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