• Symptoms
• Other diseases that can be confused with Gummy Stem Blight
• Disease spread
• Control Measures
• Double Cropping
• Climate Management
• Crop-end Activities
• Fungicides

Gummy stem blight (GSB) is an important disease of greenhouse cucumber. It is caused by the fungal pathogen Didymella bryoniae, previously known as Mycosphaerella melonis. In favorable weather, the pathogen can infect all parts of the cucumber plant during all phases of plant growth. The internal fruit rot is of particular concern because it is difficult to detect at harvest. Thus, the inevitable post-harvest decay at the grocer creates a poor image for the packing house identified on the label. GSB is not easy to manage as greenhouse conditions are often favourable for disease development and spread, and there is constant occurrence of fresh wounds from pruning and harvesting. Key strategies for disease management include good sanitation, proper pruning practices, double cropping and proper irrigation practices.

Symptoms

Stems: Tan colored lesions may develop at nodes where lateral shoots and leaves were pruned out, or where cucumber fruits were harvested. These lesions become enlarged, cracked and exude a characteristic amber-red colored sap (Figure 1). In addition, basal stem splitting/cracking caused by root pressure can also be subject to infection. Lesions at the stem base may enlarge and girdle the stem, causing wilt and eventual death of the plant. Plants that are propagated in cool climate (i.e. below 23°C) often have fleshy thick basal stems that are more prone to infection.

Leaves: Lesions usually appear at the edge of the leaf, initially as water-soaked areas surrounded by a yellow halo, and then extending into the leaf as V-shaped yellow-brown lesions (Figure 2). These infections are usually triggered by guttation, or where leaves at the top of the canopy are exposed to dripping water from condensation or roof sprinklers.

Fruit: Fruit can be infected at the stem end or the flower end either before harvest or after harvest. Pre-harvest infections are characterized by the appearance of soft black-green lesions with shrunken tissues externally on fruit surfaces. Internal fruit rot is identified by a subtle tapering of the flower end of the fruit (Figure 3). Pre-harvest internal fruit rot is not always noticeable because fungal growth may be retarded by an active chemical resistance mechanism in the fruit which encapsulates the fungus (Figure 4). After harvest, this mechanism is no longer active and thus the fungus continues to spread. Internal fruit rot is characterized by yellowing and eventual rotting of the fruit.

Seedlings: The fungus can be carried in seeds. This can cause pre- and post-emergence death and damping-off of young seedlings.

	Figure 1. Tan-colored lesions with black pinpoint spore-producing structures on cucumber stem.
	Figure 2. V-shaped lesions on cucumber leaf, where spores entered through guttation droplets; not common in B.C.
	Figure 3. Healthy cucumber fruit on left, and two cucumber fruits on right with tapering end indicating internal fruit rot.
	Figure 4. Two cucumber fruits on left with early symptoms of pre-harvest internal fruit rot, and two cucumber fruits on right with severe symptoms of post-harvest rot.

Other diseases that can be confused with Gummy Stem Blight

Disease spread

The pathogen produces two types of spores: asexually-produced pycniospores, and sexually-produced ascospores. Both types of spores are short-lived once they are released into the environment. But the pathogen can survive up to 2 years as chlamydospores or mycelium on undecomposed plant debris. Both pycniospores and ascospores start new infections by germinating on plant surfaces that are wet for an hour or more. Wounded tissues are more susceptible to infection than intact tissues.

After spore germination, conditions of high relative humidity, low light and temperatures between 20-28°C promote the growth and spread of the fungus, and the disease symptoms appear within 4 to 8 days of initial infection. As soon as the symptoms can be observed, the fungus will have begun to produce spores which will be invisibly spreading through the greenhouse.

There are two main methods of disease spread. Under humid conditions, 1) pycniospores are spread by splashing water droplets; a film of water on the plant surface is necessary for spore release and infection, and 2) ascospores are released into the environment with air-currents in the evenings between 18:00 - 21:00 hours. These spores germinate in evening condensation or guttation droplets and grow under low light conditions. High root pressure associated with excess watering and fluctuating transpiration due to weather changes promotes guttation. If there is inadequate moisture, the spores may stay dormant and germinate when moisture becomes available. Greenhouse practices such as overhead sprinkler irrigation, pruning and improper crop handling greatly influence the spread of the fungus. Spores can also be dispersed on pruning knives, wet hands and clothing.

Control Measures

There are no "silver bullets" for controlling gummy stem blight. Double-cropping can limit disease development because plants are removed before fungus levels build up. See below under "Double Cropping".

• Keep plants dry through growing season and maintain an open canopy.
• Avoid root pressure increases.
• Monitor for lesions starting in March. Remove infected plant materials from the greenhouse.
• Keep disposal piles away from the greenhouse.
• Prune to leave clean breaks that are flush with the stems. Avoid leaving stubs. Remove prunings from the greenhouse but be aware that removal will reduce the build-up of beneficial insect populations and adjust them accordingly.
• Disinfect pruning tools periodically with a 2 minute dip in 70% ethanol, or a quick dip in either 5% Virkon or 0.525% sodium hypochlorite (10% household bleach).
• Avoid pruning during periods of dark weather.
• Do not grow squash or other cucurbits around the greenhouse. They can also harbour GSB.
• Perform a good year-end cleanup.

Double Cropping

Double cropping (growing two crops per season) will result in more even production and higher quality fruit from July onward. Double cropping will extend deliveries until December, and the young crop will have few wounds which minimize the number of infection sites. Removal of the old crop before disease build-up will reduce the spore load during the fall cropping period.

Climate Management

• Avoid sudden changes in climate, especially relative humidity (RH).
• Target RH below 80%.
• Verify the RH and temperature readings by periodic calibration. Ensure their placement is near the active growing head.
• Maintain minimum pipe to provide VPD >3 grams/m³. Keep a narrow dead zone in the morning.
• During March/April, maintain a minimum pipe temperature of 45°C. Increase temperature to reduce RH especially when it is raining. Decrease pipe temperature with increasing light.
• Increase the temperature slowly in the early morning at 1°C per hour and ensure tissue temperatures are at day-time targets before sunrise.

Air Temperature	RH	Dewpoint Temperature
18°C	80%	14°C
18°C	90%	16°C
22°C	80%	18°C
22°C	90%	20°C

• Put a grow pipe in the canopy at a maximum of 50°C. Decrease pipe temperature with increasing light.
• Vent early with 1/2 inch minimum once outside temperatures reach 5-6 °C. Do not overvent. Cold air coming into the greenhouse can promote GSB.
• Consider venting at night to avoid soft plants.
• Avoid spraying insecticides in the evening, as plant surfaces may remain wet long enough to allow fungus spores to germinate. If you must spray in the evening, turn up pipe heat after spraying.

Avoid High Root Pressure:

• Use higher EC levels (3.0) especially in the development of the plant to the wire. This encourages the development of stronger stem tissues which will be less prone to infection at leaf scars.
• Do not water early in the morning. Start watering 2 hours after sunrise or later on cloudy mornings, and overdrain by third or fourth dripturn. Extend day watering period as the days become longer. Do not water after midnight.
• Reduce irrigation under low fruit loads. Keep overdrain to the third or fourth dripturn..
• Even, regular pruning reduces problems with root pressure. Remove no more than 2 actively growing green leaves at one time.

Post-Harvest Handling

• Store fruit at 10 to 12°C shortly after harvest. Do not let cucumber fruit be stored with apples, tomatoes or bananas.

Crop-end Activities

• At the end of each crop, remove all crop debris from the greenhouse. Pay close attention to wires and temperature sensors.
• Take debris off site, compost or bury it.
• Clean/disinfect picking carts, machinery, totes, sprayers etc. at the end of each crop.
• Disinfest the structure (refer to the Crop Clean-up Factsheet) at the end of the fall crop.

Fungicides

Apply 4 days apart at the first sign of infection or under conditions of high RH, low light and high inoculum levels:

• Rovral 50 WP (2 days PHI, crop cannot be exported to the U.S.)
• Nova 40W (2 day PHI)
• Dyrene 50 WP (7 day PHI) 
  Note, Dyrene has been discontinued.  Existing stocks may be used until Dec. 15, 2005.

Adapted from R. Young’s Greenhouse Vegetable Factsheet "Gummy Stem Blight of Greenhouse Cucumber"1995 with input on double cropping from Dr. Kees van den Berg.