Formation, production and viability of oospores of Phytophthora infestans from potato and Solanum demissum in the Toluca Valley, central Mexico

Aspects of the ecology of oospores of Phytophthora infestans were studied in the highlands of central Mexico. From an investigation of a random sample of strains, it was found that isolates differed in their average capability to form oospores when engaged in compatible pairings. Most crosses produced large numbers of oospores but a few yielded none and some yielded only a few oospores. The results reveal that oospore production and fecundity is dependent on both isolates and the combining ability of a specific combination of parental strains. On average, 14% of the oospores produced were viable as determined by the plasmolysis method. Viability ranged from a low 1% in one cross to a high of 29% in another cross. Oospores were found in 10-20% of naturally infected Solanum demissum leaves from two different collections, and leaflets with two lesions per leaflet produced more oospores than did leaflets with 3-5 lesions per leaflet. There was no consistent trend for preferential mating between isolates from the same location or host

Sexual reproduction in Phytophthora infestans

Potato late blight, caused by the oomycete Phytophthora infestans, is one of the most devastating plant diseases worldwide. It is becoming increasingly difficult to control the disease in Sweden, leading to an ever more intensified use of fungicides in the potato production. This unsustainable development could be caused by changes in the pathogen due to the introduction of sexual reproduction. These changes could result in an enhanced capability of the pathogen to adapt to different factors, e. g. weather, cultivar resistance, chemical control or other cultivation measures. This might manifest itself in earlier infections and a late blight that spreads faster in the potato fields requiring increasing efforts to control. Different approaches were taken to clarify the role of sexual reproduction in P. infestans in the epidemiology of potato late blight. The formation of sexually formed oospores and their ability to serve as primary inoculum was studied, both as field observations and by determining within-field genotypic diversity. To further estimate the importance of reproduction on a large spatial scale a population study based on molecular and phenotypic markers was performed on P. infestans isolates from the whole Nordic region. The variation in the aggressiveness of Nordic populations of P. infestans was studied by determining different components of aggressiveness, e.g. sporulation capacity, lesion growth and infection efficacy. The aggressiveness study was combined with an analysis of the phenotypic structure of the Nordic population of P. infestans. The results clearly indicate that the Swedish populations of P. infestans are influenced by sexual reproduction. The facts that both mating types are found all over the country in near 1:1 proportions and that oospores are commonly formed in field crops and serve as inoculum under field conditions support this. Studies of the genetic diversity also indicate that sexual reproduction has an effect on the population structure of P. infestans in Sweden and the Nordic region as a whole

Oospores of Phytophthora infestans in soil provide an important new source of primary inoculum in Finland

There have been numerous indications since the 1990s that oospore-derived primary infections play an increasing role in the epidemiology of potato late blight. The aim of this study was to verify that oosporederived epidemics actually occur in Finland. For this purpose, 20 suspected foci of oospore-derived potato late blight were scouted in 2000–2002. All of these were located in fi elds in which late blight had been observed in at least one of the four previous years. Primary symptoms in these foci occurred always on the lowest leaves near or touching the ground. Leafl ets typically showing numerous primary infections or lesions were in direct contact with the soil. In the former, oospores were observed after incubation. Soil samples from two experimental fi elds, marked by severe epidemics in most years during the last decade, caused infections in a bioassay. Both mating types were on every occasion present in groups of single lesion isolates collected from foci and the bioassay. Oospores survived over the winter, as shown by soil samples taken during the spring that infected potato leafl ets in the bioassay. The results presented indicate that oospore-derived epidemics occur in Finland. This paper also discusses the role of oospores in causing blight epidemics in Finland compared to more southern countries

Biologically active Phytophthora mating hormone prepared by catalytic asymmetric total synthesis

A Phytophthora mating hormone with an array of 1,5-stereogenic centers has been synthesized by using our recently developed methodology of catalytic enantioselective conjugate addition of Grignard reagents. We applied this methodology in a diastereo- and enantioselective iterative route and obtained two of the 16 possible stereoisomers of Phytophthora hormone α1. These synthetic stereoisomers induced the formation of sexual spores (oospores) in A2 mating type strains of three heterothallic Phytophthora species, P. infestans, P. capsici, and P. nicotianae but not in A1 mating type strains. The response was concentration-dependent, and the oospores were viable. These results demonstrate that the biological activity of the synthetic hormone resembles that of the natural hormone α1. Mating hormones are essential components in the sexual life cycle of a variety of organisms. For plant pathogens like Phytophthora, sexual reproduction is important as a source of genetic variation. Moreover, the thick-walled oospores are the most durable propagules that can survive harsh environmental conditions. Sexual reproduction can thus greatly affect disease epidemics. The availability of synthetic compounds mimicking the activity of Phytophthora mating hormone will be instrumental for further unravelling sexual reproduction in this important group of plant pathogens.

Strategies to control late blight in potatoes in Europe

In Europe an aggressive genetically diverse population of potato late blight is present which regularly causes problems in all potato growing regions. It is therefore of the utmost importance that blight is managed in an integrated way by combining a range of measures. Hygiene measures can keep the number of primary sources of inoculum low. In a number of European countries campaigns are ongoing to increase the awareness of growers to reduce the risk of early inoculum sources such as dumps, volunteers and infected seed. The use of varieties with stable resistance for foliar and tuber blight is also a part of an integrated control strategy. In intermediate resistant varieties possibilities are investigated to reduce the input of fungicides. Fungicides still have a key role to play in the integrated control of late blight. In order to optimize the use of fungicides it is important to know the effectiveness and type of activity of the active ingredients to control blight. The use of fungicides should be targeted by using information on infection conditions based on weather data, disease pressure and fungicide characteristics. Decision Support Systems (DSS) can be used to integrate and organize all the available information required for decisions to control late blight

Epidemiology and integrated control of Potato Late Blight in Europe

Phytophthora infestans, the causal agent of late blight, is a major threat to potato production in northwestern Europe. Before 1980, the worldwide population of P. infestans outside Mexico appeared to be asexual and to consist of a single clonal lineage of A1 mating type characterized by a single genotype. It is widely believed that new strains migrated into Europe in 1976 and that this led to subsequent population changes including the introduction of the A2 mating type. The population characteristics of recently collected isolates in NW Europe show a diverse population including both mating types, sexual reproduction and oospores, although differences are observed between regions. Although it is difficult to find direct evidence that new strains are more aggressive, there are several indications from experiments and field epidemics that the aggressiveness of P. infestans has increased in the past 20 years. The relative importance of the different primary inoculum sources and specific measures for reducing their role, such as covering dumps with plastic and preventing seed tubers from becoming infected, is described for the different regions. In NW Europe, varieties with greater resistance tend not to be grown on a large scale. From the grower’s perspective, the savings in fungicide input that can be achieved with these varieties are not compensated by the higher (perceived) risk of blight. Fungicides play a crucial role in the integrated control of late blight. The spray strategies in NW Europe and a table of the specific attributes of the most important fungicides in Europe are presented. The development and use of decision support systems (DSSs) in NW Europe are described. In The Netherlands, it is estimated that almost 40% of potato growers use recommendations based on commercially available DSS. In the Nordic countries, a new DSS concept with a fixed 7-day spray interval and a variable dose rate is being tested. In the UK, commercially available DSSs are used for c. 8% of the area. The validity of Smith Periods for the new population of P. infestans in the UK is currently being evaluated

Factors affecting Pythium ultimum oospores : production, germination, and virulence to cotton seedlings in soil

In a field test, severity of cotton (Gossypium hirsutum) seedling disease was not affected by nitrogen fertilization. There were no differences in disease severity in seedlings grown in soils that had received 0, 90, or 180 kg/ha, either in the form of NH4CL or NaNO3. Oospore production by Pythium ultimum (Trow) was determined on five liquid culture media. Higher numbers of oospores were produced on a basic mineral medium containing lecithin and on a com meal medium than on three other media evaluated. Numbers of oospores increased progressively during the 30-day incubation period on four of the media. Maximum oospore formation occurred in a medium containing 1.7 g/L of lecithin in a soybean oil formulation. The addition of oospores of ultimum to soil significantly increased cotton seedling disease in previously sterilized and in nonsterilized soil which had been air-dried for 1 year, but not in oospore-amended fresh field soil. A significant amount of infection occurred in sterilized soil amended with 10 oospores/g, and very severe disease occurred in sterilized soil containing 100 oospores/g. Conversion (change of thick walled spores to thin ones) and germination of oospores were studied with the use of an agar-slide technique, in which oospore-amended agar was solidified as a film on a glass slide. When agar-slides were placed in soil, oospores began the conversion process immediately, and some thick walls were converted to thin walls after 24 hours of incubation. Most of the oospores added to soil had thin walls after 16 days of incubation. Oospores did not germinate in soil during the three month incubation period. P. ultimum oospores in soil immediately adjacent to the primary root of cotton germinated within 48 hours after cotton seeds were planted. Germination occurred at a greater distance (within 1.6 mm) from secondary root surfaces. All germ tubes observed grew toward root surfaces. Roots were mostly infected in the root hair zone and occasionally the root tips. Germ tube infection of the region of elongation was rare. The agar-slide technique was very efficient for studying oospores of Pythium in soil, and could be adapted for determining the fate of propagules of other plant pathogens that cause root disease

Specificity of soil-borne pathogens on grain legumes

Specificity of soil-borne legume pathogens on pea, lupin and faba bean is currently investigated in fields where grain legumes are intensively cultivated. The study has so far lead to the following conclusions -Legume host-pathogen interactions demonstrate specificity of pathogen populations particularly in pea and lupin. -A. euteiches rot root was specified to pea in Denmark as root rot symptoms and oospores of the pathogen never were observed in roots of faba bean and lupin -F. oxysporum followed by F. solani were most frequently isolated from plant roots in plots highly infested by lupin pathogens -F. avenaceum was most frequently isolated from plant roots in plots highly infested by pea pathogens -Pathogenicity tests showed F. solani followed by F. avenaceum to be the most pathogenic Fusarium species on pea while F. avenaceum was the most destructive pathogen on faba bean. In contrast F. avenaceum was non-pathogenic on lupin