Gummy stem blight, also known as black rot when affecting fruits, is caused by the fungal pathogen Didymella bryoniae (anamorph = Phoma cucurbitacearum).

Gummy stem blight affects all cucurbit crops such as cucumber, watermelon, melon, cantaloupe, squash, and pumpkin.

Stems, vines, leaves, and fruits are affected by gummy stem blight, which can result in yield reduction, damaged fruit (black rot), and plant death.

Gummy stem blight causes ovate stem and vine cankers, usually starting near the nodes, with a characteristic brown gummy exudate (Figure 1). Within advanced lesions on dead tissue, small black fungal fruiting bodies, known as pycnidia, can be seen with a 10x hand lens (Figure 2). Pycnidia contain pathogen conidia (Figure 3). Leaves can exhibit dark brown circular spots starting at margins and rapidly spreading to entire leaf. These foliar lesions can vary in color from light brown to nearly black (Figure 4). Black rot infected fruits show small water-soaked spots that advance into large brown spots, sometimes also exhibiting the brown gummy secretion as found in stems (Figure 5).

Figure 1. Split stem with brown gummy exudate.

Dr. Lina Quesada, NC State Vegetable Pathology Lab

×

Figure 1. Split stem with brown gummy exudate.

Dr. Lina Quesada, NC State Vegetable Pathology Lab

Print Image

Figure 2. Stem with fungal fruiting bodies known as pycnidia.

Shawn Butler, NCSU PDIC

×

Figure 2. Stem with fungal fruiting bodies known as pycnidia.

Shawn Butler, NCSU PDIC

Print Image

Figure 3. Pycnidia containing pathogen conidia.

Shawn Butler, NCSU PDIC

×

Figure 3. Pycnidia containing pathogen conidia.

Shawn Butler, NCSU PDIC

Print Image

Figure 4. Necrotic, brown spots on leaf margins.

Dr. Lina Quesada, Dr. Lina Quesada, NC State Vegetable Pathology Lab

×

Figure 4. Necrotic, brown spots on leaf margins.

Dr. Lina Quesada, Dr. Lina Quesada, NC State Vegetable Pathology Lab

Print Image

Figure 5. Squash fruit with black rot.

Gerald Holmes, Valent USA Corporation, Bugwood.org

×

Figure 5. Squash fruit with black rot.

Gerald Holmes, Valent USA Corporation, Bugwood.org

Print Image

Gummy stem blight and black rot can look like other plant diseases that affect cucurbits; however, this pathogen can be distinguished by stem and vine lesions near nodes, black fruiting bodies on dead tissue along with the characteristic brown gummy exudate from wounds near lesions. The following are images to help distinguish gummy stem blight from those diseases that can have similar symptoms during some life stages:

• Anthracnose, Colletotrichum orbiculare, note the yellow halo around infected spots (Figure 5).

• Blossom end rot, abiotic, note water-soaked spots and only located opposite to fruit stem (Figure 6).

Figure 5. Anthracnose on cucumber.

Dr. Lina Quesada, NC State Vegetable Pathology Lab

×

Figure 5. Anthracnose on cucumber.

Dr. Lina Quesada, NC State Vegetable Pathology Lab

Print Image

Figure 6. Blossom end rot of watermelon.

Photo, Gerald Holmes, Valent USA Corporation, Bugwood.org

×

Figure 6. Blossom end rot of watermelon.

Photo, Gerald Holmes, Valent USA Corporation, Bugwood.org

Print Image

• High humidity and moisture (4–10 hours of persistent moisture) with frequent rains, overhead irrigation, and/or poor drainage
• Warm temperatures between 61 and 75°F
• This disease can occur early in seedlings and greenhouse transplants, as well as anytime throughout the season. Maximum damage in North Carolina is usually seen in late June and July.

Gummy stem blight can live on seeds transported from state to state. The spores can also be spread state to state by wind. Locally, gummy stem blight can be spread by asexual reproductive structures, conidia, in the gum during rains or watering. This pathogen can also survive for several years on un-decomposed cucurbit plant material and on volunteers of related species. It may also attack other host plants such as green beans, okra, tomato, and tobacco, and may overwinter on the refuse from these crops. The spores of the fungi germinate and infect young seedlings through the hypocotyl or stem. Cotyledons and young leaves of cantaloupe and watermelon are more susceptible than those of cucumber and pumpkin.

• Purchase seeds or seedlings free of pathogens from a reputable company, no resistant cucurbit varieties exist. Consider seed treatments.
• Inspect seedlings regularly for symptoms and dispose of all seedlings near infected plants.
• Deep plow infected areas post-harvest to ensure complete decomposition of all plant material.
• Rotate crops, carefully removing all volunteers of relates species. Since this disease can survive in soil and on un-decomposed plant material for 2-4 years, rotate all cucurbits out of infected areas for at least that long. Crop rotations with non-host plants such as small grain, corn, of two or more years are effective in reducing the incidence of these diseases, if disease-free seed are used.

Fungicides may be required to control gummy stem blight, especially if environmental conditions are favorable for disease. When applying fungicides, always remember to alternate products to avoid generating fungicide-resistant strains. For the latest fungicide recommendations for gummy stem blight see the Southeastern US Vegetable Crop Handbook. Fungicide labels are legal documents, always read and follow fungicide labels.

Example products for gummy stem blight control:

For example, in watermelon you can use: Bravo alternated with Viathon, alternated with Luna Sensation, alternated with Switch every 7 days.

• The NC State University Plant Disease and Insect Clinic provides diagnostics and control recommendations
• The NC State Extension Plant Pathology Portal provides information on crop disease management
• The Southeastern US Vegetable Crop Handbook provides information on vegetable disease management
• The USDA Fungus-Host Distributions Database provides information about reported hosts for fungal and oomycete pathogens

This factsheet was prepared by the NC State University Vegetable Pathology Lab in 2013.

• Like us on Facebook!
• Follow us on Twitter!