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The effects of sublethal heat treatments on heat resistance were studied forFusarium oxysporum f.sp.dianthi (Fod) andVerticillium dahliae (Vd), one isolate of each pathogen. Treatments of propagule suspensions of Fod at 55°C and of Vd at 45°C for 30 min were survived by less than 0.001% and 0.01% of the propagules, respectively. Pretreatment of suspensions of Fod at 45°C increased survival of the 55°C treatment up to 0.73% of the propagules and pretreatment of suspensions of Vd at 40°C increased survival of the 45°C treatment up to 0.40%. Induction of heat resistance was dependent on duration of the exposure to the sublethal temperature. With Fod, this duration was shorter for propagules from old cultures than for those from young cultures. Pretreatment at 45°C of a suspension of an 1-week-old culture of Fod induced resistance when lasting 30 min or longer, but not when 20 min or shorter. With Vd, the duration of the pretreatment inducing heat resistance depended on type of culture — white or black — due to differences in microsclerotia formation. Implications of induced heat resistance for control of plant diseases by thermotherapy are discussed.

Potential antagonists ofFusarium solani f. sp.pisi (Fsp) were selected from soil samples with varying degrees of receptivity to this pathogen. They were tested against Fsp isolate 48 (Fs48), in increasingly complex systems. Most species testedin vitro were able to antagonize Fs48. No relation could be establishedin vitro between the receptivity of the soil from which an isolate originated and its antagonism to Fs48. In soils naturally infested with pea root rot pathogens, which were stored humid at 4°C for a period longer than a year, various isolates ofFusarium, Gliocladium andPenicillium spp. were able to reduce root rot. After sterilization of these soils, onlyGliocladium roseum isolates, added at 105 conidia g-1 dry soil, significantly reduced disease severity and prevented root weight losses caused by Fs48 at 104 conidia g-1 dry soil. In soils in which the biota were activated by growing peas before the assays, doses of 106 and 107 ofG. roseum were required to reduce root rot. In these soils, the antagonistic effects of fluorescent pseudomonad strains from soil of low receptivity to Fsp were variable. Some strains of fluorescent pseudomonads, from soil moderately receptive to Fsp and from highly infested soils, were also able to reduce root rot. Disease suppression by pseudomonad strains was more evident in the absence than in the presence ofAphanomyces euteiches in the root rot pathogen complex. The role of receptiveness of the soil with regard to potential antagonists is discussed.

Despite the tremendous economic impact of broomrapes (Orobanche spp.) on agriculture in many countries little is known of the pattern of genetic variation within this group of parasitic weeds. The present paper describes the use of RAPD markers for the study of five Orobanche species in agricultural fields in Israel. Pronounced genetic differentiation was found between the species, and RAPD markers were raised for the identification of each of them. Southern-hybridization patterns of RAPD products of the various species were used to confirm the interpretation. The same markers were valid both for broomrapes collected in agricultural fields and for those collected in natural habitats. The validity of the markers found for O. cumana and O. crenata was confirmed on plants of the same species that were collected in Spain. Parsimony analysis of 86 RAPD characters produced a tree that clearly distinguishes between the five studied Orobanche species, separates the two Orobanche species belonging to sect. Trionychon from those belonging to sect. Osproleon, and supports the separation of O. cumana from O. cernua and of O. aegyptiaca from O. ramosa.