Citrus leaf effects caused by Phytophthora root rot

Citrus growers take meticulous care of their trees, so why are species of yield-robbing Phytophthora present in almost every grove?

The disease is highly persistent and hardy, enabling it to survive adverse conditions in the soil. Another factor involves an essential ingredient for tree survival ― water, which is also the most common source of introducing and reintroducing the disease to groves.  Last year, CA growers saw a short supply of water, creating the perfect conditions for root diseases to take hold. Citrus trees are having to ask a lot of their root system – needing them to be as efficient as possible – but with water in shorter supply, the root system is compromised and, therefore, less efficient.

Unlike insect pest threats where you can quickly and easily see the damage, Phytophthora can be a silent killer, choking off water and nutrient supply to the above-ground crop. Permanent crops like CA citrus groves are less flexible than annual crops, and the events of 1 year typically roll into the next year and the years after. If a disease threat is left untreated, it will continue to permeate the root systems. At that point, a grower is leaving a lot of yield potential on the table, not just in the current season but in seasons to come.

This gradual waning of tree health and productivity often goes unnoticed, delaying the much-needed strategies that can reverse the disease’s negative impacts. Phytophthora also shares symptoms ― thinning canopies, twig dieback and soft, discolored outer root tissue ― with other destructive forces, including drought, 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 ― and your bottom line at harvest. Because yield primarily goes to the fresh fruit market, the visual quality and flavor of CA citrus fruits needs to be there. Phytophthora can also negatively impact grove establishment and your future profits since young trees are particularly susceptible to the disease.

To minimize your risks, test annually for this tenacious pathogen. Traditionally, threshold has been 10-15 propagules per gram, but growers have been finding root rot at lower rates (1-3 propagules per gram and lower). Implement a management plan geared toward boosting root health to help bolster tree longevity. Integrating practices against compacted soils, standing water and over-irrigating is essential. When replanting or establishing new plantings, choose resistant rootstocks where possible.

Key to maintaining productivity is proactive treatments, like Orondis^® fungicide in the spring and fall. Orondis provides preventive and residual control of Phytophthora root rot and other common diseases. The sweet spot for economic advantage is 6.4 fl oz per acre, which Syngenta Agronomy Service Representative Garrett Gilcrease says consistently pays for itself, especially when the product is watered in. 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.

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