Association of Xylella fastidiosa with Yield Loss and Altered Fruit Quality in a Naturally Infected Rabbiteye Blueberry Orchard

Xylella fastidiosa Wells et al. causes disease in a number of plants in the southeastern United States, including southern highbush blueberry (Vaccinium corymbosum interspecific hybrids), but little was known concerning its potential impact in rabbiteye blueberry (Vaccinium virgatum Aiton syn. Vaccinium ashei Reade). In a naturally infected orchard in Louisiana, mean yields of X. fastidiosa–positive plants were 55% and 62% less than those of X. fastidiosa–negative plants in 2013 and 2014, respectively. Average berry weight was also lower in X. fastidiosa–positive plants. Within 3 years of testing positive for X. fastidiosa, four of nine X. fastidiosa–positive plants appeared dead. However, plants that were X. fastidiosa–negative in 2013 remained so until 2015, indicating that the bacterium did not spread rapidly in this established orchard during this time. Other factors, including soil chemistry variables, Phytophthora cinnamomi, ring nematode, and ringspot symptoms, were also investigated to determine if one of these might predispose plants to infection with X. fastidiosa or be partly responsible for observed yield loss. In most cases, interactions were not found, but associations with soil Cu and Zn suggest a need for further research on whether these elements predispose rabbiteye blueberry toX. fastidiosa infection and thereby contribute to yield losses. Researchers, extension workers, and growers should be aware of X. fastidiosa as a potential yieldand survival-impacting factor in rabbiteye blueberry. Diseases incited by X. fastidiosa are especially prevalent in the southeastern United States (Hopkins and Purcell, 2002). Proposed reasons for the bacterium’s success in this region include a long growing season and warm nights (Hopkins and Purcell, 2002). The feeding habits of the glassy-winged sharpshooter [Homalodisca vitripennis (Germar)], a vector of X. fastidiosa that is common in the southeastern United States, likely contribute as well (Hopkins and Purcell, 2002). Several X. fastidiosa–caused diseases found in the Southeast include Pierce’s disease, phony peach disease, pecan bacterial leaf scorch, and oleander leaf scorch (Hopkins and Purcell, 2002; Sanderlin and Heyderich-Alger, 2000). Since 2004, researchers in Georgia and Florida have confirmed X. fastidiosa as the cause of a leaf scorch disease of southern highbush blueberries (Vaccinium corymbosum interspecific hybrids) (Chang et al., 2009; Harmon and Hopkins, 2009). Inoculation of rabbiteye blueberry (V. virgatum syn. V. ashei) plants with X. fastidiosa resulted in no detected infection in ‘Premier’ and in local colonization of two of six ‘Powderblue’ plants. In infected ‘Powderblue’ plants, symptoms did not progress past the inoculated stem within a 14-month period (Chang et al., 2009). However, X. fastidiosa has been detected in samples from rabbiteye blueberry plants at farms in Louisiana that have experienced noticeable plant decline and death (Ferguson, 2016; R. Sanderlin, personal communication). Rabbiteye blueberry comprises the majority of the 3000 acres of blueberry plantings in Louisiana and Mississippi (E. Stafne, personal communication; USDA-NASS, 2014). Bacterial leaf scorch symptoms described on southern highbush plants include marginal leaf necrosis, thin twigs on new growth, leaf drop, stem yellowing, and plant death (Brannen et al., 2016). In FL 86-19 southern highbush plants, symptoms progressed from marginal necrosis to yellow stems and leaf loss, on both inoculated and noninoculated branches (Chang et al., 2009). Defoliation combined with yellowed stems is characteristic of blueberry plants with bacterial leaf scorch, and these symptoms help differentiate between this disease and other biotic and abiotic causes of plant decline (Brannen et al., 2016). Death of southern highbush blueberry plants typically occurs a year or more after plants first show symptoms (Brannen et al., 2016). Rabbiteye blueberry plant symptoms noted in Louisiana and suspected of being associated with X. fastidiosa include plant death, shoot dieback, marginal necrosis, leaf reddening, and extensive defoliation combined with yellow stems (D. Ferrin and R. Sanderlin, personal communication; M.H. Ferguson, unpublished data), similar to symptoms described in Georgia on southern highbush blueberry (Brannen et al., 2016). There are other factors that may cause decline in rabbiteye blueberry plants. Phytophthora cinnamomi Rands incites root rot in blueberry, which can result in yellowing of leaves and leaf loss in rabbiteye cultivars (Milholland, 1995). Ring nematode (Mesocriconema ornatum) has been observed in association with plants symptomatic for blueberry replant disease, although experiments did not reveal significant differences in rabbiteye blueberry plant growth after inoculation with them (Jagdale et al., 2013). Several viruses cause ringspot or ring blotch symptoms on blueberry plants (Martin et al., 2012). One of these, blueberry red ringspot virus, has been detected in rabbiteye blueberry (Polashock et al., 2009). It is important to consider such alternative causes of symptoms when assessing the possible impact of X. fastidiosa in rabbiteye blueberry. In some crops, such as susceptible cultivars of Vitis vinifera L. grape, death of plants infected with X. fastidiosa takes place quickly enough that the proportion of yield reduction that individual infected plants experience, while they remain alive, is likely of minor interest (Hopkins and Purcell, 2002). Southern highbush blueberry plants with bacterial leaf scorch can also die soon after symptoms are observed (Brannen et al., 2016). However, in other crops, such as peach and citrus, infected plants can decline slowly with prolonged reduced yield and altered fruit quality (Gonc xalves et al., 2011; Hartung et al., 1994; Hutchins, 1933). There are crops for which yield loss in the presence and absence of X. fastidiosa has been compared. In almond [Prunus dulcis Received for publication 21 Apr. 2017. Accepted for publication 8 June 2017. We appreciate the contributions of cooperating growers and the financial support of Louisiana State University College of Agriculture and the LSU AgCenter. This research was also supported in part by Hatch funds from the United States Department of Agriculture, National Institute of Food and Agriculture. Assistance provided by Renee Allen, Jacob Boudreaux, Phil Brannen, Tim Burks, Melinda Butler, Rock Christiano, Bill Cline, Mary Hoy, Steven Jeffers, Charlie Johnson, Rebecca Melanson, Penny Perkins-Veazie, David Picha, Randy Sanderlin, Carolyn Savario, Harald Scherm,Raj Singh, Eric Stafne, DeborahXavier-Mis, and members of LSU’s Dept. of Experimental Statistics is gratefully acknowledged. This article is based on a chapter of the first author’s PhD dissertation. Corresponding author. E-mail: mhferguson@ yahoo.com. HORTSCIENCE VOL. 52(8) AUGUST 2017 1073 DISEASE AND PEST MANAGEMENT