Features Crop Protection Inputs
WUR: how dangerous viroids in plants are spread

July 26, 2010, Wageningen, the
Netherlands – The dangerous potato spindle tuber viroid (PSTVd) mainly
ends up in
tomato plants as a result of human actions. The main source of infection
appears to be ornamental shrubs, with infection via seeds rarely if ever
playing a role.

Viroids can cause severely misshaped potatoes. Left: healthy potatoes, middle and right: potatoes of infected plants. PHOTOS COURTESY WUR

July 26, 2010, Wageningen, the Netherlands – The dangerous potato spindle tuber viroid (PSTVd) mainly ends up in
tomato plants as a result of human actions. The main source of infection
appears to be ornamental shrubs, with infection via seeds rarely if ever
playing a role. These conclusions are published in the dissertation of Koo
Verhoeven with which he obtained his doctoral degree at the Wageningen
University, part of Wageningen UR, earlier this year.

“Viroids are miniscule pathogens that consist of only one RNA molecule,”
says Verhoeven, employee at the Dutch Plant Protection Service (PD). “In
comparison: A virus is a DNA or RNA molecule covered with a protein shell.
Scientists were not able to determine the nature and presence of viroids until
the 1970s.” 

Viroids cause problems in growth and development of plants.

Verhoeven developed a molecular-biological technology that can relatively
easily determine whether tomato plants are infected with viroids. He extracts
the RNA of viroids from samples and further analyses it to determine the
presence of viroids. One of the results was Verhoeven’s discovery of a new
viroid in capsicum plants.

The main viroid in the Netherlands is the potato spindle tuber viroid
(PSTVd), which causes growth inhibitions, discolouration and deformities in
tomatoes and potatoes. Not all crops that carry PSTVd actually become diseased
says Verhoeven. “Specific ornamental shrubs such as Solanum jasminoides,
commonly known as the potato vine, are not sensitive to the viroids. As a
result it is impossible to see if plants are infected, making them a dangerous
source of infection.” 

In stark contrast, tomato and potato plants are highly sensitive to
PSTVd. Verhoeven discovered that the tomato in particular was easily infected
with PSTVd by people. Fingers which have touched leaves of viroid-carrying
ornamental shrubs can transmit them to tomato plants up to two hours after contact.
Viroids can also be spread by tools such as the small knives used to cut shoots
from plants.

In the outbreaks studied by Verhoeven, the viroids in the plants were
shown not to have originated from the used tomato seeds. Verhoeven was also
involved in research into the role of insects such as bumblebees and thrips in
the transfer from ornamental shrubs to tomato plants. The results showed that
they were not a major source of infection. “Tomato plants infected with PSTVd
were most likely infected by people bringing the viroid into the greenhouse
from outside,” says Verhoeven. 

Verhoeven’s conclusions are important to both breeders and breeding
companies. Breeders should be able to organise their activities in such a way
that the risk of spreading the viroids is as limited as possible. Breeding
companies could be more assured that their products are not a source of PSTVd
infection and use Verhoeven’s findings to prevent viroids from ending up in
seed production.

The test Verhoeven developed is of interest to the international trade
and the organisations that carry out the related phytosanitary checks, such as
the Dutch Plant Protection Service (PD) and the inspection services. Now that
it is known which viroids occur in tomatoes, what the major sources of
infection are and how the viroids are spread, all the essential data for making
a Pest Risk Analysis is available. Based on such an analysis, governments can
make more substantiated decisions in granting a quarantined status. Such a
status offers extra guarantees that the plant material on the market is not
contaminated with specific organisms. It can also result in the compulsory
destruction of crops, fruits and seeds, even if they are not visibly affected
by the infection.