Diseases of fruit trees caused by pathovars of Pseudomonas syringae van Hall are of major concern in fruitproducing areas worldwide, are exceedingly difficult to control, and result in significant economic losses. The pathogen has the ability to kill both young and older trees. Systemic infection and death of young trees is a perennial problem in nurseries, and canker development leading to the girdling and death of scaffold limbs and entire trees is a common event that can lead to the rapid demise of older orchards. For example, bacterial canker of plum caused by P. syringae pv. syringae causes annual tree mortality rates as high as 30% in Germany (43), and bacterial canker of hazelnut has resulted in the mortality of large numbers of trees in Italy and other European countries (77). Yield losses due to lesions on fruit are typically more sporadic in occurrence and variety dependent, but significant yearly losses have been reported in some instances. Finally, the pathogen's trait of ice nucleation activity also could exacerbate the importance of frost injury as a predisposing factor for infection. Frost damage in commercial orchards is a sporadic problem that facilitates colonization and the initiation of pathogenesis for these opportunistic pathogens.P. syringae and its close relatives cause diseases of monocots, herbaceous dicots, and woody dicots, and these pathogens utilize an impressive array of virulence factors such as effectors, toxins, and phytohormones to incite disease symptoms. Aspects of the systematics, ecology, and genetics of P. syringae have been reviewed and discussed by many authors (cited literature examples: 6,27,44,47,52). Extensive recent research efforts have focused on Pseudomonas diseases of herbaceous plants, and our understanding of P. syringae-host interactions has been facilitated by genome sequencing, comparative genomics, and functional studies (cited literature examples: 21,31,96).Our understanding of and ability to manage P. syringae diseases of fruit trees is relevant to our understanding of P. syringae diseases of herbaceous hosts, and vice versa. The exciting new insights into host-pathogen interactions from the model systems can give us clues on how to pro-ceed with the less genetically pliable fruit tree systems. Studies of the fruit tree systems, and comparisons with the model systems, can provide insights into the interactions of P. syringae strains with woody hosts and address relevant questions, such as: What is the role of type III secretion in infection of woody tissues? What virulence traits enable the colonization of woody tissue by P. syringae? Are there differences in the roles of toxins in diseases of woody hosts and herbaceous hosts? How did some pathogens evolve the ability to induce galls in woody hosts? Can resistance mechanisms characterized in the model-system herbaceous hosts be exploited in fruit trees?The objectives of this article are (i) to provide an overview of several Pseudomonas diseases of fruit trees, (ii) to discuss the current and emerging unde...

Several aspects of grapevine downy mildew epidemiology that are fundamental to model predictions were investigated. Simple rainfall-, temperature-, and phenology-based thresholds (rain > 2.5 mm; temperature > 11 degrees C; and phenology > Eichorn and Lorenz [E&L] growth stage 12) were evaluated to forecast primary (oosporic) infection by Plasmopara viticola. The threshold was consistent across 15 years of historical data on the highly susceptible cv. Chancellor at one site, and successfully predicted the initial outbreak of downy mildew for 2 of 3 years at three additional sites. Field inoculations demonstrated that shoot tissue was susceptible to infection as early as E&L stage 5, suggesting that initial germination of oospores, rather than acquisition of host susceptibility, was probably the limiting factor in the initiation of disease outbreaks. We also found that oospores may continue to germinate and cause infections throughout the growing season, in contrast to the widely-held assumption that the supply of oospores is depleted shortly after bloom. Lesion productivity (sporangia/lesion) did not decline with age of a lesion in the absence of suitable weather to induce sporulation. However, the productivity of all lesions declined rapidly through repeated cycles of sporulation. Extremely high temperatures (i.e., one day reaching 42.8 degrees C) had an eradicative effect under vineyard conditions, and permanently reduced sporulation from existing (but not incubating) lesions to trace levels, despite a later return to weather conducive to sporulation. In fair weather, most sporangia died sometime during the daylight period immediately following their production. However, over 50% of sporangia still released zoospores after 12 to 24 h of exposure to overcast conditions.

Clusters of Vitis vinifera and V. labrusca are reported to become resistant to Plasmopara viticola at stages of development ranging from 1 to 6 weeks postbloom. It has been suggested that resistance is associated with loss of the infection court as stomata are converted to lenticels, but the time of onset, cultivar variation, and seasonal variation in ontogenic resistance has remained uncertain, as has the comparative susceptibility of stem tissue within the fruit cluster. In New York, we inoculated clusters of V. vinifera cvs. Chardonnay and Riesling and V. labrusca cvs. Concord and Niagara at stages from prebloom until 5 to 6 weeks postbloom. Berries were infected and supported profuse sporulation until 2 weeks postbloom, and pedicel tissue remained susceptible until 4 weeks postbloom. Although berries on later-inoculated clusters failed to support sporulation, discoloration and necrosis of berry tissues was often noted, and necrosis of the pedicel within such clusters often led to further discoloration, shriveling, reduced size, or loss of berries. When the epidermis of discolored berries that initially failed to support sporulation was cut, the pathogen emerged and sporulated through incisions, indicating that lack of sporulation on older symptomatic berries was due to infection at an early stage of berry development followed by conversion of functional stomata to lenticels during latency. We repeated the study on Chardonnay and Riesling vines in South Australia and found that the period of berry and rachis susceptibility was greatly increased. The protracted susceptibility of the host was related to the increased duration and phenological heterogeneity of bloom and berry development in the warmer climate of South Australia. The time of onset and subsequent expression of ontogenic resistance to P. viticola may thus be modified by climate and should be weighed in transposing results from one climatic area to another. Our results can be used to refine forecast models for grapevine downy mildew to account for changes in berry and rachis susceptibility, and to focus fungicide application schedules upon the most critical periods for protection of fruit.

The two-component system RhpRS was identified in Pseudomonas syringae as a regulator of the genes encoding the type III secretion system and type III effector proteins (together called the T3 genes). In the absence of the sensor kinase RhpS, the response regulator RhpR represses the induction of the T3 gene regulatory cascade consisting of hrpRS, hrpL, and the T3 genes in a phosphorylation-dependent manner. The repressor activity of RhpR is inhibited by RhpS, which presumably acts as a phosphatase under the T3 gene inducing conditions. Here, we show that RhpR binds and induces its own promoter in a phosphorylation-dependent manner. Deletion and mutagenesis analyses revealed an inverted repeat (IR) element, GTATC-N(6)-GATAC, in the rhpR promoter that confers the RhpR-dependent induction. Computational search of the P. syringae genomes for the putative IR elements and Northern blot analysis of the genes with a putative IR element in the promoter region uncovered five genes that were upregulated and two genes that were downregulated in an RhpR-dependent manner. Two genes that were strongly induced by RhpR were assayed for the IR element activity in gene regulation and, in both cases, the IR element mediated the RhpR-dependent gene induction. Chromatin immunoprecipitation assays indicated that RhpR binds the promoters containing a putative IR element but not the hrpR and hrpL promoters that do not have an IR element, suggesting that RhpR indirectly regulates the transcriptional cascade of hrpRS, hrpL, and the T3 genes.

Root-associated microbes are critical to plant health and performance, although understanding of the factors that structure these microbial communities and the theory to predict microbial assemblages are still limited. Here, we use a grafted tomato system to study the effects of rootstock genotypes and grafting in endosphere and rhizosphere microbiomes that were evaluated by sequencing 16S rRNA. We compared the microbiomes of nongrafted tomato cultivar BHN589, selfgrafted BHN589, and BHN589 grafted to Maxifort or RST-04-106 hybrid rootstocks. Operational taxonomic unit (OTU)-based bacterial diversity was greater in Maxifort compared to the nongrafted control, whereas bacterial diversity in the controls (selfgrafted and nongrafted) and the other rootstock (RST-04-106) was similar. Grafting itself did not affect bacterial diversity; diversity in the self-graft was similar to that of the nongraft. Bacterial diversity was higher in the rhizosphere than in the endosphere for all treatments. However, despite the lower overall diversity, there was a greater number of differentially abundant OTUs (DAOTUs) in the endosphere, with the greatest number of DAOTUs associated with Maxifort. In a permutational multivariate analysis of variance (PERMANOVA), there was evidence for an effect of rootstock genotype on bacterial communities. The endosphere-rhizosphere compartment and study site explained a high percentage of the differences among bacterial communities. Further analyses identified OTUs responsive to rootstock genotypes in both the endosphere and rhizosphere. Our findings highlight the effects of rootstocks on bacterial diversity and composition. The influence of rootstock and plant compartment on microbial communities indicates opportunities for the development of designer communities and microbiome-based breeding to improve future crop production. IMPORTANCE Understanding factors that control microbial communities is essential for designing and supporting microbiome-based agriculture. In this study, we used a grafted tomato system to study the effect of rootstock genotypes and grafting on bacterial communities colonizing the endosphere and rhizosphere. To compare the bacterial communities in control treatments (nongrafted and self-grafted plants) with the hybrid rootstocks used by farmers, we evaluated the effect of rootstocks on overall bacterial diversity and composition. These findings indicate the potential for using plant genotype to indirectly select bacterial taxa. In addition, we identify taxa responsive to each rootstock treatment, which may represent candidate taxa useful for biocontrol and in biofertilizers.Citation Poudel R, Jumpponen A, Kennelly MM, Rivard CL, Gomez-Montano L, Garrett KA. 2019. Rootstocks shape the rhizobiome: rhizosphere and endosphere bacterial communities in the grafted tomato system. Appl Environ Microbiol 85:e01765-18. https://

Moss is common on creeping bentgrass (Agrostis stolonifera L.) putting greens, and more control options are needed. Spot treatment of sodium bicarbonate (44.2 g·L−1) was compared with broadcast sprays of carfentrazone-ethyl (50.5 or 101 g a.i./ha), chlorothalonil (8.2 or 12.8 kg a.i./ha) and a tank mixture of chlorothalonil, mancozeb, and thiram (8.2, 9.8, and 11.5 kg a.i./ha) in 2006 in Lemont, IL. Sodium bicarbonate suppressed moss growth equally as the conventional products. These results led to further experiments in 2008 in which moss suppression was evaluated within standard and alternative putting green management regimes in Manhattan, KS, and Lemont, IL. The standard approach included spring and fall applications of carfentrazone-ethyl (101 g a.i./ha) for moss control, biweekly applications of urea (46N–0P–0K) at 15 kg N/ha, and applications of chlorothalonil (8.2 kg a.i./ha) on a 14-day interval. Conversely, the alternative approach included spring and fall spot treatments of sodium bicarbonate (44.2 g·L−1) for moss control, biweekly applications of a natural organic fertilizer (8N–1P–3K) to provide nitrogen at 15 kg N/ha, and applications of chlorothalonil (8.2 kg a.i./ha) only when dollar spot reached a predetermined threshold level. Standard and alternative regimes were compared at both 3.2- and 4.0-mm mowing heights; synthetic and organic fertilizers applied alone without pest control approaches were included as controls. In Kansas and Illinois, moss coverage using the alternative management regime was not significantly different from that on greens managed using the standard regime. In Kansas, moss severity at a 3.2 mm was 1.6-fold higher than at the 4.0-mm height. In Illinois, sodium bicarbonate suppressed moss equivalently to the carfentrazone-ethyl treatment, and in the fertilizer-only controls, mowing at 3.2 versus 4.0 mm led to more moss coverage. These studies demonstrate that moss can be effectively suppressed on greens using spot applications of sodium bicarbonate and reduced moss encroachment is possible with higher mowing heights.

Meyer zoysiagrass {Zoysia japónica Steud.) is the most widely used cultivar in the transition zone of the United States due, in part, to its excellent freezing tolerance. Ten zoysiagrass progeny from crosses between Z. matrella (L.) Merr. x Z japónica or 'Emerald' [Z japónica x Z matrella (L.) Merr. var. pacifica Goudswaard] X Meyer, 'Cavalier' (Z. matrella), and DALZ 0102 (Z. japónica) were evaluated for (i) freezing tolerance, (ii) autumn and spring color, and (iii) a relationship between autumn color and freezing tolerance. Turfgrass cores were sampled from the field near Manhattan, KS, in October, December, or February and exposed to temperatures from -6 to -22°C in the laboratory. Lethal temperatures resulting in death of 50% of tillers (LTJQ) after 6 wk of recovery in the greenhouse ranged from -0.2 C to -12.2°C. All grasses were equivalent to Meyer in freezing tolerance except Cavalier on three of five sampling dates and one Z matrella x Z japónica progeny in December 2007, which were inferior to Meyer. Digital image analysis indicated that five progeny were superior to Meyer in autumn green color retention, but none were superior to Meyer in spring green color. Autumn color in 2007 was positively correlated (r = 0.40 to 0.58, p < 0.05) with LTgg in December 2007. However, autumn color was also positively correlated with color the following spring (r = 0.31 to 0.72). These progeny demonstrated good freezing tolerance and, in some cases, superior autumn color compared to Meyer. ). Abbreviations: of tillers. lethal temperature resulting in death of 50% Z OYSIAGRASS {Zoysia spp.) is a warm-season turfgrass commonly used on lawns and golf courses in the transition zone and the southern parts of the United States. Meyer {Z. japónica Steud.; Grau and Radko, 1951) has been the cultivar used predominantly in the transition zone since its release in 1951, in part because of its excellent freezing tolerance. However, Meyer is relatively slow to establish and more coarse textured than Emerald [Z. japónica x Z. matrella (L.) Merr. var. pacifica Goudswaard] or Z. matrella (L.) Merr. cultivars and has relatively slow recuperative rates (Fry and Dernoeden, 1987;Patton, 2009).Since the release of Meyer, researchers have developed and released numerous zoysiagrass cultivars, including the Z. matrella cultivars Cavalier (Engelke et al., 2002a), Diamond (Engelke et al., 2002b), and Zorro (Engelke andReinert, 2003). These cultivars have a finer leaf texture and better quality characteristics compared to Meyer; however, poor freezing tolerance has limited their use in the transition zone. In a controlled freezing chamber experiment.

The ubiquitous soilborne and plant-pathogenic basidiomycete Rhizoctonia solani, although classified as a single species, is a complex of anastomosis groups (AGs) that cause disease in a broad range of higher plants. Here, we investigated the persistent co-isolation of bacteria with R. solani from brown patch-infected, cool-season turfgrasses, and report the presence of endo-hyphal bacteria, related to members in the genus Enterobacter, in an isolate of R. solani AG 2-2IIIB. The intracellular localization of the bacteria was corroborated by fluorescence, confocal and electron microscopy, and DNA analysis. Furthermore, the Enterobacter sp., which is rod-shaped in the free-living form, exists as an L-form (spheroid) within the fungus, a phenomenon not previously reported in endosymbionts. Our findings also indicate that the bacterium is required for full virulence of R. solani on creeping bentgrass and production of wild type levels of the toxin phenylacetic acid in fungal cultures. The possible presence of bacterial endosymbionts in R. solani AG 2-2IIIB may portend the presence of bacteria in additional AGs as well as other Rhizoctonia species, and may help resolve some of the complexities of R. solani pathogenicity. A closely associated bacterium could influence aspects of plant host pathology.