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Almond leaf scorch (ALS) disease has been present in California's almond-growing regions for over 60 years. This disease is caused by the bacterium Xylella fastidiosa and the pathogen is vectored by xylem-feeding sharpshooters and spittlebugs. Currently, there are no effective management techniques that prevent trees from becoming infected. Within affected orchards throughout California's Central Valley, disease incidence and the risk of tree-to-tree spread appears to be low. Consequently, the decision to remove or keep infected trees depends on lost productivity. We compared yield and vitality between infected and uninfected almond for cvs. Sonora and Nonpareil. Sonora was examined at three sites over 3 years and Nonpareil was examined at one site over 2 years. Yields of ALS-affected trees were significantly lower for both cultivars, although yield losses of Sonora were proportionally greater than those of Nonpareil. Yields of infected trees did not decline incrementally over years; rather, they fluctuated similarly to those of uninfected trees. In addition, no infected trees died during the course of the study. These results are in direct contrast to previous anecdotal reports which suggest that yields of infected trees incrementally decline and infected trees eventually die. A simple economic model was developed to determine conditions under which rouging infected trees would increase returns. Based on the model, orchard age, yield loss due to infection, and the value of a maximally producing almond tree should be considered when deciding to remove ALS-affected trees.

Xylella fastidiosa is the causal agent of almond leaf scorch disease (ALSD), which is currently reemerging in California as a potential threat to almond (Prunus dulcis) production. We previously reported the presence of different colony morphotypes of X. fastidiosa ALSD strains on periwinkle wilt medium solidified with Gelrite and their association with genotypes or pathotypes after a low number of serial transfers. The morphotypes could be differentiated by single nucleotide polymorphisms and were classified as A- and G-genotypes. The stability of colony morphology was not studied. Yet, it is an important issue in the bacterial characterization. In this project, we evaluated the variations of colony morphology through 14 subculture passages. The G-genotype strains were consistently (>99%) associated with smooth colony morphotypes. Similarly, A-genotype strains were consistently associated with rough colony morphotypes. Rough A-genotype strains reversed to smooth colony variants at a low frequency suggest...

Xylella fastidiosa is a xylem-limited bacterium that causes almond leaf scorch (ALS), Pierce's disease of grapevines, and other plant diseases. We surveyed ground vegetation in ALS-infected almond orchards in California's Central Valley for the presence of this bacterium. Plant tissue samples were collected throughout a 2-year period and processed for the presence of X. fastidiosa using restriction enzyme digestion of RST31 and RST33 polymerase chain reaction (PCR) products and bacterial culture on selective media. Overall disease incidence was low in the ground vegetation species; only 63 of 1,369 samples tested positive. Of the 38 species of common ground vegetation tested, 11 tested positive for X. fastidiosa, including such common species as shepherd's purse (Capsella bursa-pastoris), filaree (Erodium spp.), cheeseweed (Malva parvifolia), burclover (Medicago polymorpha), annual bluegrass (Poa annua) London rocket (Sisymbrium irio), and chickweed (Stellaria media). There was a seasonal component to bacterial presence, with positive samples found only between November and March. Both ground vegetation and almond trees were most commonly infected with the almond strain of X. fastidiosa (six of seven surveyed sites). ALS-infected almond samples had an X. fastidiosa concentration within previously reported ranges (1.84 × 106to 2.15 × 107CFU/g); however, we were unable to accurately measure X. fastidiosa titer in sampled ground vegetation for comparison. These results are discussed with respect to ground vegetation management for ALS control.

Groves, R. L., Chen, J., Civerolo, E. L., Freeman, M. W., and Viveros, M. A. 2005. Spatial analysis of almond leaf scorch disease in the San Joaquin Valley of California: Factors affecting pathogen distribution and spread. Plant Dis. 89:581-589. Almond leaf scorch (ALS) disease has emerged as a serious threat to almond (Prunus amygdalus) production areas throughout California’s San Joaquin Valley. This disease is caused by the xylem-limited bacterium Xylella fastidiosa, and this pathogen is transmitted by xylophagous insects including sharpshooter leafhoppers (Hemiptera: Cicadellidae) and spittlebugs (Hemiptera: Cercopidae). Among four orchards surveyed, enzyme-linked immunosorbent assay (ELISA) and bacterial isolation followed by polymerase chain reaction (PCR) were equally effective in detecting X. fastidiosa from ALS-symptomatic trees. Disease incidence varied among almond cultivars in each orchard, with the highest mean incidence and most severe symptoms frequently encountered in ‘Sonora’. X. fastidiosa isolates consisted of mixtures of grape or “G-genotype” and almond or “A-genotype” strains present in surveyed orchards. The X. fastidiosa G-genotypes characterized from each orchard were associated with the most severely affected ‘Sonora’ trees in three of the four orchards. Both ordinary runs and simple randomization analyses revealed aggregations of ALS in three of the four orchards. Clusters of ALSaffected trees frequently occurred in the outermost orchard rows. Plots of semivariance in ALS incidence over distance varied in shape and magnitude among cultivars. Semivariance increased over distance in ‘Sonora’ and ‘Carmel’, indicating spatial dependence or aggregations of incidence best fit by a combination of spherical and linear models. These results document both random and aggregate patterns of ALS spatial distribution in selected orchards and further illustrate how cultivar susceptibility influences the distribution patterns of ALS incidence. Following the recent introduction and establishment of the glassy-winged sharpshooter, Homalodisca coagulata, the impact upon the epidemiology and spread of ALS is unknown. Additional keywords: Pierce’s disease

The spatial and temporal pattern of noninfectious bud failure (BF) expression (BFexp) was studied during seven growing seasons in a population of 'Carmelʼ almond trees originating from twelve commercial propagation sources. All progeny trees were grown in a single experimental site with high prevailing summer temperatures. BFexp increased continuously but irregularly in each nursery population as measured as the proportion of trees showing BF and as an average BFexp rating. Populations from the 12 nurseries represented increasing clonal generations from the original seedling tree and showed increasing levels of BF, as well as a decreasing shape value and increasing scale value derived by a failure statistics model. Models for development, distribution and hazard functions were defi ned for each of the 12 sources studied. Only sources from the original tree and source A demonstrated potential for com- mercial use. A signifi cant correlation was found between average yearly increase in BFexp and the average daytime temperature for the previous June. The June period coincides with a specifi c stage in the seasonal growth cycle when vegetative buds mature. Noninfectious bud failure (BF) is a genetic disorder of almond (Prunus dulcis (Mill.) D.A.Webb.) (IPM Education and Publica- tion, 2002; Kester, 1974; Kester and Gradziel, 1996). The disorder is identifi ed in individual orchard trees of certain cultivars by failure of vegetative buds to grow in the spring (BF), die-back of terminal shoots (dieback), vigorous and erratic growth from surviving buds (witches-broom) and characteristic bark deforma- tions (roughbark). Flowering and fruiting are not directly affected but bloom can be delayed and fl owering density reduced, and so crop production can be reduced. There is no evidence of a pathological cause (DiTerlizzi et al., 1998; Wilson and Schein, 1956; Fenton et al., 1988a) although some viruses mimic the symptoms of BF (Kester. 1974; Wilson and Schein, 1956). BF is equally inherited in almond x almond reciprocal crosses and is directly proportional to the BF severity in the parents (Kester, 1968a, 1968b; Kester and Jones, 1970). Individual seedlings inherit different levels of the potential for BF (BF pot ) which is manifested in the number of cycles of fu- ture growth required for symptoms to be expressed. Individual seedlings pass through a period of conversion in which the BF pot changes to expression (BF exp ). The pattern is retained during vegetative propagation and is continued during further vegeta- tive generations. BF is also inherited in almond x peach F 1 , F 2 and backcross progeny where BF exp segregation indicates that a major gene for BF occurs in almond which is absent in peach (Kester 1978; Kester and Gradziel, 2002). BF exp fi rst appeared sporadically in 'Nonpareilʼ and 'Peerlessʼ in the early years of the industry and later at a devastating rate in 'Jordanoloʼ and 'Harpareilʼ, both products of a USDA-UC breeding program (Woods, 1938). Later, BF exp has appeared in high numbers in patented cultivars such as 'Mercedʼ, 'Priceʼ

Almond trees were irrigated from March through November 1994 and 1995 with 70, 85, and 100% of potential evapotranspiration (ETc). Deficit irrigation was accomplished by delivering 70 or 85% of ETc at every irrigation (sustained) or 50% of ETc during 1 June to 31 July (70 regulated) or 1 to 15 July (85 regulated). The natural incidence of dead leaf clusters and dead spurs, twigs, and small branches, measured at harvest, lessened with decreasing amounts of water, and regulated deficits were more effective than sustained deficits in reducing disease. Fruit at early dehiscence on trees in each of the five irrigation treatments were inoculated with 0.1 ml of suspensions of 104 spores per ml of Monilinia fructicola or Rhizopus stolonifer. Monilinia fructicola caused more hull rot than R. stolonifer, and both pathogens responded similarly to the irrigation treatments. The rate of fruit maturation was monitored for approximately 4 weeks before harvest by scoring the percent abscission and dehiscence and measuring the hull moisture content of fruit on trees in each irrigation treatment. Dry kernel weight was determined at harvest. Maturation was slower and kernel weight greater in treatments receiving 85% of ETc than 70% or those under sustained compared with regulated irrigation regimes.

Chen, J., Groves, R., Civerolo, E. L., Viveros, M., Freeman, M., and Zheng, Y. 2005. Two Xylella fastidiosa genotypes associated with almond leaf scorch disease on the same location in California. Phytopathology 95:708-714. Almond leaf scorch disease (ALSD) has recently reemerged in the San Joaquin Valley of California threatening almond production. ALSD is caused by Xylella fastidiosa, a nutritionally fastidious bacterium. Single nucleotide polymorphisms (SNPs) in the 16S rRNA gene (16S rDNA) of X. fastidiosa strains were identified to characterize the bacterial population in infected trees. Genotype-specific SNPs were used to design primers for multiplex polymerase chain reaction assays of early passage cultures. Two genotypically distinct types of X. fastidiosa strains, G-type and A-type, coexist simultaneously in the same infected almond orchard. This was substantiated by restriction fragment length polymorphism analysis of a different genetic locus, RST31-RST33, which has previously been used to identify and differentiate X. fastidiosa strains. Furthermore, unique bacterial colony morphology was consistently associated with the A-type X. fastidiosa strains. To our knowledge, this is the first report of a mixed genotype infection of X. fastidiosa disease on the same location under natural environmental conditions. The concept of mixed genotype infection could affect the current epidemiological study based on the assumption that one genotype causes ALSD on one location and, therefore, the disease management strategy.

In a 2-year study, thinning Granny Smith apples to one per cluster and covering them with brown paper bags when they were the size of a golf-ball resulted in significantly fewer fruits damaged by codling moth compared with fruits hand-thinned and left untreated. Sunburned fruit were also reduced both years by bagging, and fruit firmness and sweetness were improved in 1 of the 2 years. A similar experiment is now underway with Fuji apples.

Preharvest termination of weekly irrigation prior to harvest from late June to mid-August 1990 and 1991 was studied for control of almond hull rot disease caused by Rhizopus stolonifer. When trees were irrigated within 2 wk of harvest, there was more infection of hulls by R. stolonifer, more leas strikes per tree, and in 1991, more shoot and branch death per tree than when irrigation was discontinued earlier. Percent hull split incidence and hull moisture content increased as irrigation continued through the season, but these criteria did not explain differences in disease incidence among trees in the first six compared with the last two irrigation termination dates