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Date: 26 November 2008
Source: Western Farm Press [edited]
<http://westernfarmpress.com/vegetables/wilt-lettuce-1126/>

Verticillium wilt on the rise in Salinas Valley lettuce

As the spread of verticillium wilt of lettuce accelerated this season in California's Salinas Valley, plant pathologists and breeders stepped up their quest for ways to control it.

Research leader Krishna Subbarao, University of California, said, "The wilt was very active this year [2008], and since it was first discovered 13 years ago, there was never a year in which so many new lettuce fields succumbed to it." By early October [2008], he added,
13 new fields picked up the disease, bringing infected acreage in the Valley to more than 1500 acres (607 ha). Typically, during each of the past several years only 4 to 6 new fields were found to be infected.

The majority of incidence is around Salinas, although it spans from Watsonville to King City. At least 2 fields were abandoned, and others varied in loss from 20 percent to 60 percent. Many fields have been fumigated and planted to strawberries for 2 seasons, only to have the wilt reappear when lettuce was planted. The wilt has great diversity in the Salinas Valley, and thus far sources of it are known to be Washington State, Denmark, and the Netherlands.

[Byline: Dan Bryant]

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[Serious wilting diseases caused by _Verticillium dahliae_ and _V.
albo-atrum_ affect a wide range of crops and other plants. The fungi are soil-borne and can persist in soils for many years. _V. dahliae_ wilt has emerged as a serious threat to lettuce production in California. Symptoms include wilting and yellowing of the outer leaves as well as brown streaks in the vascular tissue of crown and root. Two races of the pathogen have been identified in lettuce, and weed reservoirs as well as infected seeds have been found to be potential sources of inoculum for this crop. Airborne spread is also suspected. Resistant lettuce cultivars are being developed, but in the meantime disease management includes crop rotation and fumigation to reduce soil-borne inoculum.

Maps
USA:
<http://www.mapsofworld.com/usa/usa-road-map-enlarge-view.html> and <http://healthmap.org/promed?v=40,-97.6,4>
US states:
<http://www.census.gov/geo/www/us_regdiv.pdf>
California:
<http://healthmap.org/promed/en?g=5391295&v=36.678,-121.656,5>

Pictures of _V. dahliae_ symptoms
On cucumber:
<http://www.apsnet.org/online/feature/plantdisease/images/fig21.jpg>
On sunflower leaf:
<http://www.inra.fr/hyp3/images/6037412.jpg>
On tomato:
<http://www.bspp.org.uk/NDR/july2006/2006-26-1.jpg>

Links
Verticillium wilt of lettuce, fact sheet:
<http://www.ipm.ucdavis.edu/PMG/r441101211.html>
_V. dahliae_ lettuce strains in California:
<http://www.ars.usda.gov/research/publications/publications.htm?SEQ_NO_115=230007>
Information on _V. dahliae_:
<http://www.cals.ncsu.edu/course/pp728/Verticillium/Vertifin.htm>
Weed reservoirs and seed transmission of _V. dahliae_:
<http://repositories.cdlib.org/postprints/1023/>
_V. dahliae_ taxonomy:
<http://www.indexfungorum.org/Names/NamesRecord.asp?RecordID=196942>
Verticillium wilts, crop and management information:
<http://ohioline.osu.edu/hyg-fact/3000/3122.html>  and <http://www.apsnet.org/education/LessonsPlantPath/Verticillium/mngmnt.htm>.
- Mod.DHA]