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Research was initiated to determine interactions between Drechslera sorokiniana and Curvularia geniculata in a potential disease complex on Poa pratensis. Inoculations with D. sorokiniana resulted in the greatest disease severity at 20 and 25 °C: disease severity was minimal with C. geniculata inoculations at all temperatures. Inoculations with the combination of both organisms displayed competitive and synergistic interactions that were temperature dependent. Combination inoculations reduced disease severity below that produced by D. sorokiniana alone and increased disease severity above that produced by C. geniculata alone at 20 and 25 °C and was indicative of a competitive interaction. Inoculations with the combination of the organisms increased disease severity above that produced by either organism alone at 30 °C, indicative of a synergistic effect at higher temperatures. Reisolations of the organisms from lesions produced from inoculations with the combination of the organisms indicated that C. geniculata competes more successfully for infected tissue than D. sorokiniana but is dependent on lesions initiated by D. sorokiniana to establish successfully. Competition for tissue colonization by C. geniculata is greatest at temperatures of 25 to 35 °C. It was concluded that C. geniculata should be recognized as a weak, high-temperature, primary-leaf infecting pathogen of P. pratensis, that a true ‘disease complex’ relative to disease severity can exist between the organisms at 30 °C that is dependent on lesions initiated by D. sorokiniana and reflects the high-temperature aggressiveness of C. geniculata, that tissue invaded by both organisms is more successfully colonized by C. geniculata at higher temperatures, and that the necrotic portions of lesions produced by D. sorokiniana, and on which this organism does not sporulate well, is the probable site of establishment for C. geniculata where the organisms are found together.

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Drpo1.tr7irrrt t~fHortic~rrltrr,r. lo11.o Stote Urric~t~r~.sity. Arrrrs. I A , U . S . A . 50011 Hooc;t:s. C. F.. and J . P. M A D S~N . 1979. Leaf senescence as a factor in the competitive and synergistic interactions of Drc,c,lz.sl~~.o .sorokirrirrrltr and Crrrl,rrlorio gerric~rrlotcr on Potr pr.otrrr-.\is. Can. J . Bot. 57: 1706-17 1 I.The competitive and synergistic interactions of Dr.rc~lr.slrra .sorokiriiorro and Cr~r~rrlor~irr grrlit,rrloto on pathogenesis of leaf spot was evaluated on sequentially senescent leaves of Port ~~rrrtrr~.si.s. Leafage had no effect on the mean ni~mberoflesions produced from inoculations with D . soroki~lirttrct or C. g~t~i ( , i i l~l o alone. The combination of D . sor~okir~iorro and C . ,qt,t~it.rrl(rtti producetl 21 bignificant increase in the mean number of lesions between the youngest and oldest leaves at 20.25. and 30°C. Disease severity on leaves of all ages resulting from inoculations at 20 and 25°C was greatest in response to D . .so~.okirlitrtro alone. followed by the combination of organisms. and by C. ,qrr~ic.rrltrto alone. in descending order. No difference in disease severity occurred on leaves of different ages in reponse to the inoculations at 20°C. but a significant increase in disease occurred on the two oldest leaves at 25°C in response to inoculations with D . .ror.oki)liotim and to the combination ofolganisms. At 30 and 35°C. disease severity on leaves ofall ages was greatest in response to inoculations with the combination of organisms. D . sor.okirritriltr alone, and C. goricrrlotrr alone, in descending order. Disease severity increased from the youngest to the oldest leaves at 30°C in response to inoculations with the combination of organisms or with 0. .so~.okiilitrrro alone. Reisolations established that inoculations with D . .sorokitricitro alone resulted in successfirl establishment of the pathogen in the tissue of leaves of all ages at 20.25. and 30°C. At these temperatures, inoculations with C . grrric.rrloto alone produced a significantly higher level of reisolation from the youngest to the oldest leaves of the shoot. Reisolation of the individual organisms from leaves inoculated with both organisms showed that C. gc,rricllltrt~ established more successfillly than D . .sorokirritrrrtr in leaves of all ages as temperatures were increased. It was concluded that the synergistic increase in disease produced by the combination of I). .sor~okir~ittrlo and C . gt,r~icrrltr/ri at high temperatures is the result of increasetl high-temperature growth of C . gcrlicrrlottr which is probably more saprophytic than parasitic on senescing. heat-stressed leaves and does not reflect a high-temperature increase in virulence of this organism.