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Date: Thu 14 May 2009
Source: University of Georgia College of Agricultural and Environmental Sciences [edited] <http://georgiafaces.caes.uga.edu/storypage.cfm?storyid=3729>

Watermelon farmers fear gummy stem blight [GSB] more than any other disease. To develop better ways to manage it, University of Georgia [UGA] plant pathologists are leading a regional effort to figure out how GSB epidemics start each year. Once GSB hits a field, it can be hard to contain, especially in hot, wet weather, said David Langston, UGA. It attacks leaves and stems, severely reducing yields and sometimes killing plants.

Langston said, "If you don't know where the inoculum is coming from, it is near impossible to control a disease." Farmers typically spray fungicides 8 times during the growing season. There are many fungicides to fight GSB, but the disease is very good at becoming resistant to them quickly, Langston said -- as fast as in just a few years of the 1st use. This is a big problem.

Georgia is a top watermelon-producing state. Farmers typically plant transplants grown from seeds from Asia, South America, and Australia. Farmers in these places use fungicides to control the disease, but which ones and how often is not known in the US. Langston believes that GSB strains are becoming resistant to certain fungicides in foreign fields.

The disease could be due to the seeds, or it could be spread through a build up of [local] spores. Research will determine if US greenhouse plants are being infected by GSB spores either already inside or flying in from the outside. If GSB is seen [inside] before spores arrive in the area [outside], it came from the seeds. The GSB spores collected from infected plants and the air will be tested to see if they are resistant to fungicides.

The UGA researchers will do similar comparisons in farmers' fields using spore traps [see below] in Florida, Georgia, South Carolina, and North Carolina to determine if fungal spores are present at or during GSB epidemics. If the disease is mostly seed-borne, cooperative efforts with seed companies and their foreign growers could reduce or eliminate fungicide resistance problems. If air-borne spores are the problem, more aggressive fungicide rotation programs and management practices could be developed.

[Byline: Brad Haire]

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[Gummy stem blight of cucurbits is caused by the fungus _Didymella bryoniae_ and is the most common leaf disease of watermelon. Symptoms include lesions on leaves and stems of seedlings. The seedling may be girdled by the lesions and collapse, or be so weakened by the infection that it is never productive. Gummy stem blight requires prolonged moisture to become established and consequently is a serious problem in regions of high rainfall and glasshouse crops. The fungus also causes black rot if it infects the fruits.

Spores may be seed-borne or spread by air currents, water from rain or irrigation systems, or insects. The fungus can survive between growing seasons in an over-wintering structure, and wild cucurbits or volunteer hosts may serve as pathogen reservoirs. Disease management includes fungicides and use of certified GSB-free seed from reliable
sources. It is not clear why seed certification does not appear to be available or effective for the farmers mentioned in the story above.

Spore traps are used for determining identity and time of dispersal of fungi. Spores can be identified by morphology to genus and sometimes species. They can indicate the presence of a fungal pathogen before disease is discovered in the field because symptoms need time to develop and may initially be difficult to detect.

Maps of USA:
<http://www.mapsofworld.com/usa/usa-state-and-capital-map.html> and
<http://healthmap.org/r/009V>

Pictures
GSB on cucurbit transplants:
<http://agdev.anr.udel.edu/weeklycropupdate/wp-content/uploads/2008/05/gummystem-300x253.jpg> 
and
<http://www.forestryimages.org/images/192x128/1234191.jpg>
GSB on watermelon:
<http://www.aragriculture.org/images/diseases/image_library/Tomato_Vegetables/watermelon/
V19a_Watermelon.jpg> 
and
<http://www.clemson.edu/coastalrec/Melcast/Gsbloss.gif>
GSB on cucurbits, photo gallery:
<http://vegetablemdonline.ppath.cornell.edu/Images/Cucurbits/GummyStem/GummySB_Collage.jpg>
Spore trap:
<http://georgiafaces.caes.uga.edu/graphicsfiles/LangstonSporeCollector1.jpg>

Links
Gummy stem blight and black rot information:
<http://vegetablemdonline.ppath.cornell.edu/factsheets/Cucurbit_GSBlight.htm>,
<http://ohioline.osu.edu/hyg-fact/3000/3126.html>,
<http://www.ces.purdue.edu/extmedia/BP/BP-142-W.pdf>,  and
<http://cals.arizona.edu/PLP/plpext/diseases/vegetables/watermelon/watermelongsb.htm>
GSB review and disease history:
<http://www.apsnet.org/online/feature/pumpkin/gummy.html>
_D. bryoniae_ taxonomy:
<http://www.indexfungorum.org/Names/NamesRecord.asp?RecordID=224305>
Information on spore traps:
<http://www.burkard.co.uk/7dayst.htm>,
<http://ionicsporetrap.com/main/inside.php?page=why_a_new_spore_trap>,  and
<https://www.emlab.com/media/reports/STstandard.pdf>.  - Mod.DHA]