"Black Rot" Blight Upon our Watermellons

GUMMY STEM BLIGHT, WATERMELON - USA: NEW STRAINS
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A ProMED-mail post

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


Date: Thu 14 May 2009
Source: University of Georgia College of Agricultural and
Environmental Sciences [edited]



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]

--
Communicated by:
ProMED-mail


[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:
and


Pictures
GSB on cucurbit transplants:

and

GSB on watermelon:

and

GSB on cucurbits, photo gallery:

Spore trap:


Links
Gummy stem blight and black rot information:
,
,
, and

GSB review and disease history:

_D. bryoniae_ taxonomy:

Information on spore traps:
,
, and
. - Mod.DHA]

[see also:
2007
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Downy mildew, cucurbits - USA (02): clarification 20071109.3633
Downy mildew, cucurbits - USA: (SC), new strain 20071023.3448]
...................................dha/mj/mpp