New findings of Iris yellow spot virus in the Netherlands in 2007
The NPPO of the Netherlands recently informed the EPPO Secretariat about new findings of Iris yellow spot virus (Tospovirus, IYSV - EPPO Alert List). In August 2007, new infections of IYSV were detected in onion (Allium cepa) at different locations. In total, 31 plants out of 108 from 5 fields tested positive for IYSV. However, none of the tested plants showed any virus symptoms. In September 2007, IYSV was detected in one crop of Eustoma in the vicinity of two onion fields infected by IYSV. The origin of IYSV infections in onion remains unclear but may be attributed to the possible existence of wild host plants. Therefore, a more extensive survey in onion is planned for 2008.
In the Netherlands the following incidental findings of IYSV have been recorded since 1992.
|
Year
|
Crop
|
Number of thrips
|
|
1992
|
Iris
|
Many
|
|
1997
|
Allium porrum
|
Few
|
|
2004
|
Alstroemeria (2x)
|
Many
|
|
2005
|
Alstroemeria
|
Few
|
|
2005
|
Allium cepa (1 plant)
|
Many
|
|
2006
|
Allium cepa (3 plants)
|
Many
|
|
2006
|
Eustoma
|
Many
|
|
2007
|
Allium cepa (31 plants)
|
Many
|
|
2007
|
Eustoma
|
Few
|
On the infected crops, IYSV only caused local symptoms, with the exception of Eustoma, in which it was not clear whether the plants were infected locally or systemically. Therefore, in these hosts (Eustoma possibly excluded) the virus will not spread systemically to other plant parts, including the bulbs of onion and ornamental crops. This would mean that the virus has to be introduced into a crop by viruliferous thrips (Thrips tabaci). Therefore, economic losses caused by IYSV strongly depend on the number of viruliferous thrips present, which in itself is thought to be related to the presence of systemic hosts. In the Netherlands, the IYSV infections in Iris (1992) were recorded after lifting symptomatic leek plants (Allium porrum), and in Alstroemeria (2004) and Eustoma (2006 and 2007) after lifting onion plants, some of which had been infected by IYSV. These correlations suggest that leek and onion had been sources of infection for the ornamental crops. However, since IYSV causes local lesions on leek and onion, thrips can only acquire the virus from these local lesions. Such lesions are mainly present on older leaves whereas thrips prefer younger leaves for feeding. These two aspects considerably reduce the probability of acquisition of the virus by thrips. In Alstroemeria and Eustoma lesions are necrotic, which further minimizes the chances for virus acquisition by thrips. Therefore, the Dutch NPPO considered that eradication of all infected plants does not seem necessary for virus control in the crops mentioned above. Moreover, eradication of infected onion crops would result in similar thrips behaviour as observed during the lifting of onions at the end of the growing season. Instead of eradicating IYSV infected plants, it is felt that more attention should be given to thrips control during the period when onion and leek crops are lifted.
In contrast to reports from some other countries, IYSV was mainly found in symptomless onion plants in the Netherlands; only in 2006 were mild symptoms observed in three plants. Therefore, infestations by IYSV could be more widely spread in Europe than currently known. In addition, the main source(s) of infection for onion and leek still seem to be unknown. Therefore, the search should continue for a host plant that may be infected systemically.
The pest status of Iris yellow spot virus in the Netherlands is officially declared as follows: Present, few occurrences, little damage.
Sources
NPPO of the Netherlands, 2008-03.
INTERNET (last retrieved in 2008-03)
Website of the Dutch Ministry of Agriculture, Nature and Food Quality. Pest record. Finding of Iris yellow spot virus in the Netherlands in 2007. http://www.minlnv.nl/portal/page?_pageid=116,1640321;_dad=portal;_schema=PORTAL;p_file_id=26184