Cucumber Leaf Blight: How to Identify, Prevent, and Manage Common Leaf Diseases

Once the brown spots, yellow patches and dead leaf area appears throughout the crop, cucumber leaf blight can be a serious issue for farmers. The symptoms may decrease the production of good fruit and can have an impact on fruit quality and quantity as well. However, not all leaf spots are from the same disease pathogen and therefore determining what it is before prescribing treatment will be useful. I would be examining the affected plants (backward side and top of leaves) as the farmer, and then watch the changes develop on the plants. Cultivate resistant cultivars, enhance air circulation, minimize extended leaf wetness, eliminate the removal of infected plant debris and apply proper disease management techniques. When appropriate and locally permitted, applications of biological products, Trichoderma harzianum (fungicide for alternaria leaf spot) may be considered in integrated disease management programs such as one for the control of alternaria leaf spot, caused by the fungus.

Their effectiveness, however, is dependent upon the pathogen, product formulation and approved use. The remainder of this report will describe the common diseases of cucumber in the leaves, and suggest when a cucumber farmer should take cucumber blight control​.

Cucumber Leaf Blight Description

Farmers may observe in a field with cucumbers that there are brown or yellow spots on the plants, lesions expanding, dead spots of leaf tissue, or leaves that gradually shrivel and die. These are all given the name of cucumber leaf blight and involve a variety of different diseases. Unfortunately, many fungal, oomycete and bacterial diseases can have similar symptoms, making it important to identify the disease for a specific cucumber blight treatment.

Look at the lesions closely, paying attention to the location and if there is any sign of them. On cucumbers, small gray or brown spots resembling concentric rings can be indicative of alternaria leaf blight on cucumbers and yellow patches confined to the veins are a diagnosis of downy mildew. Angular leaf spot – Infected leaves are angular in shape with water soaked lesions, which turn dry and create hole-like spots in the leaves.

The farmer should also be mindful of the amount of rain recently fallen, humidity, how the crop is being irrigated and if comparable symptoms are being observed on other nearby plants. Applying a fungicide against Alternaria leaf spot seedlings without being certain what is causing the crop to be diseased can be a waste of money and result in an incomplete cucumber blight control.

This is a collection of the common cucumber leaf diseases that occur in the field and how to identify them

It is important that farmers consider factors other than color because several cucumber diseases cause similar yellow or brown spots. The characteristic shape of the lesion and the appearance of its margins, also the position on the plant and under leaf growth can be of great help.

It is also important to note that symptoms start on older or older leaves, occur on the entire plant or occur in scattered patches throughout the field. By checking recent trends in weather and irrigation, farmers can get better insights into those diseases more likely to attack their farm, but it's not always a sure diagnostic sign.

Alternaria Leaf Blight (ALB) and Alternaria Leaf Spot (ALS) (Alternaria spp.)

How to treat Alternaria leaf blight? The first step is to identify symptoms and determine the extent that the disease is spreading. Alternaria cucumerina is the usual cause of Alternaria leaf blight of cucumbers.  When choosing a fungicide for Alternaria leaf spot, farmers need to make sure that the alternaria blight disease problem they are dealing with is the correct one matched with the recommended fungicide and the target list.

Downy Mildew

Pseudoperonospora cubensis is one of the oomycetes which destory cucumbers by causing a honeydewly mildew. Initial symptoms are often evident as wash outs on the top surface of the leaves, as pale green or yellow bands.

Angular Leaf Spot

Angular leaf spot is a disease of angularly leaf-staining bacteria, Pseudomonas syringae pv. lachrymans. Initially, the plant takes on the appearance of small water-soaked spots on the leaves of cucumbers. 

Other (anthracnose, powdery mildew) leaf diseases

How to treat alternaria leaf blight​? Not every piece of cucumber leaves with damage have Alternaria blight, downy mildew and angular leaf spot. However, other diseases are also seen on foliage and so it is imperative that farmers do a thorough checking of the symptoms before implementing their management plan.

Leaf lesions or other symptoms are caused by anthracnose (Colletotrichum species). Powdery Mildew often is easier to observe because of the white ‘powdery' look of the leaves. Fungal and bacterial diseases of water and food carrying systems can have similar patterns of spotting, yellowing or death as diseases noted in the WFP.

Alternaria Leaf Blight

Early detection will enable growers to rapidly mobilise plant protection strategies to combat cucumber blight and take action on the prevailing environmental conditions and – if necessary – choose a registered fungicide or biological control product. What is the cause of Alternaria Leaf Blight? Alternaria leaf blight is linked with fungal disease of the susceptible foliage. Overegrown infected crops are a source of inoculum for inter-crop seasons. These spores are then dispersed by splashed water onto healthy leaves and/or wind.

After pathogen infects lesions will enlarge and may fuse under favorable conditions. The alternaria blight disease is influenced by the pathogen, the variety of cucumber, the pathogenicity of the plant as well as the length of time an optimal disease environment is maintained.

What is the worst time for Alternaria Blight? Lacking a few gaps, high canopy leaf densities may dry slowly after rain or irrigation, allowing the alternaria blight disease to work. Adding to the difficulties in management are plant stress, and infected crop debris. Close attention should be paid to older leaves—especially in hot, humid seasons for cucumber blight treatment. 

Symptoms of Alternaria blight disease

The following information may help you to identify the cucumber leaf blight in a timely manner so you can make a well-informed decision on how to treat it.

• The first place to start when looking for a diagnosis is the visible pattern of damage. To help growers rule out the different potential conditions, the following description of symptoms are presented as a sort of "tentative diagnosis," rather than a diagnosis.
• Spots over time become brown or circular in shape, often with concentric rings; possibly Alternaria leaf spot or anthracnose.
• Yellow, frequently with grayish underside spots, often with angular outlines: Take note of angular downy mildew.

When to Confirm the Diagnosis for Farmers?

How to treat alternaria leaf blight? Diagnosis in the laboratory or extending service can be especially helpful if symptoms are similar, the disease is spreading rapidly, if an alternaria leaf spot treatment has not been successful or if both bacterial and fungal diseases are suspected. Having the right product has the potential to save the grower money because he or she is not spending on a treatment that treats the wrong pathogen.

Cucumber Blight Control: An Integrated Disease Management Approach for Farmers

Cucumber blight control includes preventive, monitoring, sanitation, environmental management and appropriate treatment. Using a fungicide for alternaria leaf spot or biological product as part of management may help manage the disease, but it's not likely to address exposures to the pathogen and infected crop debris by the host caused by long periods of leaf wetness.

For farmers, this can involve altering the growing environment and so far the sources of infection must be controlled with everything else they do, even if they do use the product.

Enhance air circulation and minimise leaf dampness

Many leaf diseases can be minimized by controlling ventilation and humidity in greenhouses. The amount of direct wetting of leaves, by overhead sprinkler systems, can be minimized with drip irrigation. Where overhead irrigation is needed, water is done to ensure the leaves are dry as soon as possible. Do not handle or prune with wet plants as some pathogens may be spread by contact or splashed water from plants.

Sanitation: Clean out the barns and fields and rotate crops

When possible, remove affected leaves that are highly infected by the disease, without causing unnecessary damage to the plant. Clear up and clean tools and equipment and after harvest crop residues. If the disease is not manageable, in small plantings, a severely diseased plant may have to be taken out.

Cucumber Blight Treatment: What Works and When?

Alternaria leaf spot treatment products for the crop. Don't just select a product based on the description being a broad spectrum fungicide.

Fungicides available to control Alternaria and other fungal leaf diseases

Fungicide for alternaria leaf spot are to be chosen based on the diagnosed pathogen and the local registration/labels of those fungicides. There are products that are mainly preventive and some possess activity in early infections. Both types will not necessarily restore dead leaf tissue.

For the farmers it is important to take heed of the product label and apply the treatment when recommended. Determine if symptoms may be caused by a disease caused by bacteria or oomycetes that must be approached differently from the disease-site.

Cucumber Leaf Diseases are Controlled Biologically

Biological cucumber blight control involves using a natural microorganism or the activity of the microorganism to aid in the suppression of a plant pathogen. As part of an integrated disease-management program, these products can be used as part of other sanitation and resistance, canopy management, and appropriately selected conventional products.

The outcome can vary based upon the strain, application technique, environmental factors and pathogen. Don't offer biological products as a sure-fire cucumber blight treatment for serious, developed illness.

A study on Trichoderma harzianum - Alternaria Blight Disease relationship has been made

The ability of Trichoderma harzianum to act as a biological control agent against alternaria leaf blight has been studied. In 2003, Yacoub Batta published a study of the cucumber leaf disk that showed that formulated T. harzianum conidia decreased lesion size under the study conditions. The authors noted, however, that the results were yet to be tested on whole plants in the field before generalizations were granduated. 

The disease suppressing properties of T. harzianum and G. mosseae have also been reported in a previous study in 2022 conducted against A. alternata on C. melo under greenhouse environment. These types of studies are useful for further testing of biological control, but are not an indication of biological control efficacy under all field conditions in the cucumber crop. 

FloraEgis for Cucumber Leaf Disease Management

What Is FloraEgis?

FloraEgis is a Trichoderma harzianum strain TH17 foliar biological fungicide specially developed by Novobac. In addition to powdery mildew and Botrytis, the manufacturer mentions other diseases: Leaf spot and Anthracnose. It's product data says it can be applied to foliage and has a mode of action of biological antagonism and stimulating plant defense.

Integrating the use of Trichoderma harzianum into a Cucumber Disease Program.

The efficacy is dependent upon the type of strain, formulation, pathogen and the period and environment. Results with one strain of Trichoderma cannot be taken as evidence for another strain. Likewise, any data obtained from laboratory or greenhouse experimentals should be viewed as not necessarily representing the ability to achieve reliable control in all field situations as encountered in commercial production.

In situations in which its usage is recommended, FloraEgis can evaluate as a part of an integrated cucumber disease-management program. It should support the proper diagnosis, sanitary disposal, rather than replace it, as well as the proper irrigation and registered disease cucumber blight control measures.

When to Consider FloraEgis

A biological foliar product is best used in a management program versus in response to high levels of leaf damage. It is a responsibility for growers to keep track of crop health, screen for early signs of disease in susceptible species and apply the crop according to the approved label directions for the target disease and the crop.

How to treat alternaria leaf blight? According to Novobac, foliar application rate is 2–3 g/L water and re-apply with 7–14 day intervals based on the disease pressure. They are directions as listed by the manufacturers and are not universal directions for all cucumber crops, or for every jurisdiction. Prior to use, verify the latest label for the product, local labeling, application timing, and compatibility with other usable crop protection products. In addition, the manufacturer warns against using FloraEgis in combination with copper fungicides because this could cause damage to the microbial agent.

Cucumber Blight Treatment Program for Farmers (Practical)

With a structured approach, growers will avoid unnecessary applications and be better able to deal with outbreak if it occurs.

Select the appropriate comparative index

With a limited number of symptoms and a recent weather history, reduce the number of potential causes; but do not presume that all brown spots are due to Alternaria. Get diagnostic assistance when symptoms are similar, rapid and when the first treatment has been unsuccessful.

Control Disease Sources

Distribute crop residues and cut off badly affected leaves, if possible. Ensure tools are clean and avoid movement of wet plants and soil from dirty production areas.

Select an appropriate treatment

Choose a registered product from the register for the taxon/pathogen that is known/expected. Use fungicides for diseases they have been labeled as control and follow application practice requirements as outlined by the local area such as rate, timing of performance or intervals between application and harvest. New disease management methods may be required to manage bacterial or oomycete disease. Methods available for management of alternaria leaf blight on cucumber might not be adequate for bacterial or oomycete.

Take into account biological protection

Locally authorized, the use of a biological product with T. harzianum can be included in a preventative or early management program (like FloraEgis). 

Monitor and Reassess

Monitor new growth after treatment. Loose any infected areas that are already dead, and determine if new spots are developing and disease development has slowed. Revisit diagnostics and management plan if the symptoms persist in spreading.

Conclusion

Cucumber leaf blight is a disease that can be handled most effectively through diagnosis, rather than just selecting a fungicide. Alternaria leaf blight, downy mildew, angular leaf spot and anthracnose share the same disease symptoms but the pathogens and the methods for cucumber blight control are different.

By using good planting stock, variety selection, adequate air movement, reduction in leaf wetness and good crop residues and regular inspections of crop plants, growers can reduce the risk of infection. If an athlete requires treatment it must contain the same treatment as per the pathogn and meet requirements as described on the athlete's body as outlined in the product label.

Integrated disease management including biological control agents like Trichoderma harzianum could be part of the solution and Floral Egis is registered as a foliar biological fungicide containing strain TH17 by Novobac. Use should be based on local authorisations and on the label cucumber/target disease. A multi-facet integrated approach to prevention, diagnosis, alternaria leaf spot treatment and point-in-time monitoring is more sustainable than the use of any single product.

Reference:

• Elad, Y. "Trichoderma harzianum T39 preparation for biocontrol of plant diseases-control of Botrytis cinerea, Sclerotinia sclerotiorum and Cladosporium fulvum." Biocontrol Science and Technology 10.4 (2000): 499-507.
• Elad, Y. "Biological control of foliar pathogens by means of Trichoderma harzianum and potential modes of action." Crop protection 19.8-10 (2000): 709-714.