Citrus Scab Control: Symptoms, Treatment, Prevention and Management for Healthy Orchards

What is Citrus Scab and What Causes it?

The pathogens are members of the Elsinoaceae and can cause infestations on leaves, twigs and fruit of susceptible citrus species. E. fawcettii is able to infect wide host range of citrus and E. australis is predominantly found on sweet orange and limited to some mandarins.

It is important not to believe that in each citrus planting area the disease is confined to the one organism mentioned above in the citrus scab scientific name. If a pathogen- Host combination is correctly identified it may affect decisions of management. In India, E. fawcettii was found to be a significant cause of citrus scab which can result in severe infections causing deformity of foliage and leaf and fruit drop. 

A knowledge of how citrus scab caused by also helps to explain why scab is quite a challenge to control, because young tissues are the primary targets of scab management. Leaves of citrus are especially vulnerable in the first few days after they emerge in canopy, but are not as vulnerable as they get older. Young fruit may also become infected and infections early in fruit development may result in permanent blemishes when the fruit develops, or fruit may be severely deformed. 

When is the point that symptoms appear in crops and how does the disease spread?

Citrus scab symptoms may be hard to detect. Tiny spots on young leaves grow into pale, yellowish or brightly-colored pustules. As the infection progresses they are raised and non-uniform and produce typical warty or corky growths. Severely affected leaves can be wrinkled, distorted or malformed, and lesions will occur on young twigs and shoots. 

When fruits are infected when young, rough, raised, irregular lesions may develop. These blemishes may be noticeable and detract from the attractiveness of the fruit as it grows. Infected young fruit may produce in severe cases with poor shape and be dropped early. This is especially important for fresh market production because external appearance is a commercial quality of the product. 

Moisture highly affects the citrus scab disease cycle. Spores form on old scab lesions and spread in the water splash of rain or any other movement of water to healthy tissue. The University of Florida summarizes research indicating only slight tissue wetting will help produce spores and a few more hours will likely be enough for infection.

This is the reason that disease pressure may rise during wet seasons when flushes of plants constitute susceptible plants. Inoculum for subsequent infections can be carried over from infected leaves, twigs and fruit of the previous season. The extraction of infected material and regulation of moisture in the canopy may then minimize the potential for additional disease cycles to occur. In the coming sections, we will look at citrus scab management.

But is there any difference between citrus canker and scab?

An important distinction between citrus canker and scab is that the two diseases may cause lesions that, at first may look alike. The lesions of citrus scab are typically raised, corky, rough or warty, appearing mostly on young leaves and developing fruit. As the infected tissues proceed to maturity these lesions may become more obvious. 

Citrus canker is also a bacterial disease, and generally yields lesions with better-defined margins that may have yellow halo. There are further differences in the organism that causes the disease, the management approach and whether it is an epidemic or not. Symptoms of citrus scab can be confused with other diseases or mechanical or environmental damage; growers should not use a single symptom in making a management decision.

In commercial orchards accurate diagnosis is vital, particularly as disease pressure gets heavier. If citrus scab or citrus canker are the cause of symptoms, growers can plan a citrus scab disease management program or citrus scab disease treatment which may include use of a product designed specifically to control the disease.

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What are some common methods growers can use to avoid citrus scab in commercial orchards?

Proper control of citrus scab management program that is aimed at prevention. It is important for growers to be aware of the time when sensitive young leaves, shoots and fruit are present. Monitoring for new flushes by frequent scouting will help determine new diseases and help determine if the disease pressure is building.

An important part also is the orchard sanitation. Provided that sufficient control procedures are followed, the healthiest parts of the crop can be pruned out and destroyed. Limiting infected plant material will result in lower amounts of potential inoculum that may cause subsequent infections. Scattering thick canopies can also promote the movement of air in the canopy, which can help to prevent high humidity around sensitive tissues for extended periods of time. 

The management of water must be taken care of in the same manner. Water splash is a dispersal mechanism of fungal spores from one tissue to another, so nonsensible overhead irrigation on susceptible citrus can help minimize sources of disease. Reducing or eliminating foliage wetting (with or without active growth) is noted by the University of Florida to help reduce citrus scab severity. 

These practices are specifically applicable to citrus scab control inasmuch as they affect not only when lesions are already wide-spread, but also when conditions for the emergence of scab are present. These cultural practices can be the first step in an organic production citrus scab organic treatment approach, and then biological and other approved inputs as necessary can be added on to the organic approach.

What Are the Best Citrus Scab Treatment Options?

How to treat citrus scab? Timing is the most critical factor to convince in citrus scab disease treatment. When raised lesions become evident, infection has already taken place and prevention of infection is likely to be of more use than when there is extensive infection.

The use of registered fungicides for citrus scab suitable for a commercial disease-management program during the time of new growth that may be susceptible and under conditions that promote infection can be made. The research and extension recommendations outline the programs whose applications are coordinated with the timing of the development of spring flush and areas of petal fall followed by fruit susceptibility. Products and time, however, depend upon the variety grown, disease pressure, climate and regulations. 

Growers should, therefore, strictly adhere to the product label and local agricultural recommendations, rather than use a generic fungicide schedule, if they are thinking about protecting citrus for scab. Rotation to just compatible fungicide groups may also be significant for resistance management; especially when repeated applications are needed. Fungicide-resistant strains of E. fawcettii have been recorded, as an illustration of the fact that repeated use of a single fungicide chemistry can cause problems in long-term management. 

The same is true where plant species are citrus, treating citrus scab organically is a similar decision. Not going organic involves not taking away disease management discipline. An integrated approach may involve a combination of products, cultivar resistances and less susceptible cultivars, soil management, and water management.

How to Use Trichoderma harzianum to Aid Biological Disease Citrus Scab Management?

Trichoderma harzianum is a member of the fungus genus Trichoderma that are known as bioagents for biological crop protection and are extensively investigated. It does not have any one mode of action. The growth, ability to occupy space and resources with other microorganisms, metabolite production, enzyme production, and ability to parasitize some pathogenic fungi and induce plant defense mechanisms are potential activities of Trichoderma that may affect the application data it produces.

This suggests its application in the plant and root plantation care, but not directly in the citrus scab organic treatment, for this purpose in citrus scab a different organism, Trichoderma harzianum s.l., should be used. The evidence of scab research from citrus is particularly interesting: an Indian study has demonstrated that T. harzianum exhibits mycoparasitism against E. fawcettii under laboratory testing, and that this fungus decreased the disease incidence under field testing. 

But there can be significant variations based on the strain, formulation, application, weather and disease pressure. The use of a biological product should thus be considered as part of citrus scab controls, and not necessarily as a substitute for all other conventional disease control measures.

How does HarzShield fit into a Sustainable Citrus Disease Management Strategy?

HarzShield is Novobac's Trichoderma harzianum-based biological fungicide for citrus scab organic treatment that can be added into an orchard program for growers that want to use microbial inputs in their program. Its manufacturer's information suggests its primary use is for soilborne citrus scab management and root protection applied as soil drenching, drip irrigation and other soil-based applications. 

This is not a distinction that is significant from an agricultural management point of view. HarzShield for citrus scab treatment is not a spray to be recommended as a cure for existing lesions of citrus scab. Rather than that, its use may be part of an overall biological soil-health and root-management program. Novobac claims that its T. harzianum technology will occupy space and resources with pathogens and develop roots, helping to improve nutrient uptake. Therefore when orchard inputs involve biological input, it is most useful when combined with orchard monitoring, sanitation, orchard management, and agrochemicals for specific diseases. Microbial products can be part of a comprehensive approach to disease management, but will not be required to provide a solution for all pathogens when registered for use and crop.

Building a citrus scab management strategy – the long term view

How to treat citrus scab? Diagnosis and knowledge of the orchard scab history is the first step towards good citrus scab management or citrus scab disease treatment. Regular scouting of young flushes, shoots and fruit should be undertaken by growers, particularly when conditions are wet and productive. Implementing early detection gives more time for protecting vulnerable tissues from establishment of infection.

Practical program is the integration of a combination of measures that are compatible among each other. Properly raised planting material and appropriate cultivars can minimize susceptibility and pruning and sanitation can minimize sources of inoculum. The better the air circulation, the more manageable infection and spore movement will become, and stopping of useless overhead irrigation will help to minimize the "wetness" that is required for infection and spore movement to happen. Preventive fungicides for citrus scab applications can be considered and use of biological inputs can be used to augment other crop management and health strategies. 

Citrus scab control in the end is best achieved by disease cycle management and not just in response to visual symptoms. Knowing when the tissues are susceptible, how the spores travel and on which environmental factors they depend to infect the tissue makes more precise decisions about citrus scab treatment.

The answer for growers is still not one product or application when it comes to citrus scab – because it's not a single product or application that's needed. It's a coordinated programme, which is centred around pre-scouting, moisture management, sanitation, adequate preventive protection and if appropriate, biological measures, which include Trichoderma harzianum. When applied correctly, growers can minimise disease pressure and keep fruit appearance unchanged, orchard productivity and orchard health (over multiple year roost cycles) in good condition.
 

Reference:

•  ALAM, MD. "MANAGEMENT OF SCAB AND DIE-BACK OF CITRUS (Citrus limon L.) THROUGH BIOAGENT, PLANT. EXTRACTS AND FUNGICIDES." (2006).
• De Sousa, Marcio Alves, and Camille Eichelberger Granada. "Biological control of pre-and post-harvest microbial diseases in Citrus by using beneficial microorganisms." Biocontrol 68.1 (2023): 75-86.