4. Discussion
Only a minority of the farmers in this study can identify viruses
as the causal agent of specific disease symptoms. Even fewer know
the role of insect vectors in the spread of plant viruses, which
confirms the earlier study of Nagaraju et al. (2002) in Karnataka,
who found that only 2% knewthat the tomato leaf curl virus disease
was transmitted by whiteflies. However, farmers in Tamil Nadu
generally had more knowledge about plant viruses than farmers in
Thailand and Vietnam, as extension services and seed companies
had paid attention to this issue.
Strategies to address plant virus diseases need to include farmlevel
training to increase farmers' awareness and knowledge about
plant viruses, including disease identification, epidemiology and
management. Being able to distinguish symptoms of virus infection
from other diseases, and knowledge that most viruses are transmitted
by insect vectors (mainly whiteflies, aphids or thrips) may
encourage farmers to control insect populations and avoid the
unnecessary use of fungicides and other pesticides ineffective
against virus diseases. Yet, insect control must be practiced before
the symptoms of virus diseases have started appearing in the
cropdwhich requires a massive leap in farmers' knowledge about
the epidemiology of viruses. Careful monitoring of vector populations
for disease control is not practiced currently; for instance,
no farmers in our sample used blue or yellow sticky traps, a simple
method commonly employed for vector population monitoring in
other counties, nor did input shops sell such traps.
Symptoms of fungal, bacterial and viral diseases can be
confusing even for trained experts to identify, and may occur in
mixtures, further confounding correct identification. This study
could have been stronger if it had been combined with a diagnostic
survey to more accurately determine the incidence and severity of the different pests and diseases and to estimate the actual losses
caused. Knowledge discrepancies thus could have been recognized
more precisely. Nevertheless, it is important to know farmers'
perceptions of pest problems and losses because these beliefs
shape practices. Our study showed that current practices to manage
virus diseases are largely ineffective and probably unsustainable.
Better practices are already available, but farmers will not use them
unless they have a better understanding of the problems they are
facing.
Management options readily available to farmers include
keeping fields and surrounding areas free of weeds and volunteer
plants (alternative hosts for viruses and their vectors); ensuring old
crops are completely destroyed after harvest to remove sources of
infection; growing seedlings (tomato and chili) in net cages that
exclude virus-vectoring insects until the seedlings are transplanted
to the field; and inspecting the crop regularly after transplanting
and removing (roguing) plants showing virus disease symptoms as
soon as they are seen (to remove sources of infection for spread
within the field). Insect traps or insecticides can also be used to
control vector populations, but must be applied before virus disease
symptoms first appear in the field, which emphasizes the
importance of monitoring