Greg Douhan with University of California Agriculture and Natural Resources (UCANR) recently wrote about phytophthora diseases in California citrus. Edited excerpts follow:
There are at least four species of Phytophthora species (P. citrophthora, P. parasitica, P. syringae and P. hibernalis) associated with citrus in California. The three diseases in citrus caused by these fungal-like pathogens are phytophthora root rot, phytophthora brown rot of citrus fruits both pre-and post-harvest, and phytophthora gummosis.
These organisms are active within the field essentially all year long. One tree could have all three disease symptoms at one time, but that is not usual. These pathogens are ubiquitous in California citrus soils.
PHYTOPHTHORA ROOT ROT (PRR)
PRR is caused primarily by P. citrophthora and P. parasitica. The former is most active in the winter with respect to PRR whereas the latter is more active in warm weather. This disease can affect young to mature trees and is often associated with groves that do not have good drainage, such as those with high clay soils. For example, in the Terra Bella area of the San Joaquin Valley, there are areas with high clay soils that lead to problems with PRR due to the lack of drainage. That can also lead to additional disease issues such as dry root rot (Fusarium solani).
P. citrophthora and P. parasitica can survive for years in the soil by producing persistent spores (clamydospores). When moisture is present in the soils, these pathogens can then produce oospores which will differentiate into motile swimming zoospores — the infective spores that can decimate the citrus root system.
Trees that are infected with this disease will often show light green to yellowing of the leaves and thinning of the canopy. The disease often causes a slow decline of the tree.
This pathogen mostly infects the feeder roots below the soil line within a foot or so of the surface of the soil line.
This disease is caused by the various Phytophthora spp. and is usually associated with mature fruits. However, twigs, leaves and flowers can also occasionally be infected, which can result in death of these tissues. This disease is usually associated with cool and wet conditions. The symptoms can be seen primarily on low-lying fruit. It is recommended to skirt trees so that there is no low-lying fruit to get infected.
Brown rot can also occur after the fruit is picked. In storage, the disease can spread to healthy fruit.
Phytophthora gummosis disease is caused by the various Phytophthora spp. It is usually only seen around the soil line to a foot or so above the soil line but could produce a larger canker higher up the trunk. Once infected, the tree starts to produce compounds to combat the infection, which results in oozing of sap from small, infected cracks in the bark. It may look as if the tree is bleeding. The bark usually remains firm but dries out and eventually cracks and can slough off the trunk. Sometimes a white crust appearance will also be seen within and around the canker.
Once an infection occurs and the tree is not treated, the canker can eventually spread around the circumference of the trunk. That can lead to complete girdling of the tree and ultimately general decline and/or death of the tree.
In a field that has a history of various phytophthora issues, there is the possibility of performing a pre-plant fumigation using metam sodium or chloropicrin.
If a grove becomes infected after planting, the most common control is chemicals, usually applied through drip lines. The most common products are Aliette, Ridomil Gold and Prophyt. In the last several years, Orondis and some other chemistries have been developed to control phytophthora diseases. Micronutrient sprays that contain phosphite may also help to control these diseases. Learn more about new options for control of phytophthora root rot.
Read about management of phytophthora diseases in Florida here.
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