Gummy Stem Blight Control – Treating Black Rot Fungus In Cucurbits
By: Bonnie L. Grant, Certified Urban Agriculturist
Gummy stem blight is a fungal disease of melons, cucumbers and other cucurbits. It is a contagious disease which can spread across a field of fruits. The fungus damages the tissues of the stem at all stages of development. Find out what is gummy stem blight so you can prevent this problem in your vegetable garden.
What is Gummy Stem Blight Disease?
Gummy stem blight fungus is most active during periods of warm, wet weather. The spores of the fungus can spread in soil or by air. The fungus will overwinter in milder climates in soil and plant debris.
The leaves will get necrotic areas of dead tissue that turn brown and have a darker halo. The stems and fruit will show black, soft spots or large brown lesions that are bordered by black. The dark coloring of these lesions also lends the disease the name of black rot fungus.
Black Rot Fungus Characteristics
Stem blight forms when seeds or sites are previously infected with the fungal spores. When conditions are 85 percent humid or wet and warm, with temperatures averaging in the 60’s, (16-21 C.), the fungal spores bloom.
You should start treating black rot fungus at the first signs of the disease. Unfortunately, the first signs vary dependent upon plant species. Many get water spotting on the foliage or stems may ooze black or brown gummy beads of fluid. It is difficult to identify these early signs of gummy stem blight, which is why preparation of the seedbed, purchasing resistant seeds and rotating crops are important prequels to stem blight treatment.
Ultimately, plants affected by this disease will bear rotten fruits, which are unmistakable and inedible.
Prevention of Gummy Stem Blight
The first stages of a disease free cucurbit crop are preparation and rotation. Never plant cucumbers, melons or other susceptible plants in the same area as the previous season’s crop. The plant debris, and even seeds, left over in the soil will harbor the spores of black rot fungus.
Careful preparation of soil prior to planting removes all old organic matter. Use seeds from a reputable seed company that has a history of fungus-free seeds. Since the disease can manifest even on seedlings, inspect any that you have purchased from a nursery prior to purchase and planting. Gummy stem blight signs on seedlings are brown lesions and dry leaf edges. Do not plant suspect specimens.
Treating Black Rot Fungus
In most cases, removal of old plant debris, rotation and resistant species will prevent the appearance of gummy stem blight. In climates with warm, moist bloom conditions, the fungal spores are carried on the wind, and you may have to combat the disease even if you took preventative steps.
The most common method is the use of fungicides as a stem blight treatment. Dusts or sprays of fungicides useful for preventing and combating powdery or downy mildew have been shown to be effective against gummy stem blight disease.
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Gummy Stem Blight affects many cucurbits including watermelon, cantaloupe, cucumber, pumpkin, and some squash.  Some symptoms are common of all Gummy Stem Blight infections while other symptoms can vary depending on the specific host the pathogen has infected. Hosts can become infected at any time in their life. When the pathogen is present in a young seedling, the cotyledons will sprout appearing dark and drenched.  When older plants become infected, their leaves may appear water soaked and begin to develop dark tan lesions. The leaves begin to turn brown at the margins and necrosis progresses towards the base of the leaf. 
Cankers, which may or may not have black spots, may appear in the epidermal cortical tissue and on the stems of infected plants. Black spots, if visible, are pycnidia and/or perithecia. Black rot is a common symptom on the fruit of Gummy Stem Blight infected cucurbits. Lesions formed on the fruit start as water soaked spots that expand and exude gummy ooze. As the spots grow they develop fruiting bodies which turn the spots black.  Fruit can also rot internally, with the only symptoms being shriveling and discoloration of centrally located tissue.  Fruit rot can occur while in the field or after fruit has been harvested. 
D. bryoniae produces signs of infection such as white aerial mycelium, olive-green substrate mycelium and pycnidia. P. cucurbitacearum produces sparse serial mycelium and many pycnidia are present.  Young leaves and cotelydons of melon and watermelon that are immature are at high risk to the Gummy Stem Blight infection whereas cucumber and some squash are resistant at young age and only become susceptible once they have matured. 
D. bryoniae is an Ascomycota fungus. In spring, asexual fruiting bodies called pycnidia and sexual fruiting bodies called perithecia are formed from last year’s infected plant debris. Pycnidia are flask-shaped structures that house asexual conidia which are readily released from pycnidia through the ostiole when enough moisture is present.  Perithecia are also flask-shaped, but they are sexual fruiting bodies which give rise to bitunicate asci that contain 8 ascospores. Ascospores are readily dispersed and spread by wind after rain or during evening dew periods. 
Temperature and moisture are the most important factors for germination and development of the pathogen on the plant, with moisture being most important of all.  Free moisture must be present on susceptible leaves for at least one hour in order for germination of spores to occur.  The pathogen can enter a healthy host in a variety of ways. With enough moisture, conidia directly penetrate through the cuticle and infect healthy cucurbits.  Wounds to the plant, especially those left by feeding insects such as the striped cucumber beetle or aphids, are important passageways for the pathogen to enter in older hosts.  Other diseases, like Powdery Mildew, can also weaken a host enough to provide easy entry for D. bryoniae. After spore germination, symptoms can appear as soon as 7 days later. 
D. bryoniae survives on or in seeds, surrounding weeds, or organic debris from previously infected cucurbits.  Without a host, the pathogen is able to overwinter and survive for over a year as chlamydospores, hardened masses of hyphae that act as survival structures during dry or otherwise adverse conditions.  The pathogen is transferred from infected hosts to healthy plants via ascospores carried in the wind and by conidia that are released from pycnidia by water splash and in gummy exude. Conidia are hyaline and aseptate if produced by the anamorph, and either septate or aseptate (more common) if produced by the teleomorph form of the pathogen. 
The host must remain wet for growth and spread of the disease. Once the primary infection takes place, as long as it remains wet, the pathogen will spread to the stem where cankers form and ooze a gummy substance full of conidia.  Conidia spread from the gummy ooze to another host is considered the secondary asexual cycle.
Gummy Stem Blight occurs throughout the southern and eastern United States.  Temperature and moisture are the most important factors in the spread of Gummy Stem Blight. For watermelon and cucumber, the best temperature for infection is around 25 °Ce for melon the best temperature is around 20 °C.  Continual leaf wetness from 1 – 10 hours is necessary for germination, sporulation, and colonization of conidia.  After infection has set in, large brown lesions will retain moisture for long periods of time. Even though it takes constant moisture to facilitate the pathogen, it is highly resistant to dry conditions and can survive as chlamydospores for over a year in dry organic debris. 
There are currently no varieties of cucurbit completely resistant to D. bryoniae.  Cultural practices and preventive techniques can be taken to avoid or reduce harm done by D. bryoniae. Purchasing and planting reputable disease-free seeds is necessary for ensuring D. bryoniae will not be present at planting.  A rotation (of at least 2 years) of cucurbit and non-cucurbit crops should be performed to greatly reduce the incidence of Gummy Stem Blight.  Since Gummy Stem Blight can survive as chlamydospores in dead plant debris, it is recommended to remove or deep-plow dead cucurbit plant debris into the soil so it can fully decompose to lessen the likelihood of the pathogen overwintering.  Keeping fields pruned and weed-free will help to control Gummy Stem Blight as overgrowth promotes poor air circulation and moisture from humidity, which support D. bryoniae germination and growth.  If chemical control is needed in important cucurbit production regions, there are a variety of preventative fungicides commercially available that can be applied during the early stages of plant growth.  Effective contact fungicides include chlorothalonil and mancozeb effective systemic fungicides are Folicur®/Monsoon®, Inspire Super®, and Switch®.  These fungicides should be applied around when the vines of different crops start to grow and make contact with each other
Yield losses due to D. bryoniae exceeding 30% can occur in early season crops facilitated by wet weather conditions.  Curcurbits are important commodity crops in many parts of the world. In the United States, cucurbit production accounts for approximately 229,000 hectares with a value of $1.43 billion. 
Keep Gummy Stem Blight Out of Your Watermelons
In the past couple of weeks we have seen a few watermelon seedlings affected with gummy stem blight. Growers are advised to scout their watermelons seedlings to make sure they are not presenting gummy stem blight symptoms and to avoid introducing the pathogen into fields by planting infected transplants.
Gummy stem blight is caused by the fungal pathogen Didymella bryoniae and affects the leaves, stems and fruits of all cucurbits. Watermelons are particularly susceptible to this disease. Seedlings infected with D. bryoniae will present water soaked lesions on the stems (Fig. 1) and necrotic, brown spots on the edges of leaves (Fig. 2). When infected tissue is inspected closely under a microscope, reproductive structures of the pathogen called pycnidia, will become apparent (Fig. 3). For a detailed description of the disease, its diagnosis, and control options, see our Gummy Stem Blight of Cucurbits fact sheet.
Fig. 1: water soaked lesions on crowns of watermelon seedlings infected with gummy stem blight (Photo: Shawn Butler, NCSU Plant Disease and Insect Clinic)
Fig. 2: leaves with necrotic, brown lesions on edges due to gummy stem blight infection (Photo: Shawn Butler, NCSU Plant Disease and Insect Clinic)
Fig. 3: close-up of a watermelon seedling stem infected with gummy stem blight seen under a microscope. Note the pycnidia (round, brown reproductive structures) on the surface. Pycnidia can be seen with a 10x handheld lens as small brown to black dots (Photo: Shawn Butler, NCSU Plant Disease and Insect Clinic)
The gummy stem blight pathogen can be seedborne, therefore, treating seeds prior to planting is recommended. The disease is favored by warm (61°F–75°F), wet conditions (4–10 hours of leaf wetness), and the pathogen is dispersed by water splashing (rain, overhead irrigation), thus, using drip irrigation will help contain infections. Once greenhouse transplants in a tray become infected, it is advised to destroy the affected tray and any adjacent trays, since the pathogen will have likely spread to neighboring trays due to irrigation splashing, even if transplants look healthy. Spores of the pathogen can survive on crop residue, so sanitation is key to prevent this disease in the greenhouse and a 2-year crop rotation may be needed for fields that have experienced severe infections.
The disease has been difficult to control in recent years due to the development of fungicide-resistant populations. Nonetheless, a fungicide trial conducted in North Carolina last year revealed some products that are effective in controlling gummy stem blight on conventional operations. While there are some products labeled for organic operations, no efficacy data has been published. For a complete report of results from our gummy stem blight trial please refer to our demonstration trials.
Products and rates assayed in this trial included:
|Product||Active Ingredient||Fungicide Group|
|Viathon 5.1SC 4 pt||Potassium phosphite + Tebuconazole||3|
|Inspire Super 2.09SC 18 fl oz||Difenoconazole + Cyprodinil||3 + 9|
|Luna Experience 400SC 10 fl oz||Fluopyram + Tebuconazole||7 + 3|
|Luna Sensation 500SC 7 oz||Fluopyram + Trifloxystrobin||7 + 11|
|Catamaran 5.3SC 5 pt||Potassium phosphite + Chlorothalonil||M|
|Fontelis 1.67SC 16 fl oz||Penthiopyrad||7|
|Switch 62.5WG 13 oz||Cyprodinil + Fludioxonil||9 + 12|
|Bravo Weather Stick 6SC 2 pt||Chlorothalonil||M|
|Folicur 3.6F 6 fl oz||Tebuconazole||3|
|Cabrio 20WG 10 oz||Pyrasclostrobin||11|
|Manzate Pro-Stick 75DG 2 lb||Manganese, Zinc||M|
Products were applied every 7 days. All treatments with the exception of Manzate Pro-Stick and Cabrio suppressed gummy stem blight when compared to the non-treated control (Fig. 4). Treatments labelled with the same letter are not statistically different, so all treatments not labelled with the letter a, which corresponds to the non-treated control, presented significantly less gummy stem blight.
Fig. 4: products for gummy stem blight control on watermelon
When planing your spray program, make sure you alternate fungicide groups of products to avoid generating fungicide-resistant strains. You can find some example products on our Gummy Stem Blight of Cucurbits fact sheet. Growers are encouraged to read the 2014 Watermelon Spray Guide developed by Dr. Anthony Keinath from Clemson University, and the Southeastern US Vegetable Crop Handbook for the latest fungicide recommendations.
If you think you have gummy stem blight in your cucurbits please contact your local Extension Agent and send photos and/or physical samples to the Plant Disease and Insect Clinic for confirmation.
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Mix together 1 tablespoon of 7 percent liquid copper concentrate with 1 gallon of water in a pump-type sprayer at the first sign of phoma blight or alternaria leaf blight. Spray the all parts of the cucumber plants until covered on all sides, and repeat every seven to 10 days, as needed. You can also spray copper on the cucumber as a preventative measure when the rainy season starts.
Mix together 1 tablespoon of 50 percent captan with 1 gallon of water in the tank of a pump-type sprayer if the plants have phytophthora blight. Spray the foliage until covered on both sides, and repeat every 10 to 14 days as needed.
- Mix together 3/4 teaspoon of a fungicide containing 29.6 percent chlorothalonil with 1/2 gallon of water for every 100 square feet of growing area in the tank of a pump-type sprayer if the cucumbers have gummy stem blight or alternaria leaf blight.
Buy seeds from companies that treat their cucumber seeds with fungicides or produce their seeds in arid regions, such as California's inland valleys, to help prevent gummy stem blight and phoba blight.
Spray pesticides on a calm day to minimize drift.
Always wear chemical-proof gloves, a respirator and safety goggles when mixing and applying garden chemicals.
If you're pregnant or nursing, wear heavy-duty garden gloves when working with soil to prevent skin contact with soil-borne pathogens.
Keep garden chemicals out of reach of children.
Don't allow children or pets in an area treated with fungicides until they dry.
Don't compost plants affected by blight, but give them to your sanitation service on garden waste pickup day.
Stop using fungicides 30 days before harvest.
A.J. Andrews' work has appeared in Food and Wine, Fricote and "BBC Good Food." He lives in Europe where he bakes with wild yeast, milks goats for cheese and prepares for the Court of Master Sommeliers level II exam. Andrews received formal training at Le Cordon Bleu.
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