The overwintering ofExserohilum turcicum in Israel

Exserohilum turcicum (Pass.) Leonard and Suggs, the causal agent of northern leaf blight of corn, overwinters onSorghum halepense L. plants and on corn debris (dead leaves). Spqrulating lesions ofE. turcicum were observed on sorghum plants in the winter (February). Spores from these lesions were pathogenic to susceptible sweet corn plants cv. ‘Jubilee’. Infected sporulating leaves of corn were stored for 1 year at 20°C (40-60% relative humidity), at 5°C (60% relative humidity), or buried 5 cm underground. During the storage period, 32% and 22% of the spores formed chlamydospores, at 20° and 5°C, respectively. Leaves buried 5 cm underground were totally decomposed after 6 months. After 4 months, 25% of the spores in the buried leaves had formed chlamydospores. Spores with chlamydospores were pathogenic to corn plants. The viability of spores without chlamydospores stored at 20°, 5°C or buried underground was 0, 60 and 0%, respectively. In a parallel experiment infected leaves were stored under similar conditions and allowed to sporulate. No sporulation occurred on infected leaves buried in soil. Spores produced on infected leaves stored at 20° and 5°C were highly pathogenic to corn plants. In leaves treated with 0.1N glucose, chlamy dospore formation was significantly inhibited.     

Survival of Didymella rabiei in chickpea straw debris in Spain

Didymella rabiei grew saprophytically on pieces of infested chickpea stems and pods, and formed pycnidia and pseudothecia. The extent of saprophytic growth and production of viable spores were determined by the incubation conditions. On debris left on the soil surface under natural conditions, the fungus rapidly colonized the tissues, formed abundant pseudothecia and pycnidia, and remained viable throughout the 2 years of the study. When plant debris was buried, D, rabiei was restricted to the original lesions, in which it formed new pycnidia and was viable for 2 to 5 months. Under controlled conditions in the laboratory, D. rabiei extensively colonized plant debris spread over the soil surface. On the other hand, the fungus did not grow on buried debris, or showed only very limited development when the artificially infested debris was buried between two layers of sterilized soil. Incubation temperature was the principal factor associated with the production of conidia and especially ascospores.     

Growth and survival of the mycoparasiteConiothyrium minitans on lettuce leaves in contact with soil in the presece or absence ofSclerotinia sclerotiorum

Lettuce leaves co-inoculated withSclerotinia sclerotiorum andConiothyrium minitans and controls were placed on, or buried in, soil for a period of two weeks to study development and survival ofC. minitans. OnS. sclerotiorum-infected leaves on the soil surface, the number of colonies ofC. minitans recovered was about 40% of the number of pycnidiospores applied. When buried in the soil there was a reduction to about 2% of the spores applied. WhenC. minitans was applied on healthy lettuce leaves, which were subsequently placed on or in soil, the recovery was about 4%. It is argued that these figures indicate multiplication ofC. minitans onS. sclerotiorum-infected lettuce leaves on the soil, and good survival in all other cases.     

Survival of Clavibacter michiganensis ssp. michiganensis in infected tomato stems under natural field conditions in California, Ohio and Morocco

The survival and half-life of Clavibacter michiganensis ssp. michiganensis ( C. michiganensis ), the causal agent of bacterial canker of tomato, were determined in infected plant debris under natural field conditions in California, Ohio and Morocco using a semiselective agar medium. The organism survived significantly longer in tomato stems left on the soil surface than in stems buried in the soil at all locations studied. The pathogen was recovered in high amounts from tomato stems left on the soil surface for 314 days in Ohio and California, USA, and for 194 and 132 days in Melk Zhar and Aït Melloul, Morocco, respectively; it was recovered from stems buried in the soil for up to 314 days in Ohio, up to 240 days in California, and up to 60 days in Aït Melloul and Melk Zhar. The half-life of the pathogen in stems left on the soil surface ranged from 23·2 to 24·8 days in the USA, and from 7·8 to 12·3 days in Morocco, whereas the half-life in buried stems ranged from 14·0 to 16·7 days in the USA and from 3·7 to 9·5 days in Morocco. Based on the half-life data, the predicted survival times of C. michiganensis in stems on the soil surface in Ohio, California, Melk Zhar and Aït Melloul would be up to 822, 770, 424 and 261 days, respectively, while the predicted survival times in stems buried in the soil would be 541, 497, 305 and 128 days, respectively. These results show that the survival and half-life of C. michiganensis in plant debris are relatively long and are influenced by both tissue exposure and geographic location.     

Overwintering of brown leaf spot fungus, Mycochaetophora gentianae, in infected gentian leaves as the primary inoculum source

Overwintering of the brown leaf spot fungus, Mycochaetophora gentianae, in infected gentian leaves was studied in Iwate, northern Japan. Sporophores were produced on overwintered, infected leaves when they were sampled from January to July, but not in August after incubation in high humidity at 15 °C. Symptoms developed on gentian plants grown in soil artificially infested with overwintered, infected leaves that were either left throughout the experiments or removed before planting. Few lesions developed when plants were grown in soil infested with conidia. These results indicate that M. gentianae can overwinter in infected leaves, which act as the primary inoculum source.     

Survival of the lettuce anthracnose fungus (Microdochium panattonianum) in Victoria

Soils from anthracnose-infected lettuce fields at Keilor, Werribee South and La Trobe University caused lesions of Microdochium panattonianum to develop when inoculated on to lettuce leaves. Under field conditions conidia lost infectivity within 10 weeks in pasteurized and 6 weeks in non-pasteurized Werribee South soil, 2 weeks in Keilor red and immediately in Keilor black non-pasteurized soils. Soils remained infective for 18 weeks in non-pasteurized and pasteurized Werribee South soils inoculated with conidia and for Hand 16 weeks respectively when the same soils were inoculated with infected leaf discs.
In 1983 and 1984 M. panattonianum survived for 14 and 16 weeks respectively on infected Cos Verdi debris on the soil surface, for 10 and 20 weeks respectively on debris buried at 10 cm depth and for 70 and 58 weeks respectively on debris suspended in the air. The fungus survived for similar periods on infected Winterlake debris on the soil surface and buried at 10 cm depth. In soil the decline of infectivity was primarily influenced by duration of exposure and soil moisture, and in debris by duration of exposure and rainfall that occurred over the 2 weeks preceding each sample. The disease was not transmitted on seed produced on infected plants. Seedlings grown from healthy seed inoculated with conidia did not develop anthracnose after the seed had been stored for 24 days at 5 or 20°C. The significance of these results to the management of lettuce anthracnose in Victoria is discussed.     

Survival of Clavibacter michiganensis subsp. michiganensis in tomato debris under greenhouse conditions

Bacterial canker, caused by Clavibacter michiganensis subsp. michiganensis, is one of the most important diseases of tomato worldwide. Once the pathogen has been introduced into an area, i.e. by contaminated seeds or transplants, it survives mainly on host debris. In different geographic areas the survival time of the pathogen in crop residues under field conditions has been very variable, ranging from 2 months in Morocco to 2 years in Iowa (USA). This study took place in the horticultural belt of Buenos Aires – La Plata, Argentina, where greenhouse production prevails, and monoculture with two production cycles per year is a common practice. The aim was to determine the survival time of this pathogen in plant residues left on the soil surface or buried. During three consecutive years, by the end of both production cycles in July (winter) and December (summer), above‐ (stem, petiole) and belowground (root) tissues were placed into nylon netting bags and left on the soil surface or buried at 10 cm depth. The pathogen population was regularly quantified by dilution plating on semiselective medium. In host debris left on the soil surface, bacteria survived 120–260 days for crop production cycles that ended in winter and 45–75 days for those that ended in summer. In stems or roots buried in winter, this period was 45–75 days. It is concluded that host debris, including roots, might be an important primary inoculum source of the pathogen in greenhouses.     

棉铃疫病菌越冬卵孢子作为初侵染源的研究

棉铃疫病菌(Phytophthora boehmeriae Sawada)卵孢子经土壤埋存160天左右越冬,至少有16%～47%的卵孢子仍存活。卵孢子不依赖病残体可单独在土壤耕作层中越冬存活。越冬卵孢子萌发后可侵害棉苗。认为棉铃疫病菌卵孢子在土壤中可安全越冬作为次年病害的主要初侵染源。     

Überleben von Ralstonia solanacearum Biovar 2 (Rasse 3), dem Erreger der Schleimkrankheit der Kartoffel, in Boden und auf verschiedenen Materialien (Mikrokosmos)

Microcosm studies were carried out to test the survival of Ralstonia solanacearum biovar 2 (race 3) in soil at the permanent wilting point (wp) water content and at field capacity (fc) water content and on various material. Soils were placed at permanent ?5°C, 4°C, 15°C and 20°C and weekly fluctuating ?10/0/+10°C and the material at 5, 15 °C, 20°C with relative humidity (rh) uncontrolled or at constant 10% or 90%. In soil, survival was clearly dependent on temperature independent of water content. At 20°C Ralstonia solanacearum could be reisolated up to 364 days, at 15°C up to 290 days, at 4°C up to 209 days and at fluctuating temperatures (?10/0/+10°C) only up to 18 days. The lower the temperature, the more the population declined. At 15°C and 20°C appr. 10⁷ cfu/g soil were detected after 100 days, whereas at ?5°C only 10² cfu/g soil were detected after only 18 days. The pathogen was longer detectable in sandy-clay loam than in lighter sandy soil. It could be longer reisolated at wilting point and the populations did not decline as rapidly as at field capacity. Ralstonia solanacearum could best survive on material surfaces like rubber, plastic and varnished metal with maximum survival of 40 days at 5°C and 10% rh. In general there is a low risk of Ralstonia solanacearum overwintering under European climatic conditions when the fields are cleared of plant debris and the soil is frozen. Contamined material surfaces pose the risk of pathogen transmission to healthy tubers.     

Relative importance of different types of inoculum to the establishment of Mycosphaerella graminicola in wheat crops in north‐west Europe

The contribution of wheat debris to the early stages of septoria leaf blotch epidemics was assessed in a 3‐year field experiment. First lesions were detected very early (December) in the case of an early sowing (mid‐October), showing that the first contamination could occur as soon as the seedlings emerge. The tested debris management options (chopped debris, removal of debris followed by tillage, or tillage in absence of debris) had a strong effect, although transient, on the epidemic dynamic: the more debris present on the soil surface, the more severe initial disease was. The magnitude of differences between treatments differed substantially between years. The relative production of pycnidiospores and ascospores was measured on the chopped debris. Peaks in pycnidiospore and ascospore production coincided in October–November. Both types of spores can be involved as primary inoculum in north‐west European conditions. The local amount of pycnidiospores available on debris in the field, estimated per square metre, was 1000‐fold the local ascospore production. Moreover, inoculum production was quantified on debris exposed to different environmental conditions. Autumnal conditions, characterized by moderate temperature with alternating wet and dry periods, were favourable for the production of both pycnidiospores and ascospores, as shown by the high inoculum production on debris exposed to field or outdoor conditions. By late autumn, the canopy became the most important source of pycnidiospores, and this period, characterized by the decreasing role of debris as a local source of inoculum compared to distant potential sources, can be considered as the end of the early epidemic stages.     

Survival of Phoma koolunga,a causal agent of ascochyta blight,on field pea stubble or as pseudosclerotia in soil

Phoma koolunga is a recently recognized pathogen in the ascochyta blight complex of field pea (Pisum sativum). Unlike the other three ascochyta blight pathogens, survival of P. koolunga is poorly understood. Survival of this fungus was examined on field pea stubble and as pseudosclerotia on the surface of, and buried in, field soil. Pseudosclerotia were formed in plates containing potato dextrose agar (PDA) mixed with sand or amended with fluorocytocin. After 1 month, P. koolunga was recovered on amended PDA from 93% of stubble sections retrieved from the soil surface, 36% of buried stubble sections and 100% of pseudosclerotia buried in field soil, pasteurized or not. The frequency of recovery of P. koolunga decreased over time and the fungus was not recovered from stubble on the soil surface at 15 months, nor was it recovered from stubble buried in soil at 11 months or later, or from pseudosclerotia buried for 18 months. In a pot bioassay, most ascochyta blight lesions developed on plants inoculated with stubble retrieved from the soil surface after 1 month. Infectivity of the inoculum decreased over time. Disease on plants inoculated with stubble that had been buried or left on the soil surface for up to 6 and 5 months, respectively, and pseudosclerotia retrieved at 14 months and later from field soil did not differ from the non‐inoculated control. These results suggest that field pea stubble may play a role in survival of P. koolunga, especially if it remains on the soil surface. In addition, pseudosclerotia were able to persist in soil and infect field pea plants into the next season.     

Phytophthora infestans in a subtropical region: survival on tomato debris, temporal dynamics of airborne sporangia and alternative hosts

Little is known about inoculum dynamics of late blight caused by Phytophthora infestans in tropical/subtropical areas, particularly in Brazil. The objectives of the present study were to assess (i) the survival of the pathogen on stems, leaflets and tomato fruits, either buried or not in soil; (ii) the pathogenicity of P . infestans to mostly solanaceous plant species commonly found in Brazil that could act as inoculum reservoir; and (iii) the temporal dynamics of airborne sporangia. Phytophthora infestans survived in tomato plant parts for less than 36 days under greenhouse and field conditions. In greenhouse tests, pathogen structures were detected earlier on crop debris kept in dry than in wet soil conditions. Isolates of two clonal lineages of P. infestans , US-1 from tomato, and BR-1 from potato, were inoculated on 43 plant species. In addition to potato and tomato, Petunia  ×  hybrida and Nicotiana benthamiana were susceptible to the pathogen. Airborne inoculum was monitored with Rotorod and Burkard spore traps as well as with tomato and potato trap plants. Sporangia were sampled in most weeks throughout 2004 and in the first two weeks of 2005. Under tropical/subtropical conditions, airborne inoculum is abundant and is more important to late blight epidemics than inoculum from crop debris or alternative hosts.     

Seed survival and predation of Anoda cristata in soyabean crops

Anoda cristata is a troublesome annual broad-leaved weed in summer crops in the rolling Pampa in Argentina; seeds are the only source of regeneration of this species. Seed persistence or depletion is the result of survival and loss processes, including predation. The objective of this study was to determine survival at two burial depths in undisturbed soil and predation rates of A. cristata seeds in soyabean crops in different rotations and tillage systems. Survival was discontinuous and decreased to 25% after 35 months, after which no further reduction in survival was observed to the end of the experiment at 96 months. No differences in seed survival between seeds placed on the soil surface and buried 5 cm below the soil surface were found at 80 months, but at later times survival was lower for seeds placed on the soil surface. Predation rates ranged between 0.3% day⁻¹ and 6.7% day⁻¹. Of the models tested, a polynomial regression of the rate of predation with time gave the best representation of seed predation. From January to July, predation was higher in non-tillage plots in the wheat/soyabean rotation. There was no significant difference in predation rates between tillage systems in the soyabean monoculture and no difference between planting densities. Higher crop residue levels in non-tillage plots in the wheat/soyabean rotation was the dominant factor influencing seed predation, probably because such habitat favours the presence of seed predators.     

Seed dormancy and periodicity of seedling emergence in Veronica hederifolia L.

Emergence of Veronica hederifolia seedlings began in mid-October and continued into spring; few appeared from June to September. Ripe seeds shed in June were dormant but wben buried in soil outdoors developed a capacity for germination initially at low temperatures (constant4 C; daily alternations of 4-10° and 4-1 5 C) and later at somewhat higher temperatures, with peak germination in September-November. During winter, spring and early summer thc germination capacity declined, to increase again in late summer and early autumn. Cyclic physiological changes thus occur in seeds of V,hederifolia present in the soil, with which lhe consistent seasonal periodicity of seedling emergence is associated. In dry storage ihe capacity for germination progressively increased, but alter 12 months there was a sharp decline in germination at 4° C. Few seeds germinated at 20° C, but moistening with GA 4/7; brought about complete germination at this temperature.     

盐碱棉田地表-地下接力式滴灌下水盐分布及棉花生长特征

地表-地下接力滴灌是集膜下滴灌和地下滴灌优点于一体的新型节水控盐技术,但目前针对该技术应用效果的研究尚少。针对如何对地表-地下接力式滴灌中的地表滴灌和地下滴灌进行水量分配效果最优这一问题,设置100%地表滴灌(W1)、75%地表滴灌+25%地下滴灌(W2)、50%地表滴灌+50%地下滴灌(W3)、25%地表滴灌+75%地下滴灌(W4)、100%地下滴灌(W5)共计5个处理,比较了不同水量分配下的地表-地下接力式滴灌与单一地表滴灌、单一地下滴灌对盐碱棉田土壤水盐分布和棉花产量的影响。结果表明：(1)W3处理根区土壤含水量分布最均匀,干燥区域面积最小。(2)W4处理窄行区域淋洗范围最大,脱盐效果最显著。(3)W3处理棉花吐絮期总干物质量和籽棉产量最大,分别为112.66 g和9 147 kg·hm^-2;吐絮期总干物质量比W1和W5处理分别提高11.3%和19.1%,籽棉产量比W1和W5处理分别提高14.1%和11.9%。地表-地下接力式滴灌处理下土壤含水量得以显著改善,在对土壤盐分进行淋洗的过程中表现出接力效应,淋洗面积和淋洗效果均有所增大。相比于单一地表滴灌和单一地...     

Limited survival of Ralstonia solanacearum Race 3 in bulk soils and composts from Egypt

Survival of Ralstonia solanacearum race 3 biovar 2 (phylotype II sequevar 1) in Egyptian soils and compost was studied under laboratory and field conditions. Survival of the pathogen under laboratory conditions varied with temperature, water potential and soil type, with temperature being the major determinant of survival of the pathogen. The effects of temperature and moisture content were variable between different experiments, but survival was generally longer at 15°C than at 4, 28 and 35°C respectively. Survival was also longer when moisture levels were constant compared with varying moisture levels at all temperatures. In experiments to compare the effects of progressive drying in sandy and clay soils there was a difference in survival times between the two soil types. In sandy soils, the pathogen died out more rapidly when soil was allowed to dry out than in controls where the soil was kept at constant water potential. In clay soils there was little difference between the two treatments, possibly due to the formation of a hard impermeable outer layer during the drying process, which retarded water loss from within. Survival in mature composts at 15°C was of the same order of magnitude as in soils but shorter at 28°C, possibly owing to increased biological activity at this temperature, or a resumption of the composting process, with concomitant higher temperatures within the compost itself. The maximum survival time recorded over all soil types and conditions during in vitro studies was around 200 days. In field studies, the maximum survival time in both bare sand and clay was around 85 days at depths up to 50 cm. The survival time was reduced in field experiments carried out in summer to less than 40 days and in one study when the ground was flooded for rice cultivation, the bacterium could not be detected 14 days after flooding. The maximum survival time of R. solanacearum in infected plant material or in infested soil samples incorporated into compost heaps was less than 2 weeks. At the culmination of field soil and compost experiments, no infection was detected in tomato seedlings up to 10 weeks after transplanting into the same soils or composts under glasshouse conditions at a temperature of 25°C.     

Depth Distribution of Rotylenchulus reniformis Under Different Tillage and Crop Sequence Systems

ABSTRACT The population density of the reniform nematode, Rotylenchulus reniformis, was monitored at depths of 0 to 30, 30 to 60, 60 to 90, and 90 to 120 cm in a tillage and crop sequence trial in south Texas in 2000 and 2001. Main plots were subjected to three different tillage systems: conventional tillage (moldboard plowing and disking), ridge tillage, and no-tillage. Subplots were planted with three different crop sequences: spring cotton and fall corn every year; spring cotton and fall corn in one year, followed by corn for two years; and cotton followed by corn and then grain sorghum, one spring crop per year. The population density of R. reniformis on corn and grain sorghum was low throughout the soil profile. In plots planted with spring cotton and fall corn every year, fewer nematodes were found at depths of 60 to 120 cm in the no-tillage and ridge tillage systems than in the conventional tillage system. Population densities were lower at depths of 0 to 60 cm than at 60 to 120 cm. Soil moisture and cotton root length did not affect nematode population densities in the field. When soil was placed in pots and planted with cotton in the greenhouse, lower population densities developed in soil taken from depths of 0 to 60 cm than in soil from depths of 60 to 120 cm. Final nematode populations were similar in size in soil from the different tillage systems, but reproductive factors were higher in soil from plots with reduced-tillage systems than in soil from plots with conventional tillage. Reduced-tillage practices lowered the risk of increases in R. reniformis populations and reduced population densities following 2 years of non-hosts throughout soil depths, but population densities resurged to the same high levels as in soil planted with cotton every year during one season of cotton.     

Burial depth and cultivation influence emergence and persistence of Phalaris paradoxa seed in an Australian sub-tropical environment

Emergence and persistence characteristics of Phalaris paradoxa seeds in no- and minimum-till situations and at different burial depths were studied in a sub-tropical environment. Three experiments were carried out using naturally shed seeds. In the first experiment, seedlings emerged from May through to September each year, although the majority of seedlings emerged in July. In the second experiment with greater seed density, cultivation in March of each year stimulated seedling emergence, altered the periodicity of emergence and accelerated the decline of seeds in the seedbank compared with plots that received no cultivation. The majority of seedlings in the cultivated plots emerged in May whereas the majority of seedlings in the undisturbed plots emerged in July. Emergence accounted for only 4–19% of the seedbank in both experiments over 2 years. Seed persistence was short in both field experiments, with less than 1% remaining 2 years after seed shed. In the third experiment, burial depth and soil disturbance significantly influenced seedling emergence and persistence of seed. Seedlings emerged most from seed mixed in the top 10 cm when subjected to annual soil disturbance, and from seed buried at 2.5 and 5.0 cm depths in undisturbed soil. Emergence was least from seed on the soil surface, and buried at 10 and 15 cm depths in undisturbed soil. Seeds persisted longest when shed onto the soil surface and persisted least when the soil was tilled. These results suggest that strategic cultivation may be a useful management tool, as it will alter the periodicity of emergence allowing use of more effective control options and will deplete the soil seedbank more rapidly.     

Relationships between disease and yield in cotton fields affected byAlternaria macrospora

Application of various fungicides to cotton plots (Gossypium barbadense cv. Pima) affected byAlternaria macrospora resulted in a yield increase from 3955 kg/ha in unsprayed plots to 5207 kg/ha in plots treated with fentin acetate. Pathogen, host and disease parameters were estimated by a key of grades in the field and by precise measurements of detached plants. Measurements included the number of spores present in the field and numbers and/or areas of healthy, lesioned and shed leaves and bolls. No significant differences were found between the number of healthy bolls on variously treated plants and the number of lesioned bolls on the same plants. Disease incidence was relatively low in treated plots, with no significant difference between treatments and no correlation with the size of the pathogen population or the yield level in variously treated plots. The low proportion of lesioned leaves resulted from their tendency to shed and from the emergence of new leaves, which masked the damage. Rate of shedding was correlated with the respective yield and with the size of the pathogen population in the same plot. Leaf shedding was induced by lesions occupying an average of 1 to 3% of the leaf area. Shedding was affected by leaf position on the plant, being most frequent for the lowest leaves despite their being the least lesioned, and least frequent for the upper leaves even though these were the most lesioned.     

Temporal Dynamics of Phytophthora Blight on Bell Pepper in Relation to the Mechanisms of Dispersal of Primary Inoculum of Phytophthora capsici in Soil

ABSTRACT The effect of components of primary inoculum dispersal in soil on the temporal dynamics of Phytophthora blight epidemics in bell pepper was evaluated in field and growth-chamber experiments. Phytophthora capsici may potentially be dispersed by one of several mechanisms in the soil, including inoculum movement to roots, root growth to inoculum, and root-to-root spread. Individual components of primary inoculum dispersal were manipulated in field plots by introducing (i) sporangia and mycelia directly in soil so that all three mechanisms of dispersal were possible, (ii) a plant with sporulating lesions on the soil surface in a plastic polyvinyl chloride (PVC) tube so inoculum movement to roots was possible, (iii) a wax-encased peat pot containing sporangia and mycelia in soil so root growth to inoculum was possible, (iv) a wax-encased peat pot containing infected roots in soil so root-to-root spread was possible, (v) noninfested V8 vermiculite media into soil directly as a control, or (vi) wax-encased noninfested soil as a control. In 1995 and 1996, final incidence of disease was highest in plots where sporangia and mycelia were buried directly in soil and all mechanisms of dispersal were operative (60 and 32%) and where infected plants were placed in PVC tubes on the soil surface and inoculum movement to roots occurred with rainfall (89 and 23%). Disease onset was delayed in 1995 and 1996, and final incidence was lower in plants in plots where wax-encased sporangia (6 and 22%) or wax-encased infected roots (22%) were buried in soil and root growth to inoculum or root-to-root spread occurred. Incidence of root infections was higher over time in plots where inoculum moved to roots or all mechanisms of dispersal were possible. In growth-chamber studies, ultimately all plants became diseased regardless of the dispersal mechanism of primary inoculum, but disease onset was delayed when plant roots had to grow through a wax layer to inoculum or infected roots in tension funnels that contained small volumes of soil. Our data from both field and growth-chamber studies demonstrate that the mechanism of dispersal of the primary inoculum in soil can have large effects on the temporal dynamics of disease.