Substantial rainfall and wind from Tropical Storm/Hurricane Helen has resulted in detached fruit, broken limbs, and fallen trees in many citrus groves across southern Georgia. From a disease management perspective, the most immediate concern in the coming weeks will be the development of problems with Phytophthora spp.
Phytophthora Diseases of Citrus
Phytophthora are organisms that can cause root rot, crown rot, foot rot, and (in some cases) fruit rot of citrus. The lifecycles of Phytophthora spp. are intimately tied to water, and in areas where soils remain wet (particularly in locations where standing water may have remained longer than 72 hours) diseases caused by Phytophthora can be an issue. Phytophthora spp. persist in soil, bark or infected plant roots. Under wet conditions, spore producing structures of these pathogens produce motile “swimming” zoospores, and these wet conditions therefore encourage the spread of the pathogen from plant to plant. Young citrus trees can be particularly affected by Phytophthora root rot or foot rot, and, in previous years, we have seen the death of multiple young plants in some locations as a result of this disease. In addition, mature trees can also be significantly affected.
Another disease caused by Phytophthora spp. in citrus is Phytophthora Brown Rot. We have not previously observed this fruit rot disease to be widespread in Georgia citrus. However, the pathogens known to cause this disease Phytophthora nicotianae and P. palmivora have been observed here. While these pathogens largely live in the soil, they can spread to fruit when fruit are in contact with the soil or when soil splashes onto fruit. Once fruit are infected, these pathogens (especially P. palmivora) can produce abundant spores on the fruit that can then be spread by wind-blown rain to other fruit on the tree or within the grove. Unfortunately, in sites with substantial fruit drop after Helene, there is likely a high risk of this occurring. Once it is safe to do so, removing these fallen fruit from below the trees will be very important to reduce the chances of potential problems with Phytophthora Brown Rot.
For management of diseases caused by Phytophthora, adequate drainage is essential for good control. In production fields, infection generally occurs in low, poorly drained areas. Improved drainage may possibly help plants recover from the disease; however, improving drainage once plants are in place is often difficult. As such, control measures are generally limited to the use of chemical fungicides. Three categories of fungicides for control of Phytophthora in citrus include mefenoxam, oxathiapiprolin, and phosphonates. Some products containing these active ingredients are labelled and registered for use in Georgia. Consult the various product labels for rates, other recommendations and precautions.
Citrus Canker
In addition to concerns regarding Phytophthora, tropical weather has historically been tightly associated with the spread of other citrus pathogens in Florida. Accordingly, it will likely be important for Georgia growers to remain vigilant during next season (2025) as they monitor their groves for disease. One disease of particular concern after hurricanes in Florida has been citrus canker. The bacterium that causes citrus canker can be easily spread by wind-blown rain and it is believed that severe hurricane seasons have been responsible for spreading this pathogen around the state of Florida. In Georgia, citrus canker has been previously identified in isolated locations in Decatur and Bulloch counties. While it is not believed that citrus canker is widespread enough in Georgia citrus at this time to merit specific interventions in most locations, growers should be aware of the symptoms of citrus canker and examine trees during 2025 in case the winds and rain of the recent storm may have redistributed this pathogen to groves where this disease has not previously been found.
Other Disease Concerns
In some locations that experienced high winds, a substantial number of broken limbs, lost leaves, or fallen trees may be present in affected Georgia citrus groves. While there is, of course, no particular prescription from a disease management standpoint that will cure this type of damage, grove clean-up activities can be important in preventing additional disease issues next season. Dead wood/limbs and leaf litter on the ground are known to be sites where the fungal organisms that cause greasy spot, melanose, and Botryosphaeria dieback of citrus can grow, sporulate, and spread. Specifically, removing dead wood from the ground as well as pruning out broken or dying limbs from affected trees may help reduce future issues with melanose or Botryosphaeria. If possible, these materials should be removed from the grove and/or destroyed. Likewise, removing leaf litter from the ground below trees helps to reduce reinfection by the greasy spot fungus of new leaves/fruit in the spring. Given the large number of leaves on the ground after this storm, growers should be aware that leaf litter removal may be even more important than usual to prevent issues in 2025.
