News Cff (Curtobacterium flaccumfaciens pv. flaccumfaciens) in legumes on the radar: cause for concern?

Curtobacterium flaccumfaciens pv. flaccumfaciens, or Cff for short, is a notorious pathogen that affects legumes such as beans and soybeans. In Europe, this bacterium was virtually unknown for a long time, while in North America it has been causing damage to bean crops for over a century, with a clear resurgence since the end of the 20th century (see this plant pest). In Europe, Cff suddenly came into the spotlight in 2019 when it was included as a full quarantine organism in the European Plant Health Law. Since then, all member states have been required to take measures to prevent introduction and spread of Cff in the EU. Following several detections of Cff in the EU and the interception of infected seed lots from the United States, import requirements for seeds have recently been considerably tightened, with special attention being paid to testing for Cff before seeds are exported to the EU. In addition, the EU now requires all member states to conduct annual field surveys for the presence of the bacterium on their territory. This has led to increased vigilance within the legume cultivation sector in recent months.

What is Cff and where does it occur?

Curtobacterium is a rod-shaped bacterium that occurs in virtually every natural ecosystem on earth, especially on plants and in the soil. Some species can affect plants, but they are completely harmless to humans and animals. Cff, the Curtobacterium discussed in this article, is known as a pathogen of legumes (Fabaceae), and in particular of Phaseolus beans.

In the laboratory, Cff stands out for its striking color palette. On culture media, the colonies can be yellow, yellow with purple pigment formation, orange, salmon-colored, or red, which is why the bacterium is also called the “multicolored bacterium” (Photo 1).

Cff was first described in 1922 in Phaseolus beans in the US. The disease is still present there, especially in the central plains (including Nebraska). Since then, economic losses have also been measured in Brazil on Phaseolus beans and soybeans (Glycine max), in Iran on Phaseolus beans and cowpeas (Vigna unguiculata), and in eastern Australia on mung beans (Vigna radiata).

In Europe, the bacterium has only been found sporadically to date. In 2001 and 2005, there were a few isolated reports of infected Phaseolus seeds and plants in Spain. In 2011, Curtobacterium was isolated from symptomatic soybean plants in a variety trial in Germany. In 2018, mung bean plants and seeds tested positive in Austria. More recently, in 2024, a few Phaseolus field crops and the associated seed were found to be infected in the Netherlands and Switzerland, while in Greece the same year, the bacterium was detected in bean seeds. Just outside Europe, Curtobacterium is present to a limited extent in Turkey and Russia.

Cff has now also been found in Belgium. In 2021, the pathogen was isolated by chance from Vicia faba plants (field beans) with symptoms of Botrytis fabae, and again in 2024 from symptomatic Phaseolus plants during an investigation into the occurrence of the bacterium in variety trials with leguminous crops. In 2025, during the first implementation of the national survey by the FASFC (imposed by the EU), two asymptomatic Phaseolus samples from the field also tested positive. In each of the cases in 2024 and 2025, the source of the infection was found to be contaminated seed from the US.

How harmful is Cff and how does it spread?

Infected seeds are the main route for Cff to enter the EU. From the seed, the bacteria penetrate the plant's water-conducting channels (the vascular tissue), where they spread further. In Phaseolus beans, the first symptoms are often subtle: limp leaf edges and light green discoloration in the leaves. These evolve into brown spots with a burnt appearance, usually surrounded by a yellow edge. This typical symptom is known as “firing” (Photo 2). Eventually, the affected leaves dry out and fall off. In severe cases, plants can die completely, resulting in a significant reduction in yield. In soybeans, the infection is usually observed as “tan spot” on the leaves, resembling sunburn. Because Cff symptoms can closely resemble drought stress or infections by other pathogens, the disease is not always immediately recognized.

In addition to primary infection via seed, secondary infections can also occur. Cff can spread from plant to plant through contact (e.g., via tools) or via water (e.g., irrigation over the crop). The bacteria enter the plant's vascular system through natural openings or small wounds.

The most damage occurs in regions where conditions are favorable for the development of the bacteria: prolonged periods of hot weather (>30 °C) combined with drought. This puts the bean plants under stress, making them more susceptible to Cff infection. In addition, irrigation is often used under such conditions, which promotes the spread of the bacteria. Under our current climate conditions, such stressful situations are still limited, which means that infections often go unnoticed.

What are the consequences if Cff is detected in Belgium?

The use of healthy seed is always essential for a healthy crop, and this certainly applies to Cff. Thanks to stricter controls on seed imports, the risk of Cff being introduced is relatively low. If the bacterium is detected, it must be reported to the Federal Agency for the Safety of the Food Chain (FASFC) and specific measures must be taken.

For example, the plants in the infected area must be plowed under as quickly as possible. For the rest of the year and at least the following two calendar years, no legumes may be grown in the affected zone. Any regrowth of the crop must be removed during that period to prevent the bacteria from surviving. After two years, it is assumed that no viable bacterial cells are present. Crops from covered cultivation can also be removed and destroyed by incineration, biomethanization or industrial composting, subject to approval by the competent authorities. In that case, the cultivation ban can be limited to one calendar year after the year of contamination. If, in addition, the soil is steamed or substrate mats are steamed or replaced, Fabaceae may be grown again in the following calendar year after thorough disinfection of the site.

If the crop is ready for harvest at the time the bacteria is detected, unripe pods may still be harvested for destruction, but only for immediate processing in the food industry (preserving or freezing). All waste from the industrial process must be carefully disposed of and decontaminated using the methods described above, and the process water must be treated in a water treatment plant.

If Cff is found in a batch of seed, that batch must be completely destroyed. If the seed has already been sown, the crop must be closely monitored. Only if no symptoms are detected may the crop continue to ripen and be harvested for direct delivery to industry or for sale through auction or wholesale. In this case, reusable crates must be thoroughly disinfected afterwards. The same rules apply to the processing of harvest residues and the cultivation ban as to contaminated crops. In none of the above scenarios may seed be harvested for use as seed.

What does ILVO do?

Since the update of the quarantine status of Cff in the European Plant Health Law, various projects involving this bacterium have been underway at ILVO's bacteriology laboratory. ILVO is one of the founding partners of the European Reference Laboratory (EURL) for plant pathogenic bacteria. Within this European network, we play a pioneering role in validating and providing standardized diagnostic methods for Cff, including specific tests for detecting the bacterium in bean seeds (Photo 3). These methods are routinely applied in our research laboratory and also in our Plant Diagnosis Center (DCP). At the DCP, we analyze, among other things, the samples taken by the FASFC as part of the official monitoring program for Cff. This includes samples from national surveys as well as import controls.

In addition, ILVO, in collaboration with the Walloon research institute CRA-W, is carrying out the CurtoALERT project, which is subsidized by the Belgian Federal Public Service Health, Food Chain Safety, and Environment. This project focuses on mapping the occurrence of Cff in Belgium. To this end, both leguminous crops and seed lots in Flanders and Wallonia are being surveyed, and diagnostic methods are being further refined. In addition, experimental research is being conducted to determine how sensitive different Phaseolus genotypes are to the bacterium.

An important finding from the project is that the Cff infection level in contaminated seed lots is generally low. Furthermore, when the bacterium is transferred from the seed to the plant, the development of disease symptoms remains very limited under Belgian climatic conditions. These insights are crucial for a substantiated risk analysis and help to better tailor future control measures to the actual situation.

Would you like to read more about our research into the occurrence of Cff in Belgium? Click here.