Environmental conditions influencing Sclerotinia sclerotiorum infection and disease development in lettuce

The environmental factors that influence infection of lettuce by ascospores of Sclerotinia sclerotiorum , and subsequent disease development, were investigated in controlled environment and field conditions. When lettuce plants were inoculated with a suspension of ascospores in water or with dry ascospores and exposed to a range of wetness durations or relative humidities at different temperatures, all plants developed disease but there was no relationship between leaf wetness duration or humidity and percentage of diseased plants. Ascospores started to germinate on lettuce leaves after 2–4 h of continuous leaf wetness at optimum temperatures of 15–25°C. The rate of development of sclerotinia disease and the final percentage of plants affected after 50 days were greatest at 16–27°C, with disease symptoms first observed 7–9 days after inoculation, and maximum final disease levels of 96%. At lower temperatures, 8–11°C, disease was first observed 20–26 days after inoculation, with maximum final disease levels of 10%. Disease symptoms were always observed first at the stem base. In field-grown lettuce in Norfolk, 2000 and 2001, inoculated with ascospore suspensions, disease occurred only in lettuce planted in May and June, with a range of 20–49% of plants with disease by 8 weeks after inoculation. In naturally infected field-grown lettuce in Cheshire, 2000, disease occurred mainly in lettuce planted throughout May, with a maximum of 31% lettuce diseased within one planting, but subsequent plantings had little (≤ 4%) or no disease. Lack of disease in the later plantings in both Norfolk and Cheshire could not be attributed to differences in weather factors.     

Effects of environmental factors on discharge and germination of ascospores of Venturia nashicola

Experiments were conducted under controlled environmental conditions to study the effects of temperature, duration of wetness, relative humidity (RH) and light on the discharge and germination of ascospores of Venturia nashicola , the causal agent of pear scab in China. Discharge of ascospores from pseudothecia required free water or 100% RH. A period of soaking in water as short as 10 s was sufficient to initiate the discharge of ascospores. Temperatures from 10 to 30°C did not significantly affect the temporal trend of ascospore discharge. A greater proportion of ascospores was discharged under light than in the dark. However, a period of light as short as 10 min, either during the initial wetting of pseudothecia or interrupting the darkness, was sufficient to reduce the inhibitory effect of darkness on ascospore discharge. Ascospores were discharged within 10 min after pseudothecia were wetted and most ascospores ( c. 80%) were discharged within the first hour. The temporal pattern of ascospore discharge could be well described by a logistic model, which estimated that 50% of ascospores were discharged within half an hour of wetting. Ascospores germinated over a wide range of temperatures from 5 to 30°C, with an optimum at c . 20°C. Temporal dynamics of ascospore germination at six temperatures (5, 10, 15, 20, 25 and 30°C) were satisfactorily described by logistic models.     

Influence of relative humidity and temperature on development of Didymella rabiei on chickpea debris

Didymella rabiei grew saprophytically on pieces of artificially and naturally infected chickpea stem debris under artificial incubation conditions, and formed pseudothecia and pycnidia. The extent of growth was not significantly affected by temperature of incubation within the range 5–25°C, but was significantly reduced as relative humidity (RH) decreased from 100% to 86%, when no growth occurred. Pseudothecia matured at 10°C and constant 100% RH, or at 5 and 10°C and alternating 100%/34% RH. Under these conditions, pseudothecial maturation, assessed by a pseudothecia maturity index, increased over time according to the logistic model. For temperatures higher than 10°C or RH lower than 100%, pseudothecia either did not form ascospores, or ascopores did not mature and their content degenerated. When pseudothecia that initially developed to a given developmental stage were further incubated at a constant 100% RH, temperature became less limiting for complete pseudothecial development as the developmental stage was more advanced. Pycnidia of the fungus developed and formed viable conidia in all environmental conditions studied, except at 86% RH. However, the density of pycnidia formed and the number of viable conidia per pycnidium were significantly influenced by temperature, RH and the type of debris (artificially or naturally infected) used.     

Sporulation of Pyrenopeziza brassicae (light leaf spot) on oilseed rape (Brassica napus) leaves inoculated with ascospores or conidia at different temperatures and wetness durations

In controlled environment experiments, sporulation of Pyrenopeziza brassicae was observed on leaves of oilseed rape inoculated with ascospores or conidia at temperatures from 8 to 20°C at all leaf wetness durations from 6 to 72 h, except after 6 h leaf wetness duration at 8°C. The shortest times from inoculation to first observed sporulation ( l ₀), for both ascospore and conidial inoculum, were 11–12 days at 16°C after 48 h wetness duration. For both ascospore and conidial inoculum (48 h wetness duration), the number of conidia produced per cm² leaf area with sporulation was seven to eight times less at 20°C than at 8, 12 or 16°C. Values of Gompertz parameters c (maximum percentage leaf area with sporulation), r (maximum rate of increase in percentage leaf area with sporulation) and l ₃₇ (days from inoculation to 37% of maximum sporulation), estimated by fitting the equation to the observed data, were linearly related to values predicted by inserting temperature and wetness duration treatment values into existing equations. The observed data were fitted better by logistic equations than by Gompertz equations (which overestimated at low temperatures). For both ascospore and conidial inoculum, the latent period derived from the logistic equation (days from inoculation to 50% of maximum sporulation, l ₅₀) of P. brassicae was generally shortest at 16°C, and increased as temperature increased to 20°C or decreased to 8°C. Minimum numbers of spores needed to produce sporulation on leaves were ≈25 ascospores per leaf and ≈700 conidia per leaf, at 16°C after 48 h leaf wetness duration.     

Infection of onion leaves by Alternaria porri and Stemphylium vesicarium and disease development in controlled environments

Infection of onion by Alternaria porri and Stemphylium vesicarium was investigated under a range of controlled temperatures (4–25°C) and leaf wetness periods (0–24 h). Conidia of A. porri and S. vesicarium germinated within 2 h when incubated at 4°C. Terminal and intercalary appressoria were produced at similar frequencies at or above 10°C. The maximum number of appressoria was produced after 24 h at 25°C. Penetration of leaves by both pathogens was via the epidermis and stomata, but the frequency of stomatal penetration exceeded that of epidermal penetration. There was a strong correlation ( R ² > 90%) between appressorium formation and total penetrations at all temperatures. Infection of onion leaves occurred after 16 h of leaf wetness at 15°C and 8 h of leaf wetness at 10–25°C, and infection increased with increasing leaf wetness duration to 24 h at all temperatures. Interruption of a single or double leaf wetness period by a dry period of 4–24 h had little effect on lesion numbers. Conidia of A. porri and S. vesicarium separately or in mixtures caused similar numbers of lesions. Alternaria porri and S. vesicarium are both potentially important pathogens in winter-grown Allium crops and purple leaf blotch symptoms were considered to be a complex caused by both pathogens.     

Latent infection of winter oilseed rape by Leptosphaeria maculans

Plants of oilseed rape, cultivars Primer and Jet Neuf, were grown in a glasshouse and inoculated at G.S. 2.4–2.7 with pycnidiospores or ascospores of Leptosphaeria maculans. The plants were kept for a further 2–4 weeks at 14°C and then transferred, together with uninoculated plants, to a polythene tunnel in winter. The majority of stems of inoculated plants did not have macroscopic symptoms of L. maculans infection 6 weeks after inoculation. Examination of whole mounts of peripheral tissue and transverse sections of fixed and embedded portions of these stems revealed intercellular septate fungal hyphae, often deep in non-necrotic cortical tissue, in symptomless inoculated plants but not in uninoculated plants. L. maculans was recovered following surface sterilization of adjacent portions of the same stems. When symptomless inoculated plants were transferred to a glasshouse at 18–20°C, cankers soon developed. The significance of these latent mycelial infections to canker development in the field is discussed.     

Effects of humidity, leaf wetness, temperature and light on conidial production by Phaeoisariopsis personata on groundnut

Controlled-environment studies of conidial production by Phaeoisariopsis personata on groundnut are described. With constant relative humidity (RH), conidia were only produced above a threshold (94·5% RH) and there was a linear increase between 94·5% RH and 100% RH. Conidial production was less with continuous leaf wetness (resembling heavy dew) than with continuous 98–99% RH, but it was similar with intermittent leaf wetness and intermittent 98–99% RH (8 h at 70% RH each day). With alternate high (≥97% RH) and low humidity, daily conidial production depended both on the duration of high RH and on the low RH value. With 99% RH at night (12 h), night-time conidial production decreased with the previous daytime RH. After conidial production had started, small numbers of conidia were produced even when the RH was well below the threshold (94·5%). Conidia were produced in continuous light when the photon flux density was 2 μmol/m²/s, but production was completely inhibited with 60 μmol/m²/s. With constant RH, more conidia were produced with a 12 h photoperiod than in continuous darkness. However, more than 75% of the conidia were produced in the dark. With continuous darkness, more conidia were produced during the night (18.00–06.00 h) than during the day, but this biological rhythm was overcome with a (light-night)/(dark-day) regime. With constant 98–99% RH there was a linear increase in conidial production with temperature between 10 and 28°C, and virtually no conidia were produced at 33°C. The daily production of conidia increased with time for 2 to 6 days, depending on the treatment.     

Comparative epidemiology of Pyrenopeziza brassicae (light leaf spot) ascospores and conidia from Polish and UK populations

Despite differences in climate and in timing of light leaf spot epidemics between Poland and the UK, experiments provided no evidence that there are epidemiological differences between populations of Pyrenopeziza brassicae in the two countries. Ascospores of Polish or UK P. brassicae isolates germinated on water agar at temperatures from 8 to 24°C. After 12 h of incubation, percentages of ascospores that germinated were greatest at 16°C: 85% (Polish isolates) and 86% (UK isolates). The percentage germination reached 100% after 80 h of incubation at all temperatures tested. The rate of increase in germ tube length increased with increasing temperature from 8 to 20°C but decreased from 20 to 24°C, for both Polish and UK isolates. Percentage germination and germ tube lengths of UK P. brassicae ascospores were less affected by temperature than those of conidia. P. brassicae produced conidia on oilseed rape leaves inoculated with ascospores or conidia of Polish or UK isolates at 16°C with leaf wetness durations from 6 to 72 h, with most sporulation after 48 or 72 h wetness. Detection of both mating types of P. brassicae and production of mature apothecia on leaves inoculated with mixed Polish populations suggest that sexual reproduction does occur in Poland, as in the UK.     

Production and immunodetection of ascospores of Mycosphaerella brassicicola: ringspot of vegetable crucifers

Pseudothecia containing abundant ascospores of Mycosphaerella brassicicola were produced in vitro on Brussels sprout decoction agar at 15°C under a 16-hour photoperiod of different light regimes. Spermogonia containing spermatia were also produced on the decoction agar. Ascospores were released when cultures were misted with SDW and placed under continuous light. Germination of ascospores was highest between 20°C and 25°C and spores remained viable at relative humidities above 93.5%. Exposure of ascospores to 55% relative humidity for 24 h reduced their germination to 75%. A polyclonal antiserum raised against whole ascospores was used to detect, by immunofluorescence, the ascospore and mycelial wall of M . brassicicola , following reaction with anti-rabbit IgG FITC conjugate. Autofluorescence of spore and mycelial components of other fungal species could be eliminated using the counterstains Evan's blue and eriochrome black at 0.2% and 0.5%, respectively, in phosphate buffered saline (pH 7.2). A procedure was developed to detect, by immunofluorescence, ascospores of M . brassicicola on artificially inoculated Melinex spore tape. Coating of the spore tape with bovine serum albumin provided a suitable support medium and blocking agent for detection of ascospores in the field. The potential use of the system for selective detection of ascospores of M . brassicicola in infected crops of vegetable brassicas in the presence of other ascosporic fungi is discussed. Keywords : ascospores, immunofluorescence, Mycosphaerella brassicicola , spore production, spore trapping .     

Effect of soil temperature on disease development in melon plants infected by Monosporascus cannonballus

The effect of soil temperature on melon collapse induced by Monosporascus cannonballus was studied in the laboratory and in the field. In the laboratory, ascospore germination and hyphal penetration into melon roots were enhanced by increasing the temperature from 20 to 32°C. The optimum temperature for mycelial growth of five isolates of M. cannonballus was 30°C. In the field, the effect of temperature was studied in experiments conducted during the winter and autumn cropping seasons from 1995 to 1998. Disease progress was much faster in the autumn than in the winter crop seasons. Disease incidence reached 100% in the three consecutive autumn seasons studied. In the winter seasons, however, planting date influenced disease incidence. Early planting, at the beginning of January, resulted in a low disease incidence (6–26%, 125 days after planting), whereas planting at the end of January resulted in higher disease incidence (72–88%, 95–119 days after planting). In plots in which the soil was artificially heated to 35°C during the winter season, disease incidence reached 85%, as in the autumn season. Plants grown during the winter in unheated soil, or in artificially heated soil disinfected with methyl bromide, did not collapse. Root colonization by the pathogen was higher in the autumn and in heated soil than in the winter season in nonheated soil. Fifty per cent of root segments were colonized 35, 42 and 67 days after planting in the winter-heated, autumn and winter-unheated plots, respectively. A high correlation was found between soil temperatures above 20°C during the first 30 days after planting and disease severity. It is suggested that soil temperature during the early stages of plant development is an important factor in disease development and the expression of melon collapse caused by M. cannonballus.     

Germination of <Emphasis Type="Italic">Gibberella zeae</Emphasis> ascospores as affected by age of spores after discharge and environmental factors

The effects of age of ascospores (0–18 days after discharge), photon flux density (0–494 mol m^–2 s^–1 PAR), temperature (4–30 °C), frost (–15 °C for 30 min), relative humidity (RH; 0–100%), pH (2.5–6.5) and dryness (0 and 53% RH for up to 40 min) on the germination of the ascospores of the mycotoxin-producing fungus Gibberella zeae (anamorph Fusarium graminearum) were studied. Freshly discharged ascospores germinated within 4 h at 20 °C and 100% RH. The rate of germination and the percentage of viable ascospores decreased over time after the spores were discharged from perithecia. The time course of ascospore germination was not significantly affected by photon flux density. The period of time required to obtain 50% germinated ascospores at 100% RH was 26.90 h at 4 °C, 10.40 h at 14 °C, 3.44 h at 20 °C and 3.31 h at 30 °C. There was no significant effect of frost on the percentage of viable ascospores. A small percentage (6.6 ± 3.8%) of the ascospores germinated at 53% RH. At RH 84% and 20 °C almost 100% of the freshly discharged ascospores germinated. The time course of ascospore germination was affected by pH. The maximum rate of ascospore germination was estimated to be at pH 3.76. Ascospores lost their ability to germinate following exposure to 0% RH almost instantaneously. No germinating spores were detected after an incubation period of 1 min at 0% RH. Incubating the ascospores at 53% RH decreased the percentage of viable spores from 93 to 6% within 10 min. The data demonstrate that age of spores, relative humidity, temperature and pH, but not photon flux density, are key factors in germination of G. zeae ascospores.     

Effects of temperature and wetness duration on conidial infection, latent period and asexual sporulation of Pyrenopeziza brassicae on leaves of oilseed rape

Experiments in controlled environments were carried out to determine the effects of temperature and leaf wetness duration on infection of oilseed rape leaves by conidia of the light leaf spot pathogen, Pyrenopeziza brassicae . Visible spore pustules developed on leaves of cv. Bristol inoculated with P. brassicae conidia at temperatures from 4 to 20°C, but not at 24°C; spore pustules developed when the leaf wetness duration after inoculation was longer than or equal to approximately 6 h at 12–20°C, 10 h at 8°C, 16 h at 6°C or 24 h at 4°C. On leaves of cvs. Capricorn or Cobra, light leaf spot symptoms developed at 8 and 16°C when the leaf wetness duration after inoculation was greater than 3 or 24 h, respectively. The latent period (the time period from inoculation to first spore pustules) of P. brassicae on cv. Bristol was, on average, approximately 10 days at 16°C when leaf wetness duration was 24 h, and increased to approximately 12 days as temperature increased to 20°C and to 26 days as temperature decreased to 4°C. At 8°C, an increase in leaf wetness duration from 10 to 72 h decreased the latent period from approximately 25 to 16 days; at 6°C, an increase in leaf wetness duration from 16 to 72 h decreased the latent period from approximately 23 to 17 days. The numbers of conidia produced were greatest at 12–16°C, and decreased as temperature decreased to 8°C or increased to 20°C. At temperatures from 8 to 20°C, an increase in leaf wetness duration from 6 to 24 h increased the production of conidia. There were linear relationships between the number of conidia produced on a leaf and the proportion of the leaf area covered by 'lesions' (both log₁₀-transformed) at different temperatures.     

Relationships between temperature and latent periods of rust and leaf-spot diseases of groundnut

The effect of temperature on the latent periods of rust, late leaf spot and early leaf spot diseases of groundnut caused by Puccinia arachidis, Phaeoisariopsis personata and Cercospora arachidicola , respectively, was studied. The latent periods (LP) of rust, late leaf spot and early leaf spot ranged from 12–49 days, 13–38 days and 13–39 days, respectively, between 12°C and 33°C An equation relating the rate of pathogen development (1/LP) to temperature was fitted using daily mean temperatures to provide three cardinal temperatures: the minimum (T_mln), optimum (T_opl), and maximum (T_max), T_mln was about 12°C for rust and about 10°C for the two leaf-spot diseases. T_opt, for all three diseases was close to 25°C. T_max was 31°C for early leaf spot, and extrapolated values for late leaf spot and rust were about 35 and 40°C, respectively.
For P. personata , a temperature response curve was fitted using data only from controlled environment experiments. This curve was used to simulate latent periods from both mean daily and mean hourly temperatures in the field. There was substantially better agreement between observed and simulated latent period with hourly temperatures, provided the developmental rate of the pathogen was determined at a constant temperature.     

Effects of seed treatments and storage on the incidence of Phoma betae and the viability of infected red beet seeds

Red beet seed clusters retained a high level of germination when stored for 13 years at 10°C and 50% RH in a seed store; seed infection by Phoma betae (black leg) declined from 27–5 to 4–5% over the same period. Seed treatment before storage by soaking in thiram or ethyl mercury phosphate significantly improved germination and reduced mean infection by P. betae from 17% to less than 1%.     

Infection of ryegrass by three rust fungi (Puccinia coronata, P. graminis and P. loliina) and some effects of temperature on the establishment of the disease and sporulation

Experiments were conducted to examine the processes leading up to the infection of Lolium temulentum by crown rust ( Puccinia coronata ), stem rust ( P. graminis ) and brown rust ( P. loliina ), and the effects of temperature on these processes and sporulation. Uredia of all three rusts were produced freely if the adaxial leaf surface was inoculated, but did not form following inoculation of the abaxial surface. Light and scanning electron microscopy revealed abnormal growth of germlings on the abaxial surface which had amorphous sheet-like epicuticular waxes and very few stomata. On the adaxial leaf surface germ tubes of all the rusts orientated at right angles to the long axis of the leaf. However, the directional growth of germ tubes was often disrupted when they contacted the surface of bulliform cells at the base of leaf grooves. For P. loliina the optimum temperatures for urediospore germination and sub-stomatal vesicle formation were 12–16°C, and 8–20°C for appressorium formation. The optimum temperatures, for the same stages of fungal development, for P. coronata and P. graminis were higher. Urediospore production of P. loliina was higher at 10°C than at 25°C, but was similar at both temperatures for P. coronata .     

Quantitative resistance to symptomless growth of Leptosphaeria maculans (phoma stem canker) in Brassica napus (oilseed rape)

Quantitative resistance to Leptosphaeria maculans in Brassica napus was investigated in field and controlled environments using cultivars Darmor (with quantitative resistance) and Eurol (without quantitative resistance). In field experiments, numbers of phoma leaf spot lesions in autumn/winter and severity of stem canker the following summer were assessed in three growing seasons. There were no differences between Darmor and Eurol in number of leaf lesions in autumn/winter. However, stem cankers were less severe on Darmor than Eurol at harvest the following summer. In controlled-environment experiments, development of leaf lesions at different temperatures (5–25°C) and wetness durations (12–72 h) was investigated using ascospore inoculum; symptomless growth of L. maculans along leaf petioles towards the stem was quantified using quantitative PCR and visualized using GFP-expressing L. maculans ; growth of L. maculans within stem tissues was investigated using GFP-expressing L. maculans . There were more leaf lesions on Darmor than Eurol, although there was no difference between Darmor and Eurol in L. maculans incubation period. There were no differences between Darmor and Eurol in either distance grown by L. maculans along leaf petioles towards the stem or quantity of L. maculans DNA in leaf petioles, but L. maculans colonized stem tissues less extensively on Darmor than Eurol. It was concluded that quantitative resistance to L. maculans operates during colonization of B. napus stems by the pathogen.     

Seasonal patterns of dispersal of ascospores of Cryphonectria parasitica (chestnut blight)

Infected barks of chestnut blight cankers, caused by Cryphonectria parasitica , were collected from a naturally infected orchard and incubated at different temperatures. Cankers started to discharge ascospores about a week after incubation at 15–25°C; most ascospores were collected at 20 and 25°C. When incubated at 5, 10 or 30°C, only a few cankers released a small number of ascospores and only during the later stages of incubation. However, the rate of formation of perithecia was not affected by the incubation temperature. The number of airborne ascospores was monitored using a volumetric spore trap in a chestnut orchard during 1996 and 1997. In both years, the number of ascospores trapped daily varied greatly, but in general it increased sharply from March onwards, reached a peak in May, and then declined steeply. There was a significant correlation between daily counts of ascospores and air temperature. Time-series transfer function (TF) analysis showed a positive association of the daily number of ascospores with increasing temperature, rain events and wet/humid conditions. In general, values predicted by the TF model agreed well with the observed pattern. However, a multiple regression equation based on TF analysis failed to provide a satisfactory prediction of the daily number of ascospores.     

Role of temperature in regulating timing of germination in soil seed reserves of Thlaspi arvense L.

Summary. Most freshly-matured seeds of Thlaspi arvense L. (Brassicaceae) were dormant at maturity in May. Seeds sown on soil germinated in autumn and spring, but mostly in autumn. Buried seeds exhumed at monthly intervals and tested in light and darkness over a range of thermoperiods exhibited annual dormancy/non-dormancy cycles. However, the dormant period was short, usually only in April, but sometimes May, and in some years 1–6% of the seeds remained conditionally dormant. After-ripening occurred during summer, and seeds were non-dormant during autumn. Seeds entered conditional dormancy in winter and dormancy in late winter or early spring. When buried dormant seeds were kept at 25/15, 30/15 or 35/20°C for 12 weeks, they gained the ability to germinate to 95–100% at 15/6, 20/10, 25/15, 30/15 and 35/20°C. After burial for 12 weeks at 15/6 and 20/10°C, seeds germinated to 80–100% at 15/6, 20/10 and 25/15°C. but to only 11–64% at 30/15 and 35/20°C. After 4 weeks at 5°C, initially-dormant seeds germinated to 100% at all thermoperiods except 35/20°C, where only 15% of them germinated. However, after 18 weeks at 5°C, only 0–1% of the seeds germinated at all thermoperiods. Most non-dormant seeds exposed to 1, 5 and 15/6°C for 16 weeks were induced into dormancy; 1–15% entered conditional dormancy and thus germinated only at 15/6, 20/10 and 25/15°C. This study indicates that seeds of winter annual plants of T. arvense are non-dormant in autumn and enter dormancy in winter, while those from summer annuals are dormant in autumn and become non-dormant during winter.     

Formation and survival of oospores of Phytophthora infestans under natural conditions

Phytophthora infestans is able to produce oospores in leaves of potato and tomato plants after inoculation with a mixture of Al and A2 mating-type isolates. Various conditions for oospore formation were analysed. Under controlled conditions, oospores were produced in potato leaves at temperatures ranging from 5 to 25° C. In leaves of potato cultivar Bintje incubated at 15°C, oogonia and antheridia were observed 6 days after inoculation and thick-walled oospores appeared 3-4 days later. In field experiments oospores were found in leaves and stems of potato cultivars Bintje, Irene and Pimpernel and in leaves, stems and fruits of tomato cultivar Moneymaker within 2 weeks after inoculation. A bioassay was developed to test the survival of oospores in soil under various conditions. To determine whether late-blight infections derived from infectious soil were caused by oospwres, DNA fingerprinting was performed. DNA fingerprint probe RG-57 was suitable for distinguishing asexual progeny from recombinant progeny arising from soil-borne oospores. We demonstrated survival of viable, infectious oospores of P. infestans in soil during the winter of 1992–93. Oospores were not infectious from soil exposed to temperatures of 40°C or higher but in the range 35°C to as low as – 80°C for 48 h, oospores survived.     

Solarization and natural heating of irrigated soil amended with cruciferous residues for improved control of Macrophomina phaseolina

The efficacy of summer irrigation and soil solarization combined with cruciferous residues was tested against the dry root rot pathogen Macrophomina phaseolina in an arid climate. In irrigated amended soil, polyethylene mulching during May increased the soil temperature to 57°C and 50°C at depths of 0–15 and 16–30 cm, respectively. As a result, within l5 days the population of M. phaseolina was almost eradicated (93–99%) at both soil depths. A considerable reduction (75–96%) was also achieved by natural heating of irrigated soil (46–53°C) for l5 days after amending with cruciferous residues. Mulching alone was only partially effective (69–89% reduction). These results suggest a new approach to controlling soil-borne pathogens in hot, arid regions by combining summer irrigation with soil amendment. Amendment with residues alone or in conjunction with soil solarization also increased the population of lytic bacteria against M. phaseolina .