Mary Ferguson, Clark, Christopher A.
Mary Helen Ferguson, Christopher A. Clark and Barbara J. Smith
The bacterial plant pathogen Xylella fastidiosa causes a number of plant diseases, including bacterial leaf scorch of Southern highbush blueberry. Most blueberry orchards in Louisiana and Mississippi are planted with rabbiteye blueberry (Vaccinium ashei or V. virgatum) cultivars, while relatively few Southern highbush blueberries are grown. In Louisiana, X. fastidiosa has been detected in rabbiteye blueberry orchards, and it was of interest whether this bacterium affected yield in rabbiteye blueberry plants or perhaps just lived in them without causing harm.
Seventeen blueberry orchards in Louisiana were sampled, and X. fastidiosa was detected at only two. Over the course of two seasons, a yield study was conducted at one of these farms, and a strong association between detection of X. fastidiosa and yield loss was found. Plants in which X. fastidiosa was detected had less than half the yield, on average, as plants in which it was not detected. Because plants were naturally infected, it cannot be stated with certainty that X. fastidiosa caused the yield loss; it can only be said that it is associated with it. However, other factors that can affect plant productivity – including Phytophthora root rot, nematodes, soil pH, and soil nutrient levels – were investigated, and none appeared to be directly responsible for the observed reduction in yield.
Even though there was significant yield loss associated with X. fastidiosa, when plants were tested for three years in a row, ones that tested negative for the disease in the first year tested negative in the second and third years, too. This suggests that X. fastidiosa, which is transmitted by certain insects that feed on plant xylem sap, was not spreading rapidly among plants. However, these plants were approximately 30 to 35 years old, and it is possible that it would spread more quickly among young plants.
Symptoms observed in infected plants included early fall colors (yellowing and reddening of leaves) and a gradual dieback of plants (Figure 1). Approximately one-third of plants in which X. fastidiosa was detected appeared dead within three years of when it was first detected. Because other factors may cause symptoms similar to those observed on X. fastidiosa-infected plants, testing by a laboratory such as the LSU AgCenter Plant Diagnostic Center is needed to determine if a plant is infected. Leaves should be taken from multiple shoots of each plant tested. Bimonthly testing of infected plants revealed that late summer is the preferred time for taking samples. Even when X. fastidiosa is present in plants, it is not readily detected in early spring.
X. fastidiosa from rabbiteye blueberry plants in Louisiana was compared genetically to X. fastidiosa strains from other plant hosts to determine what plants are infected by the same or similar strains. There appears to be more than one strain of X. fastidiosa in rabbiteye blueberry, but all were within a group that has been found in a limited number of hosts. The predominant strain has been found in Southern highbush blueberry in Georgia and in two wild hosts (giant ragweed and western soapberry) in Texas. Because rabbiteye blueberry plants are infected by a strain that also infects Southern highbush blueberry, it is a potential source of infection for susceptible Southern highbush blueberry cultivars. Growers should take care to only plant clean rabbiteye blueberry plants from a reliable source, as rabbiteye plants may not always show obvious symptoms of infection. Good weed management in the orchard may also help reduce the chance of getting X. fastidiosa in blueberries by reducing the incidence of wild plants that are potential sources of infection.
Mary Helen Ferguson is a research associate and Christopher A. Clark is a professor in the Department of Plant Pathology and Crop Physiology. Barbara J. Smith is a research plant pathologist at the U.S. Department of Agriculture Agricultural Research Service, Poplarville, Mississippi.
Mary Helen Ferguson, an LSU graduate student in plant pathology, holds a rabbiteye blueberry plant inside a screenhouse on the LSU campus. Photo by Olivia McClure
Figure 1. This rabbiteye blueberry plant shows dieback and early fall colors, which were symptoms observed in association with Xylella fastidiosa infection. Photo by Mary Helen Ferguson