Is it a Year of Plant Diseases?

Gummy Stem Blight (GSB) of Cucumbers

Mohyuddin Mirza

The pictures show symptoms on stem and fruit.  GSB is caused by a fungus Didymella bryoniae.  Sometimes the disease can be confused with black rot caused by Phomopis cucurbitae and Botrytis.  Botrytis is easy to distinguish because growers have experience with this disease.  Botrytis becomes grey and fluffy under high humidity.  The picture on the right shows a mini cucumber cut up to show necrosis and browning in the middle.  On the surface the blossom end may show yellowing and some browning.

Symptoms:  GSB can occur on all parts of the cucumber plant and all phases of growth.  Mini cucumbers appear to be more susceptible to GSB compared to regular cucumbers.  It first appears as brown-grey lesions on the tips of soft growing leaf points and young developing tendrils.  Growers may be more familiar with lesions on the base of the stem with characteristics amber-red gummy exudates.  On stem tissue where pruning wounds from laterals or leaves originate, the disease produces both exudates and also dark, pin-head size structures as seen in the picture on the left.  In the early stages these lesions may appear to be on the surface but eventually the infections move inside and leads to girdling and eventually the plant dies.

Infected flowers may develop into fruit which is shrivelled at the blossom end with traces of internal, brown rotting tissue.  Fruit may also appear normal at harvest but develop an internal, post harvest rot later.

On leaves, the symptoms are shown as in this picture. The lesions are V-shaped and appear as yellow-brown.  Infection starts from the edge of the leaf and moves inwards.

Some facts about the fungus:

• The disease is spread by air borne spores which are released between 18:00 to 21:00 hours.  These spores may remain dormant until moisture is available.
• A second method of spread is by splashing water droplets including high-volume spray or by transfer from pruning knives or hands.
• Spores need only a few hours of moisture to germinate and establish infections.  It has been shown that a wet surface or moist wound site for at least one hour will enhance disease establishment and development.
• Spores can infect flowers and pruning wounds.  Spores which land on very young flowers cause the fungus to grow inside the fruit as it develops.
• The fungus develops rapidly at high relative humidity, low light and temperatures between 20 to 28C.

Cultural:

• The most important cultural control strategy is to manage relative humidity not only in the greenhouse air but in the canopy, at the base of the plant and around the plants.  Air movement is restricted due to plant density.  Find ways to get rid of moisture laden air and bring dry air in from the outside.  More greenhouses depend on natural convection for air movement so they should focus on pipe temperatures to move the air around the base of the plant and in the canopy.
• Avoid sudden changes in climate, especially relative humidity.  Look at your computer screens which show graphs of relative humidity over a 24 hour period.  If you are using very sharp pre-night dipping regimes, that is temperature drops from day setting to night setting in just one hour, then dew point can be reached.  Here is information on critical temperatures, relative humidity and dew points.  It may help in proper decision making.

• If you are using Moisture Deficit (MD) as a measurement for environmental control, then try to maintain a minimum of 3 grams/m3.
• Managing night time relative humidity is also important.  Vent if necessary to reduce the humidity.
• Avoid spraying insecticides or foliar feeding up to 3-4 hours before sunset.  It means that foliage should dry out before sunset.
• Make sure that pruning practices are proper.  
• If you look at the picture on the next page you can see leaf stubs hanging onto the stem and many of them are turning yellow.  Such stubs are easily accessible to fungus spores and infection can enter the main stem.  Growers’ argument is that it is a labor saving strategy just to remove the main leaf.  The recommended practice to “clean” break the leaves that are flush to the stem with no “stubs” remaining.  I can see the point if you can manage the relative humidity and air movement is good and those stubs dry out within a couple of days.  If GSB is an issue then this type of pruning should be examined in the context of cost and benefits.

• Maintaining proper nutritional balance in the feed and leach is important to keep plants healthy.  This means proper balance of nitrogen and potassium.  Lately I have seen some nutrient analyses where nitrogen is higher than potassium in the leach solution. This means that plants become more vegetative and could make it more susceptible to GSB and other diseases as well.
• Harvest fruit with a sharp knife and disinfect it frequently with bleach or other disinfectants.

• Good hygiene is very important to keep the inoculums to a lower level.  This can be achieved by removing infected plant material from within the greenhouse not by dragging the plants on walkways, but by bagging them and then removing them.  Disposal piles near the greenhouse can also be a prime source of spores throughout the year.
• At the end of each crop, remove all crop residue and debris from the greenhouse, especially on wires and in temperature sensors.  Avoid raising seedlings in the same area where mature crops are still around.  Avoid intercropping if GSB is a major issue.  Give a break between crops and clean thoroughly.
• Disinfect greenhouse structures at the end of the crop cycle.  Many disinfectants are available to growers for this purpose.
• Pruning tools should be disinfected daily.  Many growers use 10% bleach for one minute dipping.

• Many fungicides are available for the control of GSB. Spot spray where it is feasible.