Sheath brown rot of rice caused by Pseudomonas fu.';covaginae has been described in areas where low temperatures occur during the rice booting and heading stages. To analyse the relationship between pathogenicity of P. fuscovaginae and low temperatures, pathogenicity process in rice at booting stage was studied in a growth chamber at midrange and low temperatures. Analysis performed at 13 C, 18 C and 23 C in two rice cultivars showed that pathogenicity of P. fuscovaginae was explained by the general model of the independent action. The inoculum dose necessary to obtain 50% of diseased sheaths decreased with increase of temperature. Analysis of m planta bacterial population dynamics and mean response time pointed out that low temperatures affected pathogen multiplication in host before and after symptoms development. In consideration of our results, it was concluded that low temperatures acted negatively on the pathogenicity process of P. fuscovaginae. Therefore, occtirrence of P. fuscovaginae in areas where rice cultivation is restricted by low temperatures can not be explained by a direct effect of temperatures on pathogenicity. Zusammenfassung Pathogeoititeveriauf von Pseadomanas fuscovagimu, Ursache der Stengelbraunmule des Reises Die Stengelbraunfaule des Reises, verursacht durch Pseudomonas fuscovaginae, kommt in den Anbaugebieten vor, wo niedrige Temperaturen wahrend des Bootings und im Stadium des Rispenschiebens herrschen. Um die Zusammenhange zwischen der Pathogenitat von P. fuscovaginae und niedrigen Temperaturen analysieren zu konnen, wurde der Pathogenitatsverlauf im Reis im Bootingstadium bei mittleren und niedrigeren Temperaturen in Klimakammern untersucht. Analysen, die bei 13,18 und 23°C in zwei Reissorten durchgefQhrt wurden, zeigten, daB die Pathogenitat von P. fuscovaginae durch ein generelies Modell der unabhSngigen Aktivitat beschrieben werden konnte. Das ftir einen 50% igen Befall der Blattscheiden notwendige Inokulum wurde durch eine Erhohung der Temperatur reduziert. Eine Analyse der in planta bakteriellen Populationsdynamik sowie der durchschnittlichen Reaktionszeit zeigte. daB niedrige Temperaturen die Vermehrung des Pathogens im Wirt sowohi vor als auch nach der Symptomauspragung beeinfluBten. Die Ergebnisse deuten daraufhin, daB niedrige Temperaturen einen negativen EinfluB auf den Pathogenitatsverlauf von P. fuscovaginae austiben. Von daher konnen direkte Temperatureinfltlsse auf die Pathogenitat das Vorkommen von P. fuscovaginae in den Regionen, wo niedrige Temperaturen den Reisanbau limitieren. nicht erklaren. U.S.Cop,righ,Cta.™n» Center Code S,.»,n=«: 0931-1785/96/4410-0425 S 1 1.50/0
Mail.). The parasite can adjust its pathogenicity by means of two mechanisms: one is at the level of ascospores differenciation, the other is the result of heterogenicity among the spores resulting from vegetative multiplication of the fungus. These two mechanisms participate in the initiation and spreading of epidemics. This study will deal with the second mechanism.The following material was used: 1) host range (Table I) : two wheat cultivars, two series of triticales and one rye cultivar; and 2) fungal isolates : isolate 6 (wheat adapted), isolate 42 (triticale adapted), monopycnidiospore cultures derived from 6 and 42 and subcultures derived from a single spore, isolate collected in Lyon's area-strain "Lyon" " (Rh6ne-Poulenc), isolates collected in Israel: strains IS, and IS 2 . -The following measurements were made: 1) thousand-grain-weight (PMG) or reduction of thousand-grain-weight (RPMG), 2) rate of spread of foliar necrosis ('P maJ .3) rate of spread of epidemics.The selection pressure by host-plants was obtained with 1-4 passages through the host, using inoculation of detached seedling leaves. The parasite was re-isolated from pycnidia and inoculated on ears.Significant differences in aggressiveness, as expressed by thousand-grain-weight, were observed between pycnidia cultures isolated from the same strain (Table II), between monopycnidiospore cultures derived from a same pycnidium (Tables III, IV) or between 2 generation monopycnidiospore subcultures (Table V). Similar observations were made with tests on Triticale "Clercal" (Table VI).The difference between the dynamics of strains 6 and 42 was revealed by two tests: Fig. 1. : on two wheat lines, the one susceptible, the other with a high partial resistance level; and Fig. 2. : on two susceptible wheat cultivars, according to the number of dissemination and contamination periods.Variability of aggressiveness of pathogen strains towards their own hosts, was also expressed by thousand-grainweight in response to selection pressure by several alien hosts (Table Vlla). The range of isolates is not the same after host passaging (Table Vllb). The particular effect of genotype and cytoplasm was illustrated with series of triticale: one isoplasmic with various genomes, the other isogenic and alloplasmic (Table VIII, Fig. 3).The effect of number of passages (1-4) of strain 6 through an alloplasmic list of three Triticale, observed either on two susceptible wheat cultivars (Fig. 3) or on passaging host (Fig. 4), showed that the increase in pathogenicity occurred from the first passage onwards.Concerning the partial resistance we observed that the passage of strain 6 through the Triticale "Clercal" reduced the incubation time (LPI) and increased the rate of spread of foliar necrosis (P max ) 'These parasite variations led to an increased pathogenicity on susceptible varieties, but the behaviour of lines with a high level of general resistance remained constant (Table IX). ).All observations confirm that every Septoria nodorum isolate is a population possessing ...
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