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International Society for Infectious Diseases
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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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[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
----
Downy mildew, cucumber - Canada (ON), USA (MI) 20060803.2149
2003
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Downy mildew, squash - Italy (Latium, Umbria) 20030401.0803] |
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