Citrus leaf drop caused by Phytophthora root rot

Syngenta Agronomy Service Representative Kendra McCorkle believes 2021 had the potential to be better in terms of root mass gains, with its reduced crop load and generally favorable growing conditions, but pathogen pressure was unexpectedly high and lowered yield potential, especially in southwest FL. The Palmivora species of Phytophthora root rot surged in groves and regions where it has historically been present, which could have been caused by the record fruit drop FL saw in the 2020 growing season.

Unlike other insect pests that cause visible damage, Phytophthora can be a silent killer, choking off water and nutrient supply to the tree. The disease is highly persistent and hardy, enabling it to survive adverse conditions in the soil.

When it comes to Phytophthora, 2 notable species cause problems: P. nicotianae and P. palmivora. Both cause root rot, foot rot and brown rot. While P. nicotianae tends to be more widespread than the P. palmivora species, P. palmivora is more destructive because it’s a more prolific spore producer. And more spores mean a higher disease impact. P. nicotianae mainly impacts the fibrous root system of trees while P. palmivora is capable of invading and damaging the structural root system of trees – both damaging to FL citrus groves.

Phytophthora root rot causes a gradual waning of tree health and productivity which often goes unnoticed, delaying treatments that can reverse damage from the disease. Phytophthora also shares symptoms ― thinning canopies, twig dieback and soft, discolored outer root tissue ― with other destructive forces, including drought, citrus greening, high salinity, nematodes and root weevils, making diagnosis even more difficult.

Trees with Phytophthora root rot can survive for years. Meanwhile, overall health diminishes, growth and yield suffer, leaves drop, and senescence occurs earlier in the year, which reduces fruit quality, yield potential and your bottom line at harvest. Phytophthora can also negatively impact grove establishment and future profits since young trees are particularly susceptible to the disease.

Though Phytophthora pressure is increasing, growers are aggressively treating for the disease. In 2021, growers took a historically high number of samples for Phytophthora root rot. But the number of samples that exceeded the economic treatment threshold was the lowest recorded since 2013.

To win against Phytophthora root rot, McCorkle recommends that citrus producers:

• Sample 3 times a year: spring, summer and fall.
• Boost root health, and thus tree longevity, by implementing a robust management plan.
• Avoid compacting soils to limit standing water.
• Avoid over-irrigation.
• Choose resistant rootstocks when replanting or establishing a new planting.

Another effective component of a proactive management plan is applying an efficacious fungicide treatment, such as Ridomil Gold® SL fungicide in the spring and Orondis® fungicide (4.8-9.6 fl/oz per acre soil application) in the fall for preventive and residual control of common diseases, including Phytophthora root rot. Even at low-use rates, this fungicide protects roots by reducing propagule count and inoculum potential. By encouraging strong root development, Orondis consistently increases root mass, which leads to better nutrient and water uptake and a reduction in tree stress and pest pressure. Phytophthora root rot is a chronic problem, so changes or improvements in tree condition will be seen over time.

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