| | Explanation of some terms
Some of the terms used may not be familiar to everyone. Here is a simple explanation.
VPD (Vapor Pressure Deficit). This term is used to describe the potential of water loss between the leaf and air surrounding it and depends on relative humidity and temperature. the term refers to grams in one cubic meter of air. When using a deficit between 2 and 7, it means it is more humid when the number is lower and it is drier when the number is higher.
EC (Electrical Conductivity). It is a measure of total dissolved salts in a fertilizer solution and is measured in millimhos or millisiemens. Over 3 millimhos is considered to be towards the high side. Below 1 millimhos is considered low.
Chlorosis: Light green or yellow areas randomly scattered through green area in leaves or other parts of the plant.
Necrosis: Dead spots on leaves or other parts of plant.
Leaves
Elephant Ear Leaves- This is a term to describe unusually large leaves. Common with cultivars like Corona. Stems may also appear to be thick and fruit short.
- Plant too vegetative due to cooler night temperature and lower 24 hour average temperature.
- Maintain a night temperature between 21°C to 22°C in establishment phase and around 19°C when harvest starts.
Entire leaves brown to light copper colored - Generally towards the top portion of the plant. Sometimes the entire crop is affected when looking at the top portion.
These symptoms indicate a very high leaf temperature and happens in the top portion of the crop. This is also an indication of vapor pressure deficit (vpd) of over 7 grams/m3 of air which results from warm temperatures and low relative humidity. Try to bring the day temperature down by increasing the relative humidity, if possible. Shading of the greenhouse should be considered.
Yellowing of the lower leaves - Warm, humid conditions in the lower canopy of the plants combined with low light levels may cause the lower leaves to turn yellow. In this type of yellowing, the leaves are weak and thin and appear to not have any strength. This is due to a lack of assimilates, the food inside the leaves. Sometimes there may be patchy green areas intermixed with yellow areas. Improve the penetration of the light by a gentle pruning in the plant canopy. Growers who use proper spacing have less problems with leaf yellowing.
Nitrogen deficiency may also cause a yellowing of leaves. In such cases, healthy green leaves turn yellow within a span of 2 to 3 days. The entire leaf turns yellow including the veins; suspect a deficiency of nitrogen. A leaf test will confirm if yellowing is due to nitrogen deficiency. Adjust the amount of nitrogen in the feed solution.
Sudden wilting of lower leaves - Lower leaves are wilting but the top leaves are not. The wilting continues for a few days and then leaves show orange-reddish coloration. By this time, the top leaves start becoming smaller in size. These symptoms are consistent with those caused by the fungi Fusarium and Verticillium, which clog the water conducting system of the cucumber plant. These fungi gradually move from the growing media into the water conducting tissue and, thus, impeding the flow of water and nutrients. Cut the basal portion of the stem and check for any browning of the tissue. There are no effective fungicides registered for the control of this disease. Fusarium and Verticillium wilt has been seen on crops grown on sawdust, rockwool, coco coir and other growing media.
Wilting of upper leaves - If the upper leaves are wilting and appear to be blue green in color and are usually small in size, then suspect root loss due to the fungus Pythium. Later, the entire plant may wilt during the day and recover at night. The fruit in such plants turns very dark green to almost black and start losing turgidity. Sudden changes in weather, from cloudy to bright, may also cause similar symptoms. This is because on cloudy days the relative humidity is higher and root production is slowed down. Under bright conditions, the roots cannot meet the transpiration demand of the leaves and, thus, wilting occurs. Plants under stress or bearing high fruit loads are more vulnerable to this condition.
Pythium can be managed by avoiding water logged conditions and directing the plant to send some more food to the roots. Bring the 24 hour average temperature to the lower side by reducing the night temperature. This means making the plant more vegetative.
On cloudy days, try to maintain an active climate so that the plant is able to do some transpiration. This can be done by giving a noon temperature boost of about 2°C for about 1 hour.
Reduce the Electrical Conductivity (EC) in the feed solution so that there is less water stress on the plant. Growers say that some cultivars like Flamingo require higher EC to maintain fruit quality. That is true; but, under stress situations, reduce water stress on plants by reducing the EC. Reduced EC would mean around 1.8 to 2.5 millimhos in the feed solution and around 2.8 in the bag.
- If problem is serious then use higher EC at night, if night feeding, and water without fertilizer when the temperature is over 25°C and relative humidity is below 50%.
- Reduce fruit load as quickly as possible.
Brown tipping on the growing point leaves -
- Suspect early phase of gummy stem blight infection.
- Check symptoms on the base of the stem. It will appear as gummy exudates.
- Check for fruit infection as well.
- Ensure that there is good air circulation through the canopy and the relative humidity does not rise above 50%.
Yellow or necrotic tissue on the outside perimeter of the leaf -
- Temperature is dropping too rapidly from day to night.
- High root pressure as a result of the last watering being too close to sunset. Last watering should be about 1-2 hours before sunset.
- Raise the EC by 0.5 millimhos in the feed solution by using potassium sulfate.
- Create an active climate for the plant by heating and venting, especially in the morning.
Large light green patches spread over the entire surface of the lower leaves - This condition has been seen in summer crops when the root zone temperature goes over 25°C. Such patches do not become necrotic and can sometimes be confused with symptoms of magnesium deficiency.
- Use cooler water to irrigate.
- Use greenhouse shade to reduce the sunlight.
Leaf coming out of the fruit and the fruit is malformed and ribbed - This condition is due to an imbalance between vegetative and generative growth, with the plant being too vegetative.
- Increase night temperature to achieve a higher 24 hours average.
- Reduce the use of ammonium nitrogen in the fertilizer solution.
Stem
Short internodes - Normal space between nodes should be between 12 to 15 cm.
- Day and night temperatures are to close together and can cause short internodes.
- Maintain a temperature differential between the day (24 to 26°C) and night (21 to 22°C). This is more applicable in the early phases of crop. When harvesting begins, maintain a night temperature closer to 19°C and set venting closer to 22-23°C.
- Generally smaller differences between the average night and day temperature tend to result in shorter internodes.
Swelling at the junction of the stem and rockwool block -
- This happens as a result of slow growing conditions during the propagation stage with little difference between day and night temperatures.
- If the stem is splitting, then the cause is high root pressure. Raise the EC of the feed solution by 0.5 millimhos and adjust watering practices to reduce root pressure.
- Check for any disease infection in these areas.
Tops quit growing and may appear bunchy - Possible lygus bug damage.
- Check for a small hole at the base of the head. Lygus bugs generally bore a hole and cause the top to die off.
- Allow a side shoot to develop.
- Control lygus bugs.
Tops dying off and younger leaves cupping downward. Irregular leaf margin -
- Possible calcium deficiency due to low transpiration because of high relative humidity.
- Activate the plant by maintaining a vapour pressure deficit between 3 to 7 grams/m3.
- Foliar feed with calcium chloride or calcium nitrate at 1 gram/litre.
Head or growing point
Thin growing points with small leaves, poor flower development and poor tendril growth -
- Improve the vegetative growth by bringing the average day and average night temperatures closer together. This can be done by raising the night and lowering the day temperature.
- Once the heads thicken up (2 to 4 days), increase the average day and night spread by dropping the night temperature to 16°C for 2 to 3 days until new flowers are initiated.
- Review the calcium status in the feed and in the growing medium. Check if calcium is being precipitated out due to alkaline pH.
Stem flattened at the top - This is due a to a genetic condition called fasciation. Sometimes two fruit will also be welded together. The plant otherwise appears to be healthy. This disorder can not be transmitted plant-to-plant in the greenhouse. Such plants are not productive and should be removed.
Thick growing points on heads with large flowers, vigorous tendrils and short internodes - This is an indication that plant is too vegetative likely due to cooler night temperatures.
- Higher 24-hour average temperature is required, increase 24 hour average 2°C
- Let the day go up to 26°C for 3 to 4 hours.
- Maintain this higher average temperature as long as you see new laterals and a decrease in the number of flowers.
Heads are chlorotic or slightly mottled - Such heads are commonly seen when the plant is heavily loaded with fruit and light levels are low, such as in early spring or after several cloudy days. Under these conditions, an iron stress is set up in the shoots despite adequate amounts of iron in the growing medium.
- Increase EC in the morning.
- Foliar feed iron chelate if problem is severe.
- Remove the fruit as soon as possible.
- Natural increases in light will also correct the problem.
Heads are dark colored in the morning - Dark heads in the morning can be associated with not enough irrigation the preceding day.
- Increase the number of irrigations at the end of the day.
- Introduce 1 to 2 night waterings at about 100-150 ml/watering.
- Increase the night average temperature.
Flowers
Flower and fruit abortion - There are many causes of flower and fruit abortion and they are all stress related. Too many flowers are allowed to develop in relation to the number of leaves. Growers can use a ratio of one flower/fruit per 4 leaves as a guideline. If more than 4-6 cucumbers are allowed to develop on the main stem in spring crop, then fruit abortion is common. This is due to less food being available for the entire plant. Allow the plant to establish itself by removing fruit and maintain the proper vegetative/ generative balance. Avoid the temptation to flush fruit heavily on the stem to take advantage of the early season prices or later production will be sacrificed.
Heavy deleafing is another common reason for flower/fruit abortion. Many growers resort to drastic pruning to stimulate new growth but in this process the plant undergoes a big stress and flowering is delayed for up to 2 weeks and that means no or minimal production.
Loss of roots due to rot or other factors also contributes to flower/fruit abortion. This problem can be seen when smaller volume grow bags are used.
Too many pesticide applications can also cause flower/fruit abortion. Growers using biological controls have less problem. The use of dibrom can cause flower abortion. Furnace pollution due to incomplete combustion can cause flower/fruit abortion as well.
- Use the checklist above to reduce stress on plant.
- Restrict de-leafing to 1 to 2 leaves per week.
- Achieve a balance between the vegetative and generative growth.
Small flower with small under developed or weak immature fruit -
- Reduce 24 hour average temperature in low light periods.
- If laterals are weak, force the plant into vegetative phase with equal day and night temperatures for 2 to 4 days. In summer, it becomes difficult to do because days are longer and nights are shorter. If possible, start the night setting early, before sunset. Increase the spread between the day and night temperature to induce flower development.
Flower pale color-
- Ensure that phosphorus levels are adequate in the feed and the tissue.
- Maintain vpd between 3 and 7 grams/m3.
Fruit
Fruit short and "chubby" -
- Make sure you are growing the right cultivar. Some cucumber varieties are normally short. fruited.
- Day and night temperatures are too close together. Increase the spread from 6 to 10°C between night and day temperature.
- Decrease the relative humidity so that plant is working early in the morning.
- These strategies may not help with the mature fruit.
Fruit are dumb-bell shaped - This is caused by an acute deficiency of potassium in the fruit. Before the fruit is affected leaves will show potassium deficiency symptoms as chlorosis on the edges and then necrosis.
- Make sure that adequate potassium is available to the plant.
Light green to brown patches or scald on fruit -
- Too high fruit temperature due to direct sunlight.
- Fluctuating vpd between 2 and 7 grams/m3.
- Early infection of gummy stem blight disease. Look for gummy exudates.
Very dark green color -
- Very high EC.
- Plant under water stress.
- If your thumb leaves an impression with gently pressing the fruit, water is being lost from the fruit.
Vertical scars -
- Condensation on fruit surface.
- High root pressure. Ensure the last watering is not too close to sunset.
- Do not drop the night temperature rapidly.
- Suspect early boron deficiency.
M. Mirza and Jim Calpas
Greenhouse Coverings - June 1998 |
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