Sanitation practices that inhibit reproduction of Monosporascus cannonballus in melon roots left in the field after crop termination

Ascospores of Monosporascus cannonballus function as primary inoculum for infection of melon roots. Previous studies demonstrated that pathogen reproduction (i.e. ascospore production) occurs on infected melon roots primarily after the crop has been terminated. Therefore, the key to maintaining low soil population densities of the pathogen is to destroy the hyphae of the pathogen in infected roots as soon as possible after crop termination, thereby inhibiting ascospore production. Results from a 3-year field study demonstrated that, relative to the nontreated controls, an immediate postharvest application of metam sodium (applied via the drip irrigation system at 187 L ha⁻¹) or cultivation (which lifts roots onto the surface of the soil for rapid desiccation) significantly inhibited pathogen reproduction in infected melon roots, as shown by the number of roots subsequently bearing perithecia. Additionally, ascospore populations in plots that received either the metam sodium or cultivation treatment were significantly lower ( P  < 0·05) than populations in the nontreated control plots at the end of the 3-year study. These results demonstrated the efficacy of these postharvest treatments in the inhibition of pathogen reproduction and retardation of inoculum build-up in soil.     

Epidemiological aspects of mango malformation disease caused by Fusarium mangiferae and source of infection in seedlings cultivated in orchards in Egypt

Mango malformation, caused by the fungus Fusarium mangiferae , is one of the major diseases of this crop occurring worldwide. This study was conducted to investigate aspects of the epidemiology, survival and spread of the pathogen in general and specifically in seedlings, the majority of which are cultivated in infected orchards in Egypt. Survival of conidia of a representative isolate (506/2) declined very rapidly in soil under summer conditions (1·6 weeks for 50% population decline), but significantly less in controlled and winter conditions (17·9 and 15·0 weeks, respectively, for 50% population decline). Likewise, inoculum survival in naturally infected panicles on the soil surface declined faster than in those buried at 30-cm depths. Natural infections were evaluated on fruits and seeds in a heavily infected and a healthy orchard. In infected trees, the skins of all sampled fruits within a 2-m radius of infected panicles were infected, but the pathogen was not detected in the seeds, seed coats or flesh. The pathogen was not detected in any parts of fruits from a healthy orchard. Vegetatively malformed mango seedlings, growing under infected trees bearing infected panicles, were sampled in two locations in Egypt to determine whether infection in seedlings was systemic (evenly distributed within plant tissue) or whether the pathogen originated from malformed panicles. According to PCR-specific primer amplification, the pathogen was detected in 97% of seedling apical meristems, declining gradually to 5% colonization in roots. It was concluded that inoculum of the pathogen originates from infected panicles and affects seedlings from the meristem, with infections descending to lower stem sections and roots. Minor infections of roots may occur from inoculum originating from infected panicles, but the pathogen is not seedborne.     

Effect of crop growth and canopy filtration on the dynamics of plant disease epidemics spread by aerially dispersed spores

Most mathematical models of plant disease epidemics ignore the growth and phenology of the host crop. Unfortunately, reports of disease development are often not accompanied by a simultaneous and commensurate evaluation of crop development. However, the time scale for increases in the leaf area of field crops is comparable to the time scale of epidemics. This simultaneous development of host and pathogen has many ramifications on the resulting plant disease epidemic. First, there is a simple dilution effect resulting from the introduction of new healthy leaf area with time. Often, measurements of disease levels are made pro rata (per unit of host leaf area or total root length or mass). Thus, host growth will reduce the apparent infection rate. A second, related effect, has to do with the so-called "correction factor," which accounts for inoculum falling on already infected tissue. This factor accounts for multiple infection and is given by the fraction of the host tissue that is susceptible to disease. As an epidemic develops, less and less tissue is open to infection and the initial exponential growth slows. Crop growth delays the impact of this limiting effect and, therefore, tends to increase the rate of disease progress. A third and often neglected effect arises when an increase in the density of susceptible host tissue results in a corresponding increase in the basic reproduction ratio, R(0), defined as the ratio of the total number of daughter lesions produced to the number of original mother lesions. This occurs when the transport efficiency of inoculum from infected to susceptible host is strongly dependent on the spatial density of plant tissue. Thus, crop growth may have a major impact on the development of plant disease epidemics occurring during the vegetative phase of crop growth. The effects that these crop growth-related factors have on plant disease epidemics spread by airborne spores are evaluated using mathematical models and their importance is discussed. In particular, plant disease epidemics initiated by the introduction of inoculum during this stage of development are shown to be relatively insensitive to the time at which inoculum is introduced.     

Modelling effects of vector acquisition threshold on disease progression in a perennial crop following deployment of a partially resistant variety

Deployment of resistant varieties is a key strategy to mitigating economic losses due to arthropod‐transmitted plant pathogens of perennial crops. In many cases, the best available resistant traits for introgression confer only partial resistance. Plants displaying partial resistance have lower pathogen titres than susceptible counterparts, but remain hosts for the pathogen. As partially resistant varieties maintain yield after infection, infected plants are unlikely to be rogued (i.e. removed). Accordingly, there is a risk that partially resistant plants could serve as a source of inoculum for pathogen spread to susceptible plants. Here, a mathematical model that tracked spread of an arthropod‐transmitted pathogen in a plant population consisting of susceptible and partially resistant plants was used to identify a threshold acquisition rate from partially resistant plants that resulted in limited spread of the pathogen from partially resistant plants to susceptible plants. The acquisition threshold from partially resistant plants varied with parameters influenced by disease management decisions such as number of vectors per plant, vector turnover, replacement of susceptible plants, and proportion of plants that were partially resistant. In model simulations, effects of deploying a partially resistant variety on disease incidence in a susceptible variety depended on the extent to which pathogen spread among susceptible plants was suppressed and acquisition rates from partially resistant plants. Collectively, the results indicate that risk of partially resistant plants serving as inoculum sources could be assessed prior to deployment, thereby enabling design of complementary disease management tactics to minimize economic losses in susceptible varieties following deployment.     

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.     

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.     

Field studies of anthracnose symptoms and pathogen infection in different phases of the persimmon growing season

Anthracnose is the main disease of persimmon and is caused by Colletotrichum spp. The study of field epidemiology is essential for the development of efficient management of this disease. In this study, we investigated infection by Colletotrichum spp. throughout the persimmon growing season to understand the host–pathogen interactions better. We observed the production of primary inoculum of persimmon anthracnose and described how epidemics progress from secondary infections during the fruit crop season. The field study was carried out in an organic orchard with three susceptible persimmon cultivars, Fuyu, Kakimel and Jiro, for three consecutive seasons. Our results indicate that the pathogen survives in 1-year-old shoots, which are the sources of primary inoculum. Later that growing season, the inoculum reaches flowers and new shoots, developing symptoms and producing the secondary inoculum. Fruit drop was also observed, with or without symptoms of anthracnose, throughout the plant cycle. In some of the symptomless fruit, collected from the plant and from the ground where they had fallen, latent infections of Colletotrichum spp. were detected. Shoots, flowers, immature and ripened fruit remained infected throughout the growing season, producing conidia that could lead to new secondary infections within and among plants. The incidence of anthracnose in fruit at harvest and postharvest proved to be less relevant for disease management. Practices for chemical and cultural control of the disease throughout the persimmon growing season are discussed.     

Demonstration of secondary infection by Pythium violae in epidemics of carrot cavity spot using root transplantation as a method of soil infestation

Cavity spot of carrot (CCS), one of the most important soilborne diseases of this crop worldwide, is characterized by small sunken elliptical lesions on the taproot caused by a complex of pathogens belonging to the genus Pythium , notably P. violae . In most soilborne diseases the soil is the source of inoculum for primary infections, with diseased plants then providing inoculum for secondary infections (both auto- and alloinfection). Using fragments of CCS lesions to infest soil, it was demonstrated that CCS lesions on carrot residues can cause primary infection of healthy roots. Using a novel soil infestation method, in which an artificially infected carrot root (the donor plant) was placed close to healthy roots (receptor plants) the formation of typical CCS lesions were induced more efficiently than the use of classical soil inoculum and showed that CCS can spread from root to root by alloinfection from transplanted diseased roots. The method also demonstrated the polycyclic nature of a CCS epidemic caused by P. violae in controlled conditions. Secondary infections caused symptoms and reduced root weight as early as two weeks after transplantation of the diseased carrot. This reproducible method may be used for delayed inoculation and for studying the effect of cropping factors and the efficacy of treatments against primary and secondary cavity spot infections.     

Plasticity in plant-microbe interactions: a perspective based on the pitch canker pathosystem

What we know about the life history of fungi that cause disease in plants is commonly based on studies of the pathogen’s interaction with a susceptible host: how and when infection occurs, growth and reproduction within the host, and survival during the interval when a growing host is not available. This focus is appropriate, given the need for information that will facilitate management of disease affecting an economically important crop, but it can limit recognition of the full range of resources that may be utilized by fungi that we classify as plant pathogens. This was certainly the case for Fusarium circinatum, which causes a destructive disease of pines known as pitch canker. Although F. circinatum was initially known only as a necrotrophic, wound-infecting pathogen of coniferous trees, recent research has revealed that an isolate of this fungus that will kill shoot tissue when inoculated into a wound can also have a biotrophic relationship with roots of pine seedlings, infect and grow within grasses without causing symptoms, and cause ear rot of corn. Thus, although F. circinatum became known to science because it induced visible symptoms on pines, it has the capacity for a much broader range of ecological activities than is captured by its designation as a necrotrophic pathogen. The physiological plasticity manifested by F. circinatum illustrates the challenge of obtaining a comprehensive understanding of the life history of a plant pathogenic fungus.     

Colonization of lettuce cultivars and rotation crops by Fusarium oxysporum f. sp. lactucae,the cause of fusarium wilt of lettuce

The severity of fusarium wilt is affected by inoculum density in soil, which is expected to decline during intervals when a non‐susceptible crop is grown. However, the anticipated benefits of crop rotation may not be realized if the pathogen can colonize and produce inoculum on a resistant cultivar or rotation crop. The present study documented colonization of roots of broccoli, cauliflower and spinach by Fusarium oxysporum f. sp. lactucae, the cause of fusarium wilt of lettuce. The frequency of infection was significantly lower on all three rotation crops than on a susceptible lettuce cultivar, and the pathogen was restricted to the cortex of roots of broccoli. However, F. oxysporum f. sp. lactucae was isolated from the root vascular stele of 7·4% of cauliflower plants and 50% of spinach plants that were sampled, indicating a greater potential for colonization and production of inoculum on these crops. The pathogen was also recovered from the root vascular stele of five fusarium wilt‐resistant lettuce cultivars. Thus, disease‐resistant plants may support growth of the pathogen and thereby contribute to an increase in soil inoculum density. Cultivars that were indistinguishable based on above‐ground symptoms, differed significantly in the extent to which they were colonized by F. oxysporum f. sp. lactucae. Less extensively colonized cultivars may prove to be superior sources of resistance to fusarium wilt for use in breeding programmes.     

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.     

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.     

Survival and Extinction of Xanthomonas campestris pv. campestris in Soil

Carry-over of inoculum of X.c. pv. campestris in the soil from one cropping season to the next was studied in field experiments over three years. These studies were supported by laboratory and greenhouse experiments on quantitative assessment of bacteria by bioassay using the Most Probable Number technique, and on recovery rates of bacteria from the soil. The mean recovery rate from artificially infested soil was 58%. Extinction of X.c. pv. campestris in soil infested with infected plant debris proceeded exponentially and extinction rates depended on temperature, as did the decomposition of plant debris. In replicated field plots, over three years, infection foci of black rot disease were established. At harvest time, all plants were chopped and resulting plant debris was rotovated into the soil. The resulting soil infestation was sampled and showed clear infestation foci reflecting the original infection foci of the crop. These infestation foci decreased with time and disappeared after the winter. Follow-up crops remained virtually uninfected. The results show that in The Netherlands good crop and soil management impedes survival of inoculum from one year to the next, so that cabbage can be grown continuously. Polyetic carry-over of inoculum by debris in the soil can be avoided in The Netherlands.     

Phoma diseases: Epidemiology and control

Phoma is the most widely distributed and omnipresent genus of the order Pleosporales and the largest genus with some 3,000 taxa described so far. Of these, approximately 110 species are pathogenic and occupy varied ecological niches. The genus Phoma is polyphyletic and is not really delimited, with unclear species boundaries that make it a taxonomically controversial genus. Fungi belonging to Phoma commonly occur on crop plants that are economically important, where they cause devastating plant diseases. Pathogenic members of Phoma sensu lato species attack crop plants with symptoms ranging from leaf blight to root rot, and even wilting of the plant. In infected crop residues and field stubbles, the pathogen produces abundant pycnidia and pseudothecia that serve as the source of primary inoculum, whilst repeated crops of conidia produced inside pycnidia are the main source of secondary infection during the same growing season. After successful infection, the pathogen produces various phytotoxins that alter photosynthetic efficiency and actin cytoskeleton-based functions, and cause electrolyte leakage from cells. Controlling the diseases caused by members of Phoma sensu lato is challenging and efforts have been made to identify resistant varieties that can be used in various plant breeding programmes. Studies have also been conducted to devise cultural and biological control measures as well as to evaluate the efficacy of fungicides against members of Phoma sensu lato. In this review we aim to discuss the disease epidemiology and control measures that can be practised to protect crops from Phoma diseases.     

Integrated disease management of ascochyta blight in pulse crops

Ascochyta blight causes significant yield loss in pulse crops worldwide. Integrated disease management is essential to take advantage of cultivars with partial resistance to this disease. The most effective practices, established by decades of research, use a combination of disease-free seed, destruction or avoidance of inoculum sources, manipulation of sowing dates, seed and foliar fungicides, and cultivars with improved resistance. An understanding of the pathosystems and the inter-relationship between host, pathogen and the environment is essential to be able to make correct decisions for disease control without compromising the agronomic or economic ideal. For individual pathosystems, some components of the integrated management principles may need to be given greater consideration than others. For instance, destruction of infested residue may be incompatible with no or minimum tillage practices, or rotation intervals may need to be extended in environments that slow the speed of residue decomposition. For ascochyta-susceptible chickpeas the use of disease-free seed, or seed treatments, is crucial as seed-borne infection is highly effective as primary inoculum and epidemics develop rapidly from foci in favourable conditions. Implemented fungicide strategies differ according to cultivar resistance and the control efficacy of fungicides, and the effectiveness of genetic resistance varies according to seasonal conditions. Studies are being undertaken to develop advanced decision support tools to assist growers in making more informed decisions regarding fungicide and agronomic practices for disease control.     

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.     

Spatial distribution and temporal development of fusarium crown and root rot of tomato and pathogen dissemination in field soil

ABSTRACT The spatial distribution and temporal development of tomato crown and root rot, caused by Fusarium oxysporum f. sp. radicis-lycopersici, were studied in naturally infested fields in 1996 and 1997. Disease progression fit a logistic model better than a monomolecular one. Geostatistical analyses and semivariogram calculations revealed that the disease spreads from infected plants to a distance of 1.1 to 4.4 m during the growing season. By using a chlorate-resistant nitrate nonutilizing (nit) mutant of F. oxysporum f. sp. radicis-lycopersici as a "tagged" inoculum, the pathogen was found to spread from one plant to the next via infection of the roots. The pathogen spread to up to four plants (2.0 m) on either side of the inoculated focus plant. Root colonization by the nit mutant showed a decreasing gradient from the site of inoculation to both sides of the inoculated plant. Simulation experiments in the greenhouse further established that this soilborne pathogen can spread from root to root during the growing season. These findings suggest a polycyclic nature of F. oxysporum f. sp. radicis-lycopersici, a deviation from the monocyclic nature of many nonzoosporic soilborne pathogens.     

Theoretical approach to the dynamics of the inoculum density of Verticillium dahliae in the soil: first test of a simple model

A mathematical equation was developed that describes the inoculum densities of microsclerotia of Verticillium dahliae in the soil over a long time span. The equation was based on measurable parameters and ecologically meaningful principles. In the model, the number of systemic infections of plant roots during crop growth was related to soil inoculum density. In turn, formation of microsclerotia in debris and reduction of the amount of crop growth were related to the number of systemic infections. Finally, a gradual release and mortality of microsclerotia in the soil were included to calculate subsequent inoculum densities in the soil.
Fitting the function to experimental data of potato cvs Element, Ostara, Mirka and Astarte, flax, pea, barley, sugar beet, onion and faba bean gave a very high correlation between observed and predicted soil inoculum densities. The clear differences in inoculum production among potato cultivars and other crops were expressed in quantitative terms. The highest inoculum density after incorporation of the debris of a susceptible crop was estimated to occur at 2.3 thermal time units of 3600 degree days (base 0°C). Ten per cent of the initial input of inoculum was still present after 4.5 thermal time units. The model was used to predict the dynamics of soil inoculum densities for V. dahliae under various cropping frequency schemes and performed satisfactorily.     

Alternative hosts and plant tissues for the survival,sporulation and spread of the Ascochyta blight pathogen of chickpea

Various crop and weed species were infected naturally by Didymella rabiei (anamorph: Ascochyta rabiei) in blight-affected chickpea fields in the Palouse region of eastern Washington and northern Idaho, USA. The fungus was isolated from asymptomatic plants of 16 species commonly found in commercial crops in this region. Isolates of the pathogen from crop and weed species were pathogenic to chickpea and indistinguishable in cultural and morphological characteristics from isolates of D. rabiei from chickpea. Both mating types of D. rabiei were isolated from eight naturally infected plant species. Chickpeas were infected by D. rabiei when plants emerged through infested debris of seven crop and weed species. The teleomorph developed on overwintered tissues of seven plant species infected naturally by D. rabiei in a blight screening nursery and on debris of wheat, white sweet clover and pea inoculated with ascospores of D. rabiei or conidia of two compatible isolates of the pathogen. Didymella rabiei naturally infected 31 accessions of 12 Cicer spp. and the teleomorph developed on the overwintered debris of all accessions, including those of three highly resistant perennial species. The fungus developed on the stem and leaf pieces of ten plant species common to southern Spain inoculated with conidia of two compatible isolates of D. rabiei, and formed pseudothecia with asci and viable ascospores on six of ten species and pycnidia with conidia on all plant species.     

Relationships Between Verticillium dahliae Inoculum Density and Wilt Incidence, Severity, and Growth of Cauliflower

ABSTRACT Microplot and field experiments were conducted to evaluate the effects of inoculum density on Verticillium wilt and cauliflower growth. Soil containing Verticillium dahliae microsclerotia was mixed with various proportions of fumigated soil to establish different inoculum densities (fumigated soil was used as the noninfested control). Seven inoculum density treatments replicated four times were established, and the treatments were arranged in a randomized complete block design. Soil was collected from each microplot immediately after soil infestation for V. dahliae assay by plating onto sodium polypectate agar (NP-10) selective medium using the Anderson sampler technique. Five-week-old cauliflower was transplanted into two beds within each 1.2- by 1.2-m microplot. At the same time, several extra plants were also transplanted at the edge of each bed for destructive sampling to examine the disease onset (vascular discoloration) after planting. Cauliflower plants were monitored for Verticillium wilt development. Stomatal resistance in two visually healthy upper and two lower, diseased leaves in each microplot was measured three times at weekly intervals after initial wilt symptoms occurred. At maturity, all plants were uprooted, washed free of soil, and wilt incidence and severity, plant height, number of leaves, and dry weights of leaves and roots were determined. The higher the inoculum density, the earlier was disease onset. A density of 4 microsclerotia per g of dry soil caused 16% wilt incidence, but about 10 microsclerotia per g of soil caused 50% wilt incidence. Both wilt incidence and severity increased with increasing inoculum density up to about 20 microsclerotia per g of soil, and additional inoculum did not result in significantly higher disease incidence and severity. A negative exponential model described the disease relationships to inoculum levels under both microplot and field conditions. Stomatal resistance of diseased leaves was significantly higher at higher inoculum densities; in healthy leaves, however, no treatment differences occurred. The height, number of leaves, and dry weights of leaves and roots of plants in the fumigated control were significantly higher than in infested treatments, but the effects of inoculum density treatments were variable between years. Timing of cauliflower infection, crop physiological processes related to hydraulic conductance, and wilt intensity (incidence and severity) were thus affected by the inoculum density. Verticillium wilt management methods used in cauliflower should reduce inoculum density to less than four micro-sclerotia per g of soil to produce crops with the fewest number of infected plants.