Watermelon
Crop Information
Gummy Stem Blight Resistance
- by Gabriele Gusmini and Todd C. Wehner
- Department of Horticultural Science
- North Carolina State University
- Raleigh, NC 27695-7609
Gummy stem blight caused by Didymella bryoniae
(Auersw.) Rehm [=Mycosphaerella citrullina (C.O.Sm.)
Gross. and Mycosphaerella melonis (Pass) Chiu &
Walker] and its anamorph Phoma cucurbitacearum
(Fr.:Fr.) Sacc. [=Ascochyta cucumis Fautrey &
Roum] is one of the most destructive diseases of watermelon.
Resistance to gummy stem blight was ranked for several years
by U.S. watermelon researchers as the third most important
trait for germplasm evaluation (after bacterial fruit blotch
and Fusarium wilt). Gummy stem blight was first described
by Fautrey and Roumeguere in France as the disease caused
on cucumber by Ascochyta cucumis in 1891. In 1917,
gummy stem blight was reported for the first time in the
U.S. affecting watermelon fruit from Florida, where it is
still an important limiting factor for the watermelon industry.
One severe gummy stem blight epidemic on watermelon was
reported in the Southeastern USA, with over 15% of the watermelon
crop in South Carolina abandoned before harvest. In addition,
severe economic losses have been reported during transportation
and in storage due to the disease commonly known on fruit
as black rot.
Relevant characteristics of these disease/fungus are:
- Symptoms are evident as crown blight, stem cankers,
and extensive defoliation, with necrotic areas on the
cotyledons, hypocotyls, leaves, and fruit.
- D. bryoniae is a seed-borne, air-borne, or
soil-borne fungus capable to survive in vegetable debris
in the field during the winter
- D. bryoniae is a facultative necrotroph: the
production of exudates from lesions helps the fungus in
its growth and infection
- There is no evidence of race specialization for this
pathogen and breeders and pathologists usually use a mixture
of isolates to test cucurbits for resistance to D.
bryoniae
Methods of seedling screening
for resistance to gummy stem blight are based on spraying
the seedlings with a water suspension of spores collected
from in vitro cultures of the pathogen.
Among commercial cultivars of watermelon, 'Congo' is the
least susceptible to gummy stem blight, 'Fairfax' intermediate,
and 'Charleston Gray' the most susceptible. Dr. J.D. Norton
in the years between 1978 and 1995 developed resistant watermelon
cultivars from two crosses ('Jubilee' X PI 271778, 'Crimson
Sweet' X PI 189225) by selecting disease-resistant seedlings
from backcrossed families. 'AU–Jubilant' and 'AU–Producer',
'AU–Golden Producer', and 'AU–Sweet Scarlet'
were released with moderate to high resistance to anthracnose,
Fusarium wilt, and gummy stem blight. However, they were
much less resistant to gummy stem blight than the resistant
parents PI 189225 and PI 271778. To date, no cultivars of
watermelon, muskmelon, or cucumber have been released that
have high resistance to natural epidemics of gummy stem
blight in the field.
