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

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Sudan<br />

Historically, Sudan has been<br />

regarded as one of <strong>the</strong> countries worst<br />

hit by <strong>whitefly</strong> problems, especially <strong>in</strong><br />

<strong>the</strong> cotton production system. This<br />

scenario has changed recently <strong>and</strong> <strong>the</strong><br />

<strong>whitefly</strong> problem <strong>in</strong> cotton seems to<br />

have become less acute. Highly<br />

effective <strong>in</strong>secticide treatments, often<br />

conta<strong>in</strong><strong>in</strong>g <strong>in</strong>sect growth regulators,<br />

have reduced <strong>the</strong> breed<strong>in</strong>g success of<br />

whiteflies on cotton. None<strong>the</strong>less, <strong>the</strong><br />

<strong>whitefly</strong> has ma<strong>in</strong>ta<strong>in</strong>ed its status as a<br />

highly <strong>in</strong>jurious pest of tomato,<br />

cucurbits, pepper, okra <strong>and</strong> common<br />

bean, <strong>and</strong> <strong>the</strong> numbers of producers<br />

who have ab<strong>and</strong>oned tomato<br />

cultivation because of TYLCV are on<br />

<strong>the</strong> <strong>in</strong>crease.<br />

Farmers’ perceived yield loss to<br />

<strong>whitefly</strong> <strong>and</strong>/or TYLCV <strong>in</strong> tomato was<br />

on average 62%. Accord<strong>in</strong>g to <strong>the</strong><br />

survey, 26% of producers, ma<strong>in</strong>ly <strong>in</strong><br />

Gezira, reported total yield loss, 38%<br />

reported los<strong>in</strong>g three-quarters of <strong>the</strong>ir<br />

yield, 10% reported los<strong>in</strong>g half, 10%<br />

reported los<strong>in</strong>g one-quarter of <strong>the</strong>ir<br />

yield <strong>and</strong> 16%, ma<strong>in</strong>ly from Gedaref<br />

<strong>and</strong> North Kordufan, reported no loss.<br />

Of <strong>the</strong> producers <strong>in</strong>terviewed, 34%<br />

(most of <strong>the</strong>m <strong>in</strong> Gezira State) reported<br />

ab<strong>and</strong>on<strong>in</strong>g <strong>the</strong>ir tomato crops <strong>in</strong> at<br />

least 1 year because of <strong>whitefly</strong>/TYLVC<br />

problems <strong>and</strong> most reported that this<br />

occurred <strong>in</strong> 1997. The perceived losses<br />

can be compared usefully with <strong>the</strong><br />

recorded <strong>in</strong>cidence of TYLCV<br />

symptoms. Among all surveyed tomato<br />

producers, 92% had some virus<br />

symptoms <strong>in</strong> <strong>the</strong>ir tomato crop <strong>and</strong><br />

50% had more than 25% <strong>in</strong>cidence. It<br />

should be emphasized that 31% of <strong>the</strong><br />

tomato producers had an 80% TYLC<br />

<strong>in</strong>cidence <strong>in</strong> <strong>the</strong>ir tomato crops <strong>and</strong><br />

<strong>the</strong>se outbreaks were ma<strong>in</strong>ly conf<strong>in</strong>ed<br />

to <strong>the</strong> greater Gezira. TYLC was<br />

significantly less severe (1%-9%) <strong>in</strong><br />

western, nor<strong>the</strong>rn <strong>and</strong> eastern states,<br />

<strong>and</strong> absent <strong>in</strong> Gedaref <strong>and</strong> <strong>the</strong> Abu<br />

Habil Bas<strong>in</strong>. Most tomato producers<br />

(65%) believe that <strong>the</strong>y have <strong>whitefly</strong>/<br />

TYLCV problems every year. The<br />

majority of producers (86%) believe<br />

that <strong>the</strong>re is a direct relationship<br />

between <strong>the</strong> climate <strong>and</strong> <strong>the</strong> <strong>in</strong>cidence<br />

of whiteflies <strong>and</strong>/or <strong>whitefly</strong>transmitted<br />

disease, with 63% of<br />

producers associat<strong>in</strong>g high <strong>in</strong>cidence<br />

with hot <strong>and</strong> dry seasons, while only<br />

9% believe that <strong>the</strong>y have <strong>the</strong> problem<br />

all year round.<br />

Estimation of disease<br />

<strong>in</strong>cidence <strong>and</strong> yield losses<br />

The damage associated with B. tabaci<br />

<strong>and</strong> WTVs is always more serious <strong>in</strong><br />

summer crops, especially dur<strong>in</strong>g <strong>the</strong><br />

hot season from March to August<br />

(dur<strong>in</strong>g which <strong>the</strong>re are three plant<strong>in</strong>g<br />

periods: “early summer” <strong>in</strong> March, “late<br />

summer” <strong>in</strong> May <strong>and</strong> <strong>the</strong> “ma<strong>in</strong><br />

plant<strong>in</strong>g season” <strong>in</strong> July) <strong>and</strong> total<br />

failure of <strong>the</strong> tomato crop due to TYLCV<br />

is common. This is despite <strong>the</strong> lower<br />

prevalence of <strong>the</strong> vector <strong>in</strong> <strong>the</strong> summer<br />

season. <strong>Whitefly</strong> populations build up<br />

dur<strong>in</strong>g <strong>the</strong> w<strong>in</strong>ter (October-January)<br />

<strong>and</strong> peak <strong>in</strong> December-January. Up to<br />

100% <strong>in</strong>fection by TYLCV can occur<br />

dur<strong>in</strong>g a mild w<strong>in</strong>ter but less yield<br />

depression is associated with <strong>the</strong>se<br />

w<strong>in</strong>ter <strong>in</strong>fections.<br />

In 1998, tomato grown dur<strong>in</strong>g <strong>the</strong><br />

May-July season suffered severe attack<br />

by TYLCV <strong>and</strong> this crop failed<br />

completely. Watermelon likewise<br />

suffered severe losses from WmCSV.<br />

An exceptionally heavy ra<strong>in</strong>y season<br />

from mid-August to mid-October,<br />

however, seemed to alter <strong>the</strong> <strong>whitefly</strong><br />

situation later <strong>in</strong> <strong>the</strong> year, with<br />

population levels dropp<strong>in</strong>g sharply <strong>and</strong><br />

tomato crops grown dur<strong>in</strong>g October-<br />

November be<strong>in</strong>g comparatively less<br />

affected by TYLCV. A field experiment<br />

conducted at <strong>the</strong> University of Gezira<br />

revealed that early <strong>in</strong>fection by TYLCV<br />

reduced <strong>the</strong> total number of flowers per<br />

plant, percent fruit sett<strong>in</strong>g <strong>and</strong> <strong>the</strong><br />

quality of fruits. The <strong>in</strong>fection resulted<br />

<strong>in</strong> 67% reduction <strong>in</strong> <strong>the</strong> number of<br />

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