Mike Roberts

Phytophthora and Citrus Roots

According to the 2020-2021 Florida Citrus Production Guide, “Historically, the most damaging root pathogens in citrus have been Phytophthora.” This pathogen causes root, crown, and foot rot, and ultimately leads to the decline and death of the tree. To add insult to injury, citrus greening, caused by the Candidatus Liberibacter asiaticus (CLas) bacteria, also causes significant damage to a citrus tree’s root system. It has never been more important to focus on citrus root health. See the recommendations from experts when it comes to citrus trees, Phytophthora, and root health.

A Focus on Citrus Root Health

Phytophthora is spread by a variety of water molds, and it is found everywhere. It travels through water and infects roots, causing root, crown, and foot rot below the soil line. Recommendations for guarding citrus tree roots from Phytophthora, include both cultural practices and chemical control methods, according to UF/IFAS scientists and authors of the 2020–2021 Florida Citrus Production Guide: Phytophthora Foot Rot, Crown Rot, and Root Rot, Evan Johnson and Megan Dewdney.

Recommendations include:

Disease Exclusion Practices. Know your grove’s history. For new groves that have never been planted with citrus before, it’s unlikely that Phytophthora nicotianae would be present. In groves with a history of Phytophthora foot rot, plant citrus trees on tolerant rootstocks, such as Swingle citrumelo. Similarly, inspect nursery trees for signs of fibrous root rot, and test for it if necessary, prior to planting. Also, plant rootstocks in those soils favorable to the rootstock.

Irrigation Controls. Since Phytophthora-causing molds thrive in wet environments, it’s important to utilize precision irrigation. Overwatering and the prolonged wetting of tree trunks allows phytophthora populations to build up in the soil, increasing the chances of infection. It’s recommended to utilize early to midday irrigation schedules, especially for young trees. For these same reasons, soils should be well-draining to avoid an overly wet environment at trees’ roots.

Chemical Controls. Fungicides should be used in a preventative manner on young, susceptible rootstocks. On young, tolerant rootstocks, fungicides should be used if foot rot lesions develop. Treatments should continue for a year on tolerant rootstocks and longer on susceptible rootstocks. In mature groves, the recommendation is to use fungicide based on results of soil testing for Phytophthora. Fungicides should be alternated to avoid the development of fungicide resistance.

Pest Management. Pests such as fire ants and Citrus Root Weevils have been shown to cause damage to citrus trees, creating lesions for Phytophthora to enter and infect the tree. For instance, fire ants cause damage to tree bark when they nest under tree wraps, and Citrus Root Weevil larvae feed on and damage citrus tree roots, allowing an entry point for Phytophthora. Controlling these pests can lower the risk of infection.

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