Introduction:

Alternaria alternata has become one of the main phytosanitary challenges in Valencian citrus production. This fungal disease particularly affects certain mandarin and hybrid varieties, compromising both yield and fruit quality. In a context of tight profit margins and increasingly demanding markets, its recurring presence can significantly reduce farm profitability if preventive and coordinated actions are not taken.

Why does Alternaria Appear and When is it Most Dangerous?

Alternaria alternata thrives under high relative humidity (above 85%), persistent dew or frequent rainfall. Leaves that remain wet for several hours allow the spores to germinate, especially when temperatures range between 20°C and 30°C. Areas with morning fog or poor ventilation are particularly vulnerable.

The most critical periods are:

• Spring: Tender shoots and young leaves are highly susceptible. Humidity and rain favor infections during flowering and fruit set.

• Early summer: Newly set fruit have softer tissues, and the combination of heat and humidity increases risk.

• Late summer to early autumn: Storms and prolonged rainy periods can trigger new infections in developing fruits.

2. Visible Symptoms and Affected Organs

Recognizing early signs is key to preventing severe damage:

• Young leaves: Show brown or necrotic spots surrounded by a yellow halo. In highly sensitive varieties, this may cause premature leaf drop.

• Tender shoots: Present necrosis at the tips and on new tissue, which weakens the plant and slows its growth.

• Developing fruit: Typically shows dark, sunken lesions surrounded by a yellowish halo. Recently set fruit are the most vulnerable, and in severe cases, premature fruit drop can occur, reducing overall yield.

• Flowering: Although less frequent, petal necrosis and flower drop can appear under highly favorable conditions for the fungus.

The most sensitive varieties are mandarins and hybrids such as Fortune, Nova, Murcott and Leanri, among others.

 

3. Strategies for Control and Prevention

a) Cultural measures: the first line of defense

Proper orchard management can greatly reduce the incidence of Alternaria:

• Canopy thinning and pruning: Improves air circulation, lowers humidity, and enhances light penetration.

• Irrigation management: Avoid excessive watering that prolongs leaf wetness.

• Balanced nutrition: Well-nourished plants are more resistant to infections.

• Removal of infected plant debris: Reduces the inoculum source within the orchard.

• Varietal selection: When establishing new plantations, choose varieties and rootstocks less sensitive to Alternaria in high-risk areas.

b) Prevention and monitoring

Anticipation is essential for effective control.

• Monitor environmental conditions (humidity, rainfall, dew) to detect risk periods early.

• Track phenological stages to identify the most vulnerable phases: sprouting, fruit set, and early fruit development.

• Use predictive models and phytosanitary alerts, particularly valuable in the Valencian Community, where risk warnings are regularly issued.

c) Phytosanitary control

Treatments should be preventive and focused on critical stages:

• During spring shoots, flowering and fruit set, and initial fruit development.

• Always rotate active ingredients to prevent the development of resistance.

• Applications should be made under technical supervision, following regulations and respecting authorized residue limits.

Conclusion

In the Valencian Community, where environmental humidity remains high in certain citrus-growing zones, Alternaria alternata poses a growing challenge. However, successful management is possible through an integrated strategy that combines prevention, sound agronomic practices, and responsible phytosanitary control.

Beyond fungicide use, the key lies in understanding the disease cycle, anticipating risk conditions, and maintaining a balanced, healthy orchard. Each action — from a well-timed pruning to careful irrigation planning — contributes to keeping trees productive, resilient, and sustainable