority<br />

of farmers recogniz<strong>in</strong>g <strong>the</strong> disease as a<br />

production constra<strong>in</strong>t to cassava, <strong>the</strong>ir<br />

knowledge of <strong>the</strong> disease was weak.<br />

The <strong>in</strong>cidence of CMD was relatively<br />

high <strong>in</strong> <strong>the</strong> lakeshore survey areas <strong>and</strong>,<br />

given that moderate symptoms were<br />

observed, it is likely that significant<br />

yield losses are be<strong>in</strong>g experienced.<br />

Given that <strong>the</strong>re appears to be very<br />

limited CMD spread due to <strong>the</strong> activity<br />

of whiteflies, <strong>and</strong> that B. afer, a nonvector,<br />

is <strong>the</strong> predom<strong>in</strong>ant cassava<br />

<strong>whitefly</strong> species <strong>in</strong> Malawi,<br />

phytosanitation may provide an<br />

effective approach to CMD<br />

management. However, before an<br />

appropriate regime for <strong>the</strong> use of<br />

rogu<strong>in</strong>g <strong>and</strong> <strong>the</strong> selection of CMD-free<br />

plant<strong>in</strong>g material can be proposed,<br />

basic <strong>in</strong>formation will be required on<br />

yield losses <strong>and</strong> rates of <strong>in</strong>fection <strong>in</strong> <strong>the</strong><br />

major cassava-grow<strong>in</strong>g areas for <strong>the</strong><br />

most commonly grown varieties. These<br />

results should be used to develop a<br />

programme of participatory evaluation<br />

with farmers of both phytosanitation<br />

<strong>and</strong> new varieties. This should be<br />

closely l<strong>in</strong>ked with an educational<br />

programme targeted at <strong>in</strong>creas<strong>in</strong>g<br />

farmers’ knowledge of CMD <strong>and</strong> <strong>the</strong><br />

<strong>whitefly</strong> vector <strong>and</strong> of <strong>the</strong> range of<br />

possible management approaches.<br />

Acknowledgements<br />

The authors gratefully recognize <strong>the</strong><br />

technical support of Mr. McDonald<br />

Kavala, Mr. Elias Kamsikili <strong>and</strong><br />

Mr. Eric Mazuma. Assistance from<br />

Dr. Mohammed Ali Bob of <strong>the</strong><br />

International Center of Insect

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