Vulnerable newborn types: analysis of subnational, population-based birth cohorts for 541 285 live births in 23 countries, 2000-2021.

OBJECTIVE: To examine prevalence of novel newborn types among 541 285 live births in 23 countries from 2000 to 2021. DESIGN: Descriptive multi-country secondary data analysis. SETTING: Subnational, population-based birth cohort studies (n = 45) in 23 low- and middle-income countries (LMICs) spanning 2000-2021. POPULATION: Liveborn infants. METHODS: Subnational, population-based studies with high-quality birth outcome data from LMICs were invited to join the Vulnerable Newborn Measurement Collaboration. We defined distinct newborn types using gestational age (preterm [PT], term [T]), birthweight for gestational age using INTERGROWTH-21st standards (small for gestational age [SGA], appropriate for gestational age [AGA] or large for gestational age [LGA]), and birthweight (low birthweight, LBW [<2500 g], nonLBW) as ten types (using all three outcomes), six types (by excluding the birthweight categorisation), and four types (by collapsing the AGA and LGA categories). We defined small types as those with at least one classification of LBW, PT or SGA. We presented study characteristics, participant characteristics, data missingness, and prevalence of newborn types by region and study. RESULTS: Among 541 285 live births, 476 939 (88.1%) had non-missing and plausible values for gestational age, birthweight and sex required to construct the newborn types. The median prevalences of ten types across studies were T+AGA+nonLBW (58.0%), T+LGA+nonLBW (3.3%), T+AGA+LBW (0.5%), T+SGA+nonLBW (14.2%), T+SGA+LBW (7.1%), PT+LGA+nonLBW (1.6%), PT+LGA+LBW (0.2%), PT+AGA+nonLBW (3.7%), PT+AGA+LBW (3.6%) and PT+SGA+LBW (1.0%). The median prevalence of small types (six types, 37.6%) varied across studies and within regions and was higher in Southern Asia (52.4%) than in Sub-Saharan Africa (34.9%). CONCLUSIONS: Further investigation is needed to describe the mortality risks associated with newborn types and understand the implications of this framework for local targeting of interventions to prevent adverse pregnancy outcomes in LMICs.

Incidence of Mite-Vectored Viruses of Wheat in the Texas High Plains and Interactions With Their Host and Vector.

Mite-vectored virus diseases of wheat are common throughout the Great Plains and cause significant economic losses to growers each year. These diseases are caused by Wheat streak mosaic virus (WSMV), Triticum mosaic virus (TriMV), and Wheat mosaic virus (WMoV), all of which are transmitted by the wheat curl mite (WCM), Aceria tosichella Keifer. New wheat cultivars with tolerance or resistance to WSMV have been released recently, but their widespread cultivation and potential impact on mite-transmitted virus incidence in the Texas Panhandle was unknown. A total of 648 symptomatic wheat samples were collected from 26 counties, predominately in the Texas Panhandle, and tested by enzyme-linked immunosorbent assay (ELISA) for WSMV, TriMV, and WMoV. Samples that tested negative by ELISA were subsequently tested by real-time quantitative PCR (qPCR) for each virus. Approximately 93% of the samples tested by ELISA were positive for WSMV, 43% were positive for TriMV, and 7% were positive for WMoV. Eleven samples tested positive only for TriMV, but none were positive only for WMoV. When samples that tested negative for the different viruses by ELISA were retested by real-time qPCR, detection of each virus was significantly increased. When results of the ELISA test and qPCR were combined, 100% of the 648 samples tested positive for WSMV, approximately 94% were positive for TriMV, and 23% were positive for WMoV. This demonstrated that the incidence of TriMV in the Texas High Plains is much greater than previously reported. The fact that real-time qPCR revealed over a 2-fold increase in the incidence of TriMV and a 3-fold increase in WMoV demonstrated that the ELISA test, which is commonly used by diagnostic laboratories in the Great Plains, should not be used for studies requiring a high degree of sensitivity and accuracy in virus detection. After initial virus infection status was determined, samples that tested positive for WSMV and TriMV were further observed for WCM infestation. A total of 292 samples were inspected and a total of 101 mites were collected from 40 tillers. Individual mites and the tillers from which they were recovered were tested by real-time qPCR to determine how copy numbers of WSMV and TriMV in mites and host tissue compared, and whether the WSMV/TriMV copy number ratio in individual mites was similar to that of the host tissue from which they were collected. In all mites and tillers tested, the WSMV copy number was always higher than that of TriMV and copy numbers of both viruses were always higher in plant tissue than in mites. Although there was a significant correlation between the WSMV/TriMV copy number ratio in plant tissue and in associated mites, the correlation coefficient was very low (r = 0.31, P = 0.0248). In the majority of comparisons, the WSMV/TriMV ratio was higher in individual mites than in the tiller from which they were recovered. The reason for this increase is unknown but indicates that mites may preferentially acquire WSMV from tillers coinfected with WSMV and TriMV, a finding that could have significant implications for virus transmission and disease epidemiology.

Relationships Between Early Wheat Streak Mosaic Severity Levels and Grain Yield: Implications for Management Decisions.

Wheat streak mosaic (WSM) caused by Wheat streak mosaic virus, which is transmitted by the wheat curl mite (Aceria tosichella), is a major yield-limiting disease in the Texas High Plains. In addition to its impact on grain production, the disease reduces water-use efficiency by affecting root development. Because of the declining Ogallala Aquifer water level, water conservation has become one of the major pressing issues in the region. Thus, questions are often raised as to whether it is worthwhile to irrigate infected fields in light of the water conservation issues, associated energy costs, and current wheat prices. To address some of these questions, field experiments were conducted in 2013 and 2016 at two separate locations to determine whether grain yield could be predicted from disease severity levels, assessed early in the spring, for potential use as a decision tool for crop management, including irrigation. In both fields, disease severity assessments started in April, using a handheld hyperspectral radiometer with which reflectance measurements were taken weekly in multiple plots in arbitrarily selected locations across the fields. The relationship between WSM severity levels and grain yield for the different assessment dates were determined by fitting reflectance and yield values into the logistic regression function. The model predicted yield levels with r2 values ranging from 0.67 to 0.85 (P < 0.0001), indicating that the impact of WSM on grain yield could be fairly well predicted from early assessments of WSM severity levels. As the disease is normally progressive over time, this type of information will be useful for making management decisions of whether to continue irrigating infected fields, especially if combined with an economic threshold for WSM severity levels.

Population Dynamics of Released Potato Psyllids and their Bacteriliferous Status in Relation to Zebra Chip Incidence in Caged Field Plots.

Potato psyllids vector 'Candidatus Liberibacter solanacearum' (Lso), the putative causal agent of potato zebra chip (ZC). Currently, sticky traps are the primary psyllid monitoring tools used by growers for making management decisions. However, the reliability of sticky traps in predicting psyllid numbers in potato fields has always been questioned. In 2013 and 2014, experiments were conducted in covered field plots at the Texas A&M AgriLife Research Station at Bushland to investigate the relationships among initial psyllid numbers, psyllids captured on sticky traps and their Lso status, and zebra chip incidence. Three densities of Lso-positive psyllids (5, 15, or 30/cage) were released under 2-week-old potato canopies with four replications in plot sizes of 7.6 to 9 m by 5 rows. Psyllids were released under the first plant in the center row and monitored weekly with a yellow sticky trap from the opposite end. Number of plants with zebra chip symptoms also was counted weekly beginning one month after infestation with psyllids. The total number of psyllids captured on sticky traps and disease incidence levels generally corresponded to the levels of psyllid density treatments (5 < 15 < 30), but the differences became more apparent toward the end of the experiments. Psyllid numbers in the different density treatments fluctuated more or less in synchrony over time, which appeared to reflect periodic emergence of new generations of psyllids. Initially, all captured psyllids tested positive for Lso. However, the proportions of psyllids testing positive declined dramatically after a few weeks, which suggested that the new generations of psyllids were devoid of Lso. Over all, less than 50% of captured psyllids tested positive for the pathogen. The decline in proportions of psyllids testing positive for Lso following successive generations has significant relevance to field situations and may partly explain why there are generally low percentages of Lso-positive psyllids under field conditions.

Significance and Epidemiological Aspects of Late-Season Infections in the Management of Potato Zebra Chip.

Zebra chip (ZC) of potato is putatively caused by the fastidious, phloem-limited bacterium 'Candidatus Liberibacter solanacearum' (Lso), which is transmitted by the potato psyllid (Bactericera cockerelli). The disease, which significantly impacts both crop yield and quality, was first identified in the United States from south Texas in 2000. It reached epidemic levels in north Texas and certain production areas in Colorado, Nebraska, and New Mexico from 2004 to 2007 and it caused severe losses in fields in Oregon, Washington, and Idaho in 2011. The potato plant is susceptible to infection at all developmental stages, but disease management programs have focused on vector control through early and repeated insecticide applications, in an effort to minimize early to midseason infections which are most damaging. Growers often terminate spray programs 2 to 3 weeks prior to crop harvest due to lack of visible treatment effects on crop yield or quality. However, recent studies on vector transmission and host-pathogen interactions have revealed that late-season infections pose a significant, previously unrecognized, threat to crop quality. The pathogen can move from an infected leaf to tubers within 2 days; however, tubers infected less than 1 week before harvest will remain asymptomatic and the pathogen will be undetectable. When these tubers are placed into storage they are assumed to be disease free. However, Lso can continue to multiply in respiring tubers during storage, resulting in reduced tuber quality. Likewise, if plants become infected a few days before vines are killed, ZC can continue to develop in infected tubers before they are harvested. Perspectives on the significance of late-season infections and some of the more important issues associated with those infections are discussed.

Effects of Potato-Psyllid-Vectored 'Candidatus Liberibacter solanacearum' Infection on Potato Leaf and Stem Physiology.

The bacterium 'Candidatus Liberibacter solanacearum' is associated with zebra chip disease (ZC), a threat to potato production in North America and New Zealand. It is vectored by potato psyllids. Previous studies observed that 'Ca. L. solanacearum' infection causes potato tubers to undergo ZC-symptom-associated shifts in physiology, such as increased levels of amino acids, sugars, and phenolics. However, little is known about how 'Ca. L. solanacearum' infections caused by psyllid vector feeding may affect metabolism in potato foliage and stems. This study compared metabolism in potato plants fed upon by 'Ca. L. solanacearum'-positive psyllids with potato plants not exposed to psyllids. Foliar levels of asparagine, aspartic acid, glutamine, fructose, glucose, sucrose, a ferulic acid derivative, and quinic acid were lower in 'Ca. L. solanacearum'-inoculated than noninfected plants. However, foliar levels of proline, serine, four phenolic compounds, and most terpenoids were greater in 'Ca. L. solanacearum'-inoculated than noninfected plants. Upper stem levels of asparagine and aspartic acid, upper and lower stem levels of ellagitannins and most monoterpenoids, and lower stem level of sesquiterpenoids were greater in 'Ca. L. solanacearum'-inoculated than noninfected plants. These results suggest that many defense-related terpenoid compounds might increase in plants which had psyllids inoculate 'Ca. L. solanacearum'. This could impact progression and spread of ZC.

Zebra Chip Disease Development in Relation to Plant Age and Time of 'Candidatus Liberibacter solanacearum' Infection.

A 2-year field study was conducted to evaluate plant susceptibility to 'Candidatus Liberibacter solanacearum', the putative causal agent of zebra chip disease (ZC). Incubation period of ZC, the rate of symptom progress, and the rate of pathogen population growth were evaluated for individual plants infested on different weeks after emergence. In foliage, incubation period was between 21 and 28 days. The pathogen was detected within leaf tissue in 3 to 4 weeks, regardless of the time of infestation. The rates of foliar symptom progress and pathogen population growth were uniform among all infestations. Although symptoms were observed in only 1.3% of tubers from plants infested 2 weeks before harvest, 74% of these tubers tested positive for the pathogen. There was a positive correlation between symptom severity and titer in the foliage. Within tubers, however, the relationship was negative but nonsignificant. Pathogen titer reached detectable levels some time between 7 to 14 days following infestation. Although yield reduction was significant only in plants infested during early stages of their growth, chemical management of potato psyllids needs to be continued until at least a week before harvest to minimize ZC impact on the tuber quality.

Similarities and differences in physiological responses to 'Candidatus Liberibacter solanacearum' infection among different potato cultivars.

Zebra chip disease (ZC), putatively caused by the fastidious bacterium 'Candidatus Liberibacter solanacearum', is a threat to potato growers worldwide. However, little is known about biochemical shifts in different potato genotypes in response to 'Ca. L. solanacearum' infection. To address this, 'Red La Soda', 'Russet Norkotah', and 'FL 1867' potato were infected with 'Ca. L. solanacearum' 4, 3, 2, and 1 weeks before harvest to observe variability in cultivar responses to 'Ca. L. solanacearum' infection. ZC symptoms, 'Ca. L. solanacearum' titers, and tuber biochemistry were assessed. Red La Soda tubers exhibited greater symptoms when infected for 4 weeks than Russet Norkotah or FL 1867 tubers. 'Ca. L. solanacearum' titers did not vary among cultivars. Tuber levels of amino acids, carbohydrates, and phenolics varied among cultivars but no consistent trends were observed. Individual amino acids and phenolics were greater in FL 1867 than Red La Soda, whereas others were greater in Red La Soda or Russet Norkotah than FL 1867. Most amino acids, carbohydrates, and phenolics were positively associated with infection duration and symptoms regardless of cultivar. Associations between most of the evaluated compounds and 'Ca. L. solanacearum' titer were positive in Red La Soda. However, no associations between 'Ca. L. solanacearum' quantity and compounds were observed in FL 1867 and Russet Norkotah.

Characterization of management and environmental factors associated with regional variations in potato zebra chip occurrence.

Potato zebra chip (ZC), caused by the bacterial pathogen 'Candidatus Liberibacter solanacearum', which is vectored by the potato psyllid (Bactericera cockerelli), has caused widespread damage to U.S. potato production ever since its first discovery in south Texas in 2000. To determine the influence of environmental factors and management practices on ZC occurrence, data on management and meteorological variables, field locations, and psyllid counts were collected over a 3-year period (2010 to 2012) from six locations across the central United States (south Texas to Nebraska). At these locations, ZC-symptomatic plants were counted in 26 fields from systematically established 20 m × 30 m plots around the field edges and field interiors. Mean numbers of symptomatic plants per plot were classified into two intensity classes (ZC ≤ 3 or ZC > 3) and subjected to discriminant function and logistic regression analyses to determine which factors best distinguish between the two ZC intensity classes. Of all the variables, location, planting date, and maximum temperature were found to be the most important in distinguishing between ZC intensity classes. These variables correctly classified 88.5% of the fields into either of the two ZC-intensity classes. Logistic regression analysis of the individual variables showed that location accounted for 90% of the variations, followed by planting date (86%) and maximum temperature (70%). There was a low but significant (r = -0.44983, P = 0.0211) negative correlation between counts of psyllids testing positive for pathogen and latitudinal locations, indicating a south-to-north declining trend in counts of psyllids testing positive for the pathogen. A similar declining trend also was observed in ZC occurrence (r = -0.499, P = 0.0094).

Zebra chip disease and potato biochemistry: tuber physiological changes in response to 'Candidatus Liberibacter solanacearum' infection over time.

Zebra chip disease, putatively caused by the bacterium 'Candidatus Liberibacter solanacearum', is of increasing concern to potato production in Mexico, the United States, and New Zealand. However, little is known about the etiology of this disease and changes that occur within host tubers that result in its symptoms. Previous studies found that increased levels of phenolics, amino acids, defense proteins, and carbohydrates in 'Ca. L. solanacearum'-infected tubers are associated with symptoms of zebra chip. This study was conducted to quantify variations in levels of these biochemical components in relation to the time of infestation, symptom severity, and 'Ca. L. solanacearum' titer. Levels of phenolics, peroxidases, polyphenol oxidases, and reducing sugars (glucose and, to some extent, fructose) changed during infection, with higher levels occurring in tubers infected at least 5 weeks before harvest than in those infected only a week before harvest and those of controls. Compared with the apical tuber ends, greater levels of phenolics, peroxidases, and sucrose occurred at the basal (stolon attachment) end of infected tubers. With the exception of phenolics, concentrations of the evaluated compounds were not associated with 'Ca. L. solanacearum' titer. However, there were significant associations between biochemical responses and symptom severity. The lack of a linear correlation between most plant biochemical responses and 'Ca. L. solanacearum' titer suggests that shifts in metabolic profiles are independent of variations in 'Ca. L. solanacearum' levels.

Transmission efficiency of 'Candidatus Liberibacter solanacearum' and potato zebra chip disease progress in relation to pathogen titer, vector numbers, and feeding sites.

ABSTRACT With diseases caused by vector-borne plant pathogens, acquisition and inoculation are two primary stages of the transmission, which can determine vector efficiency in spreading the pathogen. The present study was initiated to quantify acquisition and inoculation successes of 'Candidatus Liberibacter solanacearum', the etiological agent of zebra chip disease of potato, by its psyllid vector, Bactericera cockerelli (Hemiptera: Triozidae). Acquisition success was evaluated in relation to feeding site on the host plant as well as the acquisition access period. Inoculation success was evaluated in relation to vector number (1 and 4) on the plants. Acquisition success was influenced by the feeding site on the plant. The highest acquisition success occurred when insects had access to the whole plant. The results of the inoculation study indicated that the rate of successfully inoculated plants increased with the vector number. Plants inoculated with multiple psyllids had higher bacterial titer at the point of inoculation. Although disease incubation period was significantly shorter in plants inoculated with multiple psyllids, this effect was heterogeneous across experimental blocks, and was independent of pathogen quantity detected in the leaflets 3 days postinoculation. Disease progress was not affected by bacterial quantity injected or psyllid numbers.

Assessments of the Edge Effect in Intensity of Potato Zebra Chip Disease.

Zebra chip is a newly emerging potato disease which imparts dark colorations on fried chips, rendering them unmarketable. The disease is associated with the phloem-limited proteobacterium 'Candidatus Liberibacter solancearum', vectored by the potato psyllid Bactericera cockerelli. First reported from Mexico in the mid-1990s, the disease was observed for the first time in Texas in 2000 and is now prevalent in several potato-producing regions of the United States. In this study, we were interested in investigating whether there are edge effects in zebra chip intensity that can be assessed as a "foot print" of the associated insect vector. In 2009, we conducted studies in three fields in the Texas Panhandle in paired plots of 10 by 20 m around the field edges and 100 m infield in which symptomatic plants were counted just before harvest. The number of plot pairs (edge and infield) ranged from 15 to 18 depending on the size of the fields. In a separate study, temporal disease progress was assessed in two fields around the edges of the center-pivot circle in approximately 10-by-450-m areas. In 2010, the paired plot studies were repeated in 10 potato fields in Texas, Kansas, and Nebraska. Zebra chip intensity data from the paired-plot studies for both years were analyzed using the Wilcoxon's signed-rank test, a nonparametric equivalent of the classical (parametric) paired t test. In the 2009 study in all three fields, the edge plots had significantly greater zebra chip intensity than the infield plots (P < 0.05). Edge plots in the 2010 study also had greater zebra chip intensity in all fields and the differences were significant in the majority of fields (P < 0.05). In the diseases progress study in both fields, weekly zebra chip intensity on the edges reached its maximum after the third week of its first detection, and the disease progress curves were best fitted with the second-degree polynomial (quadratic) for both fields. The 2-year study clearly demonstrated that zebra chip intensity in potato fields was greater on the edges than in the infields. This finding has significant implications for psyllid management because greater emphasis in psyllid control strategy can be directed toward the edges for better results.

Spatial Patterns and Spread of Potato Zebra Chip Disease in the Texas Panhandle.

Zebra chip (ZC) is a disease that is affecting potato production in the southwestern United States and in other countries, and which has been linked to potato psyllids (Bactericera cockerelli) that harbor the bacterial plant pathogen 'Candidatus Liberibacter solanacearum'. Until recently, the epidemiology of ZC was unknown, motivating research to elucidate the spatial and temporal patterns of ZC infections in potato fields. Studies were performed in multiple commercial potato fields located in the Texas Panhandle, wherein locations of ZC-affected potato plants were georeferenced or counted within large plots and along belt transects consisting of contiguous 10-by-10-m quadrats. By employing distance- and area-based spatial statistical methods, it was determined that locations of ZC infections in potato fields departed from a completely spatially random pattern, instead appearing as clusters comprising infected plants situated in close proximity to one another, with clusters interspersed with numerous solitary infections. Disease progress curves of ZC clusters were generally well described by exponential growth and quadratic polynomial models. Numbers of ZC infections within disease clusters gradually increased over multiple weeks, with foliar disease symptoms first appearing during the tuber bulking stage. ZC infections were not found to be continuously present across fields, because many quadrats along belt transects contained zero or only a few infections while others had numerous infections. Consequently, the frequency of ZC infections within belt transect quadrats was well described by negative binomial and zero-inflated negative binomial distributions, in agreement with observed clustering of infections and distance-based spatial statistical results.

Satellite Remote Sensing of Wheat Infected by Wheat streak mosaic virus.

The prevalence of wheat streak mosaic, caused by Wheat streak mosaic virus, was assessed using Landsat 5 Thematic Mapper (TM) images in two counties of the Texas Panhandle during the 2005-2006 and 2007-2008 crop years. In both crop years, wheat streak mosaic was widely distributed in the counties studied. Healthy and diseased wheat were separated on the images using the maximum likelihood classifier. The overall classification accuracies were between 89.47 and 99.07% for disease detection when compared to "ground truth" field observations. Omission errors (i.e., pixels incorrectly excluded from a particular class and assigned to other classes) varied between 0 and 12.50%. Commission errors (i.e., pixels incorrectly assigned to a particular class that actually belong to other classes) ranged from 0 to 23.81%. There were substantial differences between planted wheat acreage reported by the United States Department of Agriculture-National Agricultural Statistics Service (USDA-NASS) and that detected by image analyses. However, harvested wheat acreage reported by USDA-NASS and that detected by image classifications were closely matched. These results indicate that the TM image can be used to accurately detect and quantify incidence of wheat streak mosaic over large areas. This method appears to be one of the best currently available for identification and mapping disease incidence over large and remote areas by offering a repeatable, inexpensive, and synoptic strategy during the course of a growing season.

Movement of Bactericera cockerelli (Heteroptera: Psyllidae) in relation to potato canopy structure, and effects on potato tuber weights.

With the threat of new plant diseases on the increase, plant disease epidemiology requires research on pathogen vector movement. Here, releases were performed in planted potato fields of different ages and canopy structures, located in the Texas Panhandle, to evaluate the range of movement of the potato psyllid Bactericera cockerelli (Sulc.). This insect is a known causative agent of psyllid yellows disease, and is a vector of the reported etiological agents of zebra chip disease of potato, 'Candidatus Liberibacter solanacearum/psyllaurous'. Based on collections of B. cockerelli immatures along transects 9 m long radiating in four cardinal directions from release points, adult females dispersed considerable distances, regardless of plant age or canopy structure. Immature abundance declined along transects and were well described by linear and nonlinear models, but abundance patterns did not differ among the different planting dates and canopy structures. However, unequal immature abundance was detected among the four cardinal directions, with more immatures generally collected along transects to the north and west of release points, opposite of prevailing winds in the area at the time of release. Plots where B. cockerelli were released had significantly lower mean potato tuber weights than control plots with no B. cockerelli. However, few plots with B. cockerelli released in them had declining trends in tuber weights with increasing distance from release points.

Effects of Wheat streak mosaic virus on Root Development and Water-Use Efficiency of Hard Red Winter Wheat.

Greenhouse and field studies were conducted to determine the effects of Wheat streak mosaic virus (WSMV), a member of the family Potyviridae, on root development and water-use efficiency (WUE) of two hard red winter wheat (Triticum aestivum) cultivars, one susceptible and one resistant to WSMV. In the greenhouse studies, wheat cultivars were grown under three water regimes of 30, 60, and 80% soil saturation capacity. After inoculation with WSMV, plants were grown for approximately 4 weeks and then harvested. Root and shoot weights were measured to determine the effect of the disease on biomass. In all water treatments, root biomass and WUE of inoculated susceptible plants were significantly less (P < 0.05) than those of the noninoculated control plants. However, in the resistant cultivar, significance was only found in the 30 and 60% treatments for root weight and WUE, respectively. Field studies were also conducted under three water regimes based on reference evapotranspiration rates. Significant reductions in forage, grain yield, and crop WUE were observed in the inoculated susceptible plots compared with the noninoculated plots. Both studies demonstrated that wheat streak mosaic reduces WUE, which is a major concern in the Texas Panhandle because of limited availability of water.

Characterization and Epidemiological Significance of Potato Plants Grown from Seed Tubers Affected by Zebra Chip Disease.

An emerging disease of potato in the United States, known as "Zebra Chip" or "Zebra Complex" (ZC), is increasing in scope and threatens to spread further. Here, we report on studies performed to understand the role of tuberborne ZC in the epidemiology of this disease. Depending on variety, up to 44% of ZC-affected seed tubers (ZCST) were viable, producing hair sprouts and weak plants. Chip discoloration in progeny tubers of ZCST was more severe than those from ZC-asymptomatic seed tubers but varied depending on whether progeny tubers or foliage were positive or negative for 'Candidatus Liberibacter solanacearum'. A low percentage of greenhouse-grown plants produced by ZCST tested positive for 'Ca. Liberibacter'. No adult potato psyllids became infective after feeding upon these plants but they did acquire 'Ca. Liberibacter' from field-grown plants produced by ZCST. Plants with new ZC infections near plants produced by ZCST were not significantly different from healthy plants, whereas plants affected with ZC from infectious potato psyllids had significantly more ZC infections near either plants produced by ZCST or healthy plants. We conclude that, in areas where ZC is currently established, plants produced by ZCST do not significantly contribute to ZC incidence and spread within potato fields.

Quantifying wheat yield across the field as a function of wheat streak mosaic intensity: a state space approach.

Wheat streak mosaic virus (WSMV), vectored by the wheat curl mite Aceria tosichella, is one of the major limiting factors in wheat production in the Texas Panhandle. The mites are blown by wind into wheat fields from nearby volunteer wheat fields or fields supporting vegetation which harbor virulent mites. Consequently, gradients of wheat streak severity are often observed stretching from the edges of wheat fields into the center or beyond. To describe the magnitude of the spatial relationships between grain yield and wheat streak intensity across the field, studies were conducted in 2006 and 2007 in three infected fields. Wheat streak severity was quantified with reflectance measurements (remote sensing) at 555-nm wave length using a hand-held radiometer. Measurements were taken in several equally spaced 1 m(2) locations along a total of eight transects and grain yield was assessed from a 0.8 m(2) area of each location. State space analysis was used to describe the relationships in which yield data and reflectance values were used as dependent and explanatory variables, respectively. A structural time series model was formulated as a state space model where the unobserved components were modeled explicitly. In the analysis the state of yield at current location (d) was related to the state of wheat streak intensity either at current locations (d) or lagged locations with autoregressive values of the first order (d-1) or greater. There were significant cross-correlations between yield and wheat streak intensity up to distances of 150 m (P < or = 0.05). Grain yield at the current position was significantly correlated with reflectance values at the same location and/or at lagged locations. The spatial aspects of the yield-reflectance relationships were best described by state space models with stochastic trends without slopes or deterministic trends with or without slopes. The models correctly predicted almost all of the observed yield values as a function of wheat streak intensity across the field within the 95% confidence interval. Results obtained in this study suggest that state space methodology can be a powerful tool in the study of plant disease spread as a function of other variables.

Rainfall and temperature distinguish between Karnal bunt positive and negative years in wheat fields in Texas.

Karnal bunt of wheat, caused by the fungus Tilletia indica, is an internationally regulated disease. Since its first detection in central Texas in 1997, regions in which the disease was detected have been under strict federal quarantine regulations resulting in significant economic losses. A study was conducted to determine the effect of weather factors on incidence of the disease since its first detection in Texas. Weather variables (temperature and rainfall amount and frequency) were collected and used as predictors in discriminant analysis for classifying bunt-positive and -negative fields using incidence data for 1997 and 2000 to 2003 in San Saba County. Rainfall amount and frequency were obtained from radar (Doppler radar) measurements. The three weather variables correctly classified 100% of the cases into bunt-positive or -negative fields during the specific period overlapping the stage of wheat susceptibility (boot to soft dough) in the region. A linear discriminant-function model then was developed for use in classification of new weather variables into the bunt occurrence groups (+ or -). The model was evaluated using weather data for 2004 to 2006 for San Saba area (central Texas), and data for 2001 and 2002 for Olney area (north-central Texas). The model correctly predicted bunt occurrence in all cases except for the year 2004. The model was also evaluated for site-specific prediction of the disease using radar rainfall data and in most cases provided similar results as the regional level evaluation. The humid thermal index (HTI) model (widely used for assessing risk of Karnal bunt) agreed with our model in all cases in the regional level evaluation, including the year 2004 for the San Saba area, except for the Olney area where it incorrectly predicted weather conditions in 2001 as unfavorable. The current model has a potential to be used in a spray advisory program in regulated wheat fields.