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Date: June, 2008
Source: Plant Disease 92(6), 978 [edited]
<http://apsjournals.apsnet.org/doi/abs/10.1094/PDIS-92-6-0978A>

First report of the A2 mating type of _Phytophthora infestans_ on tomato crops in Taiwan, Republic of China

In a study of the _Phytophthora infestans_ population in Taiwan, samples with symptoms typical of late blight were collected from field crops in an important potato (_Solanum tuberosum_) and tomato (_Lycopersicon esculentum_) production area in the central highlands region.

Isolates were obtained, and the pathogen was confirmed as _P. infestans_ on the basis of morphological characters. Mating type was determined by co-inoculating test isolates and a reference _P. infestans_ isolate of either the A1 or A2 mating type and examination for the presence of oospores where the colonies interacted. In 2004, one isolate of 200 tested and in 2006, one isolate of 102 tested produced oospores only with A1 reference isolates and were concluded to be A2 mating type.

In vitro testing showed the 2 A2 isolates were metalaxyl-resistant, which is typical of recent _P. infestans_ isolates from potato and tomato in this area. 21 single-sporangial isolates from each were confirmed as A2. These isolates were characterized and found to have a multi-locus genotype characteristic of recent _P. infestans_ isolates from tomato and potato in Taiwan, but all previous such isolates were A1 mating type. Koch's postulates were completed, and the 2 A2 isolates were found to be highly aggressive on cultivars of potato and tomato.

To our knowledge, this is the 1st report of A2 mating type strains of _P. infestans_ in the field in Taiwan, but currently, their incidence is very low (less than one percent). One crop from which an A2 isolate was obtained also yielded an A1 isolate, while A1 isolates were obtained from crops in the vicinity of the other. The concurrent presence of the 2 mating types of _P. infestans_ poses a risk of sexual reproduction and oospore formation in tomato or potato in Taiwan.

[Byline: K. L. Deahl et al.]

--
Communicated by:
ProMED-mail <promed@promedmail.org>

[Late blight caused by the fungus _Phytophthora infestans_ is a devastating disease of potato and tomato with up to 100 percent crop losses. To a lesser extent, it can also affect other solanaceous crops such as eggplant and capsicum. The pathogen can affect leaves, stems, and fruit of tomato, causing spots, lesions, and rotting, and in potato it affects leaves as well as tubers. The disease can spread rapidly within a crop and destroy it within a few days. It is highly infectious, and once it is found in a region, all susceptible crops nearby are threatened. The disease is favoured by cool, moist conditions, and under favourable conditions, epidemics in tomatoes seem to be even more rapid than in potatoes. Spread often occurs with plant material (including tomato transplants or potato seed tubers), by wind, water, or mechanical means. Disease management requires an integrated approach and includes phytosanitary measures, removal of weeds and volunteer crop plants serving as pathogen reservoirs, limiting free moisture in the crop, preventative fungicide treatments of planting material, and fungicide sprays of crops. Crop rotation with non-hosts may reduce available inoculum.

Late blight is considered an increasing problem worldwide because new and even more virulent strains continue to emerge. Some of these can destroy a plant in a matter of hours and a complete crop within days.
Two mating types of the fungus exist, A1 and A2. Where both are present, reproduction occurs sexually as well as asexually, leading to even fitter strains with higher fungicide resistances and increased yield losses. A1 was the 1st to spread worldwide. A2 began to spread later and is currently present in northern and Central America, parts of Asia, as well as in northern Europe, where a dramatic increase in the frequency of A2 is being recorded recently in the western parts. The finding of the A2 type in Taiwan severely increases the risk posed by late blight to national agriculture.

Maps of Taiwan:
<http://www.lib.utexas.edu/maps/middle_east_and_asia/taiwan_pol92.jpg>  and via <http://eng.taiwan.net.tw/lan/Cht/map/index.asp#>

Pictures
Late blight symptoms on tomato:
<http://www.omafra.gov.on.ca/english/crops/hort/news/hortmatt/2006/23hrt06a3f1.jpg>
Tomato field destroyed by late blight:
<http://www.apsnet.org/online/feature/BioSecurity/Images/lateblightbolkan.jpg>
Late blight on potato:
<http://www.sciencedaily.com/images/2007/01/070102132649.jpg>  and
<http://www-biol.paisley.ac.uk/bioref/Chromista/potato_blight.jpg>

Links
Late blight fact sheets:
<http://ohioline.osu.edu/hyg-fact/3000/3102.html>  and <http://vegetablemdonline.ppath.cornell.edu/factsheets/Potato_LateBlt.htm>
Disease history and background:
<http://www.apsnet.org/online/feature/lateblit/>,  and <http://www.olympusmicro.com/micd/galleries/brightfield/potatoblight.html>
Late blight information and resources via:
<http://www.potato.org.uk/department/knowledge_transfer/fight_against_blight/advice_blight.html>
_P. infestans_ taxonomy:
<http://www.indexfungorum.org/Names/NamesRecord.asp?RecordID=232148>
Global Initiative on Late Blight:
<http://gilb.cip.cgiar.org/>
- Mod.DHA]

[see also in the archive:
Late blight, potato - UK: (Scotland), alert 20080430.1482 Late blight, tomato, potato - USA: (FL) 20080219.0664 Potato diseases - India: (West Bengal) 20080206.0477 Late blight, potato - Papua New Guinea: recovery 20080121.0256 Fungal diseases, potato - Bangladesh 20080107.0091
2007
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Late blight, potato - India (02): (Punjab, W Bengal) 20071221.4099 Late blight, potato - UK: new strains 20071207.3939 Late blight, potato - India: (Punjab), alert 20071116.3715 Fungal diseases, vegetable crops - Canada: cucumber, potato 20070730.2442 Late blight, potato & vegetable fungal diseases - Europe 20070708.2174 Late blight, potato - India, UK 20070509.1491
2006
----
Late blight, potato - India (Kashmir) 20060424.1200 Late blight, potato - USA (AK), Bangladesh 20060324.0911
2003
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Late blight, potato - Papua New Guinea 20030306.0554
2002
----
Potato late blight, potato - Canada (Newfoundland) 20020818.5091
2001
----
Phytophthora infestans, potato late blight - Russia 20010620.1177 2000
----
Potato late blight, global research efforts 20001031.1903 Potato late blight: global initiative 20000516.0765
1996
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Potato late blight: global threat 19960617.1123]

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