Crop residue destruction strategies that enhance rather than inhibit reproduction of Monosporascus cannonballus

Destruction of infected crop residue is a management strategy commonly recommended for the control of many plant diseases. Based upon knowledge of the biology of the root-infecting fungus Monosporascus cannonballus , this strategy is a likely candidate for use in the management of vine decline of melons caused by this pathogen. Specifically, ascospores, the primary survival structure and inoculum for root infection, are produced on infected melon roots primarily after crop termination. Thus, destruction of infected roots prior to reproduction would be a very practical method of preventing inoculum build-up in soil. Results from this study demonstrated that two plant residue destruction methods commonly used by growers at crop termination [i.e. foliar application of a herbicide (glyphosate) and mechanical destruction of vines] significantly enhanced, relative to untreated controls, the rapidity and extent of pathogen reproduction on infected roots left in field after harvest.     

A simple model of pea (Pisum sativum) growth affected by Mycosphaerella pinodes

The effect of epidemics of Mycosphaerella pinodes on crop growth, radiation interception efficiency (RIE) and radiation use efficiency (RUE) was studied in field conditions and the growth of diseased crop was modelled. Natural epidemics were simulated in field plots by dispersion on the ground of barley grains colonized by the fungus. Growth and leaf area index (LAI) were measured in healthy and diseased plots during the growth season. Epidemics affected crop growth mainly by decreasing RUE, with a slight decrease in RIE. This was probably due to the strong effect of M. pinodes on leaf photosynthesis and the lateness of Mycosphaerella blight epidemics on spring pea, which became serious when the canopy was already formed, thus decreasing RIE only by accelerating the senescence of leaves. The data were used to evaluate the performance of a crop growth model for diseased pea crops. The model comprised the decrease in photosynthesis rate in the leaves, the vertical gradient of disease intensity and the differences in photosynthetic function of the various layers of the canopy. This model, validated over 2 years in the field, accurately simulated the crop growth in diseased plots. Thus the decrease in RUE may be accounted for solely by photosynthesis losses in diseased leaves. This simple model may be used for disease management, defining damage thresholds for chemical application and criteria for tolerant variety selection.     

Effects of host resistance, temperature, leaf wetness, and leaf age on infection and lesion development of pecan scab

ABSTRACT The effects of partial host resistance, temperature, leaf wetness duration, and leaf age on infection and lesion development of pecan scab were evaluated. Trees of cultivars Wichita (susceptible) and Sumner (resistant) were inoculated with conidia of Cladosporium caryigenum and placed in mist chambers set at 15, 25, or 35 degrees C. The trees were removed from the chambers after 3, 6, 12, 24, 36, or 48 h of leaf wetness and placed in a greenhouse to allow disease development. After 8 to 16 days, disease began to develop on both 'Wichita' and 'Sumner'. Logistic regression analysis showed that the probability of a leaf becoming infected was greatest for 'Wichita' it decreased with increasing leaf age and temperature and increased with increasing leaf wetness. Leaves on 'Wichita ' were susceptible to infection between 2 and 23 days after budbreak, while leaves on 'Sumner' were susceptible to infection from 2 to 18 days after budbreak. Infection frequency, lesion size, and conidia production decreased proportionately with increasing leaf age. The magnitude of this effect was greatest on 'Sumner'. Conidia production was positively correlated with lesion size, and both were positively correlated with infection frequency on both cultivars.     

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.     

Application of fuzzy logic for the simulation of Plasmopara viticola using agrometeorological variables*

A mechanistic model called PLASMO was developed earlier to simulate grapevine downy mildew (Plasmopara viticola) and has been applied in several viticultural areas of Italy since 1988 by the collaboration of several research institutions of Firenze. In this study, a new simulation model based on fuzzy logic has been developed for the same structure (biological cycle of P. viticola). This approach allows classical quantitative information to be used together with qualitative information. Vague concepts can also be handled. Agrometeorological data is used, with an hourly time step, starting from budbreak to the end of the growing season. Air temperature, relative humidity, rainfall and leaf wetness are required. The simulated processes are the growth of grapevine leaf area and the main phases of the biological cycle of the pathogen: incubation, sporulation, germination, spore survival and inoculation. The main epidemiological outputs are timing of infection events and disease intensity. The performance of the model is evaluated and the mechanistic and fuzzy logic approaches are compared.     

小麦由水分胁迫诱导的抗条锈性

 以一批在条锈病流行条件下具有保产能力的"耐病"品种和典型感病的小麦品种为试材,研究了病株水分关系的变化。结果发现,这些"耐病"品种具有水分胁迫所诱导的抗病性。在正常水分条件下呈亲和反应,但在水分胁迫时逆转为不亲和反应。病叶蒸腾作用在初期有轻微升高,其后病叶蒸腾速率、叶片扩散阻力、相对含水量和水势渐趋健叶水平,并具有较健叶更低的渗透势和更高的压力势,保持了有效的水分调控能力。而感病品种在水分胁迫时,病叶蒸腾速率急剧升高,叶片扩散阻力、相对含水量、水势、压力势和渗透势大幅降低,完全丧失了控制水分散失和维持水分平衡的能力。     

水稻叶尖枯病初侵染和再侵染研究

 本研究首次证明水稻叶尖枯病(Phyllosticta oryzicola Hara)的初侵染来源与病残体、病稻种和带菌野生杂草有关。在老病区,病残体特别是落在田中的病叶是主要的初侵染来源。病区稻种带菌率一般为0.5~2.5%,带菌部位几乎都是颖壳。田间无芒稗、西来稗、双穗雀稗、狗尾草、李氏禾、千金子、牛筋草、虎尾草、白茅、菰、马唐和芦竹等10多种禾本科杂草均可是病菌的野生寄主。试验还表明,田间水稻叶尖枯病能够进行再侵染。     

Position of inoculum in the canopy affects the risk of septoria tritici blotch epidemics in winter wheat

The spread of septoria leaf blotch in wheat caused by Mycosphaerella graminicola is widely reported to depend on the occurrence of splashy rainfall. Previous studies have also implicated an important effect of canopy architecture on the risk of disease spread. This is because architecture affects the proximity of the yield-forming leaves to inoculum present on older diseased leaves within the crop. This study demonstrated that infection of the final three leaves of winter wheat could occur in the absence of splashy rainfall. For cvs Riband and Longbow, the final two leaf layers emerged at a position ≈ 8 cm below older leaves containing sporulating lesions. Under these conditions, infection of new leaves occurred in treatments that simulated dew and nonsplashy rainfall. These treatments resulted in disease incidences of 10–40% above the untreated control on the final two leaf layers. Within a season, the distance between diseased and healthy leaves during the period of stem extension varied substantially across a range of 30 cultivars. While the magnitude of these differences was not the same across seasons, the relative differences between cultivars were generally consistent, suggesting a strong genotype influence on lesion proximity. This study shows how knowledge of the distribution of lesion proximity within a crop can be used to estimate the risk of inoculum transfer for a given maximum splash height. A rapid crop-monitoring method for estimating the distribution of lesion proximity was developed and tested. Lesion proximity was manipulated by plant growth regulator (PGR) treatments. Significant increases in disease, between 14 and 62%, were observed on the upper canopy leaves of plants treated with PGR. The largest differences were observed in treatments where lesion proximity was most affected.     

苹果炭疽叶枯病病原Glomerella cingulata及其侵染过程

为明确苹果炭疽叶枯病病原菌围小丛壳Glomerella cingulata的侵染致病特征,在分离获得该病原菌的基础上,采用形态学观察、ITS序列分析和致病性测定对其进行了鉴定,并利用光学和扫描电子显微镜对病原菌在嘎啦苹果叶片上的侵染过程进行了研究.结果表明,在陕西咸阳地区分离获得的9株病原菌均为围小丛壳G.cingulata.25 ℃下接种9 h后,分生孢子中间产生隔膜,双胞化,并萌发产生芽管和附着胞;24 h后分生孢子的2个细胞均可萌发并形成芽管及附着胞,部分芽管顶端可产生次级分生孢子;48 h后次级分生孢子萌发形成附着胞;72 h后,附着胞下形成的侵染钉可直接入侵寄主,在表皮细胞内形成初生菌丝和次生菌丝,此时叶片表面已出现褐色斑点.接种7 d后叶片病斑处出现分生孢子盘和子囊壳.表明陕西省近年出现的苹果炭疽叶枯病病原菌为围小丛壳G.cingulata,该病菌在嘎啦叶片上的一些特殊侵染行为可能是导致该病害易在短时间内暴发的重要原因.     

Epidemiological analysis of the effects of biofumigation for biological control of root rot in sugar beet

The effects of biofumigation using a Brassica juncea (mustard) cover crop on the dynamics of rhizoctonia root rot of sugar beet were recorded in two field trials in 2007 and 2008, and analysed using epidemiological modelling. Differences between partial biofumigation, involving the pulling up of mustard plants, and complete biofumigation, involving the crushing and incorporation of mustard residues into the soil, were compared with bare soil treatment. An epidemiological model was used that includes rates of transmission of primary and secondary infection, pre‐emergence damping off, and expression of wilting symptoms (above‐ground disease) due to infected roots (below‐ground disease). The model indicated that biofumigation reduces the transmission of primary infections but affects secondary infections in a variable pattern between field trials. Likewise, the proportion of infected plants expressing wilting was significantly reduced, by 28%, in the partial and complete biofumigation treatments compared with bare soil in the trial of 2007 but not in 2008. It is suggested that the effects of biofumigation on secondary infection and the expression of disease are more variable than those on primary infection, and that this is probably due to an interplay between pathogen, antagonists, host, and environmental factors. These interactions may or may not offset the benefits afforded by a reduction in primary infection and account for the overall variable success of biofumigation to control disease.     

The influence of silicon on components of resistance to blast in susceptible, partially resistant, and resistant cultivars of rice

ABSTRACT The application of silicon (Si) fertilizers reduces the severity of blast, caused by Magnaporthe grisea, in irrigated and upland rice; however, little research has been conducted to examine the epidemiological and etiological components of this reduction. Four cultivars of rice with differential susceptibilities to race IB-49 of M. grisea were fertilized with three rates of a calcium silicate fertilizer and inoculated with the pathogen to test the effects of Si on the following components of resistance to blast: incubation period, latent period, infection efficiency, lesion size, rate of lesion expansion, sporulation per lesion, and diseased leaf area. For each cultivar, the incubation period was lengthened by increased rates of Si, and the numbers of sporulating lesions, lesion size, rate of lesion expansion, diseased leaf area, and number of spores per lesion were reduced. Lesion size and sporulation per lesion were lowered by 30 to 45%, and the number of sporulating lesions per leaf and diseased leaf area were significantly reduced at the highest rate of Si. The net effect of Si on these components of resistance is an overall reduction in the production of conidia on plants infected with M. grisea, thereby slowing the epidemic rate of blast.     

Apical leaf necrosis and leaf nitrogen dynamics in diseased leaves: a model study

Apical leaf necrosis is a physiological process related to nitrogen (N) dynamics in the leaf. Pathogens use leaf nutrients and can thus accelerate this physiological apical necrosis. This process differs from necrosis occurring around pathogen lesions (lesion-induced necrosis), which is a direct result of the interaction between pathogen hyphae and leaf cells. This paper primarily concentrates on apical necrosis, only incorporating lesion-induced necrosis by necessity. The relationship between pathogen dynamics and physiological apical leaf necrosis is modelled through leaf nitrogen dynamics. The specific case of Puccinia triticina infections on Triticum aestivum flag leaves is studied. In the model, conversion of indirectly available N in the form of, for example, leaf cell proteins ( N ₂( t )) into directly available N ( N ₁( t ), i.e. the form of N that can directly be used by either pathogen or plant sinks) results in apical necrosis. The model reproduces observed trends of disease severity, apical necrosis and green leaf area (GLA) and leaf N dynamics of uninfected and infected leaves. Decreasing the initial amount of directly available N results in earlier necrosis onset and longer necrosis duration. Decreasing the initial amount of indirectly available N, has no effect on necrosis onset and shortens necrosis duration. The model could be used to develop hypotheses on how the disease-GLA relation affects yield loss, which can be tested experimentally. Upon incorporation into crop simulation models, the model might provide a tool to more accurately estimate crop yield and effects of disease management strategies in crops sensitive to fungal pathogens.     

广东黄秋葵黄脉曲叶病样中检测到烟粉虱传双生病毒

黄秋葵是近几年来从日本和我国台湾引进的一种蔬菜作物。近期,广东的黄秋葵上发生了黄脉曲叶病。病株的典型症状表现为叶脉黄化,在叶片正面形成网络状,在叶背面叶脉肿大突起明显,病株幼叶小且向下卷曲,甚至整片幼叶黄化。植株早期被感染表现矮化。在发生黄脉曲叶病的黄秋葵田间,其病株率高达60%以上。用烟粉虱传双生病毒简并引物对随机采集的病样进行PCR检测,从这些病样中均能扩增出1条预期大小为570 bp的特异片段;基因克隆及测序分析结果表明,与该特异片段同源的均属双生病毒科菜豆金色花叶病毒属病毒DNA,其中与木尔坦棉花曲叶病毒（Cotton leaf curl Multan virus, CLCuMV）分离物G6相似性最高,为99%。这些研究结果表明,广东黄秋葵黄脉曲叶病中存在烟粉虱传双生病毒,该病害可能也是由CLCuMV侵染引起的。     

白芷黑斑病病原菌鉴定及病害发生规律研究

对四川白芷黑斑病的症状、病原菌致病性和形态特征等进行了研究,确定病原菌为当归叶点霉(PhyllostictaangelicaeSaccardo)。经两年的田间病害发生动态调查,该病在四川始发期为3月上旬,高峰期为6月份,病株率达100%,病叶率达74.51%。病害的初侵染源主要是留种株的病叶和田间病残体。病菌分生孢子借雨滴飞溅的方式传播,并在白芷生长期中不断引起再侵染。     

Opportunities and problems of control of foliar pathogens with micro-organisms

Three different strategies in biological control of diseases of above-ground plant pans are discussed. (1) Microbial suppression of infection. Undisturbed naturally occurring yeast populations will create unfavourable nutritional conditions for leaf infection by necrotrophic pathogens. Application of biological control agents, operating through nutrient competition, to healthy leaves, however, is generally not very effective, because the pathogen rapidly penetrates the leaf and escapes competition. In contrast, microbial protection of man-made wounds on fruits looks promising. Field applications of bacterial preparations or aqueous compost extract to leaves reduce disease by necrotrophs as well as biotrophs. The mechanisms are not clear, but involvement of induced resistance may make the establishment of the biological control agents in the phyllosphere less important. (2) Microbial suppression of pathogen sporulation. Suppression of the dissemination of the pathogen is, in principle, effective against diseases with many infection cycles per season. This approach allows a long interaction period between the antagonist and the pathogen. Research on the use of mycoparasites for the control of mildews and rusts is well known in this respect. Promising, and relatively new. is this approach in relation to necrotrophic pathogens sporulating on dead plant material. (3) Microbial suppression of pathogen survival. Mycoparasites may also interfere with the formation and vitality of sclerotia in infected above-ground plant tissue and crop remains. Depending on the importance of initial inoculum, originating from sclerotia, treatment of infected plant material may reduce the severity of the disease in the following season.     

Coincidence of maximum severity of powdery mildew on grape leaves and the carbohydrate sink‐to‐source transition

Grapevine leaves infected with powdery mildew are a source of inoculum for fruit infection. Leaves emerging on a single primary shoot of Vitis vinifera cv. Cabernet Sauvignon were exposed to average glasshouse temperatures of 18°C (0·23 leaves emerging/day) or 25°C (0·54 leaves emerging/day). All leaves on 8–10 shoots with approximately 20 leaves each were inoculated with Erysiphe necator conidia to assess disease severity after 14 days in the 25°C glasshouse. Two photosynthetic ‘source’ leaves per shoot on the remaining 8–10 shoots were treated with ¹⁴CO₂ to identify, by autoradiography, the leaf position completing the carbohydrate sink‐to‐source transition. There was a clear association between the mean modal leaf position for maximum severity of powdery mildew (position 3·7 for 18°C; position 4·4 for 25°C) and the mean position of the leaf completing the sink‐to‐source transition (position 3·8 for 18°C; position 4·7 for 25°C). The mean modal leaf position for the maximum percentage of conidia germinating to form secondary hyphae was 4·2 for additional plants grown in the 25°C glasshouse. A higher rate of leaf emergence resulted in a greater proportion of diseased leaves per shoot. A Bayesian model, consisting of component models for disease severity and leaf ontogenic resistance, had parameters representing the rate and magnitude of pathogen colonization that differed for shoots developing in different preinoculation environments. The results support the hypothesis that the population of leaves in a vineyard capable of supporting substantial pathogen colonization will vary according to conditions for shoot development.     

利用小种标记分析云南省两个县的小麦条锈菌群体

 小麦条锈病是由小麦条锈菌（Puccinia striiformis f.sp. tritici）引起的气流传播病害,在世界各主要麦区均有发生,也是我国小麦生产上危害最严重的病害之一。小麦条锈菌毒性变异频繁,易产生新毒性小种,导致小麦品种的抗病基因失效,引起小麦条锈病周期性流行。因此,了解小麦种植区的条锈菌群体结构组成及变异,对于制定更为有效的条锈病控制方法具有重要的意义。     

Aerial Contamination of Sugarcane in Guadeloupe by Two Strains of Xanthomonas albilineans

Two sugarcane plots were set up in Guadeloupe with disease-free tissue cultured plants in a banana growing location distant from sugarcane fields. Thirteen weeks after planting sugarcane in the field, a Xanthomonas albilineans strain belonging to serotype 3 (strain XaS3) was detected in water sampled at sunrise on the leaves in the first plot. This strain randomly invaded the sugarcane canopy. Seven weeks later, a new strain belonging to serotype 1 (strain XaS1) appeared on leaves and populations of strain XaS1 progressively increased on the leaf surface, whereas populations of strain XaS3 progressively decreased. Leaf scald symptoms were first noted 26 weeks after sugarcane planting. However, only strain XaS1 was isolated from leaves and a few sugarcane stalks showing symptoms. Both strains also colonized the second field plot, which was studied at the end of the experiment to avoid human interference of aerial contamination of sugarcane. After inoculation of three sugarcane cultivars by the decapitation technique, strain XaS1 was as virulent or more virulent than five other strains of X. albilineans isolated from diseased sugarcane plants in Guadeloupe. Although strain XaS3 colonized a few stalks, it failed to produce any symptoms and was the least virulent strain. Leaf surface colonization by X. albilineans was reproduced in a greenhouse trial by spraying the pathogen on sugarcane foliage. After 8 weeks, the pathogen was isolated from disinfected leaf blades. Although the leaf scald pathogen is thought to be mainly transmitted by infected cuttings, aerial transmission of X. albilineans is also known to occur. These results indicate the importance of sugarcane phyllosphere colonization by virulent strains in the epidemiological cycle of leaf scald disease in Guadeloupe.     

The role of seeds and airborne inoculum in the initiation of leaf blotch (Rhynchosporium secalis) epidemics in winter barley

Both airborne spores of Rhynchosporium secalis and seed infection have been implied as major sources of primary inoculum for barley leaf blotch (scald) epidemics in fields without previous history of barley cropping. However, little is known about their relative importance in the onset of disease. Results from both quantitative real‐time PCR and visual assessments indicated that seed infection was the main source of inoculum in the field trial conducted in this study. Glasshouse studies established that the pathogen can be transmitted from infected seeds into roots, shoots and leaves without causing symptoms. Plants in the field trial remained symptomless for approximately four months before symptoms were observed in the crop. Covering the crop during part of the growing season was shown to prevent pathogen growth, despite the use of infected seed, indicating that changes in the physiological condition of the plant and/or environmental conditions may trigger disease development. However, once the disease appeared in the field it quickly became uniform throughout the cropping area. Only small amounts of R. secalis DNA were measured in 24 h spore‐trap tape samples using PCR. Inoculum levels equivalent to spore concentrations between 30 and 60 spores per m³ of air were only detected on three occasions during the growing season. The temporal pattern and level of detection of R. secalis DNA in spore tape samples indicated that airborne inoculum was limited and most likely represented rain‐splashed conidia rather than putative ascospores.     

琯溪蜜柚黑斑病侵染循环及防治适期研究

琯溪蜜柚黑斑病(pomelo black spot)的病原菌为亚洲柑橘叶点霉Phyllosticta citriasiana。田间系统观察和病原菌侵染试验结果表明,蜜柚黑斑病是一种潜伏侵染病害,上年度被侵染的带菌叶片为次年病害的侵染源,蜜柚花期至幼果期是病菌侵染盛期和病害防治适期。如果错过这两个时期,将难以达到理想的防治效果。