Taxonomy

Domain	Eukarya
Kingdom	Fungi
Phylum	Ascomycota
Subphylum	Pezizomycotina
Class	Dothideomycetes
Subclass	Pleosporomycetidae
Order	Pleosporales
Family	Didymellaceae
Genus	Stagonosporopsis

Scientific Name

Stagonosporopsis cucurbitacearum

Scientific Name Synonyms

Cercospora cucurbitae

Didymella bryoniae

Mycosphaerella citrullina

Phyllosticta cucurbitacearum

Ascochyta cucumis

Phoma cucurbitacearum

Common Name

gummy stem blight (vine decline)

Author: Eric Newberry, University of Florida

Reviewed by: Jeffrey Rollins, University of Florida

Pathogen

Didymella bryoniae causes the cucurbit-rot disease known as gummy stem blight or black rot. Both the sexual and asexual stages (Phoma cucurbitacearum) can cause disease and are present in most regions where watermelon is grown (3). Didymella bryoniae infects watermelon, cantaloupe, cucumber, squash, and other cucurbit hosts.

Symptoms and Signs

All parts of the plant can display symptoms and signs but the roots. Early symptoms appear as yellowing or chlorosis of the leaf margins followed by light to dark brown necrotic spots. Pale brown or gray spots develop on the leaves, petioles, and stems. Lesions usually form at the joints on the stem, become elongated, cracked, and exude amber colored sap. The whole plant may wilt and then die. On the fruit, spots appear irregularly circular and yellowish. Spots may have gummy exudates at the center that eventually turn to a black rot. Sexual and asexual fruiting bodies are visible on the same lesions of the leaves, stem, and fruits. Pale colored pycnidia are found in closely spaced groups. Dark globular perithecia are sometimes arranged in rings (1).

Ecology and Spread

The pathogen is an asomycete fungus that produces two spore stages. Triggered by environmental conditions, flask shaped sexual fruiting bodies called perithecia form and produce ascospores, which serve as the primary source of inoculum (1). Spores germinate, and can directly penetrate through the cuticle or intercellular spaces and around the bases of trichomes. Stems are infected through wounds or mechanical damage (2). The asexual fruiting bodies are also flask shaped and are called pycnidia. Pycnidia release tendrils of conidia under moist conditions in a gummy substance, and are dispersed by water splashing. The asexual spores cause secondary infections. The fungus overwinters in plant debris, in or on seed, and on weeds as a hardened off mass of hyphae known as a chlamydospores (1).

Temperature and especially moisture are important in the germination, sporulation, and disease progression of D. byrionae. Optimum temperature ranges between 20-25⁰C for watermelon and other cucurbit hosts. Free moisture for at least one hour is necessary for infection and a high relative humidity for lesion expansion. Symptoms can be observed as soon as one week after spore germination on susceptible tissue (1). Feeding by aphids, striped cucumber beetles, and mechanical wounding provide potential areas of ingress for the pathogen. Other diseases, such as powdery mildew (Sphaerotheca fuliginea), may weaken the host, making it more susceptible to D. byroniae infection (3).

Geographic Distribution

Didymella byroniae is a common problem on cucurbits in all humid growing regions (3).

Management

Use treated seed. There are no commercially available varieties resistant to D. byoniae.
Deep plow or remove postharvest debris to reduce the likelihood of pathogen overwintering.
Rotate crops to prevent the build-up of inoculum.
Eradicate weeds and other volunteer plants before planting (1).
Consult your local extension specialist for legal and efficacious fungicide products available in your state. Remember, the label is the law and the product applicator is responsible for reading and following all chemical labeling.

Diagnostic Procedures

Incubate fresh material to allow production of spores within leaf, petiole, and fruit lesions. Black fruiting bodies (i.e., perithecia and pycnidia) produced on the lesions confirm diagnosis (1). Diagnostic features include: 1) black, flask shaped perithecia or pycnidia; 2) bitunicate asci containing eight ascospores; and 3) transparent conidia, cylindrical with rounded ends, usually single celled (2).

This fungus can be cultured on a common media such as PDA.

Resources and References

1. Paret, L. M., Dufault, S. N., Olson. M. S., 2011. Management of Gummy Stem Blight (Black Rot) in Cucurbits. PP280. Gainesville: University of Florida Institute of Food and Agricultural Sciences. http://edis.ifas.ufl.edu/pp280

2. Sitterly, W. R., Keinath, A. P., 2000. Gummy Stem Blight. APSnet Feature Story. St.Paul, MN: http://jpkc.jluhp.edu.cn/zwkx/zwbl/Improve/Graduate/APSNET/gummy.html

3. Zitter, T. A. 1992. Gummy Stem Blight. Vegetable MD Online Fact Sheet. Ithaca, NY: Cornell University.http://vegetablemdonline.ppath.cornell.edu/factsheets/Cucurbit_GSBlight.htm

Acknowledgements

National Institute of Food and Agriculture, U.S. Department of Agriculture, under Agreement No. 2011-41530-30708 as part of "Diagnostic Image Series Development for Supporting IPM in the Southern Region" (USDA-NIFA-RIPM-003351)