Infectious Inoculum Research Articles

Analysis of variation of bovine viral diarrhoea virus E2 sequence following transplacental infection of cattle

The genetic and antigenic diversity observed in field isolates of bovine viral diarrhoea virus (BVDV) is thought to occur during acute infection because of the genetic stability observed in BVDV throughout the lifetime of persistently infected (PI) cattle. In this study, 15 cows in early pregnancy were inoculated with identical challenge doses obtained from a single infectious inoculum of the virologically cloned isolate Pe515nc. In order to examine the diversity that may develop in utero in the PI foetus, the variable E2 sequence of the virus isolated directly from the serum of each PI calf was compared. A high degree of sequence similarity was demonstrated, with 0–4 nucleotide differences out of 608 bases compared. Thus, the virus showed relatively few genomic changes in any of the PI calves, although we observed that the in utero environment did provide some opportunity for genetic variation to become established.

Exploitation of soil arbuscular mycorrhizal potential for AM-dependent mandarin orange plants by pre-cropping with mycotrophic crops

Results of a nursery study on the effect of short season pre-cropping with differently mycotrophic herbaceous crops on growth of arbuscular mycorrhiza-dependent mandarin orange plants at an early stage after transplantation are presented. The study was carried out with eroded, low nutrient, acidic, brown forest soil of the North-Eastern Himalayan region of India. Mandarin orange seedling plants at 180 days after transplantation showed variation in shoot growth response to single season pre-cropping with seven different crops—maize, Paspalum millet, soybean, onion, tomato, mustard and ginger and two non-cropped fallow treatments—non-weeded and weeded fallows. Net growth benefit to the orange plants due to the different pre-crops and the non-weeded fallow treatment over the weeded fallow treatment plants showed highly significant positive correlation with mycorrhizal root mass of the orange plants as it varied due to the different pre-crop treatments. Net growth benefit due to the pre-crops and weeds varied between 0 and 50% depending upon the mycorrhizal root mass of the pre-crops and weeds, AMF spore number and infective inoculum density of the pre-cropped soils. These pre-crop variables individually and cumulatively contributed to the highly significant positive correlation between the AMF potential of the pre-cropped soils and growth of mandarin orange plants through their effect on mycorrhizal root mass development (i.e. extent of mycorrhization) of the mandarin orange plants. The results show that choice of a pre-crop from the available options practised by the farmers, grown even for a short season can substantially alter the inherent AMF potential of soils to significantly influence the performance of the mycorrhiza-dependent orange plant. The relationship between soil mycorrhizal potential left by a pre-crop and mycorrhizal benefit drawn by the succeeding AM responsive plant can be taken advantage of in the exploitation of native AMF potential of soils for growth and nutrition management of crops in low nutrient, low input–output systems of production.

Factors Affecting Pycnidium Production of Diaporthe citri on Detached Citrus Twigs.

Melanose, caused by Diaporthe citri, produces black-to-reddish brown lesions on twigs, leaves, and fruit of citrus and reduces the external quality of fruit destined for the fresh market. Inoculum for infection is produced primarily in pycnidia formed on dead twigs, and conidia are dispersed by rainwater. In laboratory studies, the effect of moisture, temperature, twig size, and melanose severity on pycnidium production on detached twigs was investigated. Pycnidium production was greatest when twigs were soaked for 3 to 4 h on alternate days three times per week and the temperature was 28°C. Production was greatest on twigs 3 to 5 mm in diameter and less on thinner or thicker twigs. Pycnidium production was related linearly to melanose severity on the twigs, and almost no pycnidia were produced on asymptomatic twigs. In the field, pycnidium production was greatest on detached, melanose-affected twigs placed in the canopy monthly during January to April than it was on twigs placed in the canopy during other months. The largest number of pycnidia was produced from May to August when fruit is most susceptible. The number of pycnidia produced was related significantly to degree-days above 20°C and weakly related to cumulative rainfall. Knowledge of inoculum production peaks may assist in timing of pruning and fungicide sprays.

Greenhouse and field techniques for testing sugar beet for resistance to Rhizoctonia root and crown rot

AbstractRhizoctonia‐resistant sugar beet varieties are the key to an integrated control strategy for Rhizoctonia root rot. Because of the unpredictable occurrence of Rhizoctonia solani in the field testing of sugar beet for resistance to Rhizoctonia root rot is difficult. The aim of the study was to develop advanced greenhouse and field techniques which allow a reliable assessment of sugar beet for resistance to R. solani. A highly infectious liquid inoculum was used for the first time in this study. It can be produced in large quantities of a standardized quality, sterile, and exactly quantified according to its carbon content. In a greenhouse trial, sugar beet grown in the same way as field grown beet was inoculated by applying a suspension of Rhizoctonia mycelium (equivalent to 10 mg carbon per plant) to the beet crown. After 3 weeks, inoculation had led to uniform and severe root rot. Disease symptoms were similar to those found under natural conditions in the field. No ‘escapes’, i.e. susceptible plants apparently expressing resistance were observed in the test. A new nine‐class disease scale was established and a Rhizoctonia index (RI) was calculated. Reliability of disease assessment was demonstrated on progeny of plants, selected from segregating populations, showing Rhizoctonia resistance more closely related to the resistant parent lines than to the susceptible ones. Sugar beet varieties could be assessed in the greenhouse within only 11 weeks. All varieties were affected by the pathogen but partially resistant varieties could be clearly recognized by a significantly lower Rhizoctonia index. Significant differences in susceptibility were also found within the group of new resistant genotypes. Infection studies performed in the field showed the superiority of the new liquid inoculum compared with a solid form and revealed the influence of inoculation date and inoculum level on the development of Rhizoctonia root rot. In field tests performed at different sites under different environmental conditions, susceptible and partially resistant sugar beet varieties could be reproducibly rated according to their susceptibility to R. solani. On average, susceptible varieties showed a Rhizoctonia index of 8 while resistant genotypes ranged from 5 to 6. The newly developed techniques allow fast and reliable evaluation of sugar beet for resistance to R. solani.

Antiretroviral spermicide WHI-07 prevents vaginal and rectal transmission of feline immunodeficiency virus in domestic cats.

WHI-07 [5-bromo-6-methoxy-5,6-dihydro-3'-azidothymidine-5'-(p-bromophenyl)-methoxy alaninyl phosphate] is a novel dual-function aryl phosphate derivative of zidovudine with potent anti-human immunodeficiency virus (HIV) and spermicidal activities. WHI-07 was active against the feline immunodeficiency virus (FIV). This study evaluated whether topical application of WHI-07 as a single agent and in combination with an organometallic vanadium complex, vanadocene dithiocarbamate (VDDTC), via a nontoxic gel microemulsion can block vaginal as well as rectal transmission of feline AIDS (FAIDS) by chronically FIV-infected feline T cells in the natural host model. Genital transmission of FIV was monitored in recipient cats by the appearance of viral antibodies to FIV Gag proteins and by virus isolation of blood leukocytes as measured by FIV reverse transcriptase activity and FIV-specific PCR. Microbicidal activity was considered effective when the treated cats did not show evidence of FIV infection for up to 18 weeks postchallenge. An aggregate analysis of 46 specific-pathogen-free cats revealed that a single dose of the infected cell inoculum efficiently transmitted FIV infection when delivered into the vagina (100%) or rectum (66%). Pretreatment of the vagina or rectum with 2% WHI-07 alone or in combination with 0.25% VDDTC significantly (P = 0.004) protected cats from genital transmission by the highly infectious inoculum (7 million FIV(Bangston)-infected feline T cells). Collectively, using the vaginal and rectal transmucosal model for FAIDS, our studies demonstrated that WHI-07 either alone or in combination with a vanadocene has clinical potential for the development of a dual-function anti-HIV microbicide for sexually active women.

Exploitation of soil arbuscular mycorrhizal potential for AM-dependent mandarin orange plants by pre-cropping with mycotrophic crops

Results of a nursery study on the effect of short season pre-cropping with differently mycotrophic herbaceous crops on growth of arbuscular mycorrhiza-dependent mandarin orange plants at an early stage after transplantation are presented. The study was carried out with eroded, low nutrient, acidic, brown forest soil of the North-Eastern Himalayan region of India. Mandarin orange seedling plants at 180 days after transplantation showed variation in shoot growth response to single season pre-cropping with seven different crops—maize, Paspalum millet, soybean, onion, tomato, mustard and ginger and two non-cropped fallow treatments—non-weeded and weeded fallows. Net growth benefit to the orange plants due to the different pre-crops and the non-weeded fallow treatment over the weeded fallow treatment plants showed highly significant positive correlation with mycorrhizal root mass of the orange plants as it varied due to the different pre-crop treatments. Net growth benefit due to the pre-crops and weeds varied between 0 and 50% depending upon the mycorrhizal root mass of the pre-crops and weeds, AMF spore number and infective inoculum density of the pre-cropped soils. These pre-crop variables individually and cumulatively contributed to the highly significant positive correlation between the AMF potential of the pre-cropped soils and growth of mandarin orange plants through their effect on mycorrhizal root mass development (i.e. extent of mycorrhization) of the mandarin orange plants. The results show that choice of a pre-crop from the available options practised by the farmers, grown even for a short season can substantially alter the inherent AMF potential of soils to significantly influence the performance of the mycorrhiza-dependent orange plant. The relationship between soil mycorrhizal potential left by a pre-crop and mycorrhizal benefit drawn by the succeeding AM responsive plant can be taken advantage of in the exploitation of native AMF potential of soils for growth and nutrition management of crops in low nutrient, low input–output systems of production.

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.

Epidemiological dynamics and the efficiency of biological control of soil‐borne disease during consecutive epidemics in a controlled environment

• A combination of experimentation and modelling is used to examine the role of epidemiological dynamics on the production and infectivity of inoculum and the efficiency of biocontrol by Trichoderma viride during consecutive epidemics of damping-off disease caused by the pathogen Rhizoctonia solani in crops of radish. • Changes in the net infectivity of inoculum at the beginning of first and second crops caused a switch in epidemiological dynamics. Epidemics of first crops were dominated by secondary infection leading to amplification of inoculum so that epidemics of second crops were overwhelmingly determined by primary infection. • The biocontrol agent reduced primary infection and hence parasitic amplification of inoculum in both first and second crops but the efficiency of control dropped from 91.7% in first crops to 64.8% in second crops, with sudden outbreaks of disease in second crops which had previously been disease-free. • We conclude that parasitic amplification can cause a rapid build-up of disease and inoculum over consecutive crops, leading to loss in the efficiency of biocontrol. This form of inoculum production is supplemented by saprotrophic infestation which can result in sudden outbreaks of disease in protected crops where control of disease had previously been fully successful.

Manejo do solo e da irrigação como nova tática de controle de Meloidogyne incognita em feijoeiro

A população de Meloidogyne incognita e sua infetividade foram estudadas em parcelas amostradas a zero, dois e 14 dias após o revolvimento do solo com ou sem irrigação, irrigadas sem revolvimento e testemunha. Na amostragem aos dois dias após a instalação do ensaio, ocorreu maior redução (P<0,01) de juvenis do segundo estádio ( J2) no solo apenas revolvido seguido pelo revolvido e irrigado. A infetividade do inóculo do solo, contudo, foi menor (P<0,01) quando se revolveu e irrigou o solo comparado com aquele apenas revolvido. Aos 14 dias, a menor (P<0,01) população de J2 foi observada no solo revolvido e irrigado, seguido do revolvido, e elevada naquele apenas irrigado e na testemunha. A infetividade do inóculo do solo aos 14 dias continuou mais baixa no solo revolvido e irrigado. Aos 45 dias após a semeadura, a população de J2 continuou mais baixa (P<0,01) onde o solo foi revolvido e irrigado. O número de ovos por feijoeiro (Phaseolus vulgaris) no campo aos 90 dias também foi mais baixo (P<0,01) no solo revolvido e irrigado. Maior (P<0,01) peso das raízes e da parte aérea dos feijoeiros no campo foi observado em plantas crescidas em solo apenas revolvido, e no revolvido e irrigado A produção de feijão no solo revolvido e irrigado posteriormente foi quatro vezes maior que a testemunha, porém todos os tratamentos diferiram-se entre si. A irrigação como prática agrícola para aumentar a eficácia do alqueive no controle de fitonematóides tem sido postulada por alguns pesquisadores, mas nunca provada experimentalmente, o que agora foi realizada.

Prophylactic and therapeutic effects of phthalocyanine tetrasulfonate in scrapie-infected mice.

The transmissible spongiform encephalopathy (TSE) diseases are rare, neurodegenerative diseases that include scrapie in sheep, bovine spongiform encephalopathy, and Creutzfeldt-Jakob disease in humans. There are no effective treatments available for clinical use in humans. We now demonstrate that, in 2 different rodent models of scrapie, multiple pretreatments with the cyclic tetrapyrrole phthalocyanine tetrasulfonate (PcTS) were as effective at delaying disease as multiple treatments starting at the time of infection. At low doses of scrapie infectivity, PcTS also protected some mice from peripheral scrapie infection, even if treatment was initiated several weeks after infection. Furthermore, PcTS completely inactivated low levels of scrapie infectivity when incubated with the infectious inoculum. Thus, PcTS has a broad range of antiscrapie activities. These findings suggest that cyclic tetrapyrroles may be useful both prophylactically and therapeutically against TSE diseases in vivo, as well as for inactivation of TSE infectivity suspended in solution.

Development of a rapid method for determining the infectious dose (ID) 50 of Orientia tsutsugamushi in a scrub typhus mouse model for the evaluation of vaccine candidates

The infectious dose (ID) of an inoculum for which 50% of susceptible mice will become infected (ID 50) with Orientia tsutsugamushi is usually determined by rechallenging mice that have already been challenged with O. tsutsugamushi to determine the lethal dose (LD) 50 titer of the inoculum. Those mice not killed by the initial challenge and which survived a rechallenge with 1000 LD 50 were considered immune and to have been initially infected with O. tsutsugamushi. Mice that succumbed to the rechallenge were considered not to have been initially infected. We have developed a more rapid method of determining the ID 50 of inocula for use in our vaccine studies based upon the observation that mice surviving initial challenge and that go on to survive rechallenge produced detectable IgG to O. tsutsugamushi antigens by enzyme-linked immunosorbent assay (ELISA). Mice that did not survive rechallenge, and therefore did not receive an initial infectious inoculum did not produce detectable IgG to O. tsutsugamushi antigens. Both original LD 50 and ID 50 titers determinations require observation of mice for 21 days post-challenge. Our new ID 50 determination does not require mice or the additional 21-day observation period for rechallenge and therefore is more rapid and cost-effective than the previous standard method of determining ID 50 titer necessary for the evaluation of vaccine candidates.

More is not necessarily better: prozone-like effects in passive immunization with IgG.

Despite a century of study, the relationship between Ag-specific Ig concentration and protection remains poorly understood for the majority of pathogens. In certain conditions, administration of high Ab doses before challenge with an infectious agent can be less effective than smaller Ab doses, a phenomenon which is consistent with a prozone-like effect. In this study, the relationship between IgG1, IgG2a, IgG2b, and IgG3 dose, infective inocula, and protection was investigated in a mouse model of Cryptococcus neoformans infection. The activity of each IgG subclass ranged from protective to disease-enhancing depending on both the Ab dose and infective inocula used. Enhanced dissemination to the brain was observed in mice given a high IgG2a dose and a relatively low inoculum. Ab administration had immunomodulatory effects, with cytokine expression in lung, brain, and spleen varying as a function of the infective inoculum Ab dose and IgG subclass. In vitro studies did not predict or explain the mechanism of in vivo prozone-like effects, because all isotypes were opsonic and elicited NO release from macrophages. IgG2a was most efficient in inducing a macrophage oxidative burst. These results reveal that an individual Ab can be protective, nonprotective, or disease-enhancing depending on its concentration relative to a challenge inoculum. Our findings have implications for the potential contribution of Ab responses to defense against microbial diseases because Ab-mediated immunity may be protective, nonprotective, or even deleterious to the host.

Survival of the fire blight pathogen, Erwinia amylovora, in calyxes of apple fruit discarded in an orchard

The survival of Erwinia amylovora from calyxes of apples discarded in an orchard over a 20-day period at flowering was determined in two consecutive years. Bacterial numbers decreased significantly in the first 4 days and then persisted at low levels for the following 16 days in both years. Spread of E. amylovora from the inoculated apples was not detected by either culture or PCR in any of the rainwater, apple flowers, leaves or insects tested over the 20-day sampling period. The results of this investigation demonstrate that populations of E. amylovora present in contaminated apple calyxes were not transferred to susceptible hosts, even when the apples were placed in close proximity to hosts that were in a receptive stage at flowering. Therefore, should populations of E. amylovora exist on exported commercial apple fruit, there is no evidence to suggest that these populations would provide inoculum for a new fire blight infection. Consequently, there is a discontinuity in the pathway for the dissemination of E. amylovora from an infested fruit to a susceptible host as the bacterial populations do not multiply and there is no demonstrated spread.

Susceptibility of Sitka spruce and grand fir trees to decay by Heterobasidion annosum

SummaryThe susceptibility of Sitka spruce (Picea sitchensis) and grand fir (Abies grandis) to decay by Heterobasidion annosum was compared on a site with a previous history of serious disease. The incidence of decay 15–16 years after planting was 1.5% in grand fir, compared with 34.8% in Sitka spruce. These results are broadly similar to those reported from other trials. More information is required for older trees in Britain but the evidence so far available suggests that grand fir is more resistant to H. annosum than Sitka spruce. Planting grand fir on severely infested sites may therefore significantly reduce the serious losses that could be expected with Sitka spruce. In 14 out of 271 decayed Sitka spruce H. annosum was absent from the middle or base of the decay column but present at the top. Approximately 3% of infected spruce showed features suggesting some degree of resistance. Resin bleeding was observed at the base of 23.2% of the infected spruce, and the mean height of the decay column in these trees was significantly greater (0.96 m) than in those without this feature (0.66 m). The inoculum for infection was provided by stumps of the previous crop, and the genets of H. annosum that were present in some infected trees were also identified in stumps. The majority of infected trees contained only a single genet but three trees contained two genets. In this young crop individual genets were smaller than those found elsewhere; the largest extended to include six trees.

The role of petals in development of grey mould in strawberries

Studies were conducted in annual crops of strawberry (Fragaria x ananassa) (cv. Elsanta to assess the relative importance of petals as an inoculum source of grey mould, caused by Botrytis cinerea and to identify during which period of flower and fruit development the presence of petals has a significant effect on development of grey mould on fruits. In 1998, the incidence of B. cinerea on flower parts was assessed, and the symptoms of grey mould on fruits were characterised with regards to their starting point. The incidence of B. cinerea on petals was 65-85% of those flowers that harboured B. cinerea. The starting point of symptoms was located underneath the sepals in 65-85% of fruits with grey mould, and petals were present at this site in about 50% of the cases. In 1999, four field experiments were conducted to assess the effect of petal removal at different stages of flower and fruit development on incidence of grey mould. The incidence of grey mould on fruits with petals retained till harvest was circa 55% more than on fruits where petals were removed or had dropped naturally by the end of flowering, regardless of planting date or inoculum level of B. cinerea. The incidence of grey mould was hardly different between treatments where petals were removed at young flower stage, old flower stage, or green fruit stage. It is concluded that petals are an effective and stable source of inoculum for fruit infection by B. cinerea. Considering the high probability that retention of petals during fruit expansion and ripening results in fruit infection, the elimination of petals as an inoculum source appears worthwhile even when the incidence of petal retention on fruits is relatively low.

Les toxi-infections alimentaires collectives

Bacterial food poisoning frequently occur in developed countries, where their sanitary and economical consequences can be dreadful. They usually provide from two consecutive mechanisms: food spoilage by bacteria and their subsequent multiplication, which result either in the production of a toxin, or in the formation of an infectious inoculum. These two events can result from defaults in hygiene that occur along the production line and following process. Each bacterial food poisoning episode requires a specific investigation whose aims are the identification of the infectious agent, of the contaminated aliment, the mode of spoilage and the factors that helped the microbial proliferation. This inquiry is necessary to take emergency action and to prevent recurrence. This investigation, which is altogether clinical, microbiological and alimentary, can be difficult. Indeed, bacterial food poisoning do not always affect the digestive tract and the etiological agent is sometimes difficult to identify. It should be initiated as soon as possible. Whatever they affect collective or familial restoration, the notification of bacterial food poisoning to DDASS or to SVD is obligatory.

Ascocarp formation and survival and primary inoculum production in Erysiphe (sect. Microsphaera) pulchra in dogwood powdery mildew

SummaryDevelopment of powdery mildew Erysiphe (sect. Microsphaera) pulchra in dogwood (Cornus florida) was assessed over a 5‐year period (1996–2000). Variations in the timing of initial infection, disease severity, ascocarp formation, and primary inoculum density were evaluated. Ascocarps formed late in the growing season (September‐November) when relatively low temperatures (< 27°C) persisted for at least 2 weeks, but ascocarp abundance was not influenced by disease severity. Studies conducted in a controlled environment showed that low temperatures triggered ascocarp formation and neither day length nor host plant age affected ascocarp formation. Ascocarps formed within 12–14 days at 18°C/ 10°C (day/night) and 23°C/15°C, but required 25 days at 26°C/18°C; no ascocarps formed at 28°C/ 20°C. Because ascocarps are an important source of primary inoculum for dogwood powdery mildew, ascocarp survival was evaluated in a 2‐year study (1998–2000). 60–80% of mature, dark‐coloured ascocarps survived at ‐10°C and ‐20°C and maintained viable spores for 4 months, but only 4–12% of partially developed, light brown ascocarps survived at ‐10°C and ‐20°C in the first experiment and only 30–40% survived in the second experiment. Immature ascocarp initials (cream‐yellow in colour) withered and disintegrated at all temperatures (24°C/20°C, 4°C, ‐10°C, and ‐20°C). Because ascocarps need time to mature, the timing of ascocarp initiation affects ascocarp maturity and thus winter survival and primary inoculum density. The evaluation of spring inoculum dispersal to spore traps and trap plants in 1999 and 2000 showed that rainfall patterns in early spring influenced primary inoculum and thus the timing of initial infection.

Modification of bacteraemia by specific antibodies and relation with mortality in a pneumococcal mouse sepsis model.

The relationship between mortality and the bacteraemic profile was investigated in a pneumococcal (serotype 6B) sepsis BALB/c mouse model where animals received protection by specific hyperimmune serum. A single intraperitoneal dose of hyperimmune serum obtained from mice immunized with the heat-inactivated strain was administered (non-diluted or diluted to 1/4 or to 1/16) to 5-mice study groups 1 h prior to intraperitoneal inoculation with the infective inoculum (3.57 x 108 cfu/ml). Blood cultures were performed daily over 15 days, with 8 microl of blood being collected from the tail vein; the samples were resuspended in Todd-Hewitt broth containing 10% trisodium citrate and plated onto blood agar for colony counting. Animals included in the control group received placebo (PBS). Mortality was 100% in control animals within the first 48 h. Hyperimmune serum decreased and delayed mortality in a dose-related trend, producing 100%, 80%, 60% and 40% survival rates at 72, 96, 144 and 360 h, with non-diluted serum. Bacteraemic profiles with maximum colony counts > or =5 x 107 cfu/ml in blood during the follow-up period were related to > or =65% probability of death, regardless of the serum dilution administered.

Field incidence of rice yellow mottle virus, genus sobemovirus on rice and a weed species in the field in Cote d'Ivoire

Field surveys carried out between 1996 and 1997 in Cote d'Ivoire on weed hosts to detect the occurrence and subsistence of rice yellow mottle virus (RYMV) in nature show that rice and Echinochloa crus-pavonis (Link) harbour the virus. There was consistent detection of RYMV throughout the sampling period in rice samples mostly from the lowland varieties. It is thus evident that RYMV subsists more on rice in nature. This could serve as a source of inoculum for infection to newly transplanted rice in the field.

The distribution and spread of sorghum downy mildew in sorghum and maize fields in Nigeria and Zimbabwe

Sorghum downy mildew (Peronosclerospora sorghi, SDM) is a damaging disease of sorghum and maize crops in Africa. Runs analysis was used to study the distribution of systemically infected sorghum and maize plants in Nigeria and Zimbabwe. The temporal and spatial development of local lesions of SDM on sorghum in Zimbabwe was investigated by assessing the local lesion symptoms caused by conidia in plots with a single point source of inoculum. With ordinary runs analysis, there was evidence of clustering of disease in some fields in the humid areas of Nigeria and the semi-arid areas of Nigeria and Zimbabwe. Clustering was found in two of the eight runs analyses performed on maize in the humid south of Nigeria, and in only one of the eight runs in Zimbabwe, which was interpreted as a predominance of random infection at the time of assessment and at the spatial scales assessed. Symptoms of local lesions of SDM developed rapidly across plots from an introduced point source of infection. After 9 days-exposure to the source of inoculum, the incidence of diseased leaves was 1.2%, and after 50 days it was 74.5%. A disease gradient which initially developed flattened as the plot became uniformly diseased. The predominant wind direction was NNE, and most rapid spread of disease was towards the SSW and WSW. In conclusion, local lesions can spread rapidly in sorghum crops, suggesting that they may be an important source of conidial inoculum for further local and systemic infections during the growing season.