Effects of inoculum density,leaf wetness duration and nitrate concentration on the occurrence of lettuce leaf spot

In Ehime Prefecture, Japan, lettuce leaf spot (Septoria lactucae) caused huge losses in marketable lettuce yields. To explore potential measures to control disease outbreaks, the effects of inoculum density, leaf wetness duration and nitrate concentration on the development of leaf spot on lettuce (Lactuca sativa) were evaluated. Conidia were collected from diseased plants in an infested field by single-spore isolation and were used to inoculate potted lettuce plants with different conidial concentrations. Lesions developed on inoculated lettuce plants at inoculum concentrations from 10⁰ to 10⁶ conidia/ml. The disease was more severe when the inoculum exceeded 10² conidia/ml, and severity increased with increasing concentrations. Assessment of the relationship between disease development and the duration of postinoculation leaf wetness revealed that symptoms appeared when the inoculated plants remained wet for 12 h or longer. The number of lesions and total nitrogen content in the lettuce leaves both increased when nitrate was applied.     

Efficiency of inorganic fungicides against the formation of Erysiphe necator chasmothecia in vineyards

BACKGROUND

A reduction in chasmothecia, an important inoculum of grape powdery mildew (Erysiphe necator Schwein.), is essential for disease control in vineyards; the use of fungicides during the formation of chasmothecia on vine leaves, late in the growing season, may accomplish this. Inorganic fungicides, such as sulphur, copper, and potassium bicarbonate, are very useful for this purpose because of their multisite mode of action. The aim of this study was to evaluate chasmothecia reduction using different fungicide applications late in the growing season in commercially managed vineyards and in an exact application trial.

RESULTS

Chasmothecia on vine leaves were reduced in commercial vineyards by four copper (P = 0.01) and five potassium bicarbonate (P = 0.026) applications. The positive effect of potassium bicarbonate was also confirmed in the application trial, where two applications showed lower chasmothecia numbers than the control (P = 0.002).

CONCLUSION

The application of inorganic fungicides reduced the amount of chasmothecia as the primary inoculum source. Potassium bicarbonate and copper are of further interest for disease control as these fungicides can be used by organic and conventional wine growers. The application of these fungicides should be carried out as late as possible before harvest to reduce chasmothecia formation and, consequently, the potential for powdery mildew infestation in the subsequent season. © 2023 The Authors. Pest Management Science published by John Wiley & Sons Ltd on behalf of Society of Chemical Industry.     

Magnaporthe oryzae conidia on basal wheat leaves as a potential source of wheat blast inoculum

Wheat blast caused by Magnaporthe oryzae Triticum causes significant losses on wheat during outbreak years in several South American countries. Despite reports of wheat blast leaf lesions on some wheat cultivars, the importance of inoculum originating from leaves in severely affected commercial fields is disputed. It is generally considered that leaf lesions and/or sporulation on leaves do not usually appear before the occurrence of spike blast in wheat. The purpose of this study was to (i) determine the occurrence of wheat blast on basal leaves, (ii) estimate the number of conidia produced on these leaves, and (iii) determine the impact of current fungicide application practices on inoculum produced from sporulating lesions on basal wheat leaves. Inoculations at the three‐leaf stage showed that certain cultivar and isolate combinations caused more disease on old wheat leaves than young expanding leaves. Under optimum conditions, M. oryzae had the potential to produce tens to hundreds of thousands of conidia on small amounts of wheat basal leaves. A mean of 1 669 000 conidia were produced on 1 g dry basal leaves of a highly susceptible cultivar under optimum conditions for sporulation. Conidia production on leaves coincided with spike emergence under both greenhouse and field conditions. When field studies were conducted under natural epidemic conditions, foliar fungicide applications reduced the amount of M. oryzae conidia on basal leaves by 62–77% compared to non‐sprayed controls. An earlier application of foliar fungicides might reduce inoculum if conidia from basal leaves contribute to wheat spike blast development.     

Spatial distribution of Aphanomyces euteiches inoculum in a naturally infested pea field

The main objective of the study was to describe the horizontal and vertical distribution of Aphanomyces root rot in a naturally infested pea field. Measurements of inoculum potential clearly indicated a horizontal distribution of inoculum among several foci in the field, these foci differing in size and disease intensity. A highly significant relationship was observed between disease severity on plants during the cropping season and soil inoculum potential. In terms of the vertical distribution of inoculum in the soil, detection was maximal at a depth of 10 to 40 cm, but inoculum was detected down to a depth of 60 cm. Generally, inoculum potential was lowest for the layers at depths of 0 to 10 and 50 to 60 cm. Inoculum distribution and the value of the methodologies used are discussed in terms of possible use for epidemiology and disease forecasting.     

Wind speed and wind-associated leaf injury affect severity of citrus canker on Swingle citrumelo

Citrus canker (caused by the bacterial pathogen Xanthomonas citri subsp. citri, Xcc) can cause severe damage to citrus. It is endemic in Florida, and occurs in other citrus growing regions. The bacterium is dispersed predominantly in rain splash. To simulate dispersal in splash, and to investigate the effect of wind speed on infection, young plants of Swingle citrumelo were exposed to sprayed inoculum at different wind speeds. Wind was generated using an axial fan, and a pressurized sprayer delivered the inoculum spray. In the five experiments, higher wind speeds (>10 m s⁻¹) consistently resulted in higher incidence and severity of citrus canker developing. By 15 ms⁻¹, there was a dramatic increase in disease. Visible injury to leaves of Swingle citrumelo due to wind was evident at wind speeds ≥ 13 m s⁻¹. The relationship between wind speed and disease, and wind speed and injury was described by a logistic model. More disease was associated with visible injury as the wind speed increased, and disease not associated with visible injury also increased with wind speed. The petiole-leaflet junction was more often infected at higher wind speeds (≥17 m s⁻¹). The concentration of the Xcc inoculum increased the incidence and severity of citrus canker in all experiments. Reducing wind speed in citrus groves with the aid of wind breaks may contribute to a reduction in the severity of an epidemic by reducing dispersal and infection events.     

Alternative inoculum sources for fire blight: the potential role of fruit mummies and non‐host plants

Fire blight is the most damaging bacterial disease in apple production worldwide. Cankers and symptomless infected shoots are known as sites for the overwintering of Erwinia amylovora, subsequently providing primary inoculum for infection in the spring. In the present work, further potential sources of inoculum were investigated. Real‐time PCR assays covering a 3‐year‐period classified 19·9% of samples taken from fruit mummies as positive. Bacterial abundance in fruit mummies during autumn, winter and spring was up to 10⁹ cells per gram of tissue and correlated well with later infection rates of blossoms. Blossoms of non‐host plants growing close to infected trees were also shown to be colonized by E. amylovora and to enable epiphytic survival and propagation of bacteria. The results indicate a potential role of fruit mummies and buds in overwintering and as a source of primary inoculum for dissemination of the pathogen early in the growing season. Non‐host blossoms may also serve as an inoculum source in the build‐up of the pathogen population. Both aspects may contribute significantly to the epidemiology of E. amylovora. The significance of infected rootstocks as an inoculum source is also discussed. Fruit mummies might be used to determine pathogen pressure in an orchard before the beginning of the blooming period.     

Identification and pathogenicity of Fusarium species associated with pokkah boeng of sugarcane in Brazil

Brazil is the world’s biggest producer of sugarcane (Saccharum spp. hybrids). Pokkah boeng is an important fungal disease in this crop caused by members of the Fusarium fujikuroi species complex (FFSC) and characterized by deformation of the aerial part of the plant and stem rot. While the occurrence of symptoms has been reported in plantations in Brazil, no official reports of the disease exist. In this study, species of the FFSC were identified that are associated with sugarcane plants with symptoms of pokkah boeng in Brazil. This was achieved using two-loci molecular phylogeny, sexual compatibility and analysis of morphological markers. The ability of strains to cause disease in plants of sugarcane, maize, sorghum and millet was also evaluated. The 39 isolates studied were identified as F. sacchari, F. proliferatum and another, still unknown, phylogenetic lineage that is sister to F. andiyazi. Crossing field isolates of F. sacchari and F. proliferatum with their respective tester strains produced fertile perithecia and viable ascospores. All three species induced symptoms of pokkah boeng on inoculated sugarcane plants and caused stem rot in maize, sorghum and millet. Symptoms on sugarcane are chlorosis and necrosis of leaves, punctured lesions, twisted leaves, reduction of the total leaf area, death of the top of the plant and stalk rot. The findings confirmed the aetiology of the disease in Brazil, generated basic knowledge for the development of strategies for diagnosis and monitoring of the disease and support breeding programmes for selecting resistant germplasm.     

Aetiology of anthracnose on grapevine shoots in Brazil

Anthracnose is an important disease in vineyards in south and southeast Brazil, the main grape‐producing regions in the country. This study aimed to identify the causal agents of grapevine anthracnose in Brazil through multilocus phylogenetic analyses, morphological characterization and pathogenicity tests. Thirty‐nine Elsinoë ampelina and 13 Colletotrichum spp. isolates were obtained from leaves, stems and berries with anthracnose symptoms collected in 38 vineyards in southern and southeastern Brazil. For E. ampelina isolates, the internal transcribed spacer (ITS), histone H3 (HIS3) and elongation factor 1‐α (TEF) sequences were analysed. HIS3 was the most informative region with 55 polymorphic sites including deletions and substitutions of bases, enabling the grouping of isolates into five haplotypes. Colonies of E. ampelina showed slow growth, variable colouration and a wrinkled texture on potato dextrose agar. Conidia were cylindrical to oblong with rounded ends, hyaline, aseptate, (3.57–) 5.64 (?6.95) μm long and (2.03–) 2.65 (?3.40) μm wide. Seven species of Colletotrichum were identified: C. siamense, C. gloeosporioides, C. fructicola, C. viniferum, C. nymphaeae, C. truncatum and C. cliviae, with a wide variation in colony and conidium morphology. Only E. ampelina caused anthracnose symptoms on leaves, tendrils and stems of Vitis vinifera and V. labrusca. High disease severity and a negative correlation between disease severity and shoot dry weight were observed only when relative humidity was above 95%. In this study, only E. ampelina caused anthracnose symptoms on grapevine shoots in Brazil.     

Characterization of monilia disease caused by Monilinia linhartiana on quince in southern Spain

This study elucidates the aetiology and epidemiology of monilia disease of quince caused by the fungus Monilinia linhartiana in Spain. Disease incidence and the dynamics of apothecial development and ascospore discharge were quantified and the pathogen was characterized using morphological and molecular methods. The pathogen did not produce conidia or apothecia on agar media but produced conidia on leaves showing symptoms and apothecia on mummified young quince fruit. Monilinia linhartiana was not pathogenic on ripe quince fruit but was readily isolated from developing, mummified fruit (pseudosclerotia). Phylogenetic analysis based on 5·8S‐ITS region sequences placed M. linhartiana in the Disjuntoriae section of Monilinia species infecting rosaceous hosts. Studies during 2004–2008 in four commercial orchards in southern Spain determined two major infection periods for the disease. The first coincided with the unfolding of the first leaves and resulted in leaf blotch and shoot blight. The second coincided with flowering and led to mummification of developing young fruit. Foliar infection was apparently initiated by airborne ascospores produced on pseudosclerotia that overwintered on the soil surface, while flower infection was probably initiated by conidia produced on leaf lesions. Incidence of diseased shoots ranged from 1 to 91% and was correlated with calculated inoculum potential, based on the density and maturity of apothecia formed on pseudosclerotia. This epidemiological study has made it possible to characterize the life cycle of monilia disease on quince in southern Spain, which will help the development of new control strategies.     

Adaptation of the New Zealand Psa risk model for forecasting kiwifruit bacterial canker in Korea

Bacterial canker of kiwifruit, caused by Pseudomonas syringae pv. actinidiae (Psa), is a severe threat to kiwifruit production in Korea. An existing infection risk model from New Zealand was adopted to respond to this epidemic. Disease incidence (proportion of diseased leaves on each vine) and weather (hourly temperature, rainfall and relative humidity) data required to develop the model were collected and analysed in the study. Disease incidence data were used to modify and validate the existing model. Because the Psa risk model was originally developed in a region where the characteristic climatic conditions are completely different from those in Korea, the temperature and rainfall functions of the existing model were modified. Analyses using statistical correlation and prediction–realization tables revealed that the modified model is valid with high agreement (a correlation coefficient of 0.85 and an accuracy of 85.7%, respectively) between the observed disease incidence and simulated disease risk from the model. The model was also found to be more highly sensitive to the presence or absence of rainfall than any other weather variable inputs. Uncertainty in simulated disease risk was measured based on the level of uncertainty in temperature input from weather forecasts. Overall, these results indicate that the modified Psa risk model can be used to provide practical and applicable information for timely disease control to the kiwifruit growers in Korea.     

Ineffectiveness of pruning to control citrus huanglongbing caused by <Emphasis Type="Italic">Candidatus</Emphasis> Liberibacter americanus

The huanglongbing (HLB) disease of citrus trees, caused by Candidatus Liberibacter asiaticus and Ca. Liberibacter americanus, was first reported in Brazil in March, 2004. The presence of the disease has caused serious concerns among growers. Pruning experiments were conducted to determine if removal of symptomatic branches or the entire canopy (decapitation) would eliminate infected tissues and save HLB-affected trees. Pruning was done in five blocks on a total of 592 3- to 16 year-old ‘Valência’, ‘Hamlin’ or ‘Pêra’ sweet orange trees showing no symptoms or with two levels of symptom severity. Ten decapitated trees per block were caged and all trees were treated with insecticides to control the psyllid vector, Diaphorina citri. Mottled leaves reappeared on most symptomatic (69.2%) as well on some asymptomatic (7.6%) pruned trees, regardless of age, variety, and pruning procedure. Presence of the pathogen (Ca. Liberibacter americanus) in all symptomatic trees was confirmed by PCR. In general, the greater the symptom severity before pruning the lower the percentage of trees that remained asymptomatic after pruning.     

Validation of predictive empirical weed emergence models of Abutilon theophrasti Medik based on intercontinental data

Good weed management relies on the proper timing of weed control practices in relation to weed emergence dynamics. Therefore, the development of models that predict the timing of emergence may help provide growers with tools to make better weed management decisions. The aim of this study was to validate and compare two previously published predictive empirical thermal time models of the emergence of Abutilon theophrasti growing in maize with data sets from the USA and Europe, and test the hypothesis that a robust and general weed emergence model can be developed for this species. Previously developed Weibull and Logistic models were validated against new data sets collected from 11 site-years, using four measures of validation. Our results indicated that predictions made with the Weibull model were more reliable than those made with the Logistic model. However, Weibull model results still contained appreciable biases that prevent its use as a general model of A. theophrasti emergence. Our findings highlight the need to develop more accurate models if the ultimate goal is to make more precise predictions of weed seedling emergence globally to provide growers with universally consistent tools to make better weed management decisions.     

Flow of Botrytis cinerea inoculum between lettuce crop and soil

Botrytis cinerea causes grey mould, a disease common on many economically important crops. Although much attention is paid to the airborne inoculum of this fungus, as it sporulates abundantly in favourable conditions, knowledge on the abundance and genetic characteristics of soilborne inoculum could help improve control strategies. In this study, the soilborne inoculum of B. cinerea was quantified in two greenhouses at different times before and after the cultivation of four successive lettuce crops. Between 0 and 1177 colony‐forming units (CFU) of B. cinerea per gram of soil were recorded. There was no significant correlation between abundance of soilborne inoculum and subsequent disease incidence on lettuce (P = 0·11). Sixty‐five isolates collected from diseased plants and 66 isolates collected from the soil were investigated for their genetic diversity. The soil strains showed lower genetic diversity than the lettuce strains when considering the unbiased gene diversity within the nine microsatellite loci, the mean number of alleles per locus and the haplotypic diversity. The genetic differentiation between lettuce and soil strains decreased over three successive lettuce crops. At the same time, the genetic structure of the two groups of strains tended to become similar. These results are consistent with the hypothesis of a flow of inoculum between the lettuce crop and the soil, and vice versa. The study shows that grey mould management should pay more attention to the inoculum of B. cinerea present in the soil.     

Genetic diversity and structure of Hemileia vastatrix populations on Coffea spp.

Coffee leaf rust is the most limiting disease for coffee cultivation in Brazil. Despite its importance, relatively little is known about the genetic diversity of Hemileia vastatrix, the rust causal agent. In this work, the DNA from 112 monopustule isolates from different geographic locations and coffee genotypes were analysed by amplified fragment length polymorphisms (AFLP). The objectives were to assess the influence of the host and geographic origin on the diversity and population differentiation in H. vastatrix. The fungal population showed a low level of genotypic diversity. Gene diversity (h) was 0·027 and the hypothesis of random mating in the total population was rejected, but evidence for recombination was found for two subpopulations (São Paulo and Paraná). The analysis of molecular variance revealed that 90% of the genetic distribution of the pathogen occurs among isolates within the subpopulation (states or host of origin). There was no correlation between geographic and genetic distance (r = ?0·024, P = 0·74), which together with the high number of migrants and the low degree of differentiation in populations of H. vastatrix, is consistent with the fact that the inoculum is probably easily dispersed by wind over long distances, allowing dispersal of the pathogen among coffee growing areas in Brazil. Therefore, it is difficult to predict the durability of resistant sources to coffee rust. The recommendation for the breeding programmes is thus to incorporate multigenic resistance as a control strategy.     

Early detection of Neophysopella tropicalis in grapevine leaves and on spore traps by qPCR

Neophysopella tropicalis, one of the causal agents of Asian grapevine leaf rust (AGLR), can cause severe epidemics in Brazil that lead to yield losses in commercial vineyards. An early detection of the pathogen by air sampling of urediniospores on spore traps or in symptomless leaves would be valuable to multiple studies, such as epidemics modelling, risk forecasting, monitoring of pathogen introductions in rust-free areas, and predicting the beginning of epidemics. This study developed a quantitative PCR (qPCR) protocol to quantify N. tropicalis urediniospores attached to adhesive tapes and in grapevine leaves before symptom appearance. A specific primer pair was designed based on the internal transcribed spacer (ITS) sequence region of the AGLR pathogen. Standard amplification curves using genomic DNA from urediniospores of N. tropicalis and from urediniospores attached to adhesive tapes were established. Grapevine leaves inoculated with N. tropicalis were collected at 2, 5, and 7 days postinoculation (dpi). One primer pair (580F/720R) amplified a 140 bp product in all AGLR isolates but did not amplify products of other rust genera, such as Phakopsora, Puccinia, Hemileia, Tranzschelia, Cerotelium, and Coleosporium. As little as 0.1 pg DNA and 10 urediniospores of N. tropicalis attached to adhesive tapes could be detected. qPCR enabled the detection of the pathogen as early as 2 dpi, before symptom appearance. This method can be used to monitor N. tropicalis inoculum in grapevine-growing areas and to quantify symptomless infections of the AGLR pathogen.     

Germinate to exterminate: chemical stimulation of Spongospora subterranea resting spore germination and its potential to diminish soil inoculum

Hoagland's solution (HS), a defined nutrient supplement for plants, has been previously reported to stimulate zoospore release from resting spores of the potato pathogen Spongospora subterranea f. sp. subterranea. This study obtained direct empirical evidence for an increase in zoospore release with HS treatment, and identified Fe‐EDTA as the stimulant component of HS. Stimulation of resting spores by HS and Fe‐EDTA resulted in greater and earlier zoospore release compared to a distilled water control, and in the presence of a susceptible tomato host plant resulted in enhanced root infection. Given the labile nature of S. subterranea zoospores, it was postulated that stimulation of premature release of zoospores from the dormant resting spores in absence of susceptible hosts could reduce soil inoculum levels. In two glasshouse trials in the absence of host plants, both Fe‐EDTA and HS soil treatments reduced S. subterranea soil inoculum levels, providing proof of concept for the ‘germinate to exterminate’ approach to inoculum management.     

Spread and development of quambalaria shoot blight in spotted gum plantations

The aim of this study was to examine the disease development of quambalaria shoot blight, caused by the fungal pathogen Quambalaria pitereka, in plantation‐grown spotted gum (Corymbia citriodora subsp. citriodora, C. citriodora subsp. variegata, C. henryi and C. maculata) in south‐east Queensland, Australia. The results showed that native spotted gums are a primary source of inoculum followed rapidly by the production of secondary inoculum from infected trees in the plantation. The rate of spread and development of Q. pitereka within plantations increased exponentially over time as additional trees became infected and produced secondary inoculum. Spore concentration was shown to play an important role in disease development, with disease severity increasing with increasing disease incidence on individual trees and incidence across the plantation.     

Spatiotemporal characterization of citrus postbloom fruit drop in Brazil and its relationship to pathogen dispersal

Citrus postbloom fruit drop (PFD) is caused by Colletotrichum acutatum and C. gloeosporioides. These pathogens attack the flowers and cause premature fruit drop and the retention of fruit calyces. This study was designed to characterize the spatial and temporal dynamics of PFD in commercial citrus‐growing areas to better understand the disease spread. Experiments were carried out in three young orchards (500 trees each) in two municipalities in Sao Paulo State, Brazil. Symptoms of PFD on the flowers and presence of persistent calyces were assessed in each of three orchards for three years. Logistic, Gompertz and monomolecular models were fitted to the incidence data over time from the trees with symptoms. The spatial pattern of diseased trees was characterized by a dispersion index and by Taylor′s power law. An autologistic model was used for the spatiotemporal analysis. The logistic model provided the best fit to the disease incidence data, which had a fast progress rate of 0·53 per day. During the early epidemic of PFD, the spatial pattern of diseased trees was random, which suggested that inoculum spread was due to mechanisms other than rain splash. As the disease incidence increased (up to 12·6%), the spatial pattern of diseased trees became aggregated. The rapid rate of disease progress and the distribution of PFD suggest that dispersal of the pathogen is possibly related to a mechanism other than splash dispersal, which is more typical of other fruit diseases caused by Colletotrichum spp.     

Latent entry and spread of Colletotrichum acutatum (species complex) in strawberry fields

Anthracnose, caused by Colletotrichum acutatum (species complex), has become a troublesome problem in strawberry production worldwide. This paper reports (i) an optimized sampling method combined with a real‐time PCR technique to detect the latent presence of C. acutatum in cold‐stored strawberry plants used as planting material in several European countries, and (ii) a study of the spread of C. acutatum following a point inoculation under field conditions. Screening of different parts of planting material suggested that C. acutatum is most likely to be present on runners and old petioles. In addition, in seven out of nine batches of planting material from different nurseries, latent infection by C. acutatum was detected in at least one of five replicate samples. Field experiments in 2009 and 2010 showed extensive latent within‐field spread of the pathogen on strawberry leaves, with a within‐row dispersal distance up to at least 1·75 m in 1 week. A straw ground cover between the rows did not decrease C. acutatum spread, probably because introduced (and/or subsequent) inoculum was confined to the plant bed (within the row) and was not present between the beds. Moreover, the number of C. acutatum spores on the symptomless leaves, as estimated using a real‐time PCR method, was significantly (P < 0·05) correlated with the incidence of fruit rot at harvest and post‐harvest (r = 0·56–0·66). These results illustrate the importance of detecting latent infections in planting material and strawberry leaves in the field.     

Temporal dynamics of root and foliar severity of soybean sudden death syndrome at different inoculum densities

Temporal dynamics of soybean sudden death syndrome (SDS) root and foliar disease severity were studied in growth chamber experiments on susceptible plants exposed to different inoculum densities (0, 10⁰, 10¹, 10², and 10³ conidia g⁻¹ soil) of Fusarium virguliforme. The monomolecular model provided the best fit to describe the progress of root and foliar disease severity over time. Disease severity and area under disease progress curve (AUDPC) both increased in response to increasing inoculum density (P < 0.01), particularly for foliar symptoms. Rate of disease progress increased as inoculum densities increased for both root and foliar disease severities. The incubation period for root and foliar disease severity ranged from 9 to 18 and 15 to 25 days, respectively. Significant differences in root rot severity were most easily detected during the early stages of infection, whereas root rot and foliar severities were only weakly correlated when both were assessed simultaneously at later stages of disease development. Root rot severity assessments performed 15 to 20 days after inoculation (DAI) were most highly correlated (r > 0.9, P < 0.01) with foliar disease severity assessments performed 30 to 50 DAI. Root biomass was reduced by up to 67% at the three highest inoculum densities, indicating the aggressiveness that F. virguliforme possesses as a root rot pathogen on soybeans.