Page 9 - Volume 2,Issue 2, July 2009

Pepino mosaic virus

53

the tomato strain in commercial tomato

crops.

As a potexvirus, PepMV is easily mechani-

cally transmissible and remains infectious

for several weeks in plant debris or on

contaminated surfaces.

Tomato fruit harvested from infected

plants contain high concentrations of the

virus and as such, they can easily spread

the virus over long distances.

Due to the absence of explicit symp-

toms, the initial introduction of PepMV into

Europe in commercial tomato crops may,

thus, have remained largely unnoticed.

Nothing is known about the possible occur-

rence of the EU-tomato, Ch2 or US1 strains

in pepino or in wild and cultivated

Sola-

num

spp. in South America and the precise

origin of these strains remains unknown. It

has been suggested that the EU-tomato and

Ch2 isolates show an increased fitness and

virulence in commercial tomato crops. This

might explain the fast worldwide spread of

the virus problem.

Although PepMV was already present in

Spain since 2000, the Ch2 strain has spread

quickly over Europe after 2005. No obvious

reasons for this have been identified. It is

also not clear if this strain is more compet-

itive than the EU-tomato strain. Both strains

often occur in mixed infections and genet-

ic recombinants between the EU-tomato

and Ch2 strain have been identified both in

Spain (Pagán

et al.,

2006) and Belgium (Han-

ssen

et al.,

2008). It is not known to what

extent these mixed infections and recom-

bination events have contributed or will

contribute to the severity of the virus symp-

toms or to the spread of the virus problem.

Seed transmission has been implicat-

ed in the spread of the virus. The introduc-

tion through infected seed of such a high-

ly contagious virus in only a few plants in a

nursery can obviously result in the infection

of many thousands of plants in a very short

time. For this reason, PepMV has a quaran-

tine status on tomato seeds in the Europe-

an Union. Directive 2004/200/EC prohib-

its the trade of seeds collected from PepMV

infected plants or from plants not officially

tested for the absence of PepMV. Recently, a

study on the possibility of seed transmission

of PepMV was conducted within the frame-

work of the EC FP6 funded research project

PEPEIRA (

‘Pepino mosaic virus: epidemiology,

economic impact and pest risk analysis’

; www.

pepeira.wur.nl). In this study, 100,000 seeds

were harvested from an infected crop. Fol-

lowing only marginal cleaning without any

disinfection, the seeds were sown and the

seedlings were tested for PepMV infection

by DAS-ELISA. Of nearly 9,000 tested batch-

es of 10 seedlings each, only 23 were positive

(Hanssen

et al

., in press). This corresponds to

a seed transmission rate of 0.026%, which is

in good agreement with earlier tests (Krin-

kels, 2001; Córdoba-Sellés

et al

., 2007).

Virus damage and control

Initially PepMV infections in commercial to-

mato crops were reported as relatively in-

significant with no apparent plant or fruit

symptoms and no or very limited yield re-

duction. Studies from the UK, however, re-

ported significant effects on fruit quality

(Spence

et al.,

2006). Smaller sized fruit with

different grades of uneven ripening and dis-

coloration and occasionally misshaped fruit

due to PepMV infection led to production of

fruit unsuited for the fresh UK market and

hence to economic damage.

Studies indicate that symptom induc-

tion by PepMV is highly dependent on en-

vironmental conditions. In Spain, PepMV-

infected tomato plants showed symptoms

only from autumn through spring and

symptoms disappeared in late spring when

higher temperatures occurred. Similar ef-

fects of high temperatures and high light

conditions have been reported from oth-

er countries. However, symptoms on Pep-

MV-infected plants can be highly variable,

ranging from very mild leaf symptoms to se-

vere leaf and stem necrosis and fruit symp-

toms. Mild, chlorotic or necrotic PepMV iso-

lates from both the EU-tomato and the Ch2

strains have been reported. This suggests

that the capacity of the virus to induce dif-