Coffee Leaf Rust: Symptoms, Prevention, Management and Biological Control Strategies

One of the most significant fungal diseases of the coffee world is coffee leaf rust. It is a disease caused by Hemileia vastatrix, which attacks the leaves of the coffee plants and has the capacity of making the plants increasingly weaker when the disease pressure is high. A problem of the disease is that if it gets to a very severe stage, it may defoliate the plants prematurely, thereby decreasing the leaf area that can be used for photosynthesis and ultimately in reducing plant vigor, flowering and fruit development. Coffee leaf rust can cause substantial losses when the diseases reoccur from season to season.

The rapid spread of the disease is closely related to the economic impact. After having experienced a significant loss of leaves, coffee plants might not have a sufficient reserve of resources to sustain growing cherries and to prepare for the next production phase. Hence, knowledge of the biology of the pathogen and early recognition of early symptoms of coffee leaf rust are crucial in minimizing the likelihood of Coffee Leaf Rust epidemic to cause damage to coffee.

Coffee leaf rust has become a devastating problem to coffee farmers in America and Central America. The disease has caused massive losses in coffee production and quality and has plunged most farms to the verge of crisis and livelihoods. Some of the recent outbreaks that we have seen in the past few years have led to the need to find a way of providing long term protection without sabotaging the environment. The introduction of the coffee leaf rust epidemic warrants the development of new approaches which emphases on coffee leaf rust, coffee leaf rust epidemic, coffee leaf rust management, coffee leaf rust control and in particular, biological control of coffee leaf rust. This is where biological control of coffee leaf rust using such products as the Bacillus subtilis fungicide and Bacillus amyloliquefaciens fungicide comes in.

What are the symptoms of Coffee leaf rust?

Coffee leaf rust initial symptoms are tiny, light yellow or yellowish spots on the upper coffee leaf surface. Later, orange-yellow to orange colored powdery spores may be seen, especially at the bottom of leaves that are infected. These spores are a typical symptom of the disease and can aid growers in differentiating rust as the cause of leaf discoloration from other causes.

Often the dense canopy generates a more suitable micro climate for disease to occur. Therefore, scouting for the symptoms of the disease regularly is very important to be able to detect the disease in time so that it does not spread to form a larger coffee leaf rust epidemic.

To prevent coffee leaf rust from occurring, organic farmers should

Prevention is especially key for growing organic coffee, since disease management needs to focus on keeping plants healthy and applying several compatible strategies. Breeding varieties of coffee that are resistant or tolerant to rust disease can be a major first line of defence. But varietal resistance must be used in conjunction with good agronomic coffee leaf rust management, as pathogen populations are likely to evolve over time.

Well balanced nutrition is also important for coffee leaf rust epidemic. The stress of nutritional limitations may result in less vigor and resilience and too much vegetative growth may result in dense canopies that keep moisture in the plant. Suitable pruning and canopy management will also help to enhance air circulation and minimise the duration of leaf wetness. In addition, adequate spacing, sanitation in the field and removal of material that is highly affected by disease when appropriate can help coffee leaf rust control.

Periodic scouting will enable growers to detect new infection in a localized stage. That is the reason why the management of coffee leaf rust should be geared towards prevention and early intervention rather than waiting until the coffee leaf rust has grown to a serious level. The use of a diversity of management strategies for coffee leaf rust can help to lower disease pressure and help to maintain the productivity of the orchard for the long term.

Explain the reasons why Coffee Leaf Rust is difficult to manage

The main problem with coffee leaf rust is the suitability of H. vastatrix spore movement under favourable environmental conditions. Spores may be carried between plants on rain and wind and the warm and humid conditions can aid in the development of the disease. As more and more leaves become infected, it becomes more difficult to further reduce the source of new spores.

However, resistance-management issues may arise if the same chemistry of fungicides is repeatedly used. The challenges that are faced by organic growers are greater, as fewer conventional chemical alternatives might be available to them. Meanwhile, shifts in temperature and rainfall can lead to variation in disease pressure, thereby aggravating the potential for unpredictable coffee leaf rust epidemics in some coffee production areas.

It is these challenges that make the coffee leaf rust management a topic that is increasingly focusing on prevention, plant health, resistant varieties and biological, along with suitably applied disease-control products. The grower should not rely on one intervention but establish a coffee leaf rust control system that will limit the opportunities the pathogen has to establish and spread.

Biological control of Coffee leaf rust?

Coffee leaf rust control  is done by the use of beneficial living organisms or produced compounds to control plant pathogens. For the disease of coffee leaf rust, the microorganisms that interact with H. vastatrix or the plant itself, and may interfere with the disease process have been investigated. These microorganisms may consist of good bacteria and fungi found naturally in agricultural areas.

The goal is not to destroy all of the rust spores at the coffee plantation. Rather, biological methods are directed at minimizing disease introduction, slowing disease progression and bolstering the overall biological equilibrium in the crop system. Biological control is of special interest with organic production systems where cultural and nutritional strategies are being sought.

Biological control should therefore be used as one tool in the disease management toolbox and not as a substitute for all other disease management practices. In addition to resistant varieties, correct shade coffee leaf rust management, sanitation, nutrition and frequent scouting, microbial methods can help to develop more sustainable methods of managing coffee leaf rust and controlling coffee leaf rust.

How do ‘beneficial microorganisms' suppress Hemileia vastatrix?

There are several ways beneficial microorganisms inhibit plant pathogens. Competition is one of the mechanisms that is important. The beneficial microbes can colonize surfaces and outcompete for nutrients that could otherwise be used by the pathogens to survive and become well established.

Other antimicrobial compounds or metabolites may be produced by some microorganisms that affect the growth or development of plant pathogens. Microbial activity can also influence the germination of fungal spores or early stages of pathogen development in certain cases. Other microbes that are beneficial to the plant may stimulate defense responses, thus enhancing crop response to pathogen pressure, through interactions with the plant itself.

The efficacy of these mechanisms varies from one microorganism to another, from one strain to another, and with formulating, application and environmental factors. Therefore, when choosing a biological control of coffee leaf rust, one should take into account the product's intended use, rather than assuming that all beneficial micro-organisms are equal in their disease suppressing capability. The inclusion of microbial products into an integrated approach to control of coffee leaf rust is an interesting element for growers.

What is the cell wall of Bacillus subtilis made of?

These interactions can be competitive, helping to suppress unwanted microbes, and beneficial, promoting better plant–microbe relationships, depending on the strain and formulation.

The use of a Bacillus subtilis fungicide, biological control of coffee leaf rust , is then a possible part of an integrated disease-management program. This microbial product can be used in conjunction with other rust prevention strategies and monitoring, and not as a cure for existing rust problems.

It is especially critical to apply a biological disease management method at the right time. Most often, microbial products work best when used as part of a program that is planned ahead, before the disease is a problem. Other crop protection inputs to consider, in addition to herbicide compatibility, are other products used by the grower and the local registration requirements. Environmental conditions include how the product reacts to the weather, and whether it is compatible with other products used in that location.

Microbubbles for biological disease management: Bacillus subtilis v. Bacillus amyloliquefaciens Biocontrol

The beneficial bacteria Bacillus subtilis and Bacillus amyloliquefaciens are two well studied bacteria with potential in biological disease management, but highly dependent on the specific strain, formulation and disease. Both organisms are capable of producing biologically active metabolites, and are also a part of plant associated environment, which may lead to competition with harmful microorganisms.

A B. subtilis product can be used in a programme for which microbial competition and pathogen suppression is sought; in addition B. amyloliquefaciens strains have been of interest due to their antimicrobial activity and support of plant defence. In this way, fungalicide products that contain Bacillus amyloliquefaciens are not all replacements for the products containing other Bacillus species.

The potential of bacillus amyloliquefaciens fungicide or bacillus subtilis fungicide is evident in research, especially for use in integrated crop protection. Growers should follow specific cultural requirements and approved label instructions as well as the specific crop and pathogen to choose a biological product, but shouldn't assume that two microbial species will have the same application.

Collaborative efforts to build an organic coffee leaf rust management program

Patterns of scouting should be the first step in a successful organic program for the coffee leaf rust. The early lesions and orange-yellow spores can be looked at on both sides of the leaves by the farmer and record kept of the development of the disease from block to block within the plantation. Early detection will give more time to respond before the pathogen becomes established.

Variety selection should be another layer of the strategy, especially in situations where varieties resistant to or tolerant of rust or other disease would be suitable to local conditions. Management of canopy may allow better ventilation and less extended leaf wetness and balanced nutrition will maintain plant vigor. Other coffee leaf rust control measures can minimize conditions conducive to disease development – sanitation and appropriate spacing.

Within these limits the microbials, which are registered and suitable for the crop and target disease, can be included under the framework of microbial products. They are most effective in conjunction with cultural practices and as such are not used as a standalone solution.

Ultimately, biological control of coffee leaf rust, multiple layers in coffee management will be essential. Coffee leaf rust management should not just be a response to high leaf rust pressure, but reducing disease pressure throughout the crop cycle. In addition to agronomic methods, biological strategies for bacillus subtilis fungicide can be incorporated into cropping systems and offer growers other options for developing a disease-management strategy.

References:

1. Dorighello, Dalton Vinicio, et al. “Management of Asian soybean rust with Bacillus subtilis in sequential and alternating fungicide applications.” Australasian Plant Pathology 49 (2020): 79-86.
2. Dimopoulou, Anastasia, et al. “Shifting perspectives of translational research in bio-bactericides: Reviewing the bacillus amyloliquefaciens paradigm.” Biology 10.11 (2021): 1202.