Coffee Rust: symptoms, causes, cycle and solutions

Orange powdery lesions containing rust spores on the lower coffee leaf surface. Creative Commons ©

Today, coffee rust is considered the most destructive disease affecting coffee (Coffea sp.) in the world. For coffee producers, it is economically disastrous. 

Coffee rust was first found in Africa in 1861. But the disease was later reported as infecting cultivated (selectively bred, as opposed to wild) coffee in Sri Lanka in 1867. Here, it ruined coffee production within ten years. Since then, this devasting disease has been reported in all major coffee producing countries. 

Light infections can result in the loss of leaves. Severe infestations may cause twigs to die back from the tips. Eventually entire trees can die. The long-term effects of the disease often result in a major decrease in yield. This leads to losses of billions of US dollars annually.

Symptoms of Coffee Rust 

The most noticeable symptoms of coffee rust are irregular shaped spots on upper leaf surfaces. They are connected with yellow to orange powdery lesions (diseased tissue) on the lower leaf surfaces where the spores are located. The spores are cells of the disease that can reproduce.

Significant rust infection and rust-induced defoliation © Creative Commons

As the leaf spots grow, they may come together or blend to form bigger spots. The leaves form large irregular shapes or lesions. They eventually dry up and turn brown. The symptoms may vary depending on a variety of factors. These include the environment, farm practices and the sensitivity of the plant to disease. Spots mostly begin to form at the leaf edges or tips where water collects. The first lesions usually appear on the lowermost leaves. The infection slowly progresses up into the tree.

Infected trees may prematurely drop infected leaves. This results in long, bare branches. It also reduces the plant’s ability to turn sunlight into energy. This problem affects both the quality and quantity of the fruit (the coffee beans).

What causes Coffee Rust? 

Coffee rust is a devasting disease caused by a rust fungus, Hemileia vastatrix. It can reduce coffee production from between 30% to 50%. Infestation levels vary depending on a variety of factors. These include an advantageous climate for the disease, the management measures adopted and the plant’s level of resistance.  

The disease thrives under conditions of high humidity. Rain is the main way that the disease spreads. However, wind, animals or people also can carry the fungus to new leaves, beginning the infection again.  

Warmer and wetter climates help to spread the disease. Drier, cooler climates limit the spread of the disease.  

Certain management practices favour the development and spread of more virulent strains of H. vastatrix. This includes practices such as growing a ‘monoculture’ of a single cultivar of coffee. 

Coffee rust is frequently spread during harvest periods. It can be moved from one plant to the next within individual plots as people move through the plantation harvesting coffee beans. It can also be moved from farm to farm by the harvester. 

Disease cycle of Coffee Rust 

Rust fungi need a living host to survive. Without this, they are incapable of reproducing. This contrasts with almost all other fungal plant diseases. The rust fungi do so through a complex life cycle of up to five different stages of spore production. 

The disease cycle begins with the infection by a microscopic spore. This spore enters the plant via a natural opening on the underside of the leaf. After entering, the spore invades other cells to obtain nutrients. This eventually kills the cells while producing new spores.  

These new spores are forced out through the leaf openings. Here, they are dislodged primarily by rain but also by wind, animals and people. This begins the infection anew. This process takes about 4 to 7 weeks. 

A single spore can produce four to six generations. An exponential increase of tens of thousands of spores is generated from the initial single infection. 

How to prevent Coffee Rust 

There are many ways to prevent the spread of coffee rust. The most effective recommended measures include:  

  1. Maintaining healthy plants and good sanitation practices. 
  2. Removing weeds that may compete with the coffee plant for nutrients or stress the coffee plants. This also contributes to maintaining tree health.  
  3. Pruning, which increases air flow through the trees and reduces humidity.  
  4. Removing plants that are weak, old or already affected by other diseases or pests.  
  5. Checking with your local agricultural advisory (extension) agent for recommended practices. Do this before applying any plant protection products. 

Coffee Rust solutions 

The use of chemical fungicides can come with some issues, such as:  

  1. High cost of the products. 
  2. Continuous and repetitive use may promote the selection of rust populations that are resistant to fungicides. So far, chemical control of coffee rust relies on only two chemical groups, which makes the products losing efficiency more probable. 
  3. Residues can prevent growers from the high-value organic coffee market. This is particularly true with systemic fungicides. These are fungicides that are absorbed into the plant. 
  4. Active ingredients in fungicides may bring harm to the environment and humans  

Fortunately, there are more sustainable strategies available for coffee farmers. These include the use of resistant varieties, crop management and biological controls, namely biopesticide products.  

Use of resistant varieties 

The use of rust-resistant coffee cultivars is considered the best method for managing the disease in the long term. A cultivar is a plant variety produced by selective breeding. However, coffee growers still have little knowledge about the advantages of new cultivars. 

Rust can affect about 10% of susceptible varieties. Conversely, the incidence of rust is very low among resistant varieties. Consequently, the usual high level of economic loss can be avoided.  

 

A coffee tree with few leaves due to the coffee rust disease
A coffee tree that has been almost completely defoliated by the coffee rust fungus © Creative Commons

Use of biopesticide products (biological controls) 

The use of biopesticides seems to effectively reduce the damage caused by coffee rust by up to 97%. These products include plant extracts, which stimulate a chemical defense reaction in the plants. Thus, these products may induce resistance against the disease. This makes them a promising alternative in disease management. The use of essential oils such as cinnamon, citronella, lemongrass, cloves, tea tree, thyme and eucalyptus has also shown promising results in coffee rust management.

Some biological products based on bacteria and fungi are already registered for coffee rust control. 

Two types of fungi are known to be particularly important for coffee leaf rust: 

  1. Mycoparasitic fungi which ‘eat’ other fungi such as coffee leaf rust.
  2. Fungi capable of living inside the tissue of the coffee plant and working as bodyguards. They protect the plant against attacks by diseases like rust.  

Both types of beneficial organisms might be exploited on coffee farms as sustainable tools for managing coffee leaf rust. 

There are several bacterial groups that form beneficial associations with plants. These mainly belong to the bacterial groups Bacillus et Pseudomonas

These bacteria can benefit the coffee plants in three main ways: 

  • Competing for space or nutrients because many bacteria live both within and on tissues of plants. This prevents germination and/or the development of the fungus causing coffee rust.  
  • Producing antimicrobial compounds that can attack the cell walls of the coffee rust fungi.
  • Inducing systemic resistance to the rust. Systemic resistance in plants is a resistance mechanism that is activated by a prior infection. 

For more detailed information about biopesticides and biological control products available in your country, please visit: Portail CABI de BioProtection 

For more information on biocontrol, see Biological pest control – what is it? 

Cultural practices 

Cultural practices are any practices that help the plant to grow in its environment. Coffee is intolerant to direct sunlight. Growing coffee under the shade of a canopy of trees is recommended, as is plant nutrition, in the management of coffee rust. 

Reducing the disease progress rate by shading could bring added value to coffee production. It could help producers maintain not only environmental, but also financial, sustainability.

Coffee susceptibility to rust is associated with its nutritional status. Thus, plant nutrition is another important aspect in rust management. Different nutritional sources can be used to fertilize coffee plants. This includes growing coffee next to Crotalaria juncea (sunn hemp) and/or using coffee husks in association with castor bean cake or swine manure. The latter has been shown to reduce diseases by 21% to 31%. 

For crops in full production, nutrition must be reinforced by including boron (Bo), zinc (Zn), manganese (Mn) and silicon-based products along with the conventional nutrients. The plants use a great deal of energy during the formation of the coffee beans, so they are become weaker and more susceptible to infection by pests. Therefore, farmers need to boost the plants’ energy by supplementing these nutrients.

Coffee rust management should be based on the use of a range of integrative measures such as using resistant varieties, cultural practices and biopesticides. Biopesticide products should be used under the guidance of technical personnel. This ensures an effective and sustainable coffee rust management strategy.

Want more information about managing coffee rust? See the CABI Invasive Species Compendium https://www.cabi.org/isc/datasheet/26865