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Monday, December 22, 2008

Whitefly

Whitefly-Transmitted
Geminiviruses

Found in tropical and subtropical regions of Asia and the Western
Hemisphere, depending on specific virus



Symptoms

Several whitefly-transmitted
geminiviruses infect
peppers, including chino del tomate virus (CdTV), pepper
mild tigré virus (PMTV), Serrano golden mosaic virus
(SGMV), Sinaloa tomato leaf curl virus (STLCV), and

Texas pepper virus (TPV). These viruses all have similar
symptoms but are biologically and genetically distinct.
Symptoms depend upon the geminivirus and the
pepper variety. Common symptoms are stunting,
curling, or twisting of the leaves, bright yellow mosaic,
distortion of leaves and fruit, and reduced yields.
Peppers with CdTV infection are symptomless or
have a mild mosaic with slight leaf distortion (s
ee below).
Symptoms due to PMTV include stunting, leaf distortion,
yellow-green mottling on foliage, and small, misshapen
fruit. These viruses are components of Tigré disease, a
severe disease complex.
Pepper plants affected by
SGMV have foliage with
a bright golden mosaic color (see next page). STLCV
symptoms include foliage with yellow to yellowish-green
mosaic, interveinal chlorosis, and various degrees of
leaf curl from moderate to severe, as well as stunting
accompanied by shortened internodes, and small fruit.



Conditions for Disease Development

The pepper geminiviruses are transmitted by the sweet
potato whitefly, Bemisia tabaci, which is commonly
found in tropical and sub-tropical regions, and in
greenhouses in temperate areas. No aphid or thrip
transmission occurs, and only the TPV and SGMV
geminiviruses are mechanically transmitted from pepper
to pepper. Pepper geminiviruses are not seed-borne.
This whitefly has a very wide host range and feeds
by sucking plant juices from the underside of leaves of
crops such as pepper, tomato, tobacco, cucumber,
sweet potato, as well as some weeds. Adult whiteflies
look like tiny white moths, about 1–2 mm in length.
They fly when the leaf is disturbed.
The whitefly can acquire the virus after feeding on
infected plants for 15 to 30 minutes, and can transmit
the virus to pepper plants after 24 hours of incubation
within the insect. A period of at least 15 minutes feeding
on the new pepper host is subsequently required for
transmission of the virus. The whitefly retains the virus
for up to 20 days and does not transmit it to the progeny.
Symptoms develop on young plants after 10 to 14 days.
Hot and dry conditions favor the whitefly and
therefore help the spread of these viruses. Whitefly
populations decrease after heavy rains.

Control

Control of geminiviruses is difficult once a crop
becomes infected. Numerous cultural practices can be
used to prevent infestations.
Grow seedlings in an insect-proof nethouse or
seedbed (32-mesh size or finer) to prevent early
infection, which leads to severe crop damage.

A barrier of maize may be planted around the pepper
crop, and mulches of straw, sawdust or yellow plastic
or UV-reflective material will reduce landing of whiteflies.
Inter-planting of pepper with ‘bait’ plants may be
useful for control of this virus but other viruses may
increase in importance. The ‘bait’ plants are then
sprayed with an insecticide.
Timing of transplanting can be effective for avoiding
high populations of whitefly and therefore reducing
infection by pepper geminiviruses. Avoid overlapping
pepper crops that allow the vector to subsist and
develop new populations. Rotation with non-host crops
is also recommended. Roguing of volunteer pepper,
tomato and tobacco plants, and controlling weeds are
important to reduce sources of virus inoculum.
Chemical control methods include the use of
systemic insecticides as soil drenches or overhead
sprays during the seedling stages. Applications in the
field may be needed to control adults that emerge after
transplanting. Rotation of insecticides is recommended
to prevent insects developing resistanceto the
chemicals. Chemical control may not be effective in
areas where disease incidence is high.
Oil sprays may also be effective in reducing levels
of infestation. Neem tree seed extracts control young
nymphs, inhibit the growth and development of older
adults, and reduce egg-laying by adults.
Pepper varieties resistant or tolerant to pepper
geminiviruses are not yet commercially available.



1 comment:

Jimi Rashdan Bin Jamaludin said...

Nak guna apa untuk serangga nie??