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New strain of downy mildew in cucurbits - Charleston County, South Carolina, USA

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A ProMED-mail post
ProMED-mail is a program of the International Society for Infectious Diseases

A ProMED-mail post
<http://www.promedmail.org>
ProMED-mail is a program of the
International Society for Infectious Diseases <http://www.isid.org>

October 15, 2007
Source: The Times and Democrat [edited]
<http://www.timesanddemocrat.com/articles/2007/10/15/business/12762204.txt>

An uncommon and damaging strain of cucurbit downy mildew has been seen in Charleston County. The disease can damage crops such as cantaloupe [rockmelon], cucumber, summer and winter squash, pumpkin, watermelon, and bottle gourd. The disease is also widespread in the middle part of the state on cucumber, summer and butternut squash, according to Anthony Keinath, Clemson University vegetable pathologist at Coastal Research and Education Center in Charleston.

Keinath said the symptoms on watermelon are different than symptoms on other cucurbits. Leaf spots on watermelon are dark brown and irregular in shape. Slight yellowing may be seen around the edges of the spots or in small patches in other parts of the leaf. Leaves infected curl inward as the leaves die. Spores are usually found on the bottom of the leaf, although spores may be formed on top of the leaf in severe infections. The best time to look for spores is in the morning before dew has dried. The spores are brownish-purple and the mold growth is white to colorless.

Keinath suggests looking at the size, shape, and position of leaf spots to distinguish downy mildew on watermelon from gummy stem blight, which is the most common leaf disease on watermelon. Leaf spots of gummy stem blight are larger than individual spots of downy mildew.

Downy mildew kills leaves and vines prematurely, which reduces the number and size of the fruit but does not cause it to rot. It also reduces the sugar content of fruits, particularly cantaloupe and watermelon. "All cucurbit crops in South Carolina are at serious risk," Keinath said. Crops must be sprayed with fungicides every 5 to
7 days. Keinath suggests that organic growers should use copper products but also should expect yield loss, because copper is not completely effective against downy mildew. Home gardeners should spray with a fungicide or copper products.

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

[Downy mildew on cucurbits is caused by the fungus _Pseudoperonospora cubensis_. Different strains with variable host preferences (pathovars) seem to exist, and it is not uncommon, for example, for cucumbers to be severely affected while watermelons nearby are not. 
_P. cubensis_ is found worldwide, but it is usually most damaging in tropical and subtropical areas. By affecting the leaves and thus the photosynthetic potential of its host it reduces yield and fruit quality, and it can eventually kill the host plant. It is an obligate parasite surviving on cultivated or wild cucurbits and is spread by wind, rain, and mechanical means. Disease development can occur over a wide temperature range under conditions of high humidity. Control strategies include the use of resistant cultivars and fungicide applications.

The related species _P. humuli_ causes a serious disease of hops.
Downy mildews in other genera affect grapevine, sunflower, and a number of vegetable crops. Generally, each fungal species is confined to a number of host species.

Gummy stem blight of watermelon is caused by the fungus _Didymella bryoniae_. Symptoms are lesions on leaves and stems of seedlings. The seedling may be girdled by lesions and collapse or be so weakened by the infection that it is never productive. Gummy stem blight is a serious problem in regions of high rainfall, and the fungus requires prolonged moisture to become established. The fungus is seed-borne.
It can become a problem in watermelon transplant glasshouse facilities where the use of clean seed is essential.

Maps
US states:
<http://www.census.gov/geo/www/us_regdiv.pdf> and <http://healthmap.org/promed?v=40,-97.6,4>

Pictures
Downy mildew on cucumber leaf:
<http://www.ag.ohio-state.edu/~vegnet/news/currentvn10-06_files/image002.jpg>
Photo gallery of downy mildew on cucurbit species:
<http://www.ces.ncsu.edu/depts/pp/cucurbit/images.php>
Gummy stem blight on watermelon:
<http://plantpathology.tamu.edu/Texlab/Vegetables/wmelon/gsb4.asp?ploc=gsb.htm>
and
<http://www.clemson.edu/coastalrec/Melcast/Gsbloss.gif>

Links
Cucurbit downy mildew, disease information and control strategies:
<http://web.aces.uiuc.edu/vista/pdf_pubs/927.pdf> and <http://www.avrdc.org/LC/cucurbits/downy.html>
Cucurbit downy mildew pathotypes:
<http://www.ces.ncsu.edu/depts/pp/cucurbit/thedisease.php>
_P. cubensis_ taxonomy:
<http://www.indexfungorum.org/Names/SynSpecies.asp?RecordID=120276>
_P. humuli_ taxonomy:
<http://www.indexfungorum.org/Names/NamesRecord.asp?RecordID=119090>
Gummy stem blight information:
<http://plantpathology.tamu.edu/Texlab/Vegetables/wmelon/gsb.htm> and <http://cals.arizona.edu/PLP/plpext/diseases/vegetables/watermelon/watermelongsb.htm>
_D. bryoniae_ taxonomy:
<http://www.indexfungorum.org/Names/NamesRecord.asp?RecordID=224305>
Clemson University Coastal Research and Education Center, Plant Pathology:
<http://www.clemson.edu/coastalrec/crecweb_25_FacKeinath.html>. - Mod.DHA]

[see also in the archive:
Fungal diseases, vegetable crops - Canada: cucumber, potato 20070730.2442 Downy mildew, cucumber - USA (OH, MI), Canada (ON) 20070704.2120
2006
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Downy mildew, cucumber - Canada (ON), USA (MI) 20060803.2149
2003
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Downy mildew, squash - Italy (Latium, Umbria) 20030401.0803]

 

CLARIFICATION

November 8, 2007

Clarification re. New strain of downy mildew in cucurbits - Charleston County, South Carolina, USA

 

 

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