CULTIVAR , AND CLIMATE ON THE ESTABLISHMENT AND PERSISTENCE OF XYLELLA FASTIDIOSA INFECTIONS CAUSING ALMOND LEAF SCORCH Project Leaders :

Almonds are one of the most widely-grown crops that can host Xylella fastidiosa (Xf), so it is useful to assess the potential for almonds to serve as a source of Xf infections in grapes, and to explain why Xf dispersal between almond orchards and vineyards is uncommon. We are comparing infection establishment and survival at two field sites and in a controlled test with potted plants, varying three factors that may influence almond leaf scorch in almonds: cultivar, genetics of the pathogenic bacteria, and winter severity. In spring and summer 2005, we inoculated grape type and two almond types of Xf into highly susceptible ‘Peerless’ and less-susceptible ‘Butte’ almond trees. After vector inoculation, Xf must survive multiple winters in an almond tree to reach sufficient populations for sharpshooter acquisition and economic impact disease levels. Therefore, field sites were selected with moderate and severe winter temperatures. We also initiated a controlled dormancy test with potted plants and cold storage rooms at Kearny Agricultural Center, Parlier, California. Almond trees in the field were inoculated with buffer or Xf belonging to the grape type or two almond types, and will be held at different chill temperatures for varying lengths of time. INTRODUCTION Because almonds are one of the most widely-grown crops that can host Xf in the Central Valley, they might serve as a source of Xf infections in grapes, although for unknown reasons Xf dispersal between almond orchards and vineyards is uncommon (A. Purcell – unpublished data). Almond leaf scorch (ALS) is caused when Xf multiplies extensively within the xylem of infected trees, eventually severely limiting nut production (Davis et al. 1980). The disease was first formally described in 1974, though outbreaks occurred in Los Angeles and Contra Costa counties in the 1950’s (Moller et al. 1974). Symptoms of ALS are similar to Pierce’s disease (PD) in grapes and include leaves with marginal necrosis and chlorosis and the lack of terminal growth. Initial infections spread slowly and often occur only on one branch, but after a few years are easily visible on the entire tree (Almeida and Purcell 2003c), reducing almond productivity (Mircetich et al. 1976, Moller et al. 1974). In both grapes and almonds, Xf multiplies to high populations (1,000,000 bacteria per gram of plant tissue) and is acquired and transmitted by insect vectors (Almeida and Purcell 2003a, Almeida and Purcell 2003c, Purcell 1980a). In laboratory tests, Xf was transmitted to almonds by 5 species of xylem-feeding insects, including the glassy-winged sharpshooter (GWSS), Homalodisca coagulata (Almeida and Purcell 2003b, 2003c). In previous studies, almond cultivars varied greatly in their susceptibility to ALS, with some developing extensive leaf scorch, and others showing little disease. Xf inoculations made from May through July had the best odds of surviving the following winter. (B. Kirkpatrick – unpublished data). We compared Xf infection establishment and survival in two cultivars, highly susceptible ‘Peerless’ and less-susceptible ‘Butte,’ both on Nemaguard rootstock. The genetic type of Xf may also impact almond susceptibility, and will certainly impact formation of PD. Three genetic types of Xf have been identified from almond trees. One type was identical to Xf from PD-infected grapevines. The other two genetic types were unique to almonds (Hendson et al. 2001). The three types were distinguished by growth on selective media and DNA digestion with restriction enzymes (Almeida and Purcell 2003c). Recent cross-inoculation studies in the greenhouse showed that the genetic type influenced the ability of the bacteria to over winter in grapes or almonds, as almond types died in grapes and grape types died in almonds (Almeida and Purcell 2003c). For this reason, we also used different genetic types of Xf in our field trials. After vector inoculation, Xf must survive multiple winters in an almond tree to reach sufficient populations for sharpshooter acquisition and economic impact disease levels. Growth chamber and field studies with grapevines showed that the degree of plant dormancy, as well as severe cold, affected the over winter survival of Xf (Feil and Purcell 2001, Purcell 1980b). To date, there is no information available on the effects of winter dormancy on Xf infections in almonds. Growth chamber and