Leaf and Flower Blight Caused by Monilinia vaccinii-corymbosi on Lowbush Blueberry: Effects on Yield and Relationship to Bud Phenology

ABSTRACT Naturally established lowbush blueberry clones in four fields were evaluated for the incidence of leaf and flower blight, proportion of mummy berries, and yield reductions caused by Monilinia vacciniicorymbosi. The relationship between the phenology of flower and leaf bud development and susceptibility also was examined. Three fields were examined over one crop year and one field was studied in two subsequent crop years. The incidence of stems with blight was correlated to incidence of leaf blight in all fields and to incidence of flower blight in one field. Incidence of leaf and flower blight and the proportion of mummy berries produced were not correlated. Lowbush blueberry clones with higher incidence levels of leaf blight had reduced fruit set and lower berry weights. For healthy stems, leaf-to-fruit ratios had no effect on berry weight in most fields. In contrast, blighted stems with higher leaf-to-fruit ratios had higher berry weights in three fields. Stems with slowerdeveloping leaf and flower buds had less leaf and flower blight, respectively, than stems with faster bud development. Some blueberry clones may avoid infection by delaying production of susceptible tissue until after ascospore release by M. vaccinii-corymbosi.     

蓝莓茎溃疡病病原菌鉴定及品种抗病性研究

在浙江及福建蓝莓产区发现一种蓝莓茎部病害,病斑呈棕黄色至棕黑色,严重时引起蓝莓整株枯死。对病样进行分离得到具有致病性的36个分离株。通过对菌株的形态学研究及系统发育分析,将致病菌鉴定为Diaporthe hongkongensis,本文系国内外首次报道D.hongkongensis危害蓝莓。对代表菌株的生物学特性进行研究表明:菌落生长最适温度为25℃;最适培养基为PDA;菌落生长对光照不敏感。采用菌饼刺伤接种活体枝条的方法对10个蓝莓主栽品种进行抗病性测定,结果显示:不同蓝莓品种对该病菌表现出敏感差异,其中‘爱国者’、‘瑞卡’、‘日出’、‘奥林匹亚’、‘都克’、‘托柔’、‘蓝丰’相对抗病;‘北陆’、‘埃利奥特’、‘达柔’相对感病。     

The epidemiology of powdery mildew on concord grapes

ABSTRACT Vitis labruscana 'Concord' is a grape cultivar widely grown in the United States for processing into juice and other grape products. Concord grapes are sporadically but sometimes severely damaged by the grape powdery mildew pathogen, Uncinula necator. Although the foliage is often reported to be moderately resistant to powdery mildew, severe fruit infection occurs in some years. We observed the seasonal development of powdery mildew on leaves, rachises, and berries of unsprayed Concord grapevines. Inoculations of flower and fruit clusters revealed a brief period of berry susceptibility and a protracted period of rachis susceptibility. The rachis remained highly susceptible to infection, and the severity of rachis infection increased throughout the growing season until the rachis formed a periderm shortly before harvest. In contrast, berries were nearly immune to infection within 2 weeks after fruit set. Rachis and berry infections were detected before the disease was observed on foliage, and the incidence of rachis and berry infection often exceeded disease incidence observed on foliage until after fruit acquired substantial ontogenic resistance. Excellent control of fruit infection, and adequate control of leaf infection, was achieved by two fungicide applications targeted at the peak period of fruit susceptibility. Although Concord is thought to be moderately resistant to powdery mildew, the rachis is highly susceptible, and may be the avenue by which prebloom infections make their way onto the developing fruit. Late-season infection of the rachis neither spread to the fruit, nor did it cause fruit to drop prematurely, and may be of little economic consequence on fruit destined for processing. Although fruit of V. vinifera cultivars have been reported to remain susceptible to infection until berry sugar levels reach 8 to 15%, Concord fruit become nearly immune to infection nearly 6 weeks before this stage of development. Because powdery mildew does not become conspicuous on foliage until late summer, it is generally regarded as a late-season problem on Concord grapes, and previous management programs have reflected this belief. However, the greatest contribution to control of fruit infection is due to fungicides applied during the peak period of fruit susceptibility, from bloom until shortly after fruit set, long before the disease is observed on foliage.     

Susceptibility of grapevine buds to infection by powdery mildew Erysiphe necator

Bud colonization and perennation of powdery mildew ( Erysiphe necator ) was studied by inoculating shoots of grapevine ( Vitis vinifera cv. Carignane) at different phenological stages. Disease incidence and severity assessments indicated that buds were most susceptible at the three- to six-unfolded-leaf stage. Incidence of powdery mildew colonies on the surface of buds collected from these shoots 7 weeks postinoculation was highest at these stages (68 and 62%, respectively), which indicates that colonization of the bud interior via the infected bud surface is likely to occur within this period. Histological analyses of buds revealed hyphae with haustoria, conidiophores and conidia on all parts of the bud interior except for the meristems. In particular, trichomes were frequently parasitized by haustoria. In total, 13·2% of all buds analysed, and 32·3% of all buds originating from shoots inoculated at the three-unfolded-leaf stage, were infected by E. necator . In the spring of the following year, buds from inoculated shoots yielded 18 flag shoots (1·6% of all emerging shoots). These primary infections caused an epidemic 28 days after the appearance of the first flag shoot. A linear regression analysis on the frequency of infections of the bud exterior, bud interior and flag shoots revealed that incidence of external bud infection in the first season is strongly correlated with flag shoot incidence in the following season ( R ² = 0·94). Hence predictions of flag shoot incidence may be reliably based on the incidence of infection on the outer bud scales in the preceding season.     

Environmental and host requirements for field infection of blueberry fruits by Colletotrichum acutatum in British Columbia

Higher recovery of Colletotrichum acutatum , the causal agent of anthracnose (ripe-rot), from blueberry tissues during the growing seasons of 2002 and 2003 was found at bloom and ripe berry than at other stages of plant development. The effects of leaf-wetness duration and ambient temperature on fruit infection frequency were determined during the growing seasons of 2001–03. Potted 2-year-old blueberry plants were exposed for 1-week periods to prevailing environmental conditions and natural inoculum in a commercial field, and grown to harvest, when fruit infection was assessed. Three peaks of infection were observed: early during bloom, mid-season during the mature green berry stage, and later in the season when berries had ripened. Weather data collected simultaneously indicated that a minimum of 10 h of leaf wetness at 11°C was sufficient for fruit infection. These conditions preceded each peak of infection. To determine whether peaks of infection in the field were also caused by changes in host susceptibility or available inoculum, groups of potted blueberry plants were artificially inoculated at weekly intervals during the growing season of 2004, exposed to prevailing environmental conditions, and fruit infection assessed at harvest. Flowers and developing fruits were found to be susceptible throughout the season, indicating that specific peaks of infection were associated with environmental conditions and availability of inoculum.     

Phenology of Apothecium Production in Populations of Monilinia vaccinii-corymbosi from Early- and Late-Maturing Blueberry Cultivars

ABSTRACT Pseudosclerotia were evaluated for differences in timing of apothecium development in four controlled experiments conducted over a 2-year period. In a separate experiment, conidia from 10 randomly selected isolates from both of the fungal populations were used to inoculate open flowers. Germination of pseudosclerotia produced from these artificial inoculations also was evaluated. The timing and rate of shoot elongation for cvs. Weymouth and Jersey were assessed in one greenhouse and two field experiments. Average development times for the fungal population from cv. Weymouth were 8 to 15 days earlier or 33 to 42% less than those for the population from cv. Jersey. The fungal population from Weymouth also exhibited less variation in development times for each developmental stage measured. Similarly, germination of pseudosclerotia produced in artificial inoculations differed between populations. On average, pseudosclerotia derived from the Weymouth population produced apothecia 16 days earlier. During spring 1995 and 1996, vegetative and truss buds on cv. Weymouth developed 4 to 16 days earlier than those on cv. Jersey. These results demonstrate that M. vaccinii-corymbosi exhibits variation in timing of pseudosclerotia germination and apothecium development within and between populations. We hypothesize that differences observed in the timing of apothecium development are related to the fitness of the populations on their original host cultivars and were selected by host phenology.     

Fusarium mangiferae localization in planta during initiation and development of mango malformation disease

Mango malformation disease (MMD), caused by Fusarium mangiferae, is a major constraint to mango production, causing significant yield reduction resulting in severe economic impact. The present study characterizes fungal localization in planta during initiation and development of vegetative and floral malformation. Young mango trees were artificially inoculated with a green fluorescent protein (GFP)‐expressing strain of F. mangiferae. Shoots and buds were sampled periodically over a period of more than a year and localization of the GFP‐expressing fungi was determined using confocal microscopy. Fungal localization appears to be epiphytic: mycelia remained in close contact with the plant surface but did not penetrate the tissue. In vegetative malformation and in young inflorescences, the fungus was confined to protected regions between scales, young leaf bases and buds. Fungal colonization was only very rarely detected on open leaves or on exposed shoot sections. In developed flowers, mycelia were localized mainly to protected regions at the base of the flower organs. Upon development of the inner flower organs, specific mycelial growth occurred around the anthers and the style. Mycelial penetration through the stylar tract into aborting carpels was observed. For several months, mycelia were confined to the surface of the organs and were not detected within plant tissues. Only at later stages, transient saprophytic growth of the fungus was detected causing the malformed inflorescences to senesce and collapse, concurrent with dispersion of conidia. Implications of the present study on MMD in natural field infections are discussed.     

Flavescence dorée phytoplasma titre in field‐infected Barbera and Nebbiolo grapevines

Flavescence dorée phytoplasma (FDP) titre in two red grapevine cultivars, Barbera and Nebbiolo, was measured over the vegetative seasons of two consecutive years in two vineyards of the Piemonte Region (northwestern Italy), with a double absolute quantification of FDP cells and grapevine DNA in real‐time PCR. The relationships of pathogen concentration to cultivar susceptibility and symptom severity were investigated. FDP titre was always higher in cv. Barbera than in cv. Nebbiolo infected vines, and this difference was significant at early and late summer samplings of 2008 and at early summer sampling of 2009. A seasonal trend in FDP concentration (low in spring, high in early summer and intermediate in late summer) was conserved for cvs Barbera and Nebbiolo in both years and vineyards. Considering both cultivars and years from both vineyards, a significant positive correlation between FDP concentration and symptom severity was found in the spring samples. Regarding the FDP strains (‐C or ‐D), no differences in pathogen titres were detected for either cultivar. Similarly, the presence of another grapevine yellows phytoplasma, bois noir, a subgroup 16SrXII‐A phytoplasma, in mixed infection with FDP strains had no effect on FDP concentration. These results demonstrate for the first time that grapevine cultivars with different susceptibility to FDP support different pathogen titres.     

Resistance of sunflower (Helianthus annuus L.) to terminal bud attack bySclerotinia sclerotiorum (Lib.) de Bary

Resistance of sunflowers to terminal bud attack bySclerotinia sclerotiorum was studied by microscopical observations of infection processes and by genetical analyses of trials showing natural infections. Electron microscope studies showed that there were no differences in leaf morphology between susceptible and resistant genotypes, and that both were contaminated by ascospores. Only on the susceptible genotype was considerable ascospore germination observed, followed by mycelial development and leaf penetration. On the resistant genotype, there was little ascosopore germination and no further development. The genetical studies of percentage natural attack observed on eight inbred lines representing a range of reactions, and the hybrids from a factorial cross of these lines, indicated that inheritance is mainly additive, with few interactions.     

Novel infection strategies of Colletotrichum acutatum on ripe blueberry fruit

The infection and colonization process of Colletotrichum acutatum on ripe blueberry fruit from two cultivars with different susceptibility to anthracnose were examined using light and confocal laser scanning microscopy. Ripe fruit from susceptible cv. Jersey and resistant cv. Elliott were drop-inoculated with a conidial suspension of C. acutatum, and epidermal peels were evaluated at selected times after inoculation and incubation. Results from pre-penetration studies demonstrated that there were significant differences in the rate of formation of melanized appressoria between the two cultivars, with the rate of formation being faster in the susceptible one. In both cultivars, penetration by the pathogen occurred via appressoria 48 h post-inoculation (hpi). However, in the susceptible cv. Jersey, C. acutatum then adopted an intracellular hemibiotrophic-like infection strategy, whereas in the resistant cv. Elliott subcuticular intramural-like infection occurred. In cv. Jersey by 108 hpi, intracellular growth of the pathogen led to the formation of numerous acervuli, with orange conidial masses. By 120 hpi, the conidial masses had coalesced covering the entire inoculated area. In cv. Elliott, acervuli were not seen until 144 hpi and contained few conidia. These results demonstrate for the first time the ability of C. acutatum to adopt a different infection and colonization strategy depending on the susceptibility of the host tissue being colonized.     

Novel metrics to classify fire blight resistance of 94 apple cultivars

Fire blight (Erwinia amylovora), a potentially devastating disease in apple, can cause floral, fruit and structural damage and even tree death. Most commercial apple cultivars are susceptible and the resistance/susceptibility of many modern cultivars has not been evaluated. Fire blight resistance/susceptibility is difficult to phenotype due to quantitative resistance, impacts of tree vigour and environment on susceptibility, and the erratic nature of the disease. Resistance/susceptibility levels were determined for 94 apple cultivars and important breeding parents. In 2016 and 2017, multiple actively growing shoots per tree (about three trees per cultivar) were challenged with E. amylovora Ea153n via a cut-leaf inoculation method. Proportion of current season's shoot length blighted (SLB) was calculated for each shoot. To classify cultivar responses, estimated marginal SLB means were compared to four controls, representing highly susceptible (HS) to highly resistant (HR), via Dunnett's tests. Cultivar responses ranged from HS to HR with estimated marginal SLB means of 0.001–0.995 in 2016 and 0.000–0.885 in 2017. Most cultivars demonstrated similar resistance/susceptibility levels in both years (ρ = 0.657, P < 0.0001). K-means clustering was used to classify cultivars into three resistance/susceptibility groups based on incidence, average severity (SLB), and maximum severity values (maximum SLB and age of wood infected). Sixteen cultivars were consistently moderately resistant (MR) to HR while the remainder ranged from HS to MR. An updated comparison of susceptibility of important cultivars is provided. Resistance/susceptibility information gained could be used to identify genetic loci associated with resistance/susceptibility and/or inform parental selection in apple scion breeding programmes.     

First report of blueberry red ringspot disease caused by <Emphasis Type="Italic">Blueberry</Emphasis><Emphasis Type="Italic">red</Emphasis><Emphasis Type="Italic">ringspot</Emphasis><Emphasis Type="Italic">virus</Emphasis> in Japan

Virus-like symptoms—red ringspots on stems and leaves, circular blotches or pale spots on fruit—were found on commercial highbush blueberry (Vaccinium corymbosum) cultivars Blueray, Weymouth, Duke and Sierra in Japan. In PCR testing, single DNA fragments were amplified from total nucleic acid samples of the diseased blueberry bushes using primers specific to Blueberry red ringspot virus (BRRV). Sequencing analysis of the amplified products revealed 95.7–97.7% nucleotide sequence identity with the BRRV genome. This paper is the first report of blueberry red ringspot disease caused by BRRV in Japan. The nucleotide sequence data reported in this paper are available in the GenBank/EMBL/DDBJ database as accessions AB469884 to AB469893 for BRRV isolates from Japan.     

A mechanistic model simulating ascosporic infections by Erysiphe necator,the powdery mildew fungus of grapevine

A new dynamic model for Erysiphe necator ascosporic infections on grapevine was developed. Between budbreak of vines and the time when the pool of ascospores is depleted, the model uses weather data for calculating, at daily intervals: curve of ascospore maturation; ascospore discharge events and relative proportion of the discharged ascospores; infection periods and their relative infection severity; and progress of latency period and time when secondary infections should begin. The model was validated over a 4‐year period (2005–2008) in 26 vineyards in Italy by comparing model predictions with actual observations of the first seasonal symptoms of powdery mildew. The model showed high sensitivity, specificity and accuracy. Proportions of true and false positive predictions were TPP = 0·94 and FPP = 0·26, respectively. Because a proportion of predicted infection periods did not result in actual disease onset, confidence was higher for prediction of non‐infections than for prediction of infections. Most of the false positive predictions occurred in the earlier growth stages of the host, when the surface area of susceptible tissue may be very small so that the probability that ejected ascospores land on susceptible tissue is low. An equation was then developed to describe the probability that a predicted infection period results in disease onset as a function of the growth stage of vines at the time of prediction. The new model should improve early season powdery mildew management by helping vineyard managers schedule fungicide sprays or schedule the scouting of the vineyard for detection of first disease signs.     

Laboratory Evaluation of Citrus Cultivars Susceptibility and Influence of Fruit Size on Fortune Mandarin to Infection by Alternaria alternata pv. citri

Young leaves of 62 citrus cultivars were inoculated with conidia of three Spanish isolates of Alternaria alternata pv. citri, the causal agent of brown spot of citrus. Hybrids with Dancy mandarin, King mandarin or their derivates as a parent, grapefruit cultivars and the mandarin cultivars Guillermina, Emperor, Clemenpons and Esbal were highly susceptible to the pathogen. Satsuma cultivar Clausellina and orange cultivars, with the exception of Sanguinelli, were slightly susceptible. Lemon and lime cultivars were not susceptible, with the exception of Mexican lime (Citrus aurantifolia), which was slightly susceptible. Although this study shows a range of potential hosts for this pathogen, to date the only affected cultivars in Spain are Fortune and Nova mandarins, and Minneola tangelo. The susceptibility of Fortune fruits decreased as diameter increased, being susceptible through the whole season. This was confirmed with field observations in autumn where fruit infections have been detected when the diameter reaches 6–7 cm.     

Identification of some Fruit Characteristics in Wild Bilberry (<Emphasis Type="Italic">Vaccinium myrtillus</Emphasis> L.) Accessions from Eastern Anatolia

Some important physicochemical and bioactive characteristics of disease free 10 wild grown bilberry (Vaccinium myrtillus) accessions were evaluated. External and internal fruit quality was assessed by standard parameters (fruit weight, fruit color, fruit firmness, soluble solids, pH and total acidity) and bioactive contents (total phenolics, total anthocyanins, total antioxidant capacity and, vitamin C) in fruit were also determined. The commercial grown northern higbush blueberry, Vaccinium corymbosum cv. Bluecrop also included in the study to make comparision with bilberry samples. The highbush blueberry cv. Bluecrop had distinctive external fruit characteristics, such as bigger and more attractive fruits. However, the wild grown bilberry accessions showed interesting characters in mesocarp, such as high total phenolic content, total anthocyanin and total antioxidant capacity. Total phenolic and total anthocyanin content was 327?mg gallic acid equivalent and 142?mg of cyanidin-3-glucoside equivalent in 100?g fresh fruit in cv. Bluecrop while it was between 576–624?mg gallic acid equaivalent and 296–324?mg of cyanidin-3-glucoside equivalent in 100?g fresh fruits of bilberry accessions. Moreover, wild accessions approximately had 2 folds higher antioxidant capacity than cv. Bluecrop. Results suggested that bilberry accessions may serve as a source of desirable genes to develop improved varieties that respond to the new needs of the market.     

Effect of Wound Position, Auxin, and Agrobacterium vitis Strain F2/5 on Wound Healing and Crown Gall in Grapevine

ABSTRACT Agrobacterium vitis is the causal agent of crown gall disease in grapevine, which can be severe in many regions worldwide. Vitis vinifera cultivars are highly susceptible to freeze injury, providing the wounds necessary for infection by A. vitis. Wound position in relation to the uppermost bud of cuttings was determined to be important in tumor development. Inoculated wounds below buds developed tumors, whereas wounds opposite the bud did not, implying that indole-3-aectic acid flow contributes to tumor formation. If auxin was applied to wounds prior to inoculation with a tumorigenic A. vitis strain, all sites of inoculation developed tumors, accompanied by an increased amount of callus in the cambium. Wounds inoculated with an A. vitis biological control strain F2/5 prior to application of the pathogen did not develop galls. A closer examination of these wounds determined that callus cells formed in the cambium during wound healing are susceptible to transformation by the pathogen. Although the mechanism by which F2/5 prevents transformation is unknown, our observations suggest that F2/5 inhibits normal wound healing by inducing necrosis in the cambium.     

Susceptibility of European bread and durum wheat cultivars to Tilletia indica

Representative European wheat cultivars were tested under quarantine containment for their susceptibility to Tilletia indica, the cause of Karnal bunt of wheat. Fifteen winter and 15 spring wheat ( Triticum aestivum ) and 11 durum wheat ( Triticum durum ) cultivars were inoculated by boot injection just prior to ear emergence to test their physiological susceptibility. Selected cultivars were then re-tested by spray inoculation after ear emergence to determine their morphological susceptibility, which is a better predictor of field susceptibility. At maturity, the ears and seeds were assessed for incidence and severity of disease. For the physiological susceptibility tests, 13/15 winter wheat cultivars were infected and the percentage of infected seeds ranged from 1 to 32%. For spring cultivars, 13/15 cultivars were infected and the percentage of infected seeds ranged from 1 to 48%. For the durum cultivars, 9/11 were infected and the percentage of infected seeds ranged from 2 to 95%. Across all cultivars, 35/41 were infected. Based on historical Karnal bunt susceptibility categories using coefficients of infection, one cultivar was classed as highly susceptible, three as susceptible, 11 as moderately susceptible, 20 as resistant and only six as highly resistant. The spray-inoculation morphological susceptibility tests broadly confirmed the physiological susceptibility results, although lower levels of infection were observed. Overall, the range of susceptibility was similar to that found in cultivars grown in Karnal bunt affected countries. The results demonstrate that European wheat cultivars are susceptible to T. indica and thus could potentially support the establishment of T. indica if introduced into Europe.     

Phenotypic selection for seed dormancy in white snakeroot (Eupatorium rugosum)

Eupatorium rugosum (Ageratina altissima), commonly known as white snakeroot, is a weedy plant that invades woodland areas in North America, Korea, and Japan. In order to examine the inheritance of seed dormancy in this species, seeds from a single population were screened for their differential germination response to stratification. After two cycles of recurrent selection, the seed from the shallow–dormant lines had 4.4 times greater germination prior to stratification than did the seed from the deep–dormant lines. The seed from the deep–dormant lines showed 3.4 times greater germination after stratification, compared to the seed from the shallow–dormant selections. This suggests that primary dormancy in the seed of white snakeroot is under some degree of genetic control. This perennial species produces overwintering rhizomes that give rise to adventitious, vegetative buds each spring. The plants selected for the production of seeds with lower levels of dormancy in the fall were observed to generate rhizomatous buds that were released from dormancy earlier in spring, compared to the plants that produced seeds with higher levels of dormancy. A statistically significant positive correlation also was observed between seed and bud dormancy in a naturally occurring population of white snakeroot. Common regulatory elements might be influencing dormancy in both the seeds and vegetative buds of this species.     

Combining sanitation and disease modelling for control of grapevine powdery mildew

Chasmothecia of Erysiphe necator form in one season, survive winter and discharge ascospores that cause primary infections and trigger powdery mildew epidemics in the next season. A strategy for powdery mildew control was developed based on (i) the reduction in overwintering chasmothecia and on (ii) spring fungicide applications to control ascosporic infections timed based on estimate risk (two to five sprays per season). Several fungicides, the hyperparasite Ampelomyces quisqualis, and a mineral oil product were first tested as separate applications in a greenhouse and in vineyards. In the greenhouse, A. quisqualis suppressed chasmothecia formation by 41 %; fungicides and mineral oil suppressed chasmothecia formation by 63 % and ascospore viability by 71 %. In vineyards, application of boscalid + kresoxim-methyl or meptyldinocap once after harvest, as well as application of A. quisqualis pre- and post-harvest, delayed disease onset and epidemic development in the following season by 1 to 3 weeks and lowered disease severity (up to the pea-sized berry stage) by 56 to 63 %. Risk-based applications of sulphur and of synthetic fungicides provided the same control as the grower spray program but required fewer applications (average reduction of 47 %). Sanitation strategies were then tested by combining products and application times (late-season, and/or pre-bud break, and/or spring). Adequate disease control with a reduced number of sprays was achieved with the following combination: two applications of A. quisqualis (pre- and post-harvest), one application of mineral oil before bud break, and model-based applications of sulphur fungicides between bud break and fruit set.