23.07.2013 Views

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

SHOW MORE
SHOW LESS

You also want an ePaper? Increase the reach of your titles

YUMPU automatically turns print PDFs into web optimized ePapers that Google loves.

Conclusions <strong>and</strong> Recommendations<br />

complexity appears greater, dual<br />

<strong>in</strong>fections are more prevalent <strong>and</strong><br />

severe disease is more widespread,<br />

although <strong>in</strong> most <strong>in</strong>stances (4/5) <strong>the</strong><br />

dual <strong>in</strong>fections were <strong>in</strong> areas affected<br />

by <strong>the</strong> East African CMD p<strong>and</strong>emic<br />

(Legg, 1999). An association between<br />

virus <strong>in</strong>fection <strong>and</strong> epidemiology is<br />

similarly apparent (Table 1). In East<br />

Africa, current-season <strong>whitefly</strong>-<strong>borne</strong><br />

<strong>in</strong>fection is greatest <strong>in</strong> Ug<strong>and</strong>a, where<br />

<strong>the</strong> “Ug<strong>and</strong>a variant” of EACMV<br />

(EACMV-Ug) was reported (Chapter<br />

1.11, this volume) <strong>and</strong> where EACMV-<br />

Ug/ACMV mixtures were common, as<br />

reported earlier by Harrison et al.<br />

(1997); whilst <strong>in</strong> Nigeria, where only<br />

ACMV was reported, current-season<br />

<strong>whitefly</strong>-<strong>borne</strong> <strong>in</strong>fection was very<br />

<strong>in</strong>frequent.<br />

The <strong>in</strong>cidence of CMD varied<br />

considerably with<strong>in</strong> sub-regions. In<br />

West Africa, disease <strong>in</strong>cidence ranged<br />

from 36% (Ben<strong>in</strong>) to 72% (Ghana);<br />

whilst <strong>in</strong> East <strong>and</strong> sou<strong>the</strong>rn Africa, it<br />

ranged from 21% (Tanzania) to 68%<br />

(Ug<strong>and</strong>a). In all countries, <strong>the</strong><br />

proportion of diseased plants that had<br />

arisen from plant<strong>in</strong>g diseased cutt<strong>in</strong>gs<br />

(cutt<strong>in</strong>g <strong>in</strong>fection) was greater than that<br />

derived from current season <strong>whitefly</strong><strong>borne</strong><br />

<strong>in</strong>fection. Many factors <strong>in</strong>fluence<br />

CMD <strong>in</strong>cidence, <strong>in</strong>clud<strong>in</strong>g <strong>the</strong> virus(es)<br />

<strong>in</strong>fect<strong>in</strong>g <strong>the</strong> plant, cultivar response to<br />

<strong>the</strong> virus(es), vector abundance <strong>and</strong><br />

transmission efficiency, <strong>the</strong> physical<br />

environment, <strong>and</strong> cropp<strong>in</strong>g practices<br />

(<strong>in</strong>clud<strong>in</strong>g vector or disease<br />

management measures). Most of <strong>the</strong>se<br />

factors rema<strong>in</strong> poorly def<strong>in</strong>ed for many<br />

of <strong>the</strong> participat<strong>in</strong>g countries, although<br />

<strong>the</strong> surveys have provided quantitative<br />

data relat<strong>in</strong>g to some aspects. In this<br />

respect it would be useful to compare<br />

<strong>the</strong> epidemiology of CMD between<br />

regions, us<strong>in</strong>g common cultivars for<br />

reference, as proposed by Legg et al.<br />

(1997).<br />

<strong>Whitefly</strong>-transmitted <strong>viruses</strong> <strong>in</strong><br />

sweetpotato<br />

Sweetpotato leaf samples with<br />

symptoms of virus disease from<br />

Ug<strong>and</strong>a, Kenya, Tanzania <strong>and</strong> Zambia<br />

were analysed. Chapter 1.12 discusses<br />

<strong>in</strong> detail <strong>the</strong> results of virus diagnoses<br />

from Ug<strong>and</strong>a, Kenya <strong>and</strong> Tanzania, <strong>and</strong><br />

Kaitisha <strong>and</strong> Gibson (1999) discuss<br />

those from Zambia. The East African<br />

stra<strong>in</strong> (S EA ) of Sweetpotato chlorotic<br />

stunt virus (SPCSV) occurred<br />

throughout <strong>the</strong> region <strong>and</strong>, <strong>in</strong> Ug<strong>and</strong>a,<br />

Aritua et al. (Chapter 1.12, this<br />

volume) have discussed distributions<br />

for <strong>the</strong> two dist<strong>in</strong>ct serotypes (S EA1 <strong>and</strong><br />

S EA2 ) described by Alicai et al. (1999).<br />

The o<strong>the</strong>r <strong>whitefly</strong>-<strong>borne</strong> virus<br />

identified from project surveys was<br />

Sweetpotato mild mottle virus (SPMMV),<br />

although this was restricted to areas<br />

around <strong>the</strong> shores of Lake Victoria<br />

(Chapter 1.12, this volume). SPVD,<br />

which results from co-<strong>in</strong>fection of<br />

sweetpotato plants with SPCSV <strong>and</strong> <strong>the</strong><br />

aphid-<strong>borne</strong> Sweetpotato fea<strong>the</strong>ry mottle<br />

virus (SPFMV), occurred throughout<br />

areas sampled. The disease was most<br />

frequent <strong>in</strong> south-western Ug<strong>and</strong>a, <strong>the</strong><br />

Lake Victoria zone of Tanzania, sou<strong>the</strong>astern<br />

coastal Tanzania <strong>and</strong> <strong>the</strong> semiarid<br />

south-western part of Madagascar.<br />

Diagnostic tests related SPVD to <strong>the</strong><br />

co-occurrence of SPCSV <strong>and</strong> SPFMV<br />

for each of <strong>the</strong>se regions (Chapter 1.12,<br />

this volume). No samples from<br />

Madagascar were tested.<br />

<strong>Whitefly</strong> natural enemies<br />

The agreed survey protocol for <strong>the</strong><br />

TWF-IPM Project provided guidel<strong>in</strong>es<br />

for <strong>the</strong> collection of parasitoids,<br />

predators <strong>and</strong> entomopathogens dur<strong>in</strong>g<br />

<strong>the</strong> course of diagnostic surveys.<br />

Limitations <strong>in</strong> <strong>the</strong> methods proposed,<br />

<strong>and</strong> <strong>the</strong> short period of time available<br />

for natural enemy collection at each<br />

sampl<strong>in</strong>g site, meant that <strong>the</strong> collection<br />

of natural enemies was less<br />

comprehensive than had been<br />

105

Hooray! Your file is uploaded and ready to be published.

Saved successfully!

Ooh no, something went wrong!