This article discusses how the torrential rains caused by the DANA have favored the proliferation of Phytophthora in citrus fields, describing its effects, symptoms, detection methods and strategies to control this devastating disease.

Introduction:

The torrential rains and floods caused by the DANA (Isolated High Level Depression) at the end of October 2024 have left a desolate landscape in numerous agricultural areas of the Valencian Community. This extreme weather phenomenon, which is occurring with increasing frequency due to the effects of climate change, has once again highlighted the vulnerability of our crops to the force of nature. One of the sectors hardest hit by this catastrophe has undoubtedly been the citrus sector, an economic and cultural pillar of the region.

The accumulation of water in the fields, prolonged water logging that exceeds the height of the rootstocks and soil saturation have created a perfect breeding ground for the proliferation of one of the most devastating and difficult to eradicate citrus diseases: Phytophthora. This silent threat can remain latent for years and manifest itself with all its virulence when it finds the ideal conditions, such as those left by the recent heavy rains.

What Is Phytophthora?

Phytophthora spp. is a genus of soil pathogenic fungi, considered by many experts as one of the most persistent and destructive enemies of woody crops, especially citrus. This microorganism mainly attacks the roots, collar and sometimes even the fruit of trees. Among the diseases it causes are collar rot, gummosis (gum exudation) and root rot.

Unlike other pathogens, Phytophthora can remain dormant in the soil for long periods without causing apparent symptoms. However, when extreme moisture conditions occur, such as after a DANA or periods of continuous heavy rainfall, the fungus reactivates, multiplies its population and rapidly colonizes vulnerable plant tissues. This makes Phytophthora a difficult threat to eradicate once installed in a field.

Symptoms And Detection: Keys To Early Recognition Of The Disease

• General tree decline: One of the most obvious symptoms is loss of vigor. Affected trees show smaller leaves, chlorosis (generalized yellowing) and premature foliage drop.
• Smaller fruit: Infected citrus trees tend to produce smaller fruit and, in many cases, these fall early, drastically reducing the harvest.
• Gummy ooze: The presence of gum oozing from the trunk, especially at the base, is a clear indication of Phytophthora infection.
• Trunk collar rot: Necrosis of tissue at the base of the tree is often accompanied by odor and darkening of the bark.
• Progressive branch dieback: In severe cases, progressive desiccation of branches is observed until total death of the tree.

Confirmation Of The Diagnosis: The Laboratory As An Ally

When infection is suspected, it is advisable to resort to a specialized laboratory analysis. Although the external symptoms may be quite characteristic, there are other pathologies with similar manifestations. An accurate diagnosis will allow the application of the most appropriate treatment and avoid unnecessary applications of phytosanitary products.

Control Strategies And Integrated Management

Combating Phytophthora should not be based solely on chemicals, but on an integrated management approach, combining cultural, preventive and curative measures. Some practical recommendations include:

• Improving drainage: It is essential to ensure that the soil has an adequate structure to quickly evacuate excess water. In areas prone to waterlogging, the installation of subway drains or the formation of ridges can significantly reduce the risk of infection.
• Organic matter and biocontrol: Incorporating quality organic amendments and beneficial microorganisms such as Trichoderma spp. can help strengthen the soil microbiota, creating a natural barrier against Phytophthora.
• Removal of infected debris: Removing and destroying affected plant debris (rotten roots, dead trunks) minimizes the spread of the fungus.
• Application of authorized fungicides: In confirmed cases, systemic fungicides such as fosetyl-Al, metalaxyl or preparations based on copper salts can be used, always respecting the doses and safety periods established by phytosanitary regulations.
• Renovation of severely affected plantations: When the infection reaches a critical level, the most advisable option may be to uproot severely damaged trees to avoid dissemination to healthy specimens. In these cases, the soil should be disinfected before replanting.

Prevention: The Best Weapon Against Future Outbreaks

Climate change is increasing the frequency of extreme weather phenomena such as DANA. Prevention is therefore becoming increasingly important. The selection of rootstocks more tolerant to waterlogged soils and Phytophthora, the planning of an adequate irrigation and drainage network, and the constant training of growers and technicians are key elements to strengthen the resilience of Valencian citrus fields.

In addition, continuous monitoring and recording of meteorological and soil moisture data through digital technologies can be great allies to anticipate risk situations and act proactively.

Conclusion: A Silent Threat Requiring Immediate Action

The consequences of the DANA go far beyond the visible and immediate damage, such as fallen trees or flooded plots. The real threat is hidden underground, advancing noiselessly, compromising the future of thousands of hectares of Valencian citrus. Phytophthora, silent but lethal, is capable of decimating entire farms if not detected and fought in time.

Protecting this emblematic crop is not only an economic issue, but also a question of cultural identity for the Valencian Community. Faced with this challenge, the union between farmers, technicians, researchers and administrations is more necessary than ever. Acting quickly, diagnosing accurately and applying sustainable management strategies are fundamental steps to stop this silent death and ensure that our fields continue to flourish generation after generation.