Production of oospores by Peronospora viciae f.sp. fabae

Oospore production inVicia faba, cv. Metissa, was quantified in the field after plants had been inoculated with a sporangium suspension of a homothallic isolate ofPeronospora viciae f.sp.fabae. Oospores were produced abundantly during the hole growing season from 3 weeks after inoculation on. Oospores were found in all plant parts above soil level, except in the seeds. Most oospores were found in the leaves. Less oospores were formed in leaves inoculated in an older stage than in those inoculated in a younger stage. Towards the end of the season, in August, numbers of oospores in pods strongly increased.Oospore production in leaves of three cultivars, Metissa, Toret and Maris Bead, was studied in growth chambers at 5, 10, 15 and 20 °C at 16 h light. Oospores were formed earlier at higher temperatures than at lower temperatures. The ultimate numbers of oospores produced in leaves were highest at 10 and 15 °C. Similar numbers of oospores were formed in leaves of cultivars Metissa and Toret. In leaves of cv. Maris Bead significant less oospores were produced than in leaves of cv. Metissa and cv. Toret. Total numbers of oospores produced were not related to the level of host plant resistance to downy mildew. The percentage of asexually sporulating leaf area, assessed in a resistance test, was largest in cv. Metissa and smallest in cv. Toret.     

Production, survival and infectivity of oospores of Phytophthora infestans

The formation of oospores of Phytophthora infestans was studied in tomato and potato crops and volunteer plants under field conditions, and in laboratory tests with leaf discs of potato cultivars differing in their level of race-nonspecific resistance. Oospores were readily detected in blight-affected tomato leaflets and fruits, and in leaflets of field crops and volunteer potato plants. Oospores extracted from blighted potato leaflets yielded 13 oospore-derived progeny. Oospores were also produced following inoculation of leaf discs of eight potato cultivars expressing different levels of race-nonspecific resistance with a mixture of sporangia of A1 and A2 isolates. The highest numbers of oospores were produced in cvs Bintje (susceptible) and Pimpernel (resistant), and the lowest in Nicola (intermediate resistance). The relationship between lesions per leaflet and oospore incidence, affected by varying A1 : A2 ratios, was explored using a simple mathematical model, and validated by comparing actual oospore production in leaflets with multiple lesions of the race-nonspecific-resistant potato clone Lan 22-21 with the predictions generated by the model. Survival of oospores was investigated after their incorporation in either a sandy or a light clay soil in buried clay pots exposed to the local weather conditions. Over 6 years these soils were regularly assessed for their infection potential using floating leaflets in a spore-baiting bioassay. Sandy and clay soils contaminated with oospores remained infectious for 48 and 34 months, respectively, when flooded. Infections of floating potato leaflets occurred within 84–92 h and ceased after 11 days. Soil samples remained infective if dried and re-flooded on two, but not more, occasions.     

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.     

The effect of temperature on colony growth by Erysiphe sp. infecting Rhododendron

The growth and reproduction of powdery mildew pathogens is generally encouraged by increasing temperatures, up to 25°C. Germination and germ tube extension of Erysiphe sp. on Rhododendron cv. Elizabeth were optimal at 20°C for conidia originally formed at either 10 or 15°C. During a 50-day period of colony growth, the viability of conidia formed at 15°C declined but for those formed at 10°C it increased. The expansion of Erysiphe colonies over an initial 12-day period was favoured by incubation at 15°C compared with 10 or 20°C. In the first 8 to 10 days of growth, secondary and tertiary hyphae formed most rapidly at 20°C. After 12 days, expansion of colonies at 20°C was limited to the area initially infested by primary hyphae, whereas in colonies grown at 10 and 15°C secondary and tertiary hyphae had extended beyond the area first colonized. Small colonies of densely packed hyphae formed at 20°C compared with open spreading colonies observed at 10 and 15°C.     

Oospore Production of Phytophthora infestans in Potato and Tomato Leaves

ABSTRACT Fungal, host, and environmental factors affecting sexual reproduction of Phytophthora infestans in planta were studied. Intact and detached leaves were coinoculated with sporangia of various combinations of A(1) and A(2) mating-type isolates; leaves were incubated under various conditions, and oospore production was estimated microscopically within whole, clarified leaflets. Some A(1) + A(2) isolate combinations were more reproductive than others, whereas some potato genotypes better supported oospore formation than others. Tomato usually supported more oospore formation than potato. To induce oospore formation, A(1) and A(2) sporangia were usually mixed at a 1:1 ratio. Ratios of 1:19 to 19:1, however, also allowed abundant production of oospores. Optimal temperatures for sexual sporulation ranged from 8 to 15 degrees C, but oospores also were produced at 23 degrees C. Oogonia developed 5 to 6 days after sporangial coinoculation, and oospores developed after 8 to 10 days. Light had little effect on oospore formation in both tomato and potato leaves provided that initial lesions were established under photoperiodic conditions. Although A1 and A(2) sporangia usually were mixed before inoculation on leaves to obtain oospores, we found that discrete A(1) and A(2) lesions produced on opposite sides of the midvein of tomato leaves also induced oospore formation in the midvein and adjacent tissues. Oospores also formed when the two halves of the leaves were cut and separated at 3 days after sporangial coinoculation, which corresponded with the appearance of late blight lesions. The continuous supply of moisture to infected leaves was essential to oospore production. No oospores or oogonia formed in severely diseased plants kept at 50 to 80% relative humidity. Such plants did allow some oospore formation when kept continuously wet for 2 weeks in plastic boxes or tents. Detached leaves floated on water supported the highest sexual sporulation. Under optimal conditions of wetness and temperature, as many as 100 oospores per mm(2) of tissue were observed.     

Differences in host range, pathogenicity to potato cultivars and response to soil temperature among Streptomyces species causing common and netted scab in France

The pathogenicity and ecology of some isolates representative of the four main Streptomyces species ( S. scabies , S. europaeiscabiei , S. stelliscabiei and S. reticuliscabiei ) identified as pathogenic to potato tubers were investigated. Three pathogenicity groups could be distinguished. Group 1 included all isolates of S. scabies , S. europaeiscabiei and S. stelliscabiei from common scab lesions of potato and other susceptible root crops. All these produced similar symptoms and were pathogenic to potato, carrot and radish. Group 2 included all isolates from S. reticuliscabiei netted scab lesions; they were pathogenic to both tubers and roots of only a few potato cultivars, and did not infect carrot or radish. Group 3 included three isolates of S. europaeiscabiei from netted scab lesions on cv. Bintje, which produced either common or netted scab symptoms depending on the potato cultivar or plant species. In an experiment on a few isolates from each of the three groups, held at various soil temperature regimes, the three from group 1 were most pathogenic at higher temperatures (20°C or 20/30°C), the two from group 2 were most pathogenic at a lower temperature (17°C). The group 3 isolate caused netted scab symptoms on susceptible cultivars at low temperatures (≤ 20°C) and deep-pitted lesions at higher temperatures. Since the groups identified differ in ecological requirements, it is important to adapt the control methods to the pathogenic species present in the soil.     

Sprinkling irrigation enhances production of oospores of phytophthora infestans in field-grown crops of potato

ABSTRACT Two field experiments were conducted to study the effect of overhead sprinkling irrigation on oospore formation by the late blight fungus Phytophthora infestans in potato. Total rain (natural + sprinkling) accumulated in treatments of experiment 1 (winter 1997 to 1998) were 765, 287, and 219 mm and treatments of experiment 2 (winter 1999 to 2000) were 641, 193, and 129 mm. Sporangia from 11 isolates of P. infestans were combined in eight pairs, seven of A1 and A2 and one of A2 and A2 mating type, and were sprayed on field-grown potato crops (42 plants per plot at 7 m(2) each) and examined for their ability to form oospores in the host tissues. In experiment 1, oospores were recorded in a total of 132 of 1,680 leaflets (7.9%), 24 of 105 stems, and 2 of 90 tubers. In experiment 2, oospores were recorded in 40 of 519 leaflets (7.7%), but not in any of the 90 stems or the 45 tubers examined. Both the proportion of leaflets containing oospores and the number of oospores per leaflet increased with time after inoculation and were dependent on the rain regime, the position of leaves on the plant, and the isolate pair combination. In both field trials, increasing the rainfall significantly enhanced oospore production in leaves. Leaf samples collected from the soil surface had significantly more oospores than those collected from the midcanopy. Two pairs in experiment 1 were more fertile than the others, whereas the pair used in experiment 2 was the least fertile. The total number of oospores per leaflet usually ranged from 10 to 100 in experiment 1, but only from 2 to 10 in experiment 2. Maximal oospore counts in the field were 200 and 50 in experiments 1 and 2, respectively, but ranged from approximately 2,000 to 12,000 oospores per leaflet in detached leaves in the laboratory. We concluded that P. infestans can produce oospores in the foliage of field-grown potato crops, especially when kept wet by regular overhead sprinkling irrigation, but production was far below that in the laboratory.     

Factors affecting germination of oospores of Peronospora viciae f.sp. pisi in vitro

The effects of host plant exudates, light and temperature on germination of oospores of Peronospora viciae f.sp. pisi in vitro were investigated. Seed and root exudates did not increase percentage germination, whereas light inhibited germination. The first germ tubes appeared after 4, 7, and 14 days of incubation at 15, 10 and 5 °C, respectively. The eventual level of germination was highest and had similar values at 5 and 10 °C. At 20 °C germination was poor and at 25 °C no germination was observed. Oospores placed on membrane filters were incubated on soil. When oospores were retrieved from the membrane filters after six days and placed in water at 10 °C, they germinated within 2 days. On soil significantly less oospores germinated than in water. Germinability of oospores stored in the dark at 5 or 20 °C at 30 or 76% RH was studied over a two-year period. Germinability generally increased over time, but fluctuations were observed indicating the occurrence of secondary dormancy. Time courses of germinability were generally similar for oospores stored at several temperatures and humidities. No effect of light on time course of germinability was found when oospores were exposed to alternating light-dark periods or stored in continuous dark for 140 days. Percentage germination observed in a germination assay was correlated with percentage infection determined in a bioassay.     

In vitro effects of temperature and wet periods on infection of oilseed rape by Alternaria brassicae

The influence of temperature and duration of wet periods on infection of oilseed rape by Alternaria brassicae was studied on detached leaves and pods, leaf disks and intact seedlings. Infections increased with age of leaf and the interaction between temperature and leaf age was highly significant. On older leaves infection was optimal at 25°C. There were many infections also at 15, 20 and 29 C but relatively few infections at 10°C. On pods most infections were observed at 20 C, the highest temperature studied. Infection at each temperature increased progressively with duration of surface wetness. The minimum wet periods for infection of leaves were 3 h at 20–25°C, 4 h at 15°C, 6–9 h at 10 C and 12–24 h at 5 C and for infection of pods, between 6 h and 9 h at 10°C and 6 h (or less) at 15°C and 20 C. On leaves, dry periods interrupting wet periods limited lesion development to that obtained with the initial wet period only; on pods some further infections developed when pods were re-wetted. Dry periods of 3 h and 6 h following the inoculation of pods reduced subsequent infection but there was no further reduction by longer periods of drying to 48 h.     

Effects of wetness period and temperature on development of dark pod spot (Alternaria brassicae) on oilseed rape (Brassica napus)

In controlled environment experiments, when oilseed rape pods or leaves were inoculated with spore suspensions of Alternaria brassicae, the maximum disease incidence (proportion of pods or leaves diseased) increased as wetness period after inoculation increased from 4 to 24 h and as temperature increased to 20°C. There was a clear relationship between disease incidence on pods and incidence on leaves with the same wetness/temperature conditions. Logistic equations described the effects of wetness period after inoculation on disease incidence (number of pods or leaves infected) or disease severity (number of lesions on pods or leaves) using temperature-dependent and tissue-dependent parameters. The time from inoculation to the appearance of the first lesions was shorter on pods than on leaves at temperatures ≤15°C and wetness periods ≤12 h. Two-dimensional response surface equations or simple interpolations from one-dimensional equations were used to develop contour maps of expected disease incidence and severity, respectively, on leaves or pods to estimate the effects of different combinations of wetness period during infection and temperature on disease development.     

Effects of humidity and air speed on sporulation of Phytophthora infestans on potato leaves

An apparatus is described for the control of humidity, air speed and temperature around detached potato leaflets and whole plants. Leaflets and plants inoculated with Phytophthora infestans were incubated in air at 15°C. Abundant sporangia were formed in an air speed of 0.3 × 10⁻³ m/s when the ambient humidity was 90–100% RH, but not at 85 or 80% RH. At air speeds of 5.5 × 10⁻³ and 13.7 × 10⁻³ m/s there were numerous sporangia at 100% RH, but not at 95–80% RH. The number of sporangia formed on leaflets kept in air flowing at 5.5 × 10⁻³ m/s with alternating humidities of 80 and 100% RH each for 12 h per day was intermediate between numbers formed on leaflets incubated at each humidity continuously, and higher than numbers formed at a constant 90% RH. Removing leaflets from plants before inoculation did not affect the number of sporangia produced.     

Effect of temperature of production of Botrytis allii conidia on their pathogenicity to harvested white onion bulbs

Botrytis allii colonies incubated at low temperatures have been reported to produce larger conidia that germinate faster and give rise to longer germ-tubes than those grown at room temperature. The present study compared the effect of conidia produced at 20°C and at 0 and –2°C on their pathogenicity to artificially inoculated white onion bulbs, and the effect of conidial concentration (5×10³ and 5×10⁴ conidia/mL) on disease incidence, lesion area, incubation and latent period during storage at 20, 5 and 0°C. At all storage temperatures and periods tested conidia produced at −2°C caused a higher disease incidence and larger areas of rot than those produced at higher temperatures. When the conidial production temperature was raised to 20°C, the duration of incubation on the bulbs inoculated with 5×10⁴ conidia/mL was more than doubled during storage at 0°C, tripled at 5°C, and took 50% longer at 20°C. The incubation period was not significantly affected by conidial concentration at 20°C, and only slightly at 5 and 0°C, but at low temperatures the latent period was longer because of the delay induced in sporulation. These data are consistent with the packers' opinion that cross-infection of spring onions by long-term refrigerated onions in grading lines caused earlier and heavier rotting.     

Components of quantitative resistance to powdery mildew (Erysiphe pisi) in pea (Pisum sativum)

Components of quantitative resistance in pea ( Pisum sativum ) to Erysiphe pisi , the pathogen causing powdery mildew, were investigated. Conidium germination, infection efficiency, latent period and conidium production dynamics on cv. Quantum (quantitatively resistant) were compared with those on Pania and Bolero (susceptible). There was an additional comparison in conidium germination experiments with the resistant cv. Resal. Quantitative resistance in Quantum did not affect conidium germination, but infection efficiency of conidia on this cultivar was 34% less than on the susceptible Pania. More conidia germinated on 5-day-old leaflets than on 15-day-old leaflets but the age of the plant did not affect percentage germination or infection efficiency. The length of the latent period did not differ between cultivars. Total conidium production (AUC) per unit leaflet area on Quantum was 25% less than on Pania. The maximum conidium production per day (CMAX) per unit leaflet area on Quantum was 33% less than on Pania. The time to maximum conidium production per day (TMAX) was 10% longer on Quantum than on Pania. The cv. Bolero, reported to be susceptible, also showed some degree of quantitative resistance, but this differed from that of Quantum. Total conidium production was less on Bolero than on Quantum, but the conidia on Bolero were produced sooner, and for a shorter period, than on Quantum. The stability of these responses was tested by analysing components in three different temperature regimes and testing for interactions with temperature, and with leaflet age. Temperature affected all conidium production variables. AUC per leaflet area was nearly seven times as great and CMAX nearly 15 times greater at 23°C than at 13°C. TMAX increased by 1.5 times when temperature increased from 13°C to 18°C or 23°C. Several interactions occurred and these are described.     

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.     

Viability, germination and infection potential of oospores of Phytophthora infestans

Matings between five A1 and five A2 wild-type isolates of Phytophthora infestans from potato and tomato crops in the United Kingdom produced oospores in vitro in all cases examined. Oospores from the majority of crosses germinated, albeit at a low level (max 13-4%), after extraction from agar cultures by high-speed blending and treatment with novoZym 234. Viability of oospores from 20 crosses was tested by three methods. Two methods involved stains, either tetrazolium bromide (MTT) or phloxine B, and the third measured plasmolysis in 2 M NaCl. Both staining methods indicated a high percentage viability but gave false-positive results with heat-killed oospores. The plasmolysis method gave a lower percentage viability but no false positives. Oospores produced in vitro and stored either in sterile H₂O or in soil at temperatures between 0 and 20 C survived for between 5 and 7 months, the length of the experiments. Oospores buried in non-sterile field soil survived for up to 8 months (January-August). Inoculation of potato with zoospores of AI and A2 isolates produced oospores in stems but not in leaf tissue. In some, but not all cases, rapidly growing potato shoots (15-mm long) were successfully infected with oospores produced in vitro.     

Effects of temperature and light on non-race-specific resistance of potato leaflets to late blight

Effects of temperature and illumination on colonization by Phytophthora infestans of detached leaflets of five potato cultivars differing in field resistance to blight were investigated using an ELISA system to quantify the pathogen. Leaflets of cvs Teena and Shelagh, and in one experiment cv. Brodick, were more resistant to colonization when infected leaflets were incubated at 10°C than at 20°C, but temperature conditions before inoculation had little effect. Both photoperiod and light intensity during illumination of intact plants before inoculation interacted with genotype to determine subsequent colonization of infected leaflets incubated in darkness. Leaflets from plants of cv. Teena were more resistant to colonization after exposure to low, rather than to high, light intensities but photoperiod had no apparent effect. Leaflets of cv. Shelagh grown in a 20 h day were more resistant than those grown in a 10 h day but light intensity had no effect. Leaflets of cv. Brodick were more resistant after a 20 h day than a 10 h day and after exposure to low, rather than to high, light intensities. Leaflets of cv. Bintje were extensively colonized and those of cv. Torridon remained relatively resistant to colonization, irrespective of temperature and lighting conditions. There was a high level of unexplained variation in all the experiments.     

Growth in vitro and infectivity of Colletotrichum coccodes on potato tubers at different temperatures

To investigate the ability of black dot symptoms to develop on infected potato tubers during storage, the growth of Colletotrichum coccodes was followed in vitro on malt agar at temperatures ranging from 5–27°C, and in vivo on artificially infected potato tubers kept at 5, 10 and 15°C. In vitro , 13 isolates from different geographical origins grew at all temperatures tested; growth started with a delay of 10 days at 5°C and of 4 days at 10°C, and was fastest at 27°C. All isolates had similar growth patterns and produced conidia and sclerotia at all temperatures. Minitubers were successfully infected at 5, 10 and 15°C by depositing either a mycelial plug or a drop of conidial suspension on the tuber surface. Sclerotia were observed after 7 days at the point of inoculation. Symptoms extended in all cases, although more slowly at 5 and 10 than at 15°C. Latent infections were detected in up to 21% of tubers without black dot symptoms at harvest. These results show that latent infections by C. coccodes are probably quite frequent, and that the pathogen is able to develop at low temperatures in controlled conditions. This suggests that black dot symptoms can increase during storage if stores are not adequately managed.     

Effects of temperature and leaf wetness on Pyrenophora teres growing on barley cv. Sonja

Conidia of Pyrenophora teres germinated only in the presence of liquid water and at temperatures above 2°C. The speed with which germination occurred was inversely proportional to temperature measured from a base of 2°C, up to the maximum temperature tested of 21°C. Once conidia on leaves had been wetted, about 40% of all infections that would eventually occur were established within 100°C-hours. Subsequent lesion extension was rapid, with area doubling times of about 1 day between 10 and 20°C.
If conidia germinated, up to 80% formed successful infections on young, susceptible leaves. On older leaves fewer spores germinated and the proportion that then infected was smaller.
The latent period, defined as the time before which sporulation did not occur under any wetness conditions, ranged from about 25 days at 5°C to 11 days at 20°C under dry conditions. Under continuously wet conditions it was about 20% shorter at all temperatures. Its inverse had a curvilinear relation to temperature.
Spores were produced after one to several days of humidity above 95%. The precise period decreased with increasing temperature, but at 25°C spores never appeared. The drier a dead leaf was, the longer the pathogen in it look to produce spores.     

International Symposium on the Evaluation of Biological Activity,Wageningen, April, 1975

Abstract

A method was devised for estimating large populations of cowpea aphid Aphis craccivora Koch (Hemiptera: Aphididae) on cowpea. The densities of the aphid on stem, leaves and pod were first estimated in the laboratory for use in the field later. The densities were estimated in three arbitrary classes of infestation, viz. heavy, medium and low, determined visually based on the intensity of colonization. The densities of the aphid per cm² on stems were 108.50, 49.77 and 2503, while on pods they were 80.68, 44.46 and 23.20, in the three classes of infestation respectively. The aphid did not show any preference for the thickness of stems whereas young pods were preferred to old pods for colonization. The mean aphid densities on leaflets of topmost leaves were 62.10, 36–35 and 18–50, while on leaflets of top 2nd and 3rd leaves they were 124–60, 69–35 and 38.20, in the three infestation classes respectively. Comparisons made between the aphid densities on different parts of cowpea plant showed that the aphid colonized them in descending order of preference as stem > pod > top most leaf > top 2nd and 3rd leaves.     

Production and Germination of Oospores of Phytophthora infestans (Mont.) de Bary in Morocco

One hundred and eight isolates of Phytophthora infestans were collected from infected potato and tomato crops in the middle-north of Morocco during 1997–2000. Pairings of these isolates with tester isolates of mating type A1 and A2 revealed that 60% of the isolates were mating type A2 (65/108) and 40% were mating type A1. After 10 days incubation at 20 °C and a 16-h photoperiod, approximately 25% and 18% of the oospores produced in-vitro germinated in potato soil extract and potato root extract, respectively. Oospores were observed in potato leaf tissues in pairings that were fertile in-vitro. Maximum production of oospores was obtained in potato leaves of cultivars that were moderately susceptible (Desirée, Nicola) after 10 days of incubation at 15 °C and a 16-h photoperiod. These results confirm the presence of P. infestans strains that are sexually compatible under Moroccan climatic conditions. Production of oospores constitutes a threat for these crops because of the occurrence of recombinants with new virulences which may be difficult to control and as a consequence survival of oospores in absence of the host plant in the soil.